Renewable heating

So far, in my bid to have a modern twenty-first century lifestyle that's leans towards being sustainable, I've acquired solar panels to supplement my electricity consumption, and I have an electric car in order not to burn any more fossil fuel than I have to while getting about.  Now I want to take the next step because I want to deal with our house's heating.

Here's the picture thus far. We live in an average house built around 1970 when insulation was barely thought about. It was extended in 1990. We have fully insulated the loft and the roof space above an extension room. Our floors are not insulated and, because access to the underfloor space is extremely difficult, the only way this is going to happen is to empty all the downstairs rooms, lift their carpets and then their floorboards. Where we can, we have had the cavity walls insulated.

16.5 years ago, we installed a 24kW Vokera Linea combination boiler and water heater. This burns gas to drive a central heating system of 13 radiators on a hot water circuit; pretty standard stuff in the UK. One of those radiators is permanently switched off. The water heater comes on when it senses demand, burning gas to heat water as required. Because of this, we dispensed with the hot water storage tank that the house had at that time.

The boiler has been annually serviced by British Gas on a Homecare agreement since it was new and it is reasonably reliable. However, I'm not prepared to wait until it fails on me before getting it replaced because that will force me to make that replacement rashly. In the knowledge that the boiler is approaching the wrong end of the 'Bathtub Curve', we have decided to start planning for the changeover now.

For those unaware of the bathtub curve, this is a theoretical curve that plots the reliability of complex mechanical systems as time goes on. At first, there is likely to be a rash of failures. These will mostly be due to manufacturing or installation defects and it is these that the one-year guarantee is meant to catch. It is the steep end of the curve but then we enter a long period of reliable operation. The systems are doing what they were designed to do, within the timeframe they were expected to do it. But as they age, the probability of failure gently begins to rise as parts wear out or extreme circumstances exceed the ability of the system to cope. The shape of the graph is like the cross-section of a bathtub.

In the case of the boiler, heat exchangers are likely to corrode, gas burners may clog up, sensors might fail. Moreover, an old boiler is never as efficient as a new one, given the improvements in boiler design over the years. The Vokera Linea was 77.5% efficient when new. A new Worcester boiler is over 90% efficient.

The big question then is how to upgrade the system. There are lots of options but I'm tending to focus in on two.

Boiler changeover

The easiest solution is simply to swap my old combi boiler for a new condensing one. The Worcester brand seems to have gained a high reputation in this regard. If I do follow that path, then I'll likely follow the advice I've been given to get one that has a 30kW rating instead of my current 24kW. The feeling seems to be that the Vokera is struggling, given the size of the house.
Although a new condensing boiler will reduce the gas we consume for heating, there are solutions that can take this reduction much further. Most seem to be impractical for our house but there is one that might work.

Solar thermal

This consists of an array of evacuated glass tubes mounted on a roof. The tubes house collectors for the Sun's infra-red output and the resulting hot fluid usually heats a storage tank of water. That's as fine as far as it goes but it would mean installing a new storage tank. Further, it would not be capable of heating radiators and, worse still, it works well at the wrong time of year. From May to October, when the Sun is most capable of heating water, I've got no need for it because my central heating system is shut down.

Environmentally sourced heat

The environment around us is brimming with heat energy. If it wasn't, we'd all be frozen blocks at zero Kelvin (-273°C). Even on what we call cold days, there is plenty of heat around us. It is just that it is low-grade warmth. If we could gather this energy and concentrate it, it would provide an inexhaustible source of heat for our homes. Amazingly, we can achieve this by using the same principle of heat energy transport that is used in refrigerators and air conditioning units. These work by having a compressible fluid and a compressor. The fluid flows around a circuit absorbing heat in one place and giving it up in another.

If a gas is compressed, it will heat up. A good example is a bicycle pump with the outlet blocked. As a volume of air, maybe 25cm long, is compressed into a much smaller volume, the end of the pump can become seriously hot. This is because the quantity of heat that allowed that air to be at, say, 15°C ambient temperature has been forced into a much smaller volume. The energy is concentrated and the temperature will rise, maybe enough to scald.

If this hot, compressed gas is allowed to cool to ambient while being held compressed, then what happens when it is allowed to expand out to its original volume? The small quantity of heat energy remaining is spread through the larger volume and as a result, the air's temperature will fall precipitously.

Refrigeration works in the same way. A compressor squeezes a fluid which warms up as the heat within becomes concentrated. That warmth can be given up to the room via a radiator. The fluid is then allowed to expand within the fridge or freezer's interior. It cools markedly and begins to absorb the heat within before being recompressed and giving up that heat to the room. By the same principle, we can capture heat from the inexhaustible supply in our environment, and transport it into our houses. There are three common ways to achieve this.

1. If a body of water, preferably flowing, is to hand, then huge quantities of heat can be extracted from it. Water has an astonishing capacity to store heat energy, even when it is cold. Unfortunately, no such river runs near to our house.

2. Were the land around the house to be blessed with great green lawns, we could install a network of pipes beneath the surface that would be 'warmed' by the low grade heat within the ground. An appropriate fluid can be used to transport or pump that warmth into the home. Unfortunately, we do not possess a suitable amount of land, and anyway, such 'ground source heat pump' systems can be expensive to install.

3. Like the water and the land, the air around us can be used to supply sufficient heat to warm a house. Furthermore, it is easily accessible to an 'air source heat pump'. I'm going to investigate this source of heat as a possibility for our house.

Air source heat pump

On my Facebook feed, I've been seeing adverts from a Glasgow heating company pushing air source heat pumps so I got in contact with them looking for information on boiler replacement. The first guy that answered immediately launched into a sales spiel and I ended up with a quote for a replacement Worcester combi boiler for £3,384, including the replacement of two radiators in the coldish extension room. This seems a little high for me, but I pressed on.

As I began to query him about heat pumps, I learned some interesting stuff: A system would cost in the region of £8,000-plus. However, the UK and Scottish governments are keen to promote this technology to reduce carbon emissions and have a system of 'Renewable Heating Incentives' (RHI) which are paid out quarterly to offset the overall cost. Further, they offer interest-free loans to help with the initial outlay. It was suggested that the two schemes roughly cancel out as the RHI can cover the loan repayments and so an air source heat pump system can be installed for about £2,000. This is promising.

I was then passed to a gentleman who had a much more practical tone instead of the sales hyperbole and he explained the system to me a second time. As best as I can make out, the Daikin system they sell consists of two units. An external unit looks like an air-con right down to the large fan that dominates its volume. The fan draws air through a heat exchanger, cooling the air and warming the refrigerant which is then compressed to concentrate the heat. The extracted heat is then pumped into the house. There, it can be used to warm the central heating circuit. The system also has an internal unit that essentially consists of a compatible combination boiler so that if there isn't enough heat coming from the heat pump, it can be topped up from the burning of gas.

We now have an appointment in a week's time to assess the house and give us an accurate quote for the installation of an air source heat pump system.

Since then, I've gone searching for more information on YouTube and managed to learn a few things that may or may not be relevant. Apparently, because the warmth from the heat pump is at a lower temperature, the house needs larger radiators to get this heat into the rooms. I'll need to see if this applies to the Daikin system or whether the gas can be used to supplement the temperature.

Some of the videos I've watched show an external unit that is rather large – about the height of a man. As we would want to position ours over a narrow path, we will need to see if the Daikin unit is as large and if it can be mounted 2.5 metres above the path to maintain access.

Finally, I'm not sure this is going to be cheaper to run in terms of my fuel bill. While the gas bill drops, the electricity bill must rise. It takes 1kW of electricity to produce 2.5 to 3kW of heat. But electricity is 3 to 4 times more expensive than gas per kilowatt. At least some of the electricity would come from the solar panels.