Farmhouse solar hydronics

Sth view of finished collector rack400px

In issue 116 we visit Ian Hill’s 1970s home which has been retrofitted with solar-powered water and household heating. Here’s is a detailed version of that article, with more of the nitty gritty on system design for those about to embark on such a project.

Nearly nine years ago we made a tree change to active, semi-retirement. We bought a farm in West Gippsland, left behind seaside Frankston, and went niche beef farming for a change in lifestyle. We’re happy to say it was a good decision.

The farm came with a large home—16 rooms over five levels with two open-plan living and entertaining areas—the main selling point being we liked this style years ago.

The three concrete slabs stepping down the rolling red soil hills already had hydronic in-slab tubing, heated from a diesel furnace along with the tap water. Cooking was done by bottled gas, and there were three slow-combustion wood heaters in the two living areas.

Philosophy and design

We are very keen on sustainability and want to minimise our carbon footprint, both in the home and our farm’s beef production. We were prepared to spend some money converting the heating system for a large reduction in running costs and emissions. The farm has many large trees and limbs are always falling, so using solar-powered heating and hot water, boosted by a wood-fired heater, seemed like a sensible idea.

We found the solar collectors we wanted and set system parameters. Our plumber designed and built the conversion, changed the skylights, re-flashed the house and updated much of the water collection. We added ideas as it was built over several months.

The home

The climate is cool temperate with few frosts and the house is sited on a southern slope in foothills.

We are at latitude 38.005°. There is some unavoidable morning shading in winter from a roadside glider possum habitat of magnificent eucalypt trees over 40 meters high, 30 meters away and uphill on the northern road boundary. The outside temperature ranges between 2°C and 42°C but the house has such a large thermal time constant that living areas stay between 17°C and 22°C in winter, and no more than 25°C after a run of hot days.

The house design is classic 1970s double brick with rough-sawn, exposed beams in eight meter cathedral ceilings.It had hardwood french and other windows with single-glazing throughout, and sad plastic-vented skylights. Everything was coloured mission brown.

The designer ignored the 16 kilometre view to the Strzelecki Ranges and the valley below, and modern principles of house alignment for passive heating and cooling. However, at least the sun does not load up the interior. There are a few, small windows to the east, with excellent shading from canvas blinds, and large french windows to the west. These are shaded by a pergola and close battens. The home’s north face is stepped into the hill and the windows are totally shaded by a brick cloister with archways: a very sensible design providing a great spring breakfast area.

The kitchen, living and lounge rooms, study and billiards rooms are open plan and interconnected on three levels, which does create air currents, especially with such high ceilings. We have been slowly renovating, as one does with a retirement income.

The aim is to convert major glass in living areas to high-efficiency glass and to install as much double glazing as we can afford. So far our plumber has retrofitted seven double-glazed, openable skylights. The local glazier replaced 11 clearstory windows and made three panels openable to draw up cooling air from the lower levels when a summer cool change arrives.

We use pressurised tank water for most of the home, buildings and farm animals. There are 245,000 litres available in concrete tanks, linked by a 50mm buried ring main. Our local irrigation contractor built a fire sprinkler system for all buildings on the farm, which was essential when the wind changed during the Black Saturday bushfires in 2009. Our 100-year average rainfall is 1100mm; we received 1178mm in 2010 so we always have an excess of stored water.

There is a 1.5kW solar power system on the roof, bringing an income and well offsetting the minor energy drain from the small pumps moving water into the hydronic heating and up to the solar collectors.

Hydronic system components

Our heating system has three sources of heat:

• 36 solar-collecting vacuum tubes (rated at 5.8kW)
• wood-fired, slow combustion fire, with flue water jacket
• two gas-fired instantaneous hot water boilers.

The service courtyard where most of this hybrid heating and hot water system is hidden looks Heath Robinson, but it is a credit to our local green-accredited plumber at Baw Baw Plumbing and his team. He always knows about the latest efficiency innovations and was a terrific speaker at our Landcare group’s Green Energy field day.

Heat storage

A custom made 1000 litre stainless steel tank with 75mm of insulation and a small header tank is the heat storage. It’s similar to those made for the local dairy industry. Hot water is not drawn from this water but via three heat exchanger coils in the tank, with the solar one at the base, the hydronics one at the centre and the hot water service at the top. Each is 11 metres long.

Solar collectors

I contacted eight retailers advertising evacuated tubes. Disappointingly, not many responded.

From sellers’ claims, the most efficient collectors I could find were Ritter (labelled APR), a Chinese-made German design imported by Sunplus CPC. We bought 1.6m long evacuated tubes in banks of six, with parabolic mirrors to direct extra sunlight under the tubes. Our budget limited us to triple the normal number of tubes recommended for hot tap water and a 1000 litre hot water storage tank.

The collector bank of 36 tubes is fixed to a 1.6m by 4.4m aluminium frame facing 15° west of north. I asked for it to be tilted steeper than the roof’s 17° at a calculated winter solstice angle of 60° to collect maximum energy for winter. This reduces excess summer yield and steam problems.

Importantly, the plumbing route for the tubes allows us to add more down the track.

The north roof with evacuated tube collectors for the heating system and a 1.5kW solar power system.

Inside the solar control and storage area

Solar-heated water pumping

This is a rainwater-filled closed loop heat-exchanger. Water from the storage tank coil is lifted about five metres up to the solar array by a 3-speed 30 watt 240v hot water pump with throttling valve, giving infinitely variable flow. It usually runs at 0.2 bar boost. A Zilmet model 20013 50 litre cylinder stores system over-pressure up to four bar from summer days, backed up by a blow-off valve to save water loss on hot days.

Relief valves

Four high spots in the solar array and wood heater circuits have auto air-bleed valves, allowing only air and steam to escape.

Wood-fired slow combustion heater

We changed the third wood heater to a gas unit for quick response to a cold home.

After the first winter with the new system, an existing free-standing Saxon unit in the living room was retrofitted with a 550mm tall stainless steel heat exchanger in the first part of the flue. It burns quietly from late autumn to early spring on wind-fallen mountain ash and blackwood harvested around the farm. Water to the flue exchanger is drawn from the base of the storage tank and delivered back to the top. Piping is about 25 metres long and rises about 3 metres. It is insulated with 25mm thick foam tubing and cased in colorbond. A 240v thermostat in the output pipe in a wall behind the heater senses output temperature and controls another small circulating pump at the storage tank, moving two litre slugs of hot water at 50 °C into the storage tank every few minutes. This heater provides around half our total hydronic heating in winter.

Gas boilers

Tap water is delivered via an instantaneous Rinnai V1500 gas boiler which adds heat if stored water is not 50°C. There are no adjustments for the home owner. Electronics in this unit can be damaged by our emergency home generator, so we cannot run the hot water when mains power fails, which it does for several hours at least four times per year.

Hydronic water supply is delivered to the mixing valve via a Sime Format 34e instantaneous gas boiler, rated at 11.2kW to 34kW, large enough to heat the whole home on its own. It adds heat if needed and has user-adjustments for output temp (set to 35°C). Its instruments display output temperature and pressure. The pump within this unit is also triggered by the thermostat in the master bathroom, sending heated water to a Hydrotherm P-600 Platinum tower rail, 2.2m by 600mm wide, helping provide some extra hydronic heating to the bathroom.

Both boilers stay on in summer as they do not use any gas unless heating water.

Heat users

The house is heated by hydronic coils in five zones in three concrete slabs at descending levels in the house, plus a fan-assisted radiator in the living room. Two manifolds are fed from a mixing valve, and water circulated by five, 240v Grunfos thermostatically-controlled 3-speed pumps.

We have only activated the outer coil on the lower slab coils. We are very fortunate that it flows via the master toilet and bathroom, laundry, kitchen, two guest bedrooms and to the living room on the lowest slab.
Hot tap water runs throughout the house with all piping insulated with 25mm thick foam tubing. External piping is further encased in 90mm stormwater piping.

Water delivery controls

Hydronic water is blended by the original tempering valve supplying two hydronic manifolds. Tap water is held to 50°C by a Reliance Heatguard Ultra tempering valve. This setting can be altered.

The three room thermostats in the home are very clever Honeywell model CM 907. They can be programmed in time blocks for every day of the week, can be over-ridden for one time block, set to a fixed temperature and adjusted for daylight savings. The lower slab thermostat in the living area also masters the upper slab in the entertainment area. The second thermostat in the upper level study controls the mid slab. The third thermostat in the master bathroom controls water to the towel rail.

Operation
Solar control

The electronic differential controller, made by Whitnic Services of NSW, gets its data from 10volt thermistors, one at the array output and one at the storage tank top. It has three modes and a red light indicates the pump is on, which I positioned to see from the back door.

Gas supply

Gas was originally supplied by a bank of 40kg cylinders. These were replaced by a 190kg truck-filled tank, with pressure reducers at two boilers, and a circuit supplying the guest kitchen and fast-response gas heater in the living room.

Owner adjustments and monitoring

I wanted to monitor input and tank temperatures, so I bought three $10 electronic indoor/outdoor thermometers with remote sensors and mounted them next to the differential controller. I can feel the input arriving from the solar array, with one attached to the lowest hot connection on the storage tank, indicating roughly how much hot water is in the tank, and the other reads water delivery to the taps. These have max/min displays as well, useful for checking array performance or pump adjustments. An old clock-type dial indicator measures the temperature of water returning from the hydronic system, a rough indication of how much heat is in the slabs.
The electronic gauges are particularly useful to know how much heated water is available for a big load such as a spa fill or running the lounge room radiator. Monitoring incoming temperatures from the array allows me to tune up the flow rate for best performance just below steam occurring, and tells me if we have any problems when it’s pumping. An improvement would be digital readings from the differential controller’s thermistors.
We can adjust slab heating times in two zones and towel rail temperature, and boost heat in the lounge room by activating the fan-assisted radiator. We can control the temperature of the water leaving the fire water jacket. We cannot alter the temperature trigger points for the solar array. It might be useful to keep it pumping above 80°C to stop a steam blockage occurring.

Current settings

Thermostats have six available time block settings, with the initial settings for the slab thermostats listed below:
TIME      TARGET TEMP
6am          20°C
8                18°C
Midday   18°C
5.30pm   18°C
8.40pm   13°C
10.30pm 8°C

When there’s a run of low solar-energy days we run the wood fire hotter. When there’s sunny days predicted, we can use less wood, or not light it.

As autumn starts, we open the hydronic valves and drive the wood heater hard to put as much heat as possible into selected slabs prior to cold snaps and overcast winter days. On a run of overcast days we open the damper on the wood heater.

Fine tuning and problems

We run the collector pump at the lowest of three speeds and fine-tuned the flow to 1.5l/min on the advice of the plumber. We’ve learnt that in summer we need to double the flow rate to avoid excessive pressure build-up.
The original thermistor on the solar array burnt out after one year and the surrounding insulation was charred! The new importer tells me the replacement thermistor is a tougher type.

Anything that stops the circulating pump while there’s sun on the vacuum tubes can create a blockage in the circuit that the circulating pump cannot overcome. When the thermistor on the roof fried, and when we lose power when the sun’s on the tubes, pressure builds up and the closed loop finally drops below it’s 0.2 bar pre-set pressure. This stops circulation for that day and we lose a little water as steam. When the pump is alive again it fails to get water circulating if the array is in sun. So if the system pressure gauge is zero, I know circulation has stopped and must be topped up. To fix it we fit the garden hose onto the fill point just below the pump, and run cold water until there are no bubbles passing the sight gauge. Our plumber has suggested an automatic supply for this.

Maintenance

Particle filters in the inlets to the tap boiler and both tempering/mixing valves need to be cleaned annually, the latter by removing the fitting gland, which is not a good design.

The Zilmet pressure storage tank needs its quiescent air pressure checked annually, and the whole tank replaced every five years. Pressure cylinders on my Citroen last indefinitely, with re-gassing, so we’ll see. The system needs to be de-pressured for accurate pressure checks.

The solar collector array needs to be hosed periodically to remove leaves.

Costs

Our gas costs about $730 for 550 litres per year, but my urban mate pays a fraction of our price! We really only use significant gas when we have guests, then it goes through the litres when the large boiler is doing a lot of the home heating. We average about 66 mjoules of gas per day in winter, and as little as 19 at other times. The Elgas truck doesn’t come from October to late April. In 2010 we used half the gas of 2009, mainly due to better windows and remembering to keep bedroom doors closed. We will get further significant reductions when our window conversions and internal glass partition are finished.

Total changeover cost, including towel rail and some bathroom alterations, was about $11,500 against an estimated $17,000. The local shire gave us a rebate of $250 and we received another $6900 in rebates. If hydronic slab heating was built into a new home, it may not be any more than other hot water and heating systems. Our 44 RECs were not sold because the supplier did not have an approved system with the tank size we used, so we missed out on around $1500. That’s a little plus for the environment as energy companies had to find an extra 44 RECs somewhere else.

Changed family habits

The dog is often asleep on the hottest sections of the hydronic loop, always in doorways or on the top of stairs. The cats love the laundry benches in winter.

To minimise the generation of greenhouse gas and gas bills we use most of our hot water first thing in the morning, giving the solar array the first opportunity to recover hot water lost. We built a wooden, pull-down rack below the laundry ceiling which now dries much of our cold weather washing.

We need to shut off the hydronic valve when spring is well-entrenched and must remember to open it when the first cool weather is predicted after Easter.

What next?

We are part-way through replacing most open-plan area windows with double glazing, with low U and SHGC value glass and argon gas in the space.

At the moment glaziers are installing a glass, openable air barrier at the top of the living area. This will zone the home into separate living and entertaining zones, reducing wood demands and cold air currents up the kitchen.
Stopping heat escaping is next. After a government-funded home assessment, this air entrapment work was to be financed by the now defunct Green Loans scheme. Another task is resealing all doors, and chasing air leaks along the brick-ceiling interfaces throughout the living spaces and external walls. This is to stop bushfire embers and smoke ingress; the home is to be a refuge as we’ve spent a lot of money on a 10-hour fire sprinkler system for all buildings.

I’m also planning to have the roof re-pointed; it’s amazing how much heat escapes from the fabric of the cathedral ceiling when you remove a capping tile on a cold day.

Much of the living room slab could be heated, in cooler weather, by direct sunlight, and possible when we replace dark green fibreglass on the pergola outside with clear sheets and retractable shade cloth.

I’d like an automatic system to over-ride the pump control in the main gas boiler, so the rail can be heated when the slab hydronics are off. This will probably involve some extra 240v relays to override the pump’s under-temperature and gas supply controls, which stop the pump when the hydronics are not on.

Due to firebox corrosion we will soon replace the wood heater. The next one will have a wet-back for more hydronic capability.

Suppliers

Green-accredited plumbers—Baw Baw Plumbing, Buln Buln East

Solar equipment suppliers—Phazer, Warragul

Glaziers—Walkies’s windows and glazing, and Warragul glass and glazing

Flue heat exchanger, gas room heater—Cosy heaters, Warragul

Monitoring thermometers—Dahlsens, Warragul

Fire protection system—The Farm Depot, Warragul

Gas heater installation—West Gippsland gas services, Warragul

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