Frequently asked questions

1. Which is best, air-source or ground-source?

The choice depends mainly on the application, so the answer to this question requires some thought – Before we delve into that, ……one initial thought – in small very well-insulated houses, it might be hard to justify the extra cost and upheaval of ground source. The difference in running cost between air or ground would be small. In general, larger houses are more likely to lend themselves to ground source.

Advantages of Ground Source (GSHP)

The main advantage with Ground Source is that the ground below about 5 ft. (1.5m) deep maintains its heat during mid-winter. This is a particular advantage in climates with dramatic changes (swings) in air temperature. GSHPs are great during severe winters. It is also more of an advantage at the beginning of winter when the summer’s heat is still in the ground.

Disadvantages of Ground Source

Digging horizontal trenches in your garden can be very disruptive (though the growth does recover). Boreholes may be less disruptive, but are likely to be more expensive. The whole GSHP installation is likely to be expensive and take time. You also need a suitable position for the heat pump inside the house that is not adjacent to a bedroom or quiet room. There are potential pitfalls at design stage. The ground collector needs to be large and deep, else the energy-efficiency will tend to reduce towards the end of a long winter.

Advantage of Air Source

Air source is far easier and cheaper to install. The unit would be sited outside the house. This might be welcome for smaller houses where there is no room for equipment inside. During autumn and spring, a good ASHP is likely to be equally as efficient (same running cost) as Ground Source.

Disadvantages of Air Source

At the coldest times of the year, the efficiency of the device is at its minimum. This is just the time when maximum output is required. So during severe winter, some alternative supplementary heating may be needed. ASHPs have a defrost mechanism that clears frost build-up on the heat-exchanger when the air is below around 6°. However, units have advanced considerably over the last decade or so, so this issue is minimised. The defrost mechanism typically reduced the output by only about 10%.

The unit will need outside space which is not always available. The operating noise can be relatively low, but this could be unwelcome for some. Being outside, the unit will be affected by the weather. It is therefore not likely to last as long as its kept-in-the-warm GSHP counterpart.

Type of building

Interestingly, older solid-stone (high thermal mass) buildings could utilise a relatively small ASHP quite well by utilising the thermal storage of the building’s fabric. Due to the slow response of these ‘heavy’ buildings, the heat pump could be set to run more during the warm day than the cold night. The average efficiency of the heat pump is improved whilst the outside air is warmer. The average inside temperature may remain stable due to the heat storage in the stone. That said, it can be extremely difficult to configure this with currently available controls.

Supplementary heaters

An ASHP would match well with some sort of supplementary heating device to help cope with times of very cold weather. Many heat pumps can be used alongside boilers (bivalent). The ASHP can provide all heating on the average winter’s day, but ‘call upon’ the boiler for assistance during very cold times. The most environmentally-sound option could be a combination of ASHP and wood fuel. If the wood stove is used during the coldest times, the ASHP could automatically reduce itself – hence the heat pump’s load is reduced at the times when it would be least energy-efficient.

See slightly old article on Air Source – It’s worth reading

2. How do I ensure that my system is energy efficient?

A heat pump can heat water up to around 55°C, 130ºF, (although this will vary depending on model type). It is very important to understand that the particular characteristics of a heat pump means that the hotter the water, the poorer the energy-efficiency, so running at a lower temperature will save a lot of energy. The following figures, for a typical ground source heat pump system, illustrate this:
(note – the larger the COP the better)

Water heated to 55°,    COP = 2.4 ( e.g. a 6kW heat for 2.5kW in)

Water heated to 45°,    COP = 3.2 ( e.g. a 6kW heat for 1.9kW in)

Water heated to 35°,    COP = 4 ( e.g. a 6kW heat for 1,5kW in)

(COP is the energy efficiency ratio. See Glossary for better description).

By keeping the operating temperature low, a high energy-efficiency can be maintained. This is generally achieved by having large radiators or good underfloor heating.  That said, inefficient high temperatures are often due to incorrect settings and adjustments. Good system design along with correct setting of the heat pump controller is vital.

I see many heat pumps that are badly set-up, so are running less efficiently than they could. The operating water temperature can sometimes be dropped by attending to various things – For example – by operating for longer periods at lower temperatures, by ensuring that the water is circulating adequately, e.g. floor loops should not be throttled too much, and by ensuring that any mixing valves are operating correctly.

3. Why is underfloor heating coupled with a heat pump so highly rated? Can I use radiators?

The lower the temperature of the heated water, the better the heat pump’s efficiency, and lower the running costs.

Radiators traditionally used hot water (60 deg.C, 140°F), but with better-insulated building and larger radiators, this can be dropped to around 45°C, 113°F.

In simple terms, the bigger the heat-emitting surface area, the ‘cooler’ the water needs to be. Indeed, a good underfloor heating system may require water that actually feels cool to touch. However, such low-temperatures are only achieved with a good underfloor design. In simple terms the more pipe the better (it’s not rocket science), so long as the pressure drops and circulation pump power is kept within limits.

For energy-efficiency, a concrete screed systems with tiled surfaces promise to be the best – better than a timber floor that act as an insulator. Carpets can be used, but reduce the system efficiency when a heat pump is used.   However, the pipe design can be altered to part-compensate for the timber or carpet covering.

One thing to be mindful of is underfloor surface temperatures. For old poorly-insulated buildings, the floor might need to be uncomfortably warm. By contrast, in a highly insulated new-build, the heated floor could actually feel cold. For this reason, some choose a timber floor in these super-insulated houses simply to keep feet feeling warmer.

So, back to the question, can radiators be used? Yes, however, they might need to be very big. If the house is highly insulated, it is far easier to achieve nice low efficient radiator operating temperatures. Furthermore, given the improvements to energy efficiency of heat pumps, it may be more acceptable to operate radiators a little warmer than it was previously, so they may not need to be as big as they were in former years.

Given that heat pumps generally operate at lower temperatures, they tend to require higher flow rates. Make sure your pipe work is not too restrictive.

4. Why are heat pumps better suited to well-insulated houses?

One might think that there would be more to be gained (saved) by fitting a heat pump to an old building with very high fuel bills. However, in poorly-insulated buildings, it is more difficult to dissipate sufficient heat without running the radiators or underfloor at a relatively high temperature (hence lower COP and higher running cost).
By contrast, low radiator or underfloor temperatures are far easier to achieve in well-insulated houses, so the COP can be considerably higher, which is desirable.

However, mains electricty has de-carbonised considerably over the last few years. This shifts the goal posts, and means that heat pumps in old buildings, with mediocre COP, may now show much better carbon savings compared to common methods of heating.  Added to this, the generous RHI payments can mean that its now affordable to install expensive and efficient emitter systems (e.g. underfloor) that can achieve good COPs in  older buildings.

Contrary to popular belief, this issue has nothing to do with heat being lost through walls etc. Its all down to the water temperature that the heat pump operates at.

5. I am getting conflicting information about sizing. Why is this?

Traditionally, boiler systems have been sized to be plenty big enough so that they can raise the temperature of a building in a relatively short period of time. However, as discussed in the items above, low operating water temperatures are advantageous for heat pumps, so it is far better to operate a small heat pump for much longer hours with lower water temperatures. Furthermore, big heat pump installations are expensive to buy compared to boilers. All-in-all, this ‘steers’ us away from large-capacity heating systems using heat pumps.

A small heat pump system, that may operate almost continuously 24/7 on the coldest day in winter, could be cost effective all-round. This system may however have an exceptionally slow response time such that it is incompatible with traditional time/temperature control methods, and requires a different user attitude. The slow response issue may become less of a problem in fully-occupied houses and well-insulated buildings where the temperature can be maintained constantly at one level. As you can see, the compromise between the traditional methods and more heat pump friendly methods are open to differing opinions, and all hinges on good controls and knowing how to operate them. It’s no wonder that sizing can be a contentious issue.

One further issue to be mindful of is the possible need for direct-electric back up heating to supplement an overly small heat pump. Direct electric heating is a bad thing in many ways, but from my experience, it is sometimes a lack of understanding of control techniques that is responsible for excessive use of electric back-up heaters, rather than the fault of heat pump being under-sized.

In Sweden and other countries, they have successfully used small heat pumps, but he MCS scheme (Microgeneration certification Scheme) does not allow heat pumps that are smaller than the building’s needs on the coldest day.

In my personal opinion, a lot of heat pump units are too big, and some heat emitter (e.g. radiators) circuits too small.

6. If I leave my heat pump on continuously, will it cost a lot to run?

Firstly, Heat pump systems are (hopefully) self-controlling, so they automatically turn themselves on and off. If we leave the unit ‘enabled’ (switched on). It is not necessarily using power all the time; especially if it is turned to ‘low’.

Here is one factor to consider – “the circulating water temperature has a very considerable impact on energy-efficiency. The hotter the water, the lower the heat pump’s energy-efficiency”.

Here is another factor to consider -“there is no point keeping a room hot all day if it is unoccupied” – this is truer for boiler systems than heat pumps.

Let us consider this further – a radiator may achieve the same room-comfort by being either ‘warm’ all day, or ‘hot’ for just a few hours. Given that the energy efficiency of a heat pump (COP) will improve considerably if the radiator temperature can be kept low, it follows that continuous low-temperature radiators (or underfloor) should be better. BUT, the building will lose more heat if it is warmer for longer. On balance, the extra energy required (with hotter water) to rapidly heat a room can be greater than the energy to keep it warm all the time (with lukewarm water)… to keep it ticking over. All this depends on the type of building (heavy or light-weight) and the occupancy, but experience shows that leaving heat pumps on a low continuous setting can give lower running costs for many situations. They are also far simpler.

7. How should I set Thermostatic Radiator Valves (TRV)?

TRV valves were developed with boilers in mind. If too many radiator valves are ‘throttling’ the flow, the energy efficiency of a heat pump can be impaired in some cases. Its best to keep the main area’s TRVs set high, and use TRV valves to limit temperatures in bedrooms and extremity rooms. You might find it worth doing some tests – if you open most TRVs up, I assume the house will become hotter than you need. If so, try adjusting the water temperature on the heat pump controller down. You will need to reduce the ‘heating curve’ or simply reduce the heated water setting. If you can get the house to desired temperatures, all well and good, now use the trv’s to ‘trim’ the temperatures of bedrooms etc.

8. What about underfloor heating zoning?

This is a similar issue to trv valves. Room zones tend to open/close sporadically, but a heat pump would ideally prefer to heat most rooms together at one time. Given that there is a degree of natural self-regulation with under-floor heating, it is possible to heat well-insulated buildings by keeping many of the zones open all the time. This is again an issue where manufacturers have differing opinions. Keeping your setting on the heat pump (the heating curve) low helps to keep zones on for longer – this can be beneficial since the heat pump has a larger load, and ‘sees’ water at nice low efficient temperatures.

9. Can a heat pump also heat the domestic hot water?

It certainly can, but whilst heating to the high temperatures required, the energy-efficiency reduces. However, even low efficiencies are far better than an electric immersion heater. Most of the latest heat pump units have the hot water function built in, so it is usual to use this facility. As the insulation levels in buildings increases, the room-heating demand drops, but the hot water demand is, if anything, increasing. It is therefore becoming more important to optimise the hot water function. i.e. the size and design of the hot water cylinder is very important.

10. Do I need a buffer tank?

The manufacturers do not always agree on this point, but it is suggested that you go with their specific recommendations. A buffer tank is simply a quantity of water that can help to reduce the number of times the heat-pump has to ‘cycle’ (i.e. times it has to stop and start). It is particularly necessary in a larger property where many heating zones are involved. In well-insulated and open-plan houses a buffer tank may not be needed. In these cases, the floor itself can act as the buffer. However, the floor must have sufficient pipe in it with good thermal contact within a thick screed. High water-content radiators can also act, in part, as a buffer, and may be a benefit in some buildings.

Air source heat pumps will have a defrost mechanism. This reverses the heat flow for just a few minutes to melt the ice. If the radiators do not have a large volume, then a small buffer tank might be advisable

The relative size of the heat pump also has a bearing here; if the heat pump is relatively large, it is more likely to need a buffer cylinder than a continuously-enabled small one. Furthermore, many systems now have variable ‘inverter’ compressors. These ‘modulate’ their output, so rarely need a buffer cylinder.

In summary; having a buffer tank is playing-safe, and recommended if the radiator or underfloor system is unknown, or un-matched. With well-designed house and well-designed emitter circuits, you might be better off without one. I try to design the buffer tank out such that it is not needed

11. What is ‘Weather Compensation’?

As you now know, it is important to keep the heated water as low as possible if high efficiencies are to be attained. It is sensible, if not vital, to adjust the water temperature depending on outside conditions. i.e. your system might require water for the floor at 40°C when at -5°C outside, but require only 32°C when it is +5°C outside. This can be adjusted manually on the heat pump over the seasons. However, Weather Compensation does this automatically, and is an integral part of almost all ground source heat pumps. This is the ‘heating curve’ setting. Not only can this control save energy, but interestingly, a well-adjusted control like this can sometimes achieve accurate-enough room temperatures without any thermostats.

For ASHPs, the advantage of this facility is less clear. It may be the most practical method, however, it can also lead to the heat pump ‘revving up’ during cold nights. If the building has high mass, it might be more energy-efficient to operate on a fixed daily temperature based on the average daily outside temperature.

Its worth noting that a well designed emitter system can help here in systems with no buffer. If the emitters are large, they can ‘hold down’ the water temperature naturally, but still be giving out the same average heat quantity.  The low water temperature improves the COP. 

12. Is a vertical borehole better than a horizontal pipe trench system?

The heat from either if these systems is mostly stored solar heat in the material of earth near the surface. Either system will produce similar results, and the warmer (or ‘less-cold’) the fluid in the pipes, the better performance we achieve. Choice of type is usually a matter of cost and practicality. e.g. if land is available a similar-performance horizontal trench system will usually be cheaper to install than a borehole. Since excavator costs are generally a fraction of the cost of borehole drilling, it should be cost-effective to size trenches well (large), and make them deep. Large deep trenches can sometimes out perform boreholes.

If summer cooling is required, then the borehole may prove to be a better option. Free-cooling or passive cooling is likely to be far better from a borehole.

13. How much ground do I need?

The more the better, but this strategy could be un-economic. The average garden is often too small to get efficient heat output from a horizontal trench. However, as houses become better-insulated, then ground collectors don’t need to be so big. Do not underestimate the upheaval of digging trenches, but when the grass and plants have grown back it will all seem worthwhile. This component of the system should last well over 50 years, so extra pipe loops should eventually pay for themselves since the efficiency of the system will be better. Ground conditions will also have an effect on performance. For example, wet conditions assist the heat transfer process. Dry, sandy ground is inferior, so would require far more pipe-work and a larger area.

There is some debate about the depth, and 2m may be ideal if you are trying to achieve the highest efficiencies. However, excessive cost and health & safety regulations usually mean that shallower trenches are used, and 1.2m to 1.5m seems typical. During deep cold-spells in mid-winter, deeper trenches would be beneficial, but arguably, the extra cost of deeper trenches may not be cost effective.   If the collector area is small, there may be little advantage by going deep.

15. How long will a heat pump last?

Most good water-to-water type heat pumps will far outlast even the best quality boiler. They should operate for over 20 years, and with minimal maintenance.

Air source systems are usually exposed to the elements and have a slightly harder life, so may have a similar life to a gas boiler.

Sadly heat pumps can break down, and occasionally are very expensive to repair. Thankfully such instances are very rare.

16. Can I power a heat pump from a renewable energy source like a windmill or solar panels?

Yes you can, but it seems sensible to consider things separately at present. The middle of winter, when most heat is needed, coincides with times when days are short and solar-electric (PV) output is low.  It is unlikely that a PV array would contribute much for the heat pump in the depth of winter.  Conversely, in summer, the PV output can be vastly greater than any heat pump’s needs at that time, so usually the mains-grid is used to export PV electricity.  There are of course a lot of cold sunny days  when PV output could power your heat pump, but dont underestimate the difficulty of matching these two up.  In time, software driven systems will no doubt control this nicely

Again, wind is variable, so would be mains grid-linked. Whilst it may be preferable not to have all your eggs in one basket, it can be expensive having several technologies, and debatably better to concentrate on one properly.

There is a further thought here – most PV electric panels are only up to 20% efficient, so even when coupled with a heat pump, they have similar efficiency’s to solar thermal. (solar thermal at low temperature can be over 60% efficient). That said the electricity from solar PV is far more versatile than warm water from solar thermal. PV also have other advantages, e.g. they can be positioned anywhere, unlike solar thermal that needs to be close by

There is also interest in re-charging a ground collector using Solar thermal. This could allow the ground collector to be smaller in area. The advantage could however be minimal if it’s not done well, and it could sometimes be akin to running your rain gutters into the sea to collect water. Unless the heat pump actually ‘sees’ a higher source temperature in winter due to re-charging in summer, there would be no advantage. This method could be an excellent way of improving a system that was fitted with collector loop that is too small (as was common on some older systems). You could use low-cost un-glazed pool solar panels here.

17. Is there a real environmental benefit?

A decade of more ago, when a large chunk of our electricty was derived from coal, the environmental benefits of some heat pump systems were marginal. At that time, it was only highly energy-efficient heat pumps that showed very good enviromnemal benefits.   However, over recent years, the electricity used to power them has got far ‘greener’. This is in part due to highly efficient gas power staions, and in part due to the increase in renewables. This means that you don’t need to have an exceptionally high COP to be able to show a good Low Carbon advantage.