Ground to Air Heat Exchangers

A worthwhile investment?

First of all, what the proponents say:

‘The Supply Air Duct is laid at a depth of 1.5m, where ground temperatures remain at 7-12° all year round. The air supply to the MVHR Unit can therefore be 7-9° warmer than atmosphere in winter and 12-14° cooler in summer, with pollen filtration included. As a result, significant heating & cooling gains are enjoyed, carbon emissions reduced and comfort levels greatly improved.’

Yes, that’s true. But are these gains worth the money?

Let’s take winter. The supply air to the MVHR system will be increased by up to 9°. This doesn’t mean that the air coming into your living rooms will be 9° warmer.

To keep it simple let’s say the outside temperature is zero and you keep your house at 20°, that’s a difference of 20°. The heat exchanger in your MVHR unit is 90% efficient (say; some are slightly more and some slightly less). So the temperature of the air being delivered to your living rooms will lose 10% of 20 or 2° and emerge at 18°.

Now let’s preheat the supply air to the MVHR unit to 10° via the ground to air heat exchanger (that’s being very generous and by the end of winter that will probably be 5°). So the temperature differential is now 10°. A 10% loss of heat is 1° so the incoming air to the living rooms is now 19°.

A 250sq m house requires a constant flow of air at 75 litres per sec. So by having air coming in at 1° warmer we are saving the amount of heat required to raise 75 litres of air per second by one degree C. And this is 73 watts. Over a 24 hour period this is 1.75kwh. If the cost of electricity is 15p per kwh the saving per day is just over 26p. Assuming that we get 90 days per year when the temperature is at zero degrees over for the full 24 hours (most unlikely) the cost saving will be £23 per year.

Assuming the cost of materials and installation of the ground to air system will be in the order of £8 to £10,000 the payback period will be over 400 years! Yes, but the system is going to be used half the time for cooling. So the payback period for the heating side is only 200 years.

Summer is really where the system comes into its own and you will get full benefit from the cooling effect of the ground. Remember, though, that current levels of insulation, as well as keeping heat in in winter, keeps the heat out in summer. A well designed house should not need additional cooling in summer in the UK climate.

Using a good design, enhanced construction details and additional insulation for much less than £10,000 may be worth considering.

For details of enhanced construction details go to: http://www.energysavingtrust.org.uk/Professional-resources/Housing-professionals/New-housing/Enhanced-Construction-Details