Many people who come to see me at the Build It Live exhibitions are keen to learn the best way to heat the houses they’re about to construct.
Quite understandably, they want to be eco-friendly and reduce their impact on the planet – but they’re faced with a raft of competing information from suppliers about very different systems. These include heat pumps, woodburners, biomass boilers and solar panels.
Many self-builders are also concerned with cost, of course; though this isn’t always the top priority. The conversation will normally start with people telling me they’re considering installing some sort of renewable technology – but they’re not sure which will suit them best and want impartial advice.
My first move is to ask whether they have access to mains gas. If the answer is yes, then I’ll tell them to stick with a gas boiler. At this point, eyebrows are often raised. So in this article, I’m going to explain why somebody who has built eco homes, writes books about sustainable houses, and has generally been there and got the T-shirt when it comes to eco construction, would recommend this route.
It’s possible to design and build new houses in such a way that they need little or no space heating. This is due to an improved understanding of heat flows through materials, better design details and more effective insulation.
As our demand for space heating falls ever lower, the environmental impact of providing that energy on a day-to-day basis becomes very low – even when added together over time. The influence of the heating system itself and its installation thus becomes relatively much higher in terms of sustainability over its life cycle.
Some years ago I visited an eco-development in Devon where the architects told me that they could have heated the entire terrace of houses (super-insulated with Warmcel) using a single gas combi boiler. They had of course put a boiler in every home, as we Brits are not very good at sharing, but the point was well-made.
So, to put it bluntly, if your household only has a very low heat demand – and this should be the case for anyone building a new sustainable property – then ask yourself: why go to the bother and expense of installing something like a ground source heat pump (GSHP), when you can do the job much more simply and cheaply with a gas boiler?
The most common reason given for wanting to install a heat pump is that it will supposedly save on carbon emissions. Putting aside for a moment the fact that a heat pump will have a much higher embodied energy impact than a gas boiler, we need to examine the case purely based on running the two systems. To do this requires delving into some pretty dry sums, so bear with me.
Gas is a fossil fuel. It emits 0.185kg of carbon dioxide (CO2) per kilowatt-hour (kWh) of heat generated. If we allow for distribution losses, this could be increased to 0.203kg/kWh. To meet a heat demand of 10,000 kWh per annum using a 90% efficient modern gas boiler would thus generate 2,255kg of CO2 emissions per annum.
Heat pumps, on the other hand, are powered by electricity. This is much more carbon-intensive than gas, and it is still made largely by burning fossil fuels – so there are conversion losses.
Allowing for these and losses in distribution, electricity emits an average of 0.527kg of CO2 per kWh of energy delivered to the consumer.The attraction of heat pumps is that they generate several units of heat for every unit of electricity input.
This ratio is known as the coefficient of performance (CoP). An appliance with a CoP of three will deliver three units of heat output for each unit of electricity input.Manufacturers will often indicate efficiencies much higher than this, and in ideal conditions they may indeed be able to achieve these.
However, in field trials by the Energy Saving Trust, the average efficiency of air source heat pumps (ASHPs) was well below 2.5 and GSHPs were under 3.0. In both cases this assumes that the pump is supplying hot water as well as space heating (we’ll come back to this later).
Putting all of this together, to meet a heat demand of 10,000 kWh per annum using an air source heat pump with a CoP of 2.4 would thus generate 2,195kg of CO2 emissions per annum. This is almost identical to the figure we just calculated for a gas boiler – so there are no tangible savings here.
Ground source heat pumps are slightly more efficient than ASHPs because the source of warmth (the ground) is at a higher temperature than the air during the heating season. This means GSHPs offer a better CO2 saving in use. However, the embodied impact of fitting a ground source heat pump is much higher, as is the cost of fitting it.
There’s an alternative in the form of biomass boilers. These offer much lower in emissions than gas appliances or heat pumps as the fuel itself can be considered low carbon – though only if the source is being replanted.
There are also emissions associated with processing and transporting biomass, which is a much more difficult fuel to move than mains gas or electricity. So unless it is grown, processed and replanted on site, it is not carbon neutral.
Another way people expect a heat pump will benefit them is by delivering savings in running costs when compared to a conventional gas boiler. The price of electricity and gas varies widely by region and according to what tariff you are on.
In general, however, electricity costs about three times as much as gas per kilowatt-hour. A good guide price for gas is 4p/kWh, while electricity might be around 12p/kWh.
Based on the figures in our example above, using a gas boiler to deliver 10,000 kWh of heat would cost £400 per annum (£0.04 x 10,000). By comparison, using an air source heat pump with a CoP of 2.4 to deliver that same amount of heat would cost £500 per year (£0.12/2.4 x 10,000), which is more than the gas boiler.
The cost of running a biomass appliance will vary according to the fuel used: logs are cheapest; then chips; then pellets. It also depends on the degree of competition in local supply chain and the volume of delivery. As a rule of thumb, running costs are likely to be similar to or slightly higher than natural gas, so no gains there.
The caveat to this comes if your installation is eligible for the Domestic Renewable Heat Incentive (RHI), which pays out a sum per kWh of energy produced. The RHI only lasts for seven years and is normally viewed as offsetting the capital costs of renewable systems, which are much higher than mains gas boilers. If you have free fuel from your own woodland, that changes the equation, of course.
It might surprise you to know that money is not a bad guide to environmental impact. If you’re spending it, then you’re consuming resources, one way or the other – for instance on materials, transport etc.
A gas boiler might cost £2,500 installed, while an air source heat pump or a biomass unit is likely to come in over £7,000 including fitting, and a ground source heat pump will cost well over £12,000.
This would imply that a gas boiler uses less resources to construct and install than an air source heat pump or a biomass boiler. While this is probably true, gas boilers are common use while the market for heat pumps and biomass boilers does not yet have anything like the same volume of installations per year or the same level of competition. So, the difference in embodied impact is not as great as the price differential might indicate.
Gas and biomass boilers are very good at raising the temperature of water (hence the name). A heat pump, meanwhile, is fine for achieving the lower-grade warmth required for space heating – especially in tandem with UFH (which runs at around 35°C-40°C).
But it becomes much less efficient when asked to raise temperatures up to those we need for domestic hot water (55°C -60°C). This is the water we consume in bathing, washing and showering. Depending on the proportion of the appliance’s output that’s used for this, it can have quite an impact on the overall efficiency of a heat pump system.
Given the relatively low space heating requirement in modern houses, the hot water demand is becoming increasingly significant as a proportion of total heat energy demand (after all, we are not taking fewer showers).
Gas appliances are relatively efficient at this side of things and, unlike biomass boilers, a gas combi can provide hot water without storage on demand – avoiding the standing heat losses associated with using a storage cylinder.
At present, gas boilers still score best on capital costs and embodied environmental impact. They are also similar to heat pumps and biomass boilers in terms of running costs, while in terms of carbon emissions they are similar to heat pumps but not as good as biomass.
As heat demand is so much lower now, the overall difference is marginal and the higher maintenance associated with biomass boilers offsets this lower carbon impact to some degree. So gas appliances will be much cheaper to maintain and to replace when the time comes around. Overall my advice remains if you have gas, stick with it.