Looking for a setup to deliver on all your space heating and hot water needs? Here are some renewable options
If you're building your own home, you have a great opportunity to create something that's sustainable to construct and live on. One of the key considerations will be how to keep the house at a consistently comfortable temperature (as well as provide sufficient hot water). A variety of renewable options are available, but for many of us the goal will be to find a solution that's as simple and cost-effective as possible.
Going fabric first
A good first step is to reduce heat loss to an absolute minimum. Many would agree that the best solution for a comfortable, efficient living environment is to build in such a way that a house requires as little heating as possible.
It's actually possible to achieve levels of insulation and airtightness that are so high, you can rely on the warmth generated from normal occupation (ie from our bodies, cooking, lighting etc) to maintain a pleasant temperature. In this scenario, the provision of fresh air becomes a significant source of heat loss - so you'll need to provide domestic hot water.
Relatively few of us will be self building to Passivhaus or similar ultra low-energy standards, however - so specifying the right heat source becomes a critical choice for most projects. A gas-fed boiler can be a cost-effective and surprisingly low-carbon option. But if your property is off the grid or you want to fulfil demand with a renewable heat source, there are a couple of strong contenders to consider. As a rule, the best route is to look at either a biomass boiler or heat pump to deal with the basics, and supplement these with solar thermal panels for your hot water.
The importance of hot water
In modern homes domestic hot water is likely to be a major component of heat demand - so it's vital to factor this into your plans. While we've got better at insulating buildings and space heating requirements have fallen, the amount of hot water we use has remained constant. DHW is delivered at 50-60°C, while underfloor heating in a well insulated new property can be run at 40°C or less. This can have a big impact on your choice of systems. Heat pumps, for example, run less efficiently when they are required to raise the temperature to a higher level - so their coefficient of performance will drop if they're having to provide hot water rather than just space heating. Biomass boilers, or simple stoves with back boilers, can deliver higher temperatures at a lower environmental impact - but they have their own pros and cons.
Solar thermal panels can of course be used to provide hot water when it's warm enough (about half the year) and can be combined with standard gas boilers (not combis) heat pumps or biomass boilers. They operate on the basis of stored hot water, which has the advantage that more than one shower or tap can be used at full pressure at the same time - which you don't get with a combi boiler.
Biomass heating
Biomass boilers can and do provide an all-in-one, year round space heating and water solution. They run on wood-based fuels (logs, chips or pellets), which are considered to be carbon-neutral.
Timber harvested for this purpose must be replaced so that the equivalent CO2 emitted during combustion is reabsorbed by new trees as they grow - so where your fuel comes from is important. if you're building in a smoke controlled area then you need a DEFRA-approved appliance.
Domestic biomass boilers are most commonly fuelled by wood pellets. As they have a higher energy density, they require fewer deliveries and less space to store than chips or logs. It still takes work to feed the appliance, but pellets can be used in automated hopper setups, making this type a reasonably straightforward alternative to a gas boiler. The pellets can be delivered loose direct into the hopper, or supplied in sacks which you need to manually handle. The ash that's produced is relatively small by volume and makes a useful addition to the compost heap.
The caveat is that fitting a biomass boiler costs several times the price of an equivalent gas appliance. If your setup qualifies for the Renewable Heat Incentive, you will receive cash back of 6.74 pence per kWh of heat generated - which can offset the price of the fuel and allow you to start to make inroads on cost of installation.
If you own a small, well-insulated home, you may be able to opt for a woodburning stove for space heating, with a back boiler to provide your domestic hot water. Combining this with solar thermal panels to cover your water need in summer may well give you the lowest-carbon solution. However, you'll need some kind of fossil fuel backup for times when you're not running the stove, because the sun isn't always strong enough for hot water demand.
Heat pumps
Another technology that can provide an all-in-one heating system, heat pumps use electrical energy to concentrate low-grade warmth to a temperature where it can be used for space heating and domestic hot water.
Air source heat pumps cost more to install than a gas boiler and are currently similar in terms of the running costs. Ground source heat pumps are more expensive again, but slightly better terms of overall performance as the temperature of the energy source stays relatively stable.
The performance of this technology will be determined by the amount of kWh of heat it delivers for each kWh of electrical input. For instance, depending on the system you might get around three units of heat for each unit of electricity. The appliances work at their best in tandem with low-temperature distribution systems such as underfloor heating, which is common in self-builds.
When comparing the CO2 emissions of heat pumps to those of natural gas, it would be tempting to simply look at the annual figures for the carbon intensity of electricity, divide this by the coefficient of heat pumps and conclude that they provide a saving compared to the carbon intensity of gas. However, it's important to bear in mind two things:
Carbon intensity is expressed as an annual figure. In the winter, when space heat demand is at its highest and the UK's solar fleet is all but inactive, the electrical power required to run your heat pump is more likely to be met, at least in part, by fossil fuels.
The performance of an air source heat pump drops markedly in winter, and all pumps are less efficient when providing domestic hot water - which as mentioned earlier, can represent a high percentage of the total heat demand in a new, well-insulated house. If you're off the grid, heat pumps are a fit-and-forget option and less work than biomass boilers - but higher in carbon emissions, and similar in running costs.
Conclusions
The appropriate solution for you depends on your specific situation. This includes how large your property is and what levels of insulation and airtightness you are building to, whether you're on the gas grid, whether it's a good solar site, your budget and priorities, and whether you're able to handle solid fuel.
It's always worth thinking about building in flexibility. For example, you could install a twin-coil cylinder so that solar water heating could be added later. Or you could design your home to include enough storage space should you decide to switch over to biomass in the future.
If you're aiming to get closer to independence from fossil fuels, at the least possible running cost, then the best way to do this is probably to combine several different sources. For example, you might use a biomass boiler or stove for space heating and backup hot water, with solar thermal for GHW in the summer. The most efficient systems usually employ a thermal store that can accept multiple heat inputs - but that may not be an all-in-one solution!
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