One of the key questions for the self builder and renovator is: how do I go about designing the most efficient heating system for my new home? Designing a heating system is a complicated process, so it is best to leave the technical stuff to those properly qualified and consider the broader issues.
Issues to be Considered
At the most basic level, there are only three things to consider:
- Insulation and airtightness of the house — this will determine how much heat the house will need, the key efficiency issue.
- The heat source — do we want to use a boiler (whether gas, oil or biomass), a heat pump and/or solar energy?
- Heat distribution — do we want underfloor heating (UFH) or radiators? In efficiency terms UFH wins, but not by such a margin as to make it essential.
A possible fourth factor is whether there is mains gas available. Gas is currently very cheap (compared to other fuels) and likely to remain so for a while at least. While it does nothing in terms of improving efficiency, there is a cost comparison that needs to be considered. In addition, there are other issues that we need to be aware of.
Heat Demand for Each Room
There is a need to calculate the heat demand for each room. This will tell us the size of radiator needed or the size of pipe runs for an underfloor heating system. It will also indicate areas where improvement is possible if you’re designing a system for an existing building.
There are a number of calculators available online (Google ‘radiator sizing’ to find some) but approach with caution. They take the least possible information and assume standard construction and insulation levels. They provide a useful initial guide but the final calculation needs to be done by an expert and the heating system suppliers will probably do this for you.
Heating Distribution Methods
UFH vs Radiators
The decision is a matter of personal choice: UFH is a little more expensive to install and radiators a little more expensive to run, but the differences are not deal breakers one way or the other. The issue is more to do with control (see below).
The location of radiators is another factor to consider. Warm air meeting a cold surface (a window, say) will cool and fall — in some cases, to the point where it becomes a draught, which is why radiators are typically positioned below a window. This is not an issue with underfloor heating — the warmed air rising tends to cancel out the cooled air falling. With windows that reach down to floor level (bi fold doors, for example), in a room heated with radiators the problem still exists but can be solved with a trench heater at the base of the window.
UFH allows for zone control, so that we can control the temperature of each room, and when we want the room at that temperature. We can do something similar with programmable radiator thermostats but as the thermostat is so close to the radiator it is difficult to accurately control the temperature of the whole room. The normal result of this is that we tend to heat the room for longer and to a higher temperature.
The weather constantly changes, and so does the heat load required to warm the house. A good heating system will have a weather compensator to deal with this. The sensor registers the actual outdoor temperature and adjusts the heat supplied to the house.
The problem can be with the location of the sensor. It is typically placed on the external wall nearest to the boiler but needs to be on a north-facing wall and out of the prevailing wind.
Incidental and Solar Heat Gains
One of the key issues in efficient heating design is knowing where the heat is coming from. The heat supplied by the boiler is easily calculated. That arising from incidentals like big, south-facing windows, an Aga, woodburning stove or the like, is far more difficult to calculate, but can have a disproportionately high impact.
We know that an Aga (or similar) will put a lot of heat into the room when it is being used, but we (the designer) don’t know when it will be used. It is a similar situation with woodburning stoves. The best we can do is to install a control system that will shut off, or reduce, the heat being sent to that room when the other heat source is in play. There is also the potential to use a mechanical heat recovery system to move any excess heat to other rooms.
Solar heat gain is relatively easy to deal with, in that the requirement is to stop it getting into the house (in summer) with an awning or a brise soleil.
Domestic Hot Water
The proportion of the total energy demand used for sanitary purposes is steadily increasing as the amount of insulation in the house rises. Big houses can have a big impact, as bathroom taps can be a long way from the heat source, meaning that each time we turn the tap on we drain away a lot of cold and tepid water while waiting to get the hot water we want.
There are a couple of solutions to this: a continuously pumped system where hot water is circulated around the system and back to the hot water tank. We get immediate hot water at the tap and the hot water in the pipe cools only slightly before it gets back to the tank.
An option, especially suited to taller, three or four-storey houses, is called a ‘riser return’. This is a second pipe laid parallel to the hot water supply pipe, connected to the supply pipe at its highest point and back to the hot water cylinder. When the tap is turned on, hot water is delivered and when it is turned off gravity takes over and draws the hot water back down the return pipe to the tank. The point is that an efficient heating system is not limited to space heating. Hot water needs energy, too.
The big question, and the point at which we generally start, is whether to use a boiler or a heat pump. As an aside, an entirely solar-heated house is possible, and it has been done, but it is still fairly specialist and not for the faint-hearted. Having said that, solar energy needs to be included in virtually every new-build design.
The decision over the heat source will be influenced by all that has gone before. There is no right or wrong answer but going through the process will lead to the right decision for that project. There are constraints that can exclude options:
- If mains gas is not readily available, the cost of getting a pipe laid can be prohibitive.
- The extremely volatile price of heating oil makes this at best a brave decision.
- Insufficient garden space would exclude a ground-source heat pump.
- The work needed for a biomass boiler may be impractical.
It is often the case that when we have established what is needed and eliminated the options that cannot be used, the right answer will present itself.
Read more about off-mains heating
Conclusion: the Key Design Issues for the User
- Get a detailed and accurate heat demand calculation done.
- Decide on the heating distribution; if this is radiators think carefully about where to position them.
- Make sure incidental heat gains are properly calculated and included in the design.
- Consider the positions of the hot water storage and the hot water taps.
- Big houses may be best suited to a biomass boiler but in terms of efficiency the question is: can a heat pump be used? Is there enough ground for a ground-source heat pump or is the house better suited to an air-source heat pump?
- And finally, where should the solar thermal panels be placed?