Ochiltree Castle - now with ground source heating
We had been talking about ways of reducing our heating bills and improving our 'green' credentials when two factors coincided in 2008 to help us make a decision to put in ground source heating. Firstly, we had already decided to put in a driveway to improve the access to our house; we would therefore have workmen and equipment on site to do the required groundworks. Secondly, the price of LPG, which heated the house, the AGA and our hot water supply increased dramatically. The economics of installing a ground source heating system therefore improved markedly.
The price of LPG, which heated the house, the AGA
and our hot water supply increased dramatically
We had seen ground source heating being installed in a couple of 'Grand Designs' programmes on Channel 4, and had heard a good report about it from the Carsons when we visited Rusco with the SCA. Our researches backed up the claims that the installation costs could be recouped in ten years. We had been warned about the problems some people had run into with inexperienced suppliers. We therefore looked at the relative merits of buying our system from the two main suppliers in the UK. Costs were comparable so we chose one based on our gut feelings about their respective presentations at the Ideal Homes Exhibition in Glasgow.
What is Ground Source Heating?
There are two sources of heat from the ground - Geothermal; this gets its heat from deep in the ground and involves specialised drilling to an appropriate depth depending on how much heat the house requires.
Solar; this involves digging trenches a metre deep into which long pipes filled with a glycol water (antifreeze) mixture are buried.
These fluids merge in a manifold before going back to the house. An alternative method is to lower weighted pipes into a lake.
The anticipated cost of deep drilling was considerably greater than the expected cost of putting cables into trenches so we opted for this latter method. We also looked at air source heating but decided against it because it was less efficient at that time and was considerably noisier than ground source heating.
How does it work?
Irrespective of which heat source is used, the heat pump works like a refrigerator in reverse. It thus depends on the phenomenon of latent heats of evaporation and condensation – remember them from your school physics? The pump takes low level heat from the ground and converts it to high grade heat for the house. For every 1KW of electricity consumed in running the pump, 4 KW of heat is supplied to the home.
The heat pump works like a refrigerator in reverse
The compressor within the pump is driven by an electric motor. It compresses the refrigerant to form a high temperature, high pressure gas which passes into a condenser on one side of a plate heat exchanger; water from the house's heating system flows through the other side of the plate. Thus heat is transferred from the hot refrigerant gas to the cooler, heating system, water. As the refrigerant cools, it condenses and changes state from a gas to a liquid. After it has left the condenser, the refrigerant passes through an expansion valve. Its pressure therefore drops suddenly, and it passes into an evaporator. Here, the refrigerant (on one side of another plate heat exchanger) takes heat from the water / glycol mixture (which circulated through the ground loops on the other), boils and turns back into a gas.
The refrigerant returns to the compressor where the cycle begins again.
The ground loop antifreeze mixture cools and is pumped back out to take more heat from the ground.
The heating system water is pumped around the house where it cools before returning to the heat pump.
The preliminary analysis of our requirements (taking account of the size and thermal (in)efficiency of our house) showed we needed a 22 KW system. We agreed to install two 11KW heat pumps working together and controlled by a small computer to minimise strain on the machines. The computer monitors the external and internal temperatures and adjusts the use of the pumps to maintain the desired temperature in the house, irrespective of the conditions outside. This proved to be a good choice in the two severe winters we have had since the system was commissioned.
We had also had a survey done by the suppliers in order to establish the length of piping we would require, and their distribution.
The initial preferred layout was not suitable because we encountered rock close to the surface in one of the areas near the house we had hoped to use. A re-evaluation indicated that we could not get the required 1m depth of soil till we were about 80 metres away from the house (where the manifold would go). Thereafter we needed 1200 m of 40mm (instead of the usual 32mm) diameter thermal piping, laid in 'slinky' coils in four trenches, each 75m long and 1.5m wide. Each of the heat pumps would be connected to the pipes in two of the trenches, and there would therefore also be two separate systems of pipes going out and coming back into the house, each containing their individual antifreeze.
Our electrician and plumber all had instruction from the firm's representative before the work started, and some fine details of the work they would have to do were worked out. The groundworkers were already digging the trenches as designed. Unfortunately, they encountered first sandstone then shale and then a small amount of coal as the trenches progressed but these areas had simply to be dug out to allow the rest of the work to proceed.
Laying the pipes in slinky coils (1.5m diameter and 0.75m apart) in the trenches proved to be a challenge, because they came in rolls, the material from which they were made had a memory and therefore unrolling them to make loops of the required size was difficult initially. By the time we had completed two of the trenches, I had worked out how to make the work easier. Electrical cables come on a former whereas our pipes were held in rolls by ties. When these ties were cut, the piping became 'unruly'! I therefore made a wooden former onto which I put each of the remaining rolls of piping before cutting the ties. I suspended the wooden frame of his former from a swivel hook, which was in turn hung from the arm of the JCB. The pipes uncoiled easily thereafter so that we laid the pipes in the remaining two trenches in a morning whereas the first two had each taken a day. We had also to join some pipes using connectors (supplied by the firm) whose integrity depended on not disturbing the rubber 'O' rings which formed a seal over the pipes as the overlying plastic rings of the connectors were tightened up. This was not easy to achieve. The integrity of the joins had to be tested using a hired electric water pump set at 4 bars (the ground source system operates at less than 2 Bars). Redoing unsatisfactory joins was more difficult than the first attempts had been. The air was blue on many occasions.
The slinky coils were laid on 100mm of sand and another 100 mm was put on top. The area was then watered well for heat transfer from soil to pipes requires an aqueous environment. The trenches were then filled in.
Castle Heating System
The firm's engineer came to commission the system after our plumber and electrician had installed the pumps and connected them to the pre -existing hot water system. This involved pressure testing the complete system before filling the pipes with the antifreeze mixture, and making sure that as much air as possible was out of them, before making the necessary electrical connections and starting the pumps. Hallelujah! This work was done the day before the snow started in December 2009.
Was it worth it?
The answer is 'Yes', irrespective of whether one considers this from the perspective of comfort or cost benefit.
Before we installed this system we, like most people, put our central heating on for a short time in the morning then for a longer period in late afternoon and evening. The ground source heat pumps (and the pump which takes the water around the house) run all day, but the water in our heating circulates at a lower temperature than we needed previously. Our house quickly became warmer and more comfortable; getting up in the morning or during the night is no longer colder than going to bed late in the evening.
The walls absorb some of the heat and so feel markedly warmer than they did before. We switch the system off for about 3 months in the summer. We do not get the full benefit which the system can confer since we decided to keep our pre-existing hot water system run from the AGA which is still powered by LPG. We could not face redoing some of the extensive work we had done over the years in order to relay pipes from the heat pumps to the hot water tank.
Even allowing for:
This personal decision (which will have limited the cost effectiveness of our installation); The changes in fuel prices since our system was completed; The (considerable) extra costs we incurred dealing with the ground conditions we encountered and in restoring the garden back to an acceptable state, our fuel bills are half of what they were before the work was done.
We are also comfortable all the time over the autumn, spring and winter, which we seldom were before December 2009.
We are therefore supporters of this form of heating where it can be installed more easily than proved possible with us. Would we do it again? Yes, but we would try to get much more competitive quotes for geothermal heat.
We received money back from the heat pump supplier after the installation (about 10% of the pump prices).
The UK government plans to introduce their Renewable Heat Initiative (RHI) payments for the domestic market in 2012, but the details are not available at the moment. People who install ground source heating now are eligible for a grant called the Renewable Heat Premium Payment subject to a requirement to monitor its impact on their household using specified methodology.
They will also be eligible for the RHI payments when they are introduced.
Article by John and Hazel Hunter, Ochiltree Castle. Linlithgow