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1. Which is best, air-source or ground-source?
Air-source does not promise the highest energy-efficiencies for year-round heating in the UK (particularly in the North). The main reason being that the efficiency drops in mid winter when you need most heat.

A good ground source system (with large trench) maintains a constant heat output unaffected by the daily temperature changes above. However, air source systems are much easier and cheaper to install.

In recent years, air source units have improved considerably. That said, there are still cheap (less efficient) units on the market, and it's worth bearing in mind that air-source generally have a shorter life-span than ground source since they have to work out in the elements.

In situations where a conventional heating system is already installed, or wood stoves are used, a relatively small air source system could be worthwhile addition.

See recent article on Air Source - It's worth reading: http://aecb.net/news/2011/07/air-source-heat-pumps-friend-or-foe/

2. How do I ensure that the system is energy efficient?
A heat pump can heat water up to around 55°C (although this will vary depending on model type). It is very important to understand 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:
Water heated to 55° COP = 2.4
Water heated to 45° COP = 3.2
Water heated to 35° COP = 4
(COP is the energy efficiency ratio).

For a COP of 2.4, you get only 2.4kW for 1kW electrical input.
For a COP of 4, you get 4 kW of heat for 1kW electrical input.
(See Glossary for better description).

By keeping the operating temperature low, a high energy-efficiency can be maintained. This is achieved by good system design and correct setting of the heat pump controller.

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 can be. Since standard radiators can reach as high as 70 deg.C (160°F) - far hotter than a heat pump can achieve, you will have to significantly increase the area (or number) of the radiators to be able to utilise a working temperature of around 45 to 50 deg.C (113-122°F), from a heat pump.
However, if the water temperature can be further reduced to around 35 deg.C or less, then there is a very considerable energy benefit. Underfloor heating pipes can give sufficient heat to a room at these low temperatures and are therefore a good match with a heat pump. This system works best in insulated homes where the heat required is less than 50watts/sq.m.
MAKE SURE THAT THE UNDERFLOOR IS SPECIFICALLY DESIGNED FOR THE LOWEST PRACTICAL WATER TEMPERATURES. Also, make sure that there is enough insulation below the floor to minimise heat-loss into the ground, which can be significant. Concrete screed systems with tiled surfaces are by far the best - better than wood. Be mindful of floor coverings. Carpets can reduce the system efficiency significantly when a heat pump is used.

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 a building with very high fuel bills. However, in poorly-insulated buildings, it is very difficult to dissipate sufficient heat without the radiators or underfloor needing to be relatively hot. By contrast, low radiator or underfloor temperatures are far more easily achievable in well-insulated house. The energy efficiency is therefore likely to be considerably better simply due to the lower operating temperatures of the heat-emitter circuits.

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. For heat pumps, on grounds of high installation-cost and high running-cost (due to lower COP), this strategy is not viable, so heat pumps tend to be smaller.

A very small heat pump system, that may operate continuously 24/7 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.

6. If I leave my heat pump on continuously, will it cost a lot to run?
Firstly, Heat pump systems are generally 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 -"there is no point keeping a room hot all day if it is unoccupied" - this is truer for boiler systems than heat pumps.

Another factor - the circulating water temperature has a very considerable impact on efficiency.

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 (COP) will improve considerably if the radiator temperature can be kept low, it follows that continuously-heated radiators (or underfloor) should be better. All this depends on the type of building, but experience shows that leaving heat pumps on a low continuous setting is likely to give lower running costs for most house types.

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 can be impaired. Its best to keep the main area's TRVs set high, and use TRV valves to limit temperatures in bedrooms and extremity rooms.

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 low helps to keep zones on for longer - this can be beneficial.

9. Can a heat pump also heat the domestic hot water?
It certainly can, but whilst heating to the high temperatures required, the 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 common 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 act as a buffer.

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-running small one. Furthermore, many air source systems now have variable 'inverter' motors. These 'modulate' their output, so rarely need a buffer cylinder. This is in contrast to fixed-speed air source where the heat output will vary greatly depending on the air temperature outside, so buffers are often recommended here.

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.

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. you 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 most ground source heat pumps. Not only can this control save energy, but interestingly, a well-adjusted control like this can achieve reasonable room temperatures without any thermostats.

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 mass of earth near the surface. Either system will produce similar results. It 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, and make them deep. This can compensate for it being in close proximity to the cold winter above.

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

13. How much ground do I need?
The more the better, but this strategy is expensive. 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 more pipe-work and a larger area.

There is some debate about the depth, and 2m may be ideal. Excessive cost and health & safety regulations usually mean that shallower trenches are used, and 1.2m to 1.5m seems typical. During long cold-spells in mid winter, deeper trenches would be beneficial, and if the recent extreme winters become more common, we might need to reconsider collector sizing in the UK. The pipe-work length, diameter and manifolding is usually calculated carefully so as to minimise the required pumping power to circulate the fluid.

14. Can my plumber install a system?
Yes, but there are some potential pitfalls to avoid. Pipe connections are usually larger in diameter than rule-of-thumb sizing. A heat pump cannot simply be fitted in place of a boiler as there are some fundamental differences in the operation. But the actual handy work is exactly the same. To get RHI money or any available grants, your system will need to be designed/installed by an MCS accredited company.

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.

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. The output from solar-electric (PV) occurs when little heat is needed, so usually the mains-grid is used for export/import. 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.

17. I have an average town house, with a small garden. Can I install a heat pump?
Installing ground-source is often very difficult in this situation. Boreholes are expensive and under-floor heating not always practical. You can probably do better things with your money to save energy and fuel costs. Don't neglect the obvious draught-stripping etc. Consider investment in some serious insulation in the form of either internal or external wall cladding. This is not as interesting as a heat pump but it will save you energy for the lifetime of the building.

If you have dealt with heat loss then you could fit an air-source system. Unfortunately it will not necessarily be cheaper to run than a very well controlled condensing natural-gas system at present. However, if your only fuel option is oil, then a well-optimised air-source system could save you a significant amount of money and benefit the environment. Be mindful that these units can be a little noisy, will not last as long as a ground source system, and are not as efficient in mid-Winter. You will need planning permission since units can potentially cause a nuisance to your neighbours.

18. Is there a real environmental benefit?
Currently, a heat pump with a COP of 2.2 (efficiency ratio) has a similar CO2 polluting effect as a gas boiler. We believe that systems should have COP's of 3.5 and above to give a significantly large enough environmental benefit over gas. Oil and coal are, however, not as clean, so heat pumps generally compete well here. If comparing with electric heating, then even a poor heat pump would be an improvement since electric heating is the most polluting form of heating. Heat pumps are rarely better than wood burning, but there are many issues here to consider.

It is worth noting that the carbon emissions due to mains electricity are likely to reduce as we attempt to de-carbonise the grid. This means that heat pumps should become 'greener' in the future.

19. What is the 3-phase electric supply issue?
Heat pumps are driven by fairly large electric motors. Such motors work best with a 3-phase electrical supply. Unfortunately this supply is not common for houses in the UK. Having said that, small heat pumps (say up to 8kW output) work just as well on single phase, and multiple-compressor systems are available. A 3-phase unit will debatably last longer, or may be a little more reliable than a single-phase unit. Most people however have no option other than single-phase, so 3-phase heat pumps are becoming quite rare in the UK.

20. I have heard of heat pump systems providing heat for under £50/ year. How can this be achieved?
The reason that such a house is so cheap to heat is that it is super-insulated. Sorry, but there is no magic box that can reduce fuel costs so dramatically.

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