A few people lately have presented me with the problem of whether it is worthwhile building an energy efficient house as they have access to a source of cheap fuel, i.e. wood.

In a different case Mrs Client has made an irrevocable decision to install an electric Aga. The design we were working to called for a space heating energy demand of some 10,000kWh per year but the Aga emits closer to 14,000kWh per year. Although we can redistribute some of that heat around the house, we can’t stop or even reduce the output. Modern Aga’s do all sorts of clever things in terms of switching on and off and heating only at the right time, but the simple truth is that if you put in a given amount of electricity you get a given amount of heat. What we have to do is get to some sort of balance so that the Aga, with a heat recovery ventilation system, is making a useful contribution to winter heat demand and then deal with the excess in summer.

But that is a special case. In other cases the question is clearer in that ready access to cheap, sustainable, low (or zero) carbon fuel rather takes away the point of investing in building a highly energy efficient house.

A couple of clients are looking at the same options – a mix of biomass boiler and solar energy – as the means of supplying heating and hot water. And again it is a matter of balance. Both clients want a wood burning stove with back boiler but are relying on solar energy as the principal means of heating. In both cases the solar power will not be enough to meet peak winter demand and the stove boiler will be needed to top-up but also for the comfort factor of a living fire in the lounge. In both cases the problem is that if the stove boiler is used to its full capacity it has the potential to overheat the lounge.

It still makes sense to design in good levels of insulation and air-tightness, but there is no real point in going the extra mile with very high levels of air tightness, super efficient glazing and the like. Reducing the overall heat demand allows the solar element to make a larger contribution but the wood burner will always be needed as a top-up but maybe not used to its full potential.

In fact it will seldom be used to its full potential. A stove boiler rated at 5kW output to the boiler side, say, will only achieve that 5kW when the stove is stuffed full of good, well seasoned, hard wood. Change any of those factors and the heat to the boiler will be lower – as will the heat to the room side. So the answer is to design around the principal energy source and be circumspect in the use of the top-up.

Worst case, if the lounge gets too hot to live in, you can always open a window.

  • Percy Fulger

    Thanks for this Tim. Some really useful info here. And I had no idea Agas used that much energy!

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