From the archive: Use the sun to heat the house

man and collectors 400px

In ReNew 116 we look at a variety of hydronic-based heating systems. In this article from ReNew 95, Michael Harris gives a good overview of hydronic space heating systems that use evacuated tube collectors for solar boosting. We hope it gives good background to the hydronic heating feature in ReNew 116.

The idea of using the sun to provide heating for your house is very attractive. In the southern states of Australia space heating is the biggest energy user and in country properties heating can be very expensive if you are using bottled gas and hard work if you are using firewood. Solar energy is free, and produces no greenhouse gases. It could be a great solution.

The increasing use of hydronic heating has the potential to make solar heating easier. Hydronic heating systems distribute heat through a house by running hot water through pipes to radiators in each room or to coils in the concrete slab. It is easy to shut down the radiators or coils that you do not need and the system can be quite efficient. The water can be heated by a gas or wood-fired boiler or by solar.

So the solution to your heating needs sounds simple. Put in a hydronic heating system and bung some extra solar panels on the roof. Whoopee, we have solar heating. However, unfortunately it is not that easy.

For many years solar water heating in Australia has been done by using flat plate collectors. These collectors are basically an insulated box with a glass top and a sheet of metal with pipes attached inside the box. The sun shines, the inside of the box gets hot, the sheet of metal gets hot and the pipes containing the water get hot. It works well when it is sunny.

However it does not work so well in cloudy conditions. And when it is winter and it’s cold, when you need the system to perform at its best, these collectors provide very little energy input. Although the flat plate collectors would give you some benefit, the cost of the extra collectors far outweighs the benefits.

So what has changed?

Affordable evacuated tube-based collectors have come onto the Australian market. These work differently to flat plate collectors and are much more efficient in cold and cloudy conditions. The tubes have a double glass wall like a thermos flask. In between the two walls is a vacuum which is an excellent insulator and minimises heat loss. On the outer wall of the inside tube is a selective surface which maximises the absorption of solar radiation. When faced north the curved outer surface of the tubes will effectively collect heat from the sun at all times of the day because reflection off the glass surface is minimised.

So you end up with a number of benefits; a selective surface that absorbs more heat, a vacuum that stops that heat escaping and a round surface that reduces reflection hence collecting more heat. The result is a solar panel that collects more heat, especially in winter.

How it works

At the time of writing, one green plumber had been installing systems using the Sunplus CPC Solar brand of evacuated tubes. The typical system consists of eight 12 tube panels, a 1200 litre stainless steel tank with two heat exchange coils, a controller and solar circulator pump, mixing valves, expansion tank, a combination boiler to back up the system, and the components for the hydronic system (pipes, pump, valves and radiators or floor coils). A system of this size would be capable of heating a 25 square house.

When operating, the sun heats the water in the pipes in the evacuated tubes. A sensor detects when the water reaches the appropriate temperature and switches on the circulation pump. The pump circulates hot water to the heat exchange coil in the bottom of the storage tank. As the water heats up it expands and pressure in the circuit builds up. Rather than vent the pressurised hot water (which would waste water) the pressure is taken up by the expansion tank.

A coil in the top of the tank heats up with the water in the tank. When the pump for the hydronic heating system is turned on, heat is transferred to the water circulating around the hydronic heating circuit. If the solar system does not heat the water in the hydronic loop adequately, the boiler comes on and boosts the temperature. Domestic hot water for general household use can be heated by a separate heat exchange coil in the tank.
A critical aspect of the system is the mixing valves. The evacuated tubes are capable of generating high temperatures. Reliably regulating these temperatures is essential for both safety and reliability reasons. Boiling water can burn someone who touches a radiator, crack a concrete slab with embedded heating coils, or kill the boiler that boosts your system.

What does it cost?

Hydronic heating systems are not cheap. As a rule of thumb, the cost per radiator (including piping) is about $1000. The alternative, an in-slab floor coil, can be around $4500.

The solar components to heat a 25 square house would work out something like this; evacuated tubes $9000, custom-built tank with coils $4500, and combination boiler $2500. Installation and miscellaneous hardware add about $2500. So you can be looking at total costs for the hydronic heating of between $6000 and $10,000, and the solar heating system could be close to $19,000.

Some government rebates also apply to these systems. The solar heating system above would receive a $1500 rebate in Victoria, bringing the cost down to around $17,500.

Although this is quite a lot, remember that these systems also supply domestic hot water. A solar water heater typically costs around $5000 so the actual additional cost for the solar boosting of the heating system may only be around $12,500.

Savings

So is it worth it? To answer that, you need to take into account the life of the system. The chief cost components—the evacuated tubes, tank and piping—should have a very long life, 20 to 30 years would not be unreasonable. The pumps and valves may need replacing during that time but they are a relatively small part of the cost.

Running a hydronic heating system such as this would be likely to cost around $600 per annum in the city or about $1700 per annum in the country using bottled gas. This means the heating costs over 20 years in the city would be $12,000. In the country it would $34,000.

If the solar boosting provided half of the hot water needs then it would save $8000 in the city and $22,644 in the country. The savings are even greater when you add the domestic hot water savings and take into account the likely increases in energy costs over the next 20 years.

Performance

These systems are new in Australia so there has not been enough time to see if they will deliver what they suggest. But the results look promising. Lets look at the experience of some people who have put these systems into their homes.

Gordon, from Arthur’s Creek in Victoria, has installed a system with 15 panels of six tubes each, connected to a 1100 litre storage tank. Last winter his gas boiler was consuming two bottles of gas every five days, at a cost of $160! ‘When we realised how much it was costing us to run our heating we stopped using it. We only turned on the heating when we were desperate.’

Gordon’s new system was installed last spring so it has not yet had the chance to run through a winter. But based on how it’s been performing it looks promising. ‘The system started to operate in spring when the daily top temperature was typically 18 to 20 degrees. On the first day the tank temperature went from 15 to 45 degrees and within a few days was over 80 degrees and went off scale on the temperature gauge. Ever since it has been boiling and sitting at close to 100 degrees Celcius.’

Mitch from Seymour has put in a system to assist the heating of his 65 square house. He was first inspired by an item in ReNew on evacuated tubes. Later he was staying in South Georgia (near the Falkland Islands) and was astonished to see the very same evacuated tubes on the British Antarctic Service buildings. South Georgia is close to the Antarctic and experiences very low temperatures and strong cold winds. Mitch reasoned that if these evacuated tubes worked there, they would certainly work in central Victoria. His system uses 18 six-tube panels, storage tanks with a capacity of 1,400 litres and a combination of floor coils and radiators to heat the house.

Glen from Greendale installed a small system with six panels with six tubes each, and an 880 litre tank. Glen wanted to test his system performance so he installed sensors in the tank and panels. The system was installed in September and was providing plenty of hot water for domestic use, (the hydronic system was not being used because it was summer).

During Christmas Glen went away on holiday. When he came back the system was not working. He checked and found the sensor in the tank had melted and popped out of the tank and the collector sensor had not only failed but the heat shrink on it had melted off. He has since killed several more sensors while playing around to see what temperatures he could get from the system.

While the thermal performance of evacuated tubes in cold conditions and low light conditions suggests that these collectors may make solar hydronic heating a viable option, we will need to see how these systems perform through a full winter. It is not hard to get hot water in summer—winter is the real test.

It is also important to remember hydronic heating systems need to be installed by an experienced professional and adding a solar component increases the complexity of the system. Readers thinking about trying this kind of system should be cautious and do their homework.

This article was first published in ReNew 95

EOFY ReNew 2017

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