The Beginning

This is a story about our experiences of substituting a heat pump for our venerable-but-reliable solid fuel heating system in our ancient (supposedly 16C) thick-stone-walled house in rural Northamptonshire, UK. I hope it will help others who might also be looking at this.

Right now (June 2010) nothing new is installed, but the first actual action (drilling the holes for the ground loop) is due next week.

Why are we doing this at all? Well I am a physicist by training and I understand the way that heat pumps work - they pump heat out of the ground into the house, using electricity in the process, a bit like a backwards fridge (you will note the coils behind your fridge that get warm). The trick is that the amount of heat it pumps is about 4 times the energy needed to do the pumping (which also ends up as heat). So 80% of the heat energy actually delivered to the radiators in the house is renewable (in fact mostly solar heat accumulated in the ground - you have to go kilometres down round here to get much geothermal!). The other 20% comes from the electric power used by the heat pump. Even using electricity that is a big Carbon gain compared to our solid fuel system.

Why do we use solid fuel? Well there is no gas in the village and when we put the system in 25 years ago solid fuel looked better than oil. And it does mean that I know exactly how much Carbon we use, as I carry it in from the store in hods twice a day! (about 5 tons pa). So if we are away the heating gets shut down, and even then the boiler will go out if left for more than a weekend. We estimated a peak heat output of our solid fuel boiler at about 15.5kW based on daily fuel usage.

So a replacement seemed a good idea! And we have some advantages. The heavy walls mean that changing the temperature during the day is hardly possible - it takes literally days to change much. And because solid fuel systems deliver hot water at the same temperature as the radiators we have bigger radiators designed to work at 55 degrees C. This is OK for a heat pump which gets much less efficient as the temperature difference it is pumping into goes up (as is also true for your freezer). 35C undefloor heating would be better still but impractical for our retrofit.

So we did some research on systems and grants (basically £1.5k to help fit - which does not go very far - and perhaps £2k pa support under the renewable energy scheme from next April). We aimed to get quotes from 3 suppliers selected from the certified installers list (http://www.lowcarbonbuildings.org.uk/Microgeneration-for-your-home/Certified-installers-and-products). These were otherwise selected on location, apparent track record (eg old building experience etc) and in one case because a friend in the next village who is also installing a heat pump recommended his supplier.

The quote bases varied (there seems no way to know this without asking). Two quoted for the heat pump and associated bits supply, and commissioning and 'design'. They could suggest the builders, electricians and plumbers also needed (insofar as this is not DIY) but this system seems little used. The third did a site survey (the only one who did pre-quote, the others quoted from plans) and would have quoted an all-in price. But in fact he never actually quoted at all, in spite of promises. The other two quotes were similar.

So we accepted one of the first two, as it happens the one also being used by our next-village friend, mainly because they were much more convincing and quick to answer questions. We had done 2-3 weeks of checking things and asking questions before signing up and starting to pay, for the following reasons:-

Electricity supply. The systems we needed, with a nominal output of up to 17kW or so (5kW electric power) required, all needed a 3-phase electricity supply. We, like most people in the UK, only have single phase, though there is 3 phase buried out in the street. This proved fairly complex - our supplier (Central Networks) can fit the supply for £3k to dig all the holes, bring it across the street, control the traffic etc. But they could not bring it in where the old supply comes in because the rules now require it to come in underground (under the thick stone wall) which is OK except that this passes over the house heating pipes and that is not good anymore. So we will come in to a box on the end of the house, which looks OK. We will also pay a builder and electrician to install this (I did the original wiring) to get the system in quickly.

Ground loop - that is the plastic pipe that draws the heat from the ground, supplied as part of the heat pump system. You could spread this around in trenches about a metre underground, and this is what most people do - it is cheapest. Our friend in the next village has a convenient field and has dug the trenches (with a digger!) himself. We drew up maps and we could just get the required 250m of widely[5m]spaced trenches into our lawns, but access is very difficult and the disruption would be impressive. On the other hand we have a flat 13m-square drive-plus-lawn area in front of the house and have chosen instead to have 4*75m deep 6" holes dug in it instead. This is more expensive (~£12k) but of course needs less space. The driller is recommended by the heat pump supplier but in effect we end up project managing this - arranging delivery of the pipes and setting up the skip(s) for the 7 yards or so of spoil (the holes are refilled with 'grout' supplied by the driller). The driller was very helpful in terms of supplying sizes for his machines so we know we can get them in and commenting on maps from us of service and drainage arrangements (one needs to keep clear in case they got damaged, or froze later - the ground should not freeze but in winter and at shallow depth it might). So far so good!

The friend's system is working!

We have chosen only to use the heat pump for heating; not for hot water, which will continue to be heated by an existing immersion heater, which we use anyway in summer when the boiler is shut down. The problem is that the 'standard' hot water tank supplied by (each of) the heat pump suppliers is vast, ~300 litres against 180 standard in the UK. This is compounded by the strange design which adds another 180l of water in a surrounding heat jacket. The result is a vast and extremely heavy unit, bigger even than the heat pump itself (which is about the size and shape of a big upright freezer). This presumably reflects a different way of life somewhere else! At all events we could only accommodate this in an outhouse >10m away, and then we would have to pump-circulate the hot water loop (like a hotel) to avoid long delays in getting hot water from the tap and then... it all seemed not worth it. We might put solar hot water in later. And FWIW we did check our actual hot water usage over a week using our 'alertme' energy monitor and found we only spend about £250 pa, which is also much less than estimated (this seems to be because we power-shower rather than bath and have a cold-fill washing machine).

So - so far so good! We have the pipe coming on Monday and the drillers on Thursday (they were very quick once asked - 50% paid in advance). They expect to be out and clear by the following Wednesday at the latest - leaving not much mess and a few pipes sticking out of the ground. As we have a big family party that weekend we hope they are right! We are expecting a quote from our builders for connecting these to a buried manifold and thence into the washroom where the heat pump will go. (We had the heat pump surveyor & the builder with his electrician and plumber in together). The builder is known to us for stone repair work and seems reliable - it emerges that they have done heat pumps before.

I will report!