Starting from scratch provides the perfect opportunity to create an efficient, cheap-to-run, easy-to-use heating system. In this guide we explain the ideal heating systems for self-builders.
The basic heating system in a new house is based around the basic choice of boiler, emitter and hot water storage. For many self-builders, particularly those on mains gas, this can still be a very simple affair: a regular system boiler, a lorry-load of radiators and a copper cylinder. However, building a house from scratch is a huge opportunity to create a heating system that is more efficient than the minimum required. It may be that you have to build ‘eco’ as a result of a condition of your planning approval, or it may be that your plot is not suitable for mains services. Either way, you’ll almost certainly want to consider creating an energy-efficient heating system which revolves around two principles: minimising the amount of heat you need, and minimising how much energy you use in creating it.
However, renewable energy is not like conventional heating. Specifying a gas or oil-fired boiler is a well-trodden path: the trusted plumber turns up, takes a look at the house and, probably with no calculations at all, will know exactly which boiler to install. He might suggest a boiler with 25kW capacity in the sure and certain knowledge that it will be enough. The boiler will be capable of producing anything from 5kW to 25kW (the modulation range), and the requirement will be something in that range. The peak space heating load for the average modern house will be something around 10kW; add another 4kW or 5kW for domestic hot water and the 25kW machine is still ample. The mantra is to over-specify and be sure. As an example of how this thinking pervades the industry, the Worcester Bosch Greenstar range of condensing boilers (very good boilers!) start at 12kW and go up to 30kW. The reason being that the range covers virtually the whole of the housing market. The price difference between these machines is trivial (relatively) and the efficiency is broadly the same whatever the output.
On the other hand, although a few heat pumps offer a degree of modulation, generally a 10kW heat pump will output 10kW. An oversized heat pump will be significantly less efficient than one that is the right size. To some extent the same issue applies to all renewable energy heating systems — an oversized solar thermal system will do well in winter but will need to dump heat in summer. It is therefore critical to calculate exactly how much heat the house needs before selecting the heat source.
(MORE: Specifying Heat Pumps)
What is the ideal system?
Once we move away from gas or oil-fired boilers, there are choices to be made: to do with suitability, capital vs. running cost, convenience, cleanliness, CO2 emissions and, not least, what will work well in that house and for those people.
The options are solar, biomass or heat pumps. Increasingly the answer is a combination of these options.
In every case the decision starts with working out the heating needed for the house and for domestic hot water. It is useful to keep the two figures separate as it may be that they come from different sources. Don’t rely on the SAP assessment for these figures — SAP is an assessment, not a calculation, and makes assumptions that may or may not be accurate. Use a heating engineer to calculate the figure based on the plans, specification and U-values.
A heating engineer well versed in renewable energy will be able to say what will work and what won’t. It is then for the owner to consider budget and what sort of technology will suit — and what they are happy to live with. The decision-making process starts with the design of the house, moves to the heating system options and often returns to the house design. Bear in mind that we are designing a system and that the house itself is part of that system. Improving the insulation and airtightness of the house will obviously reduce the size and cost of the heating system and its running costs. But again, there is a balance.
If the wall U-value is improved from 0.19W/m2 (standard for SIPs construction and 36% better than Building Regulations) to 0.11W/m2 and airtightness increased from 7m3/hr (again standard and better than Building Regs) to 3m3/hr it would reduce the peak heat load from around 9kW to around 6kW; domestic hot water is related to the number of people and would not change. So we can reduce the size of the heat pump or biomass boiler and reduce running costs by around 30% per year — but there is a cost. Increasing the house’s inherent structural energy efficiency (often called the ‘fabric’) by this amount for a SIPs build is likely to cost £8,000 to £10,000 (for a 200m2 house) in extra insulation and work on airtightness. The saving in running costs is likely to be less than £200 per year, so it would take 40 to 50 years to get that capital investment back, making it a far from sensible option in financial terms.
That is not to say that improving fabric efficiency is a bad thing. It is a matter of balancing the improvement with the impact it has. Heating with renewable energy is not a well-trodden path — it relates much more closely to the house and the people in it and requires informed decision making.
Hot Water Storage
In any renewable energy heating system involving more than one technology, the hot water cylinder will be a key component. It will be the heart of the system, controlling which technology is used, starting with the cheapest. A good thermal store (basically a cylinder with a heat exchanger which can feed the heating and hot water systems rather than just storing hot water) can reduce running costs by up to 10% and integrates disparate technologies into a single system.
And, in this case, size is important. With heat pumps it’s less so, but if solar energy is to be used then, for a four-person household, a 250-350 litre cylinder will be needed to ensure the maximum amount of solar energy can be used. A biomass boiler will need in excess of 1,000 litres as the boiler tends to fire only once each day and the cylinder needs to be big enough to store all that heat. So choose your cylinder to match your renewables. You see? Things just got complicated.
(MORE: Hot Water Storage Options)
Three Typical Green Solutions
1: The Standard: The virtually default renewable heating system is a ground-source heat pump teamed with solar thermal panels and a woodburning stove with back boiler. This combines convenience with efficiency. Capital cost (for the standard house) will be around £15,000, running costs around £300 per year and it will benefit from RHI, potentially for all three elements.
2: Low Capital Cost: Where capital cost is the main consideration, a wood pellet stove boiler would be the option. Perhaps, if budget allows, add solar thermal panels to pick up the domestic hot water load in summer. Capital cost (for a standard 200m2 house) could be under £6,000, running cost around £400 per year and it will benefit from RHI. On the downside it needs feeding with fuel every two to three days and ash has to be emptied.
3. Money No Object: With a free choice the ideal solution would be solar photovoltaic thermal (PVT) panels with an integrated heat pump and a woodburning stove with back boiler. This option will produce both heating and electricity and provides the best solution in terms of running cost and convenience. The woodburning stove will top up the domestic hot water when the solar panels are not doing so well and the heat pump will minimise the number of PVT panels needed. Capital cost will be £20,000+. Running costs will be less than £150 per year and the system would benefit from FiTs and RHI.
Everything on Justin and Lindsay Stead’s new self-built home was backwards-engineered from their desire for a ground-source heat pump and cavity wall construction. By achieving a green structure (U-value of 0.2, with an airtightness of 3.4m3/hr) they were able to minimise their heat requirement, taking into account the limited capacity the heat pump could achieve owing to garden size.
The Steads specified:
Boiler: Danfoss 10kW ground-source heat pump; Hot Water Storage: 300-litre Ecocat thermal store with solar coil; 6kW immersion heater; Also: Three-panel solar thermal array; underfloor heating; and mechanical ventilation with heat recovery system.
Heat Pumps: Assessment
The efficiency of ground- and air-source heat pumps is a matter of debate. It all hinges on the claimed efficiencies of the products — called Coefficient of Performance (COP), which is a way of describing the amount of heat energy produced for the amount of electricity used to produce it (so a COP of 4:1 will indicate a system of 400% efficiency).
While most people enjoy genuine savings and are very pleased with their heat pump’s performance, there have been instances where homeowners were disappointed (largely in the electricity bills that result). As a consequence, the Energy Saving Trust completed a field-trial, year-long survey of 83 heat pump installations in 2010, with the results being examined by the Department of Energy and Climate Change (DECC) in March 2012, showing an average COP of 1.8-2.6.
The DECC analysis of the report highlighted cases where heat pumps had been successful, and also analysed why in some cases the results were disappointing. They were:
- undersizing of heat pump (reductions of up to 1.5 in COP);
- undersizing of borehole/ground loop (0.7);
- poor insulation of pipework/cylinders (0.3-0.6).
The type of emitter specified was also found to have an impact, with the average COP of a ground-source heat pump falling from 2.58 when used with underfloor heating to 2.23 with radiators.
The lesson? Make sure your heat pump is sized correctly, and that the house is designed with the heat pump in mind — perhaps using an energy consultant to advise.
(MORE: Heat Pumps – The Real Cost)
The Least You Should Know
- If you’re starting from scratch, try and reduce the amount of heat your house will need by adding extra insulation and airtightness into the structure.
- Your choice of renewables might well demand a bigger-than-expected hot water cylinder (or thermal store).