Kensa’s Technical Director and co-founder, Guy Cashmore, discusses how installers can ensure the very best efficiencies with ground source heat pump systems, as featured in Renewable Energy Installer magazine (November 2017).
Contrary to what the marketing and adverts may say, getting the best possible energy efficiency from a ground source heat pump system is all about the design and commissioning of the system it is connected to, rather than the actual rated efficiency of the heat pump itself.
It is surprisingly easy to take one of the latest A++ rated ground source heat pumps and yet get very poor efficiency from it; equally it is possible to get very good results from an older design, less efficient unit by connecting it to a really good system.
Another word of caution – MCS SCoP ratings (which are derived from ErP product ratings) should not be leant upon as being the sole indicator for what is the most efficient system and therefore the most affordable in terms of running costs for your client; sadly MCS SCoP ratings do not take into account many efficiency saving steps discussed in this article. Whilst a good MCS SCoP rating offers great Renewable Heat Incentive (RHI) returns, this does not necessarily mean the end user is going to have the most efficient system with the lowest possible running costs; to achieve this, and to achieve positive customer referrals on this basis, this article outlines the steps you can take to deliver the very best efficiencies with ground source heat pump systems.
In essence, it is the two temperatures that the heat pump is operating at which sets the efficiency; the further apart the two temperatures are, the less efficient the heat pump will be. For both air and ground source units we commonly call these temperatures the ‘source’ and the ‘load’.
Know your sources
With an air source heat pump an installer might say ‘well I can’t do much about the outside air temperature’, but actually the choice of location is really important to ensure that the discharge air from the fan gets cleanly away from the unit, otherwise a circular air flow can develop and the unit ends up getting the same air passing through it over and over again, which will really mess up the efficiency.
With a ground source heat pump, the temperature reaching the heat pump is very much a function of the ground array design. With a borehole system the installer can’t do much except design it well; over sizing can raise the source temperature a little but can easily reach the point of negative returns, so the cost of extra drilling is never recouped in energy saving.
Horizontal ground arrays tend to be cheaper to enlarge. Assuming the land is available putting in an extra circuit or two really doesn’t cost that much, it also partly future proofs the system should the building ever be extended and need more heat.
Combining horizontal ground arrays with soak away’s not only save digging cost but can also lift the average temperature reaching the heat pump; rain tends to come during slightly warmer periods of winter weather, the soak away’s carry this slightly warmer temperature into the ground, ready for harvesting by the heat pump.
Open loop systems where the water comes from deep underground can yield surprisingly high and consistent temperatures, with many parts of the South West having 15°C+ ground water all year round. Care needs to be taken though; if the water is very deep below the surface the energy used by the pump to lift it up can quickly exceed the energy saved because of the higher temperature. The usual point to start asking questions is any more than around 50 meters lift.
But for most installations, the lion’s share of any efficiency improvement is likely to come by making changes to the heating distribution system and even the building itself. Let’s start with the building fabric, everyone knows that improving the insulation of a building will reduce the heat demand. With a conventional boiler that’s where it stops; the fuel burnt by the boiler will reduce in a fairly linear fashion as insulation is added. With a heat pump the electricity used will reduce by a greater amount – why – because not only is the quantity of heat required reduced, but that energy can now be effectively delivered at a lower flow temperature, improving the actual heat pump’s efficiency; it’s a double win, the boiler only gets a single win.
Heating distribution systems
Source temperatures and building fabric improvements however pale into insignificance compared to a good or bad heating distribution design, it really is where efficiency gains can be won, or indeed lost.
In round numbers for every 1°C you can reduce the heating water temperature leaving the heat pump you will save 2% on energy. The key words to note here are ‘temperature leaving the heat pump’ because this temperature and the temperature actually reaching the heat emitters (radiators or underfloor typically) can easily be two very different numbers!
Let’s start with one of this authors pet hates: buffer tanks. Or to be more accurate, buffer tanks that use four connections – two flows in and two flows out – with an awful lot of water mixing going on inside. These have absolutely no place on any standard UK installed system. They are regularly seen to cause a temperature reduction of 10°C across them, so the heat pump has to operate 10°C hotter than is actually necessary to heat the building; an installation like this has just added 20% to the running costs, permanently, for no particular gain. Don’t do it.
Buffer tanks aren’t all bad however. With good design many systems can operate beautifully with no separate buffer tank at all; simply using the water in the distribution pipes as the buffer, this is probably the best option but can be difficult to achieve especially on retrofit installations. Where a separate buffer tank is deemed necessary it should only be connected with two pipes and should function as a bypass route rather than having full flow passing through it all the time; connected this way it doesn’t cause any mixing and only gets any flow through it when actually needed.
Any component on the heating system that allows warmer flow water to mix with cooler return water, without actually taking some useful heat from it in the process, is usually bad news for efficiency of a heat pump system. The next most common culprit is certain designs of underfloor heating manifold that feature a thermostatic mixing valve and a pump. With one or two exceptions these should be changed so they can’t act as a permanent bypass route. In general, thermostatic mixers aren’t ever required, the only exception is where the underfloor is a screeded type and the system also has radiators on it, without a thermostatic mixer the radiators will probably never get warm enough. The choice of heat emitters is many and varied, it really doesn’t make any difference, the only question that always needs to be asked is “what’s the lowest water temperature it will still heat the building with”; the answer is the lower the better, and that’s basically the end of it!
As heat pump efficiencies go up, so the energy used by water pumps on the system become a greater % of the total electricity used. Fortunately water pump manufacturers have also been busy improving their products, but all too often pumps are still over specified either through ignorance or because poor pipework design requires them. As a general rule the total heating distribution pump power should not need to exceed about 1% of the thermal output, so 100W for a 10kW heat pump. On the ground side about 1.5% should be sufficient.
This author dislikes multiple small circulator pumps dotted around a building; one correctly sized and controlled larger pump is almost always more efficient and is also quieter. All pumps need to be correctly wired and controlled, it is still common to find system pumps wired to a fused spur and left running permanently; even a little 60W pump if left running all year will consume about £75 in electricity, correctly controlled on a heating system this would be around £20.