G : GSHP's

FAQs About GSHP's


1. Can the heat pump heat the whole house?

Yes it can. Newly built or well-insulated buildings can be easily heated with a heat pump. Old, badly insulated buildings can however be a problem in cold weather, with heat pumps struggling to satisfy them. In these buildings, the temperature of the radiators or underfloor heating may have to exceed the efficient working temperature levels for the heat pump. Fires or boilers are often necessary to ensure adequate heating in the winter. Even in very well insulated houses, it is not uncommon to retain a fire or stove for extra comfort in cold weather. Air source systems require more back-up heat than ground source systems.


2. There is a big variation in efficiency claims. Who do I believe?

There are a great variety of heat pump applications and uses, resulting in a wide range of efficiencies. Just be mindful that there is sometimes a tendency to quote best-case figures.


3. What is the environmental benefit?

We believe that only systems having COP's (efficiency) of 3.5 and above 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 not good for the environment. Wood-burning is usually the best environmental heating solution.


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

Yes you can. A hydro-powered heat pump is probably the most viable renewable power-source since it is fairly constant. Solar or wind inputs would be difficult, especially on a small scale. The outputs are variable and dependent on the weather, so there can be problems matching the input power requirements. The very large number of photo-electric cells required to power a heat pump would probably make this option impractical.
Powering a heat pump from grid electricity derived from a renewable source (e.g. wind-farm or hydro) is a good option for the environment. However, there is some debate about the real overall benefits of these schemes.


5. What is the best system?

In a nut shell, a spring water sourced system heating a well-insulated building through underfloor heating. Since springs are uncommon then ground source with boreholes into the water table would be the next best option. If a back-up is required to support the heat pump, then wood is the best option, but it is labour intensive. An oil or gas boiler would be a good back-up, but this adds significant cost. A direct electric back-up is cheap to install, but not ideal environmentally.


6. Why is the size of the heat pump that I have been quoted for so much smaller than my existing boiler?

Boilers are usually larger than they need to be, and are often used with time switches to quickly heat a building. It makes sense for a plumber to play safe to avoid any complaints that the house is not warm enough. However, it is expensive to install an oversize heat pump system, therefore, the heat output is more accurately matched to heat demand. Furthermore, heat pumps are happy running continuously without a rest. Don't forget you are saving energy with a smaller system. This is also true for boilers, but to a lesser extent.


7. I have an average town house, with a small garden. Can I install a heat pump?

Installing ground-source is often difficult in this situation. However, our borehole drilling machinery is very compact and can access surprisingly tight spaces. You can however, also do other things with your money to save energy and fuel costs. Don't neglect the obvious, draught-excluders etc. Consider investment in some serious insulation in the form of either internal or external wall cladding. This is not as exciting 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 consider fitting an air-source system. Unfortunately it may not 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 may require planning permission, can be a little noisy, will not last as long as a ground source system, and are not very efficient in mid-Winter.


8. Why is underfloor heating coupled with a heat pump so highly rated?

The lower the temperature of the heated water, the better the heat pump's efficiency. Since standard radiators can reach as high as 80 deg.C (175 deg.F), far hotter than a heat pump can achieve, you will have to significantly increase the area of the radiators to be able to utilise the working temperature of 50 deg.C (122 deg.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 considerable energy benefit. Underfloor heating pipes can give sufficient heat 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. of floor area. For the average older house, the floor would have to be too warm to provide full heating. It is important to realise that heat pumps like lower temperatures and higher water flow within the underfloor pipes. This means more pipe and a lower pressure drop than average. Make sure that the underfloor heating installer respects the characteristics of the heat pump. 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 are by far the best, far better than wood. Be mindful of floor coverings. Carpets can reduce the system efficiency significantly when a heat pump is used.


9. 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 can produce similar results. However the use of slinkies, i.e. horizontal trenches is more likely to lead to problems associated with freezing of the manifold and ground (Permafrost) and subsequent loss of efficiency. However, it is a matter of cost and practicality. e.g. if land is available the horizontal trench system may be cheaper to install than a borehole. If cooling is required, then the borehole will prove to be better.


10. How much ground do I need?

Do not underestimate the upheaval of digging trenches. As a rough guide, you will need 55-65m of slinkies per 1 kW. So for example a 7 kW heat pump with good ground would require 385m of slinkies. More if dry sandy soil, less if very wet. There is some debate about the depth, but 2m deep is ideal. Many heat pump installers are now recommending against the use of trenches and "slinkies" because of increasing complaints relating to permafrost, or icing up of the area above the coils and in and around the manifolds.

Borehole space requirements are different. As a rule of thumb, one 100m deep borehole will deliver 5 kW (taking a COP of 3.6 into account, enough to run the same 7 kW heat pump). If more than one borehole is required then the distance separating boreholes should be around 6m to avoid thermal coupling. They should also be a minimum of 3m from building foundations. 


11. Is an air-source system a viable option?

Air-source can be easier and cheaper to install. However it is very visible, often requires planning permission and makes a noise. Also, it does not promise very high energy efficiencies for year-round heating in the UK, particularly in the North. The main reason being that the efficiency drops when you need most heat. A back-up from a boiler at such times is usually required. Ground source, by contrast, maintains a constant heat output unaffected by the daily temperature changes above. Smaller air systems may give reasonable overall results if used alongside a boiler, but only if used when air temperatures are not too cold. Air source units usually have a shorter life since they have to work out in the elements.


12. Can my plumber install a system?

Unless he has had special training, e.g. from the heat pump manufacturer, we would not recommend this, as there are some potential pitfalls to avoid. A heat pump cannot simply be fitted in place of a boiler as there are some fundamental differences in the operation, it is specialist skill to ensure you get the heat pump working as efficiently as possible.


13. How long will a heat pump last?

Most good ground source heat pumps will far outlast even the best quality boiler. The manufacturers give a guide line of 25 years, and with minimal maintenance. Despite this projected long life, heat pump manufacturers will usually only guarantee their heat pumps for 2 years or so. Why? Simply because the diligence and cleanliness of the installing engineer is critical to the expected life of the unit. This area is outside the control of the manufacturer.
Air source systems are usually exposed to the elements and have a slightly harder life, so may have a similar life to a boiler.


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

Yes it 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 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 facility. i.e. the size and design of the hot water cylinder is very important.


15. 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 have the water temperature vary dependent on outside conditions. i.e. if you require water for the floor to be at 40°C when at -5°C outside, you may require only 32°C when it is +5°C outside. Weather Compensation does this automatically, and is an integral part of most heat pumps.

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