Tuesday, November 20, 2007

Heat Pump: Part 1 - ground loop

As previously mentioned we have opted to use a ground source heat pump to provide both space and hot water heating. With no mains gas, an adjacent field and the opportunity to design a well insulated house from scratch with underfloor heating, the heat pump was the lowest carbon and lowest running cost option for East Cambusmoon.

For anyone considering a heat pump for their own house there aren't that many situations where it is the most cost effective and efficient choice, and certainly if you have the luxury of a mains gas supply a high efficiency gas boiler, coupled with say solar thermal panels will likely be a lower capital cost and running cost option than a heat pump doing both. If not on mains gas, then the costs need to be compared to LPG or oil and the decision will largely depend on the type of heating distribution system you already have, ie. radiators or underfloor heating. For the same heat input to a room, radiators need to be run at a higher temperature than underfloor heating simply because the heat emitter is concentrated into a relatively small wall hung panel, rather than the entire surface area of the floor. The problem with heat pumps is that their efficiency rapidly decreases in proportion to the heating medium temperature, such that running small radiaors from a heat pump is a bad idea. This can be alleviated to a certain extend by increasing the size of radiators so they can be run at a lower temperature.

Then there's the heat collection system, in our case this being a 300m length of 40mm diameter pipe buried 1.4m underground. This particular aspect of our build was a separate 'client item' from the main build contract so Debs, me, Stewart the digger driver and £60k's worth of band new JCB set to for four days of hard graft to get this pipe buried in the ground to in such a way as to absolutely maximise every joule of energy that could be sucked out of it .....which of course would be continuously be replenished by the sun. In effect, a huge solar panel!

In designing the collector system, we followed the heat pump manufacturer's (http://www.nibe.com/) advice on pipe depth, separaton and layout. Our supplier (http://www.ecoliving.info/) also offered advice on site and once certain of our plan, we got cracking! We ended up digging three 40m trenches, each 2m wide and 1.5m deep at 4m centres. We also imported 35 tonnes of sand to cover the pipe below and above to avoid the possiblity of damage from sharp rocks when backfilling and to ensure good contact with the ground. Bfore covering and backfilling, the pipe was pressure tsted and to finish off, fed into the house via the 'slow bend' duct pipes already built in to the floor slab.


Jane said...

Debs, i find this really interesting - a heat pump was one ofthe things we investigated very partially in connection with the extension but decided, based on back of an envelope type calculations, that it wouldn't really work out.
What you have written makes me feel that I was right and that it wasn't just a worry about digging up the garden!. We have bought reclaimed oak floors which will be laid throughout the house so underfloor heating isn't an option.

Debs said...

As an alternative it is possible to take the loop vertically by sinking a borehole, although this doesn't make it more efficient. Another option is to use a heat pump in tandem with an exisitng boiler but you need to be very careful about cost/benefit.

Egon said...

How much did you pay for your NIBE unit? I can't seem to locate any prices for their units on the net.

Usually no advertised prices mean large, but often negotiable markups.