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Semi-detached property in Edinburgh

Ground source heat pump in a 1920 - 1944 property, hosted by Steven.

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3 bedrooms

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Ground source heat pump

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Installed by McInnes Group, Inverness - through the BEIS Warmworks Electrification of Heat project

  • 8 kW Vaillant flexoTHERM ground source heat pump
  • 2 x 100 metre deep boreholes
  • 300 litre volumiser buffer tank
  • 200 litre hot water cylinder
  • K2 and K3 radiators with TRVs plus towel rail

  • No solar panels
  • EV charger (3.6 kW Ohme)
  • Waste water heat recovery for the first floor shower - preheats cold water supply to shower and to hot water cylinder
  • Cavity wall insulation
  • Under ground floor insulation
  • Extra loft insulation
  • Existing double-glazing

We took part in the BEIS Warmworks 'Electrification of Heat' (EoH) research project and had a ground source heat pump installed in June 2021, including remote monitoring equipment. 250 different heat pumps were installed across SE Scotland as part of the project to see how they behaved in various retrofit situations. We maybe could have had an air source heat pump, but there were very few ground source heat pumps being installed and we thought it would be an interesting challenge to install one in a small suburban garden. There are 2 x 100 m deep boreholes. One borehole was drilled in the garden, right beside some bushes, and the other was drilled under what is now a small driveway, about 10 metres away.

Our house is quite small with few spare internal spaces suitable to install the extra plant for a heat pump, so I built a small, insulated lean-to shed against the side of the house as a plant room for the heat pump, buffer vessel, hot water tank, borehole manifold and other pipework, meters, etc. 

Inside the house, all of our pipes and radiators were replaced as part of the EoH project. The radiators are all larger in order to operate at much lower temperatures, which greatly improves heat pump efficiency. The system is designed to operate at a maximum radiator water temperature of 45°C flow and 40°C return at -3°C outside, but reduces as the outside temperature warms up (weather compensation). We tend to keep the room thermostat (in the hall) at 18.5°C, but the main rooms sit around 20°C. At night we set the heating back to 17.5°C. It is important not to change the room temperatures too much as the system operates very slowly compared to a standard heating system. This is typical and actually means the house is very comfortable as the temperature is generally cooler overall but doesn't feel cold as the building fabric stays warm. You don't have the blasts of heat from very hot radiators, just a gentle warmth.

The installation process was quite disruptive, especially as the borehole drilling rig broke down several times and had to wait on spare parts. The garden was also flooded quite badly with muddy and oily drilling water, partly because of the problems with the rig and due to the type of ground that was being bored into. That is not typical, we were just unlucky! The install indoors was better, neater and less painful to endure, as the installers managed to get under the ground floors to run pipes in most places.

The installation came along at just about the right time as the existing gas boiler was getting to the end of its life and the hot water system was poorly performing.

Because it was a research project we did not have to pay for the install. However, because we chose to build the plant room shed, and the restoration works for the garden and some cosmetic repairs indoors, it was by no means a 'free' heat pump. That was expected, although we had some concerns that the running costs might be more than the old heating system. I think we have found that the heat pump is about the same cost to run, although with energy price increases (which we managed to avoid for a year due to a fixed tariff) this has increased a bit. The very high efficiency of the heat pump helps to protect against the higher cost of electricity. We also have an off-peak electricity tariff (Octopus Intelligent Go), which means we can heat the hot water cylinder at night and if needed, charge our electric car, for 7.5 p/kWh. Daytime running costs are 30 p/kWh plus 60 p/day standing charge. 

I have spent quite a bit of time over the following years tweaking the controls, to adjust things like weather compensation curve and other settings, which has required quite a bit of research and following 'Energy Twitter'. The system is outperforming its design estimates (MCS specifications). This is probably in part because we added more insulation to the house after the installation, particularly under the ground floor. In 2022 we had an overall Coefficient of Performance (COP) of 3.1 (310%) and in 2023 it was 3.8 (380%). This included hot water heating, which slightly reduces the efficiency - excluding hot water, for space heating the COPs were 4.1 and 4.0 respectively. The Vaillant app we have for the heat pump is very good at showing different graphs and other data for heat pump, which is great for someone like me who is a Building Services Engineer and generally a bit of technical geek!

I do however have some concerns that some heat pump systems may be a bit overly complicated for their owners to use and there would definitely be a benefit in trying to simplify this. However, in the process that also needs to ensure that the operation (and before that, the design and installation) of the heat pump is optimal.

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