Beat the winter chills: A guide to electric heating options
As we head into the colder months, thoughts turn to staying warm. What’s the best electric heating system for you? Lance Turner looks at the options, and their pros and cons.
This article was first published in Issue 144 (July-Sept 2018) of Renew magazine.
In past issues of Renew we have focused on what are arguably the two most popular energy-efficient heating options—reverse-cycle air conditioning and hydronic heating. Both have advantages and disadvantages, and both suit some people, house designs and climates better than others, so which is best? Are there other options that should be explored? When it comes to heating, there are lots of questions to answer, and making the right choices is important for a comfortable, warm home with low running costs and low environmental impact.
Firstly, we should note that we are not considering gas heating. Gas is a fossil fuel and there is simply no way to run gas appliances without greenhouse gas emissions. On the other hand, while electricity is in part generated by coal and other fossil fuels, it doesn’t have to be—you can purchase 100% GreenPower or install a solar power system large enough to cover your needs throughout the year and effectively be greenhouse neutral.
The economics of gas heating also no longer stack up in almost all cases. See Renew’s latest research report on this subject.
Now that is out of the way, what are the electric heating options available? Firstly, we will look at the two technologies we have covered previously which tend to be used for space (whole-of-house) heating—reverse-cycle air conditioning and hydronic heating, then we will look at resistive electric heaters, solar air heaters and other heating considerations.
Reverse-cycle air conditioners
Reverse-cycle air conditioners work by compressing a gas, called a refrigerant, which then transfers heat from one place to another. The technology that does this is called a heat pump. Heat pumps are all around us; for example, in your fridge, a heat pump transfers heat from inside the cabinet to outside, which is why the outside of the fridge gets warm. In a reverse-cycle air conditioner, the transfer can go either way, hence the name. In winter, heat is taken from outside and dumped inside, and in summer the opposite occurs.
A big pro for heat pumps: efficiency
The amazing thing about reverse-cycle air conditioners is how efficient they can be. While electric heating using resistive elements to turn the electricity into heat directly (covered later) can only ever be at most 100% efficient, the heat pumps used in reverse-cycle air conditioners are much more than 100% efficient, in fact, up to 600% efficient, meaning that they use a lot less energy to produce the same amount of heat. How can that be? As its name suggests, a heat pump pumps heat from one place to another. Instead of turning energy from one form (electricity) into another (heat), it uses electric energy to move heat from one place to another. Because heat is relatively easy to collect and move, heat pumps can move a lot more heat energy than the electric energy they use.
The efficiency of heat pump systems is given by a coefficient of performance (COP). This is a ratio of the heat moved to the electrical energy input. For example, if your heat pump uses 1 kWh of electricity to move 4 kWh of heat from outdoors to inside your home, then it has a COP of 4. Note that when a system is cooling a home, its cooling efficiency is referred to as its energy efficiency rating (EER)—EER and COP are effectively the same thing, but the ratings can be different for heating and cooling, hence they are specified as separate figures.
Reverse-cycle air conditioners can have a rated COP as high as 5.8. The actual running COP depends on numerous factors, including the temperature differential between outdoors and indoors, the refrigerant and compressor type used and overall system design.
Low outdoor temperatures can have a negative effect on efficiency when heating. Some heat pumps can have their COPs reduced to low levels (less than 2 in some cases) as the ambient temperature approaches 0 °C. If you live in an area that sees close to zero winter temperatures, make sure you check the efficiency curve (a graph of COP versus ambient temperature for a given output temperature) if available. Some manufacturers will just supply COP for several outdoor temperatures.
Split vs multi-head vs ducted
Reverse-cycle systems come in two main types—split systems and ducted.
Split systems have the compressor and one set of refrigerant coils in a box outside, often mounted against a wall. The part inside the home is called the air handling unit and consists of the other set of coils, a fan to force air over them and the electronic controls for the system. Multi-head units are a variant, with a single (larger) compressor sized to run multiple indoor air handling units. The outdoor unit and indoor units are linked by flexible or rigid high-pressure hoses or pipes.
Ducted systems have one (or sometimes more) large air handling units that can service a whole house, with conditioned air being ducted to different rooms of the house via insulated ducts. The compressor is still separate but is much larger than the units used for split systems (in some cases it may be integrated into the unit with the air handling unit, the whole system mounted outside at ground level).
Splits are generally the most efficient option. Multi-head splits are less efficient due to a number of factors, not least of which is that the single outdoor unit on a multi-head system must transfer more heat but is unlikely to be twice as large as an outdoor unit for a single head system (a common factor among all manufacturers). The efficiency of ducted systems is generally lower than split systems due to thermal losses in the ducts and other technical reasons—larger systems generally have lower COP than smaller systems.
A potential disadvantage with the single split system approach is that you need an outdoor compressor for every room you want to heat/cool. This can mean many outdoor units for large homes with many rooms, which can be aesthetically unappealing and take up a lot of outdoor space. However, many homes can manage with just two or three units so this may not be a problem. It’s also the reason they are more efficient, so there’s an advantage to the separate units as well!
Multi-head and ducted systems get around this problem, but, as already noted, at the cost of reduced efficiency/lower COP. Thus, they’ll generally cost more to run.
Another advantage with split systems (single and multi-head), aside from efficiency, is that of zoning, as you’re only heating the rooms or areas occupied at the time. With smaller individual split systems you have complete control over which ones are running, and you can even select different temperatures for different rooms—great if one occupant feels the cold more than another.
Ducted systems vary in their zoning ability. In the simplest systems, you can simply close off the vents you don’t want running, but a minimum number should be open for efficient operation. Also, ducted systems will tend to have one or two thermostats located in the zones most likely to be heated, such as lounge rooms, meaning rooms further away from the thermostat may not get as warm as desired. Ducted systems are generally less flexible than multiple splits, although modern ducted systems with electronic controls and more than one thermostat help compensate for this.
Another advantage with split systems is that you can start small, such as heating just one or two rooms, and add more units as budget permits. With a ducted system, it’s all or nothing, with a large upfront cost.
Ducts are prone to vermin damage or coming adrift, greatly reducing efficiency. Also, should the system fail and require repairs, the whole home is without heating, whereas with individual splits, the worst you will see is one or two rooms unheated.
Other advantages of split systems over ducted systems are that the air handling unit only needs a couple of small holes in the wall for piping and cabling, and the separation of indoor and outdoor units eliminates leakage of heat from the hot side to the cool side, thus improving system efficiency.
But will it keep me warm?
One of the questions many people considering a shift to reverse-cycle split systems ask is whether it will keep them as warm as, say, a ducted heating system or a hydronic system. The biggest perceived negatives for using a reverse-cycle system for winter heating is whether the movement of air has a cooling effect, reducing the effective comfort of the heater. This applies to any form of convective heating, but is perhaps exacerbated with some split systems with less than ideal outlet design which causes high air flow velocities.
This causes many people to set the heating level to higher than they otherwise would, increasing energy use. However, there is one, often unconsidered, advantage of this constantly moving air—it makes for excellent clothes drying when clothes must be hung up inside. The air handling unit will also remove the extra humidity produced—something other heating systems don’t do.
Many manufacturers have addressed this issue, at least to some degree, with wider and/or multiple air outlets with lower air velocities and directional outlets which allow airflow to flow down along walls and floor or across the ceiling (for cooling—what some manufacturers refer to as the Coanda effect, which describes how airflow attaches itself to a surface and flows along it, even if that surface curves away from the airflow). A minimally restricted path will allow airflows to reach further across a room, more effectively distributing warm (or cool) air. By reducing airflows directly onto occupants by flowing air along surfaces, the undesired cooling effect of moving air when heating the room is reduced. By selecting the correct air handling unit and location for it, you can reduce cold spots or effects.
Many units also feature adjustable airflow patterns for when cooling in summer—random airflow changes are supposed to simulate the natural variations of a cooling breeze.
Split unit placement
Air handling units are usually ‘wall hung’ but there are other types, including floor-mounted and ‘cassette’ types mounted in the ceiling. If your needs are predominantly for heating, then you should consider an air handling unit that is designed to be mounted on or near the floor. This allows the heated air to flow across the floor, providing immediate warmth. The same applies if you go for a ducted system—in-floor ducts will warm you more quickly than ceiling ducts, although many homes don’t have underfloor cavities for ducting.
The main disadvantage with floor-mounted air handling units is that they generally have a lower overall system efficiency than wall-mounted units, but it isn’t clear why this should be the case—there is no technical reason for it as the floor-mounted indoor units are at least as large or larger than their wall-mounted cousins, so heat transfer and airflow rates should be at least as good if not better. We have been unable to find a valid explanation as to why this is the case, but suspect that, because wall-mounted units are by far the most popular, they have been developed further than the other designs for maximum efficiency. This is most likely also why they are cheaper—they are manufactured and sold in much greater numbers.
Will it be noisy/blowy/run all the time?
Virtually all high-efficiency split system and many ducted heat pumps are of the inverter type—which means that instead of the compressor motor simply being on and off (remember the clunk when your old box air conditioner switched its compressor on and off), the compressor is controlled by a variable-speed drive or inverter. This allows the compressor to only run as hard as required, making the system more efficient and reducing electricity use.
Inverter-based systems are generally less noisy than simple on-off types, and indeed noise levels of indoor units on many, if not most split systems is very low, barely above a whisper, although this will depend on fan speed. Many range from 20 decibels on low, up to 40 or 50 decibels or more. Of course, with ducted systems the air handling unit is well away from the rooms, either in the ceiling or outside, and so is not really audible.
Outdoor units (where the compressor resides) vary in noise level, so compare the noise level ratings on potential purchases. Noise levels for outdoor units can also be reduced by smart placement (away from bedroom walls, for example) or by surrounding the unit with greenery.
The harder the unit has to work, the more noise it will make, so don’t buy a system that is barely adequate for your needs as it will be running hard a lot of the time. Setting the temperature set-point higher than necessary also means the unit will run harder and more often, and will also waste energy and cost you more to run.
By the same token, if your house is a thermal poor performer and allows excessive heat flows in and out due to draughts and poor insulation, the heat pump will run harder and more noisily than necessary, wasting money in the process—improving the home’s thermal efficiency can provide many advantages!
Other features to look for
All reverse-cycle air conditioners can include features such as long-life filters that only need washing every six months, air ionisation (to disinfect the air), high efficiency fan designs to reduce energy use and fan noise, remote controls with timer functions, adjustable airflow patterns, economy modes, infrared sensors to reduce operation when rooms are empty, humidity sensors, wi-fi or bluetooth connectivity for smart home integrations, and many other features, which may or may not be of use to you.
Some features, such as humidity control, can be worthwhile if you find that air conditioners dry out the air too much for you. The ideal humidity range is between 40% and 60%, and there are air conditioners designed to address this issue, such as the US7 (Ururu Sarara 7) range from Daikin. Generally you shouldn’t have to think about this, just turning humidity control on means the heat pump will do what’s necessary to maintain a desirable humidity level.
An interesting feature on the Daikin Nexura is a radiator panel on the front of the indoor unit. This heats up to emit a low-level warmth similar to a hydronic radiator, so the indoor unit provides both warm air and some radiative heating. However, the radiator panel is relatively small and only heats to 55 °C, so the radiative effect will not be as high as a full-sized hydronic radiator or electric radiant heater.
Another useful feature is an inbuilt timer. Most units have these now, and they allow you to turn on heating before you wake up, or turn on cooling before you get home from work. Many systems only have one set of timer set-points, so you can’t set different times for different days, such as setting it to turn on later on weekends to allow for sleeping in, or multiple time events in a day. If control flexibility is a requirement, then you should check the details of the timer’s capabilities.
Smart devices that have internet connectivity will also tend to have more flexible control systems, and you may be able to program multiple timers or have the system react to other parameters, such as starting up depending on the indoor temperature. These sorts of capabilities vary widely and can also be enhanced by the use of external controllers and services, so if fine-level adaptability and control is a requirement, you will need to do some homework.
You can even upgrade an air conditioner with only basic timer capabilities with a smart remote such as the Sensibo, giving you multiple timer cycles; see our article on p. 47 in ReNew 144 for more.
However, features such as timers and internet programmability are not necessarily a positive for energy reduction, potentially allowing the system to run for longer each day. While a smart system can let you turn off the heating from outside the home if you forget when leaving, it also encourages householders to turn on their systems before they get home—although if this ‘pre-heating’ is covered by solar and the house is well sealed and insulated, it could be a smart choice.
Improving heat pump efficiency
While a good quality reverse-cycle air conditioner will have high COP and EER ratings, these only hold true if the installation of the unit is well considered. The smaller the temperature differential between the indoor and outdoor units, the more efficiently the system will run and the less energy it will use to move a specified amount of heat.
For example, if the unit is primarily used for heating, then it will perform better if the outdoor unit is located in full sun, so it can harvest as much warmth as possible. To maintain summer efficiency, which would otherwise be compromised by the sunny location, planting a small deciduous shrub or tree in from of the unit will shade it in the hotter months while allowing it to be warmed by the sun in winter.
Filters inside the air handling unit should be cleaned regularly, although some systems are now self cleaning or have filters that only need to be washed every six months. They can usually just be washed with warm soapy water, rinsed and dried; instructions will be in your unit’s manual, or it’s worth getting a primer on this from your installer.
Heat pump hydronic heating
Hydronic systems consist of a water heating unit (called the boiler) to heat water, and one or more pipe circuits which have the heated water flowing through them. Each circuit incorporates one or more radiators, which may be wall-mounted radiators or in-floor pipe coils, which emit warmth into the room.
Many boiler systems include a storage tank for thermal storage and to provide a buffer, which allows the boiler to run flat out and then turn off for a while. However, some boilers are tankless in that there is no storage tank (although there is usually a small expansion tank to allow for expansion of the water as it heats): they simply heat the water as it circulates through the system.
Hydronic boilers can use a variety of fuel types—there are gas boilers, electrical resistive heating and heat pump. There are even solid fuel systems, which use wood or wood pellets.
The cleanest and most efficient boiler type is the heat pump, which is what we cover here. Like reverse-cycle air conditioners, they heat the water by harvesting external heat, so run at greater than 100 % efficiency, and can run as high as 400 % (a COP of 4) or more.
Most hydronic systems have multiple circuits, so you can heat all or only part of a home, allowing you to leave unused, closed-off rooms unheated to reduce energy use.
Water is circulated through the system using low-pressure pumps, and circuits are turned on/off by electrically operated valves, usually controlled by an electronic controller. The controller enables a system to be programmed to heat certain parts of a home at particular times—for example, heating the living areas during the evening and the bedrooms just before bedtime.
Because the heat from hydronic systems is either underfoot or close to it (through the use of skirting radiators or panel radiators mounted at floor level), you get the feeling of warmth with lower ambient room temperatures than with space heating. In addition, because there is generally very little air movement with hydronic heating, the potential cooling effect of airflows produced by convective heating such as reverse-cycle air conditioners is virtually eliminated.
Like reverse-cycle air conditioners, some heat pump hydronic systems can also provide cooling in summer. However, not all heat pump systems can do this, so if you desire cooling then you will need to select the appropriate system components (or rather, your system designer will). This may make for a more expensive system as the heat pump will need to be the reverse-cycle type, but many systems can perform cooling without additional costs, except maybe a fan to circulate cooled air in each room. Cooling with hydronics is best for in-floor systems but may also be done with radiator-based systems. Alternatively, you may choose to move away from a hydronic system and use reverse-cycle air conditioning instead, which can readily cover both cooling and heating.
Just add hot water?
Some hydronic boilers also provide domestic hot water, eliminating the need for a separate water heater.
An interesting aspect of hydronic systems is the wide variety of radiator types available. These include the traditional standing radiators, towel rails, bench seats, skirting boards, mirror surrounds, in-floor trench convectors (radiators embedded in steel boxes fitted into the floor, covered by a grille, used where above-floor radiators are not suitable), ceiling radiators, and even decorative wall art.
Because heat pump systems tend to run at a lower temperature (usually 60 °C or so compared to 70 °C or higher for other heat sources such as gas), radiators may need to be larger for a heat pump system.
Perhaps the biggest disadvantage with hydronic systems is the upfront installation cost. Even a small complete system won’t come in under $10,000, with larger, more complex domestic systems costing up to $30,000. Costs depend on a number of factors, including the type of boiler, whether it is a tank or tankless system, the number of circuits, the complexity of the control system and the type and position of the radiators used. The cost is likely to be higher if fitting a hydronic system as a retrofit to an existing home, as pipe runs can be difficult to install due to lack of space.
Prices have dropped in recent years due to increased competition as hydronics have become more popular, but larger heat pump systems can easily exceed $20,000, and ground-source heat pump systems (where heat is extracted from the relatively stable temperature of the earth) can be considerably more than this.
Hydronic heat pump boilers usually have lower COPs than reverse-cycle air conditioners, because the temperature differential between the outside ambient temperature and the target water temperature is usually higher compared to the temperature differential experienced by reverse-cycle units (efficiency of a heat pump decreases as the differential in temperature increases—see Figure 1). This is partially offset by the improved perceived levels of warmth from hydronic systems over reverse-cycle air conditioners.
Consider the complexity
Hydronic systems can be quite complex, and this is a big part of why they are so expensive—it’s not just material costs, but also installation costs that make them so. The pipes, pumps, valves and tanks need to be located somewhere they aren’t visible, and for smaller homes this may be a problem unless the system was planned for in the house design and a space was set aside for this purpose. To neaten things up, pipework can be located inside a cupboard or a dedicated ‘partage’ box.
Another issue, that only applies to in-slab systems, is thermal lag—the time it takes from when you turn on the heating until you start to feel the warmth underfoot, or from when you turn the system off to when it cools down and stops heating the room. This occurs because of the high thermal mass of a concrete slab, which takes quite a while to heat up or cool down. So if you are away from the house for most of the day and only need heating for an hour or two in the morning and evening, an in-slab system may not be a good choice. People with in-slab systems tend to leave them running all the time due to the lag, and may even resort to opening windows to cool the house off, so often use heating more than they need to. A system using wall or skirting radiators allows you to turn the heating on and off at will and get heat within a minute or so from these types of radiators. And don’t forget that in-slab systems need a fully-insulated slab, both underneath and around the edge—otherwise, energy loses to the outside air and soil under the slab can be considerable.
You can have underfloor hydronics even if you don’t have a slab. In this case, you have hydronic coils placed in aluminium heat spreader boards, which have insulation underneath. These can be placed on any sound subfloor, with your choice of flooring placed over the top. The Tiemme H28 system is an example of this sort of product.
Configuring a system
Radiators run at higher temperatures than in-slab heating, so a system using these will be configured differently. In-floor heating usually uses water temperatures up to 40 °C or so, whereas wall-mounted radiators may run at 70 °C or more (or 60 °C for heat pump based systems). It is possible to have a mixed system with both water temperatures running in the same system, but not all boilers and system designs can accommodate this. Your hydronic system installer will be able to provide more information on mixed temperature systems.
Because heat pump efficiency drops off at the higher temperatures required for radiators, the most efficient heat pump based system is one that uses in-floor heating and runs at the lower temperatures. That’s not to say heat pumps can’t be used for systems using radiators, but some installers may prefer to do only in-floor heating using heat pumps.
There is also the possibility of adding solar thermal boosting, by adding roof-mounted collectors that provide a proportion of the water heating, while the rest of the heating is done by the heat pump. Bear in mind, though, that heating is required at times of the year that provide the least solar input, so a solar system really can be thought of as a solar-assisted system, and the heat pump may well do the majority of the heating.
If you already have a gas hydronic system in place but running costs are too high due to it being an older, inefficient system, then you should be able to have your system’s boiler replaced with a heat pump. Just select a boiler with a heat output similar to what you currently have. However, the lower temperatures from a heat pump boiler may mean that existing radiators are undersized and may need to be replaced as well—ask your installer, they should have the experience to be able to assess this.
Resistive electric heating
Other than heat pumps, electric heating is all done using resistive elements, often combined with a fan. Electricity passes through the elements, which heat up due to their resistance to electric current. The heated elements can emit heat in the form of infrared radiation (the lower the running temperature of the element, the longer the wavelength of the infrared emitted), or a fan can circulate air over the element, expelling the heated air into the room.
Resistive radiant heaters
Heaters that work by radiating infrared are called radiant heaters, oddly enough. Radiant heaters are designed to heat you directly, much the way that sunlight feels warm.
The most common form of radiant heater is the bathroom heater, which usually combines a fan, light and heating bulbs into one unit. The glowing bar type heater, which uses a resistive wire coil wrapped around or sitting inside a ceramic rod or tube, is also very common, and both are often used for a quick burst of heat for a relatively short duration.
The main advantage of high-temperature radiant heaters is the rapid warm-up time and the lack of moving parts. Radiant heaters can operate for many years with minimal maintenance, although replacement elements are less available than they were a couple of decades ago as these types of heater are so cheap that they are usually replaced when an element fails—although replacing elements rather than the whole device is obviously a far better option environmentally.
A relatively new form of radiant heat is the far infrared heater. We have looked at these briefly previously, and there are still only a few suppliers of these heaters, with prices reflecting the lack of suppliers. These heaters also use a resistive element, but their large surface area compared to the amount of energy flowing through them means they heat to much lower temperatures, usually below 120 °C, although some can run at over 200 °C. The lower temperature means they emit longer wavelength infrared radiation that penetrates clothing quite well, heating you directly. They produce a slow, gentle heat that tends to warm you without you realising it until it occurs to you that you are no longer cold. Most far infrared heaters are in the form of a flat panel or elongated strip, but other designs are available, such as the Ionmax Ray ION801 (see Products in ReNew 144).
Far infrared panel and strip heaters are usually mounted on a wall or the ceiling, rather than being portable, so you have to fit one in each room that you want heating in. This can become an expensive proposition.
Another form of resistive radiant heating that runs at an even lower temperature is in-floor heating film. This consists of wire or plastic film which has the resistive element printed onto it. It can be laid under carpet or other floor coverings, where hydronic in-floor heating can’t be fitted. It is also far cheaper to install than hydronic heating, but usually much more expensive to run.
Resistive convection heaters
The other type of electric heating is the convection heater, which uses the element to heat a flow of air. Like reverse-cycle air conditioning (heat pump), this is a form of space heating, which is designed to heat all of the air in a room. This will take a lot longer to have an effect on the thermal comfort of a room than a radiant heater, as you have to wait for a large volume of air to be heated, all the while some heat is being lost through walls and ceiling. As the limit for a typical convection heater is 2400 watts (the maximum you can draw from a power point), resistive convection heaters will take a lot longer to heat a room than a heat pump, while using a lot more electricity in the process.
The most common electric convection heater would be the ubiquitous fan heater. These come in all sizes and shapes, and power ratings from 200 watt personal heaters to 2400 watt room heaters.
Some heaters combine both radiant and convection to give a quick feeling of warmth while slowly heating the room. Convective panel heaters are common nowadays, both in free-standing and wall-mounted units, and ‘oil’ heaters work this way—they use electric elements to heat oil inside the heater, which circulates to distribute the heat to the outer surface of the heater, radiating heat while also heating the air.
Running costs of resistive heating
All resistive-based electric heating is potentially expensive to run. One simple way to estimate running costs of heaters is by multiplying the kW power rating by number of hours used and electricity cost per kWh. For example, a 2 kW heater run for five hours at 30 cents per kWh will cost 2 x 5 x 0.30 = $3.
However, this assumes the heater is running flat out for that period. Most convection/combination type heaters have an in-built thermostat that allows them to switch off when the room reaches the desired temperature, so to really know how much energy they are using, you need to use an energy meter to measure them over a fixed time period.
Radiant heaters, on the other hand, draw their rated power continuously, so the calculation really is as simple as stated above. However, many radiant heaters have more than one element, such as the simple two or three bar heaters. To get their rated output per bar, just divide their maximum rating by the number of bars. For example, a three-bar unit rated at 2400 W would use 800 watts per bar, assuming they are all the same size (a few heaters have bars of different ratings).
Like heat pump systems, whether they be reverse-cycle air conditioners or hydronic heat pumps, resistive heaters can be used to make best use of any excess solar PV generation on a sunny winter’s day. If you live in an area that has a good number of such days, then using a heater that approximately matches the exported excess can mean you are heating for significantly lower cost. Of course, using that energy in a heat pump system of some sort is much more efficient, providing a lot more heat for the same energy use, but in some cases it may not be convenient (for instance, if you are in a room not serviced by the air conditioner or hydronic system). This isn’t likely to be that common a scenario, but as PV systems steadily get larger, many homeowners are finding that they do have an abundance of PV generated electricity at least on some winter days. Even owners of smaller PV systems can gain this advantage if they use personal heating devices rather than space heating or larger radiant heaters.
Solar box heaters
Of course, if it is cold but sunny, you are not only producing good levels of PV electricity, but there is another source of free heat available—the sunlight itself. Many homes will have suitable north-facing windows that can admit considerable light, but even a home with poor orientation and design can gather solar heat using a solar air heater. These come in a few shapes, sizes and configurations, but all work in the same way. They consist of a flat insulated box with a glass top, painted or coated with a solar absorbing surface on the inside. Incoming sunlight heats the air in the box which can then be transferred by a low-power fan into the home, either directly or via a heat exchanger.
Unfortunately, solar insolation is at its lowest at the time of year that it is needed most for heating—during the winter months. However, in places that see a lot of sunny winter days, and in other places that see relatively sunny shoulder months (late autumn and early spring), where some heating is needed, solar heating systems can make a real difference to the internal temperature of a home—provided they are sized correctly of course.
One often overlooked method of solar heat collection is the sunroom. Homes with a properly designed sunroom, that can be isolated and shaded during summer to prevent heating the home in the hotter months, can benefit enormously on sunny winter days—to the point where no other heating may be needed. When designing a home, especially in colder climes, a sunroom, preferably with openable double glazing and adjustable external blinds or shades, should be considered.
Sizing a whole-of-house system
All hydronic boilers and reverse-cycle air conditioners have a rated heating capacity (and cooling capacity, for reverse-cycle), so you need to have a basic idea of how much heat is flowing into and out of your home.
Doing such an assessment is beyond the scope of this article and is really something an energy assessor should help you with. There are many assessors available who can provide such services, and a number of online resources available to help you find one in your area, such as the ABSA website (www.absa.net.au) or the NatHERS site at www.nathers.gov.au.
However, if an assessment is not in the budget, then you can make an educated guess with a bit of basic knowledge.
For instance, if you are heating a room and find that a 2400 watt fan heater can keep up with heat losses in that room, then you know the minimum heating capacity required. Indeed, as crude as it sounds, this is actually one of the simplest ways to find out how much heat you need. Set up a fan heater or two on a cold day and see how it goes. If the room is still cold after half an hour then you have some more insulating and/or sealing to do. If it is nice and toasty warm then simply buy the most efficient system with a rated heat output of at least that of the fan heater(s).
Sizing a hydronic system can be a fairly involved task as it usually means sizing a system for the whole home while matching radiators for each room size. Hydronic system sizing is probably best left to the system designer/installer.
The most suitable heating
You want your house to perform as well thermally as possible—with minimised heat loss through the building envelope and draughts. Without retrofits many homes will only perform averagely at best—it makes good financial sense to seal as many gaps as possible, and install insulation where you are able (most easily in the roof cavity, but consider wall, underfloor and window insulation as well).
That all being done, which is the most suitable heating: convective (space) heating, radiant heating, or a combination?
Really, that depends on the occupants. Everyone has their own preference—some like the air to be warm, while others will prefer direct radiant heat. Also, some people are happy heating just the room they are in, while others, who may move from room to room regularly, may want to heat the whole house.
If you are happy heating single rooms, then a small radiant heater, far infrared heater or a high efficiency small split system air conditioner in all rooms to be heated might be the best option. For room-to-room heating, you need something that makes you feel warm quickly, so systems that are slow to heat, such as in-floor hydronics, are not going to be suitable.
For homes where the level of air changes per hour is relatively high (i.e. draughty homes), you don’t want to be doing a lot of space heating, as you lose that heated air quite rapidly. For these situations, radiant heaters may be the best option, with far infrared probably the best due to their effective warmth from relatively low power levels.
For well-sealed, well-insulated homes, space heating may well be preferred, with reverse-cycle air conditioning being the best option regarding energy use, and a heat pump hydronic being a fairly close second. For a small, very well-insulated open-plan home, a simple fan heater may be all that’s required. While they use a lot more energy for the heat provided, when the heat required is small anyway, more expensive options like reverse-cycle and hydronic systems may never pay for themselves in energy savings—simpler may be best in this situation.
In bathrooms, where heating is usually needed only for short periods, radiant heating is usually the simplest and most effective solution, unless the house has a whole-of-house ducted or hydronic system. Combination radiant heater/light/fan units have been the go-to solution for bathrooms for decades, because they are simple and effective, but they do have the disadvantages that the radiant bulbs are often used as extra lighting, and they allow draughts between the room and the roof cavity, although some (such as the IXL Tastic Eco Sensation) now have in-built draught excluders.
If a bathroom has a separate light and fan, then a simple wall-mounted strip radiant heater can provide fast warmth when needed.
Heated towel rails are another common bathroom fitting, and may be part of the hydronic system or separate resistive electric units. However, the latter should be placed on a timer as they are easy to leave on 24/7. A 100 watt towel rail will consume 2.4 kWh per day if left on.
The cheapest heating
You’ll want to consider both the upfront cost and running costs when considering which is the cheapest heating for your needs. This particularly applies when heating needs are low. Spending $2500 on a reverse-cycle air conditioner is not a good investment when a $100 fan heater will do the job—provided that heating needs are low enough such that the extra energy cost of the fan heater is around 5% to 10% of the difference in price of the two systems. Of course, you may prefer to reduce energy use to reduce your greenhouse gas emissions (assuming you don’t buy accredited GreenPower), so consider your priorities.
Even for whole-of-house heating systems, don’t forget to weigh up possible variations that may swing the decision one way or the other. For example, you could spend $20,000 on a hydronic system, or you could spend that same amount of money on several reverse-cycle split system units, plus a 10 kW solar PV system. The PV system will help offset some of the heating costs in winter, while generating a huge amount of energy over the rest of the year, producing a large amount of energy credit towards heating for the next winter. In short, the PV plus reverse-cycle will be effectively ‘free’ to run and the hydronics won’t be unless you spend extra on the PV system.
It may sound like hydronics doesn’t stack up so well financially, and in reality that is usually the case. Hydronic systems are complex, with pumps, valves, many metres of piping, and often a great deal of work required to install them. Complexity means higher cost—there is no way around that, at least not if you are paying someone else to do the installation. The one thing that often sways homeowners towards hydronics is the feeling of warmth underfoot, but it can be an expensive feeling to have installed!
However, one more thing that can go in hydronics’ favour is that hydronic systems tend to feel more comfortable at lower temperatures than reverse-cycle air conditioners as the heat is at floor level, and there is no cooling effect from air movement. With heat pump hydronics, you might also be able to access a cheaper off-peak tariff to heat the water, at least for part of the day. This will depend on your system’s design and your energy company’s tariffs, but it’s something to look into when weighing up whole-of-house heating options.
Even cheaper heating
So, you think you have found the cheapest form of heating, but have you really? One area many people overlook is that of personal heating—heating house occupants, not the house itself. Radiant heaters are a step towards this, but they still heat all the objects in the room. The ultimate low-cost heating is purely personal heating, where the occupants use devices to keep only themselves warm.
There are active and passive methods for this: active meaning small personal heaters, such as heated garments or heated blankets, while passive methods simply mean rugging up. In this modern world of amazingly warm textiles, it is perfectly reasonable to keep yourself nice and toasty using nothing but your own body heat.
For one ReNew reader’s interesting experiments with personal heating for his entire family, see the article ‘Heating people, not spaces’ in ReNew 144.
Rental homes often perform poorly thermally, and while some landlords are open to improvements, many are not, so how can renters stay warm without spending a fortune?
Many of the options discussed above apply to rental homes as well. Obviously, renters need to take a more focused approach to heating themselves, and personal heating is certainly the cheapest option. Small personal heaters can also work, as can far infrared heaters. It’s relatively simple to attach one of these panels to a free-standing frame for use as a semi-portable heater.
If you tend to spend a lot of time in one room, you may choose to simply heat that room and nowhere else. In this case, insulate the room as well as possible. Thick rugs on the floor and full-length wall drapes covering as much of the walls as possible will help retain heat. Adding bubble-glazing (bubblewrap taped to the glass) or another form of insulation to windows will also dramatically cut down heat loss. Finish off with draught excluders on doors.
While renters can’t install split system air conditioners unless the landlord allows it, they can still gain some of the efficiency benefits of reverse-cycle air conditioning with the use of a portable air conditioner. While these usually have lower COPs than a good split system, they can still reach COPs of around 3 if used correctly. They only require an openable sliding window (sash windows are best) to install the outlet duct and water drain hose. Portable reverse-cycle air conditioners usually come with an adjustable plate or seal system designed to let these pipes exit a window with minimal air leakage. Once fitted and with the window closed as far as possible, judicious use of closed-cell foam allows you to seal the window pretty much completely. Some units even come with flexible seal systems that allow you to use the units with other window types, such as hinged windows.
Note that portable air conditioners usually have only one large outlet duct—the air exhausted from this duct actually comes from the room. This also reduces their possible efficiency. There are some (a very few and hard to find) portable units that have two ducts, to allow air from outside to both enter and exit the unit for heat transfer. These work much like a split system, except that both halves of the unit are indoors, and they tend to have the highest COPs of all portable units.
We should mention that some ‘reverse-cycle’ portable air conditioners are in fact not. They will have a heat pump cooling system, but heating is done with simple resistive elements. These are easy to spot—if the energy consumption is the same as the heating output when in heating mode, they are using resistive heaters. However, true reverse-cycle units are readily available and cheap—one large online retailer has reverse-cycle portables with outputs up to 4.1 kW of heating for around $500. Of course, reliability is hard to know, so do your research and look at reviews, especially from those who have had the units for some time; Choice has reviewed these types of unit. Also check noise level ratings, as portable air conditioners can be quite noisy.
Like any appliance, heaters can pose a hazard if used incorrectly, and there are certain rules that you absolutely must adhere to when using certain types of heaters, especially high-temperature heaters.
Firstly, keep heaters far enough away from combustibles that there is no chance of the heater setting anything alight. This includes when used near furniture—your dog can easily knock a cushion off the couch onto a radiant heater. It sounds unlikely, but it happens (I’ve seen it), so never underestimate the possibility of fire risk. Never leave a high-temperature radiant heater running unattended. Also avoid high-temperature heaters when you have children or pets, as they can easily cause burns in an instant to the unwary. Use low-temperature heaters (with outlet air temperatures or surface temperatures below 50 °C) or wall-mounted heaters out of harm’s reach.
Also, never cover any heater with clothing or towels, even low-temperature panel heaters and oil-filled heaters, as they may overheat and damage the heater itself (depending on how well the heater is designed), even if no fire occurs. The only exception is those designed for the task, such as heated towel rails, or heaters that have a fixed temperature heat source, such as hydronic radiators.
Basically, heater safety is just common sense—assume that worst possible scenario is just waiting to happen, and make sure you avoid it!
This article was first published in Issue 144 (July-Sept 2018) of Renew magazine. Renew 144 has a focus on smart home technology and smart heating options.
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