Heat pumps work brilliantly if they are asked to produce a flow temperature of 350C rather than the 650C or 750C that a biomass/gas/oil boiler naturally generates. This favours underfloor heating systems and oversized radiators that can warm the room, despite the fact that they don’t feel really hot to the touch. Add in a well insulated home and a heat pump can ‘waft’ this warmth into the building in an extraordinarily efficient and manageable way. They are most often run on a continual basis (rather than intermittent) where the heat of the building is rarely allowed to fall below a comfortable temperature and is allowed to gradually rise to the required level as required.
Run in this way, the efficiency (or coefficient of performance – CoP) of the heat pump comes into its own. Whether the source is air, ground or water, the heat pump is able to extract the energy and multiply it to provide the useful warmth you want.
Combined with a PV system to supply some of the electricity or a solar thermal system to take over the hot water production in the summer months, a heat pump can be a really economical way to heat your home. What’s more, the maintenance costs are much reduced in comparison to gas and oil.
The Boiler Upgrade Scheme is cash incentive to make householders migrate away from fossil fuels. Whether you have an aging gas boiler that needs replacing or are about to start a whole-house refurbishment project, now could be a great time to take advantage of this £5,000 grant.
Heat exchange is no robbery
Fundamentally, all heat pumps work in the same way, despite the fact that their source of energy is different. The warmth from the air, ground or water runs through an heat exchanger where it meets a liquid/vapour mixture that boils at around -150C. The refrigerant turns to steam then is compressed producing superheated vapour which runs through another heat exchanger, giving away the energy to the central heating or hot water circuits. The cooled vapour is allowed to expand and condense once more before beginning the whole process once more.
It is this change of state which creates the additional two, three or even four units of heat from the single unit of electricity that powered the process. Indeed, it is how the technology counts as renewable, despite the fact that the electricity may have been generated using hydrocarbons as a fuel source. If the CoP is 3:1, the two additional units of heat are regarded as carbon neutral.
Creating heat from fresh air
Air source heat pumps (ASHPs) are the most common type of heat pump since the air around our homes is both accessible and free. The installation costs are lower and take less time but the seasonal efficiency (SCoP) is generally lower that the other types of heat pump as the air temperature can drop significantly in the winter. Remarkably, however, an ASHP continues to work even at -50C as even this temperature can boil the refrigerant in the heat pump.
Creating heat from fresh air
As the name suggests this system takes the heat from the air and transfers it to your central heating system. Best of all, it would run through underfloor heating circuits (UFH) which can run at 350C and produce a lovely feeling of overall warmth in your rooms. If that’s not convenient, oversized or fan assisted radiators can be run at 450C to keep your rooms nice and warm too. Calculating the heat loss of the building is crucial as is determining how the heat should be distributed in each part of the building.
Air-to-air heat pumps
In this system the warm air created by the heat pump is blown through the building using wall mounted or ceiling mounted fans. Conveniently, this means there is no disruption putting in a central heating system but these heat pumps can’t produce hot water so tend to be used as additional heat sources rather than the sole provider.
Digging for victory
For anyone lucky enough to have a large garden or paddock (plus accessibility for digging machinery) a ground source heat pump (GSHP) is a very efficient way to heat your home. The soil around your home absorbs the heat from the summer sun and stores it through the winter months. Even when there’s snow lying deep on the ground, the soil a meter below is likely to be at 80C or 90C – easily warm enough to boil the refrigerant in the heat pump. And because the earth is reliably warmer than the air, the seasonal efficiency of a GSHP is almost always better than an ASHP.
Loops or bore holes?
If there’s enough available surface area of land the pipes that are buried can be ‘ploughed’ under in straight lines or laid in loops (slinkies) in broad trenches. All of the ground loops are brought together in a manifold and the consolidated flow brought to the heat pump.
If land area is in short supply, it’s possible to drill vertically down into the earth but this can be expensive – especially if only a few holes are required. In new build projects the bore holes can sometimes be combined with any piling required for the foundations, amortising the costs over two sections of the project. The depth of the bore hole varies and it’s possible to have a small number of very deep holes (100m+) or a larger number of shallow holes (20m+). Calculating the heat requirement of the building then extracting sufficient energy from the ground to meet that demand is clearly crucial. We need to examine the soil type and complete test ‘digs’ to ensure the method we recommend fits the bill.
For ultimate efficiency, dive into the deep end
Access to a lake, a river or even a big stream can put you at the top of the seasonal efficiency lists. Water benefits from solar gain in the same way as soil and can be remarkably consistent across the seasons. The pipe loops in the water bring the energy to the heat pump in the same way as the GSHP does but there will be less groundworks as they will be limited to the distance between the lake/stream and the plant room.
In static or slow moving water, the heating loops are weighted down (and sometimes caged) so they stay in the correct position within the body of water. The size of the water source is crucial as if there’s not a big enough thermal mass, the heat pump can extract so much heat it freezes the water.
In a big stream or river, it’s possible to use multiple coils or a special heat exchanger that the water flows over. In both cases the length or size of the heat exchange mechanism is much reduced since the water passing over it constantly refreshes its ability to produce heat.
Heat pumps work more efficiently if they can provide their heat indirectly via a buffer, accumulator or thermal store. They are more efficient if the flow and return temperatures are kept low and the store can smooth out any abruptly changing calls for heat. The store protects them from this and allows the heat pump to work for longer, more continuous periods.
The buffer can sometimes be called upon to provide the energy to allow the heat pump to go through its defrost cycle. A diverter valve in the heat pump is switched and the warm water is used to melt the frozen parts of the pump.
Domestic hot water (DHW) cylinders
Making really hot water is one of the most difficult things for a heat pump to do efficiently as the CoP really dips at high temperatures. Generally, cylinders are only heated to 550C and an immersion used to heat the last 50C. This might only be once a week but is an important addition as it combats bacterial growth (legionella).
Each heat pump manufacturer has their own DHW cylinder and are keen that matching pairs are used. The heat transfer coils in the cylinder have a large surface area (2 or 3 square meters rather than 0.75 for a conventional cylinder coil) and it’s important that the flow rates work effectively across both devices.
Essential reading if you’re considering a heat pump
Aside from the fundamental differences in air/ground/water heat pumps, there are some common elements worth understanding:
1. Heat pumps work most efficiently when they’re making warm water rather than hot water (350C to 450C rather than 600C).
2. A well insulated, carefully draft proofed house is essential to get the best out of your new system.
3. Specifically designed underfloor heating is a perfect ‘emitter’.
4. ASHPs are getting quieter all of the time but you (and your neighbours) should be aware of the noise it will make in the desired position.
5. Removing a conventional gas boiler and simply replacing it with a heat pump is likely to produce disastrous results!
Many modern, well insulated homes make ideal places to use heat pumps. Ideally, they’d already have underfloor heating downstairs leaving just the radiator circuit to enlarge so the reduced flow temperatures can effectively heat the rooms. There are big financial savings to be had over oil or LPG combined with the satisfaction of seeing your carbon footprint drop dramatically
An extension or other building works can provide the perfect opportunity to migrate from a hydrocarbon burning fuel to a heat pump. Changes to the heating circuits and the insulation levels can be incorporated into the build, minimising the cost implications. If the foundations require piling, the loops for a GSHP can be incorporated into them. A large extension that wraps around the original house can sometimes reduce the heat requirement for the home, despite the fact that the floor area is greatly increased.
Not only are commercial heat pumps bigger than their domestic cousins, they can be linked together to create a huge heat source. Mitsubishi have a Monobloc ASHP which cascades up to 688kW – powerful enough to supply office blocks, factories or multiple residence apartment blocks. Once again, insulation levels and low temperature emitters are key but with this much power, the only limitation is the imagination of the designer or architect.
Schools, sports clubs, community centres – anywhere people come together for activities, is a good place to consider a heat pump. The economics of this scale of heat production coupled with the Government’s Renewable Heat Incentive means you can turn a liability into an asset with the added bonus of a huge feel good factor. But you’ll have to act fast, the RHI has only a limited time left in its current form and it’s difficult to predict what forms of incentive might be waiting in the wings.