The ground source heat pump – a year on…

It’s just over a year now since our IVT Greenline HT+ E11 11kW ground source heat pump (GSHP) was installed so I thought it would be the perfect time to reflect on our experiences of owning and running a system like this, and also importantly to do some quick analysis on how much electricity we’ve used over that period compared with previous years.

If you’re interested in previous posts relating to the heat pump, take a look at these…

A bit of background…

First, though, it’s worth a re-cap on why we decided to install a ground source heat pump  in the first place. During the latter part of 2011 we extended our house considerably by converting an adjoining timber-framed thatched barn into additional living space and creating an enlarged entrance hall, a new open plan living and dining space, large kitchen and utility room. Our existing central heating and hot water system was an electrically heated wet system with radiators designed to work with low-rate electricity tariffs. Although electrically powered (which sounds frighteningly expensive!), this had worked reasonably well and efficiently over the previous 12 years since installation but it was nearing end-of-life, the manufacturer had gone out of business and it was simply not big enough to satisfy the increased demands of the enlarged property. So it had to be replaced with a new system.

After looking at all the options available to us, particularly as we are off the gas grid, and comparing various traditional technologies with newer ones, a ground source heat pump emerged as the ideal choice for us.

At the time we decided on a GSHP we also planned to add a solar thermal installation to supplement the hot water. As it eventually turned out, we didn’t install solar thermal and instead opted for a 4kW (peak) solar PV array which, we were told, would be a very good companion to the heat pump.

Installation and commissioning

Our GSHP system was supplied by ICE Energy, probably the UK’s biggest and most experienced supplier of GSHPs. It was important to me that we chose a company with a lot of experience given the general lack of knowledge and even awareness of this technology, and we definitely got that with ICE Energy. Their sales, engineering and technical support staff were very knowledgeable, efficient and professional at all times so I would highly recommend them to anyone else looking for a heat pump.

The installation of the heat pump, cylinder and associated plumbing was pretty straightforward for the plumber working on our barn conversion, even though he had never installed one of these before. The work associated with the installation of the ground collector pipework loops was a lot more than I had originally imagined, with some 300 tonnes of soil being moved during the course of the work: digging the three trenches, 50m long x 1m wide x 1m deep, laying 200m of coiled collector pipe “slinkies” in each trench, creating a concrete inspection chamber to house the pipework manifolds, filling the trenches back in and making good the paddock again. Having said that, it was all completed within about 4 days and even though my paddock looked like a barren wasteland for a few weeks, it didn’t take long for the re-seeded grass to grow back and look as good as it had previously.

The commissioning of the system, which included checking the basic installation, filling the collector ground loop with the heat transfer fluid and powering on the system for the first time was carried out by an ICE Energy engineer. He also talked through the basic operation of the system, how to tweak the controls and what maintenance tasks needed to be performed. There was a lot of information to take in but I also had the full manual which I read several times to make sure I absorbed the important detail.

The early days

Initial impressions of the system were very favourable. It was very quiet in operation, which was an important factor given that it is housed in specially made oak framed cupboards in our entrance hall, immediately adjacent to our main living area.

The first few months involved a lot of tweaking of the system settings in order to find an acceptable level of comfort while still trying to keep it running as economically as possible. The primary settings of the heat pump include a heat “curve” which dictates how much heat the heat pump should generate given the current outdoor temperature and the temperature of the water returning from the underfloor/radiators. The steeper the curve, the more heat it will generate at lower temperatures. There is also a “fine tune” setting which raises or lowers the heat curve (not adjusting the slope of the curve) to increase or decrease the target temperature. One other setting I tweaked was that of the domestic hot water temperature, which was initially set to 49°C. I lowered this to 47°C which was still perfectly acceptable for hot water but would squeeze a little bit more efficiency out of the system by not being required to maintain quite as high a temperature.

The installation has been problem free apart from one occasion during the first couple of weeks after installation where our new underfloor heating wasn’t reaching the desired temperature, and this turned out to be because the pressure in the unvented system had dropped. Once this was corrected we had no further problems like that.

It’s fair to say that the house, particularly in the old part with traditional radiators, was cooler than it had been previously during this initial running-in period but we eventually found the settings that worked for us under all conditions and we’ve not changed them since. In fact, that is one of the selling points of a system like this – once it is set up, you shouldn’t ever need to touch it again!


There is virtually no maintenance to be carried out on the GSHP and I believe this is true for the whole life of the system. You do have to check a couple of particle strainer filters a couple of times a year, one external to the heat pump unit on the ground loop circuit, and one internal to the unit. This is a straightforward task and should only take around 30 mins at most to complete.

Electricity consumption

So, we finally get onto our electricity consumption over the last year. Bearing in mind the house is now almost twice as big as it was previously with a larger space to heat and more appliances and lighting, I’m very happy to report that our electricity consumption has actually gone down over the last year by around 30%! I’ve got to say I am very pleasantly surprised by this and I had feared quite a few times that we would be using more electricity.

Our solar PV system will have had some impact on these figures due to the fact we will have used our own generated electricity some of the time rather than having to import it. But this is probably not a significant factor as the times when we were generating the most electricity were a few hours during sunny Spring and Summer days when the heat pump wasn’t on much. Looking at seasonable usage, I suspect the savings are largely down to the efficiency of the GSHP when compared with our previous system. I haven’t had chance to calculate the coefficient of performance (CoP) for our system yet but this is something I would like to do in the future.

Don’t get me wrong – we still use a lot of electricity, much more than a lot of households use I would imagine – but the important and re-assuring thing is that we are now using considerably less than we did before. And given that our only fuel is electricity, this bodes well for the future.

electricity-consumptionSavings and subsidies

Finally, I just wanted to say a little about the financial aspects of this system. In total, it probably cost us around £15K for the installation of the GSHP, which is quite a sizable up front investment, however this was offset slightly by a £1,250 installation grant under the Renewable Heat Premium Payment (RHPP) scheme operated by the UK Government. In addition to this, if all goes to plan, the new Renewable Heat Incentive (RHI) scheme should start in Summer 2013 which will provide an annual payment towards the cost of the installation, similar to the Feed-in-Tariff (FiT) scheme for Solar PV installations, over a proposed 7 year period (compared with the 20+ years for solar). This would more than cover the cost of installation of the GSHP system, and when combined with the solar FiT payments we are already receiving, will cover the cost of all of our energy usage over the next few years.

The heat pump throws a wobbler!

All has been well with my new ground source heat pump system since it was commissioned at the beginning of January although it has needed a fair bit of tweaking of controls to get everything working as desired. It’s taken me a couple of weeks to really understand how all the different settings work together to produce the end results, but I think I’m almost there with it now…

That was until the underfloor heating simply refused to get warm a couple of days ago!

The target temperature being shown by the heat pump for the underfloor circuit looked perfectly reasonable yet the actual temperature reading from the sensor next to the shunt / mixer valve controlling the underfloor supply was way too low. And it simply refused to get anywhere near the target temperature despite regardless of any settings changes I made. After previously thinking I’d got the heat pump operation and control completely sussed, this was a bit of a let down.

After speaking with the excellent technical support team at ICE Energy they decided it would be best if an engineer came out to check the shunt valve, so an appointment was booked.

Shortly afterwards I just happened to be looking over the heat pump installation and I noticed that the pressure gauge for the unvented system had dropped from the normal of around 2 bar to just over 0.25 bar. This wasn’t right so I corrected it and lo and behold, the temperature of the underfloor circuit started to rise!

I called ICE Energy back and explained what I had found and they told me that an air lock or low pressure would cause the symptoms I’d been experiencing, which was reassuring.

So fingers crossed, that little hiccup is fixed now. And, as equally important I am back to being confident I know how the system works again!

The ground source heat pump ground loop gets installed!

If you’ve read any of my previous posts, you’ll know that we’re installing a ground source heat pump as part of our barn conversion. Well, the installation of the ground loop collector pipes started in the last couple of days and I siezed the opportunity to shoot some short timelapse videos of the work being done.

The videos linked below show how the 50m x 1m x 1m trenches are dug and how the 200m of pipe loop in each trench is laid as 1m diameter coiled “slinkies” on a bed of 100mm sand. The videos are only short (they were shot with a 15 second interval between each timelapse frame) but are HD so make sure to select the 720p HD option on playback.

My journey into the world of green energy microgeneration and ground source heat pumps…

Over the last few months I’ve been researching and planning a reasonably large building project to convert a thatched barn adjoining my house into additional living space. This new living space will obviously require space heating and domestic hot water (DHW) and so part of the project research has been concerned with investigating and choosing the system to be used.

As our village is off the mains gas grid, the main options open to use are:

  • oil
  • gas (LPG)
  • electric
  • some form of renewable such as air/ground source heat pumps (GSHP), solar thermal, solar photovoltaic (PV), biomass or a combination of these

Our current heating and DHW system is based around electrically heated water driving a traditional wet radiator distribution system. Although powered by electricity, this system has operated relatively efficiently over the last 12 years due to it using a special low rate tariff for the majority of the time. However, this is a bespoke system produced by a company that went out of business several years ago so knowledge and spare parts are pretty scarce, and this coupled with the fact that it would not be large enough to satisfy the requirements of the new complete house means that continuing to use this system would not be viable. We could choose to continue using the existing system for the old part of the house and install a completely new, separate system for the barn conversion but I feel the right thing to do would be to install a completely new system to cover the whole house, old and new. Scared by the thought of ever increasing fuel costs, the idea of a green / renewable energy based system started to appeal more and more. When also taking into account the generous benefits of the Renewable Heating Premium Payment (RHPP) and Renewable Heating Incentive (RHI) schemes (more on this later), it starts to sound even more attractive.

The ground source heat pump

Looking at our particular situation the main technology standing out was a ground source heat pump (GSHP). I had vaguely heard of this technology before but after doing a lot of research it became apparent that this would be ideal for our requirements, particularly because we have a large paddock immediately adjacent to the house and garden in which the ground loop collector pipes could be buried. If you are unfamiliar with what a ground source heat pump is, here is a very brief overview…

The ground below us stores energy from the sun as heat and from a depth of around 1m below the surface the temperature remains pretty constant all year round (between around 7°C to 13°C in the UK). It is possible to extract some of this heat energy using liquid filled collector pipes buried in the ground. The liquid is pumped through the pipes and this warms up as the heat from the ground is transferred to the liquid. Only a few degrees difference in temperature between the flow and return is enough to drive a heat pump unit which can be simplistically described as a refrigerator in reverse! It uses a system of compressors, condensers and heat exchangers to convert the small temperature differentials in the liquid coming from and going to the ground collector pipes (low grade heat) into hot water which can be used for space heating and domestic hot water (high grade heat). Once the heat energy has been extracted from the collector pipe liquid, the cooler liquid is pumped back into the ground collector pipes for the whole process to be repeated.

So, the only “fuel” used in this system is the solar energy extracted from the ground (which is free to us!) and the electricity used to drive the heat pump system. For a system sized correctly and operating efficiently you can expect it to be 400% efficient, that is for every one unit of electricity used to power the system you will get four units of heat energy out of it. Sounds good, doesn’t it! The heat pump can generate hot water for both heating and DHW. It works best driving underfloor heating due to the lower maximum temperature of the hot water it produces (up to 65°C) when compared with traditional carbon fuel based water heaters. However, it can drive radiator based distribution systems albeit at a lower temperature than traditional installations.

Ground loop collector pipes

There are different strategies for the installation of ground collector pipes depending on the land available. Possible options are:

  • straight pipe loops in horizontal trenches
  • coils of pipes, also known as “slinkies”, in horizontal trenches
  • “compact collectors” (which look like plastic radiator panels) oriented either horizontally or vertically in horizontal trenches
  • straight pipe loops in deep vertical boreholes (anything up to 200m in depth!)
  • horizontal pipe loops under water (in a lake or river)

The most common approach is to use horizontal trenches of either straight pipe loops, where enough land is available, or slinkies where there is less available space. In our situation, the most cost effective and suitable option is to use slinkies.

The heat pump and cylinder

The heat pump itself is typically a tall fridge/freezer sized unit with various ancillary expansion vessels and pipework. The flow and return pipes to and from the ground collector pipe system connect into the heat pump as do the heating and DHW pipes. A separate highly insulated cylinder stores the heated water ready for its use. One attractive option for the cylinder is that it can incorporate a separate pipework coil internally which can be connected to a solar thermal installation. This means that hot water from the solar thermal panels can feed into the heating/DHW system thereby offsetting the amount of heat required from the GSHP which in turn increases the overall efficiency of the system (remember, the solar thermal heated water is free – ignoring the installation cost – too!)

Choosing a supplier and a specification for the system

Having decided that a GSHP was the technology to go for the next decision was to find a supplier and installer. I was very keen to find a company who had significant experience of this technology, a good reputation and good levels of support as it is vital that the system is sized and installed correctly for it to operate efficiently. The company would also have to be MCS (Microgeneration Certification Scheme) accredited in order for the installation to qualify for the various government grants available.

After a lot of research I decided to go with ICE Energy given their excellent reputation and long history of supplying and installing GSHP systems. My ICE Energy sales contact Richard Floyd was very knowledgeable and helpful throughout the whole process and provided answers to my many (and there were a lot!) and varied questions. After an initial assessment of the requirements, including details of the property, it’s construction, levels of insulation, number of rooms, ratio of space heated by radiators to that to be heated by underfloor heating, land available for ground collector pipes etc. Richard produced a specification for a system comprising an 11kW IVT Greenline HT+ E11 heat pump, 600m of ground collector pipe installed as 3 x 50m trenches of slinkies and an IVT STS280 280 litre cylinder with solar coil. A further site survey was carried out shortly afterwards by a very knowledgeable and friendly engineer Adrian Rawle to ensure that the initial system specification was suitable and appropriate, and to finalise any fine detail specific to my particular installation. ICE Energy are responsible for the specification of the system, the supply of all components, unlimited technical support during installation and final commissioning of the system once installation is complete. The actual installation of the ground collector pipes will be carried out by my main building contractor responsible for the barn conversion and the installation of the heat pump, cylinder and associated plumbing and electrics will be carried out by the plumbing and electrical sub-contractors working on the barn conversion.

Government grants – RHPP and RHI

It’s worth saying something about the government grants associated with the installation of renewable heating systems as it was a big factor taken into consideration when determining what system and technology to go for. The UK government has made a big commitment to green and renewable energy and one aspect of this is encouraging the take-up of such technologies in the commercial and domestic sectors. For domestic installations this translates into providing two grant and subsidy schemes known as the Renewable Heat Premium Payments (RHPP) and the Renewable Heat Incentive (RHI). RHPP is a short term scheme (scheduled to finish in April 2012) which provides a grant towards the installation of qualifying renewable heating systems. Ground source heat pumps are a qualifying technology which attracts a grant of £1,250. More interestingly, RHI is a longer term scheme scheduled to start in October 2012 which will result in guaranteed index linked payments for a period of 23 years (in the case of GSHP installations) based on the amount of heat energy a qualifying system is generating. The final tariffs have yet to be determined but the estimate at this time is that our particular installation would attract payments of between £1,500 – £2,000 a year!

What next?

The barn conversion has already started (look out for details of progress in other blog posts) so at some point in the next few weeks the work will start to install the ground loop collector pipes in the paddock.

Watch this space!