Air Source Heat Pump Questions

Q. Why are ASHP ‘green’ when they run on electricity?
Q. What type of building is suitable for an air source heat pump?
Q. Heat pumps in existing buildings
Q. Will I be able to use my existing radiators?
Q. What should I do before I decide on an air source heat pump?
Q. Does an air source heat pump make financial sense?
Q. What financial incentives are available for heat pumps?
Q. What is COP and what does it mean?
Q. What is SPF and why is it important

Glossary Of Terms

Q. What type of building is suitable for an air source heat pump?
We can design heat pump systems for most building types - domestic and commercial; new and older builds. The ideal situation in which to extract maximum return from a heat pump is a new building (or newly renovated building that's up to current building regulations) that is properly insulated and has underfloor heating.

However, it is still possible and practical to install a heat pump into an older, less well insulated building - even one without underfloor heating. Bear in mind though that this is not the most ideal situation for obtaining optimum return from the heat pump.

Most radiators would need to be either doubled in size or converted to aluminium or fan assisted radiators to compensate for the lower water temperature and increase output temperature.

Q. Heat pumps in existing buildings?
It's possible to retro-fit a heat pump into an existing property - it'll certainly help reduce your reliance on gas, natural gas or oil.

You can consider a wide range of systems to contribute to the heat pump - solar collectors, wood-burning stoves. You'll know that underfloor or wall heating are the best match with a heat pump; most radiators will need to be increased in size but if you want to see if your existing radiators will perform with a heat pump, wait for the colder weather, turn down your boiler to about 40C and see how you feel. This at least will indicate whether you need to upgrade your radiators.

However, at the very outset, you need to upgrade your insulation throughout the entire property. Check your attic for insulation levels, (a good depth is 300mm) put dampers in open fires, see if you can have cavity wall insulation installed and try and make the property as air-tight as possible.

Q. Will I be able to use my existing Radiators?
If retro fitting, it is highly likely you shall need to upgrade your radiators. Heat pumps work efficiently at lower flow temperatures than traditional boilers so to compensate, we need to increase the size of the radiators to create a larger surface area in order to better exchange the heat into the room. We can do this by increasing depth width and height of traditional radiators, we can also exchange for the higher grade material of aluminium radiators or highly efficient fan assisted radiators. If you are planning a total refurbishment we can even retro fit under floor heating. Solutions of under floor heating are now available at depths of as little as 15mm.

Q. What should I do before I decide on an air source heat pump?
The earlier you plan your heating system and the better informed you are, the more likely it is that you'll acquire a well designed, environmentally and commercially sound heating system that suits your needs and is best for your home or office. Try and put yourself in the designer’s shoes. He will want to know about:

Location

Amount of space to heat (size of property)

Number of rooms

Room Dimensions

Room type ie. bedroom or kitchen

Building fabric, U values and insulation

Air tightness, quality of windows and doors

Desired or current emitter type (under floor or radiators)

Floor Construction

Floor coverings if going for an underfloor system

Occupancy (number of people living or working at the property)

Number of showers

Where all the equipment is to be sited

Electrical supply type, for commercial and big domestic systems a three phase supply may be required. Three-phase electricity is desirable for heat pump installations above 12kW but absolutely necessary for heat pumps with an output of 24kW and more.

If you have a set of architects' drawings you are able to post or email to us these will be helpful.

Q. Does an air source heat pump make financial sense?
Yes, if you ‘fit the bill’
If your property lends it’s self to a heat pump installation, meaning:
If it is well insulated and can hold its heat.
If you are enquiring about a heat pump for a new build then current building regulation will make your property automatically the ideal building for an effective heat pump installation. In meeting these criteria, financially you can expect to reap the benefits in savings and RHI payments from your investment in a heat pump installation especially when considering the rapidly rising cos of fuels such as gas oil and natural gas.

Q. What financial incentives are available for heat pumps?
RHI or renewable heat incentive is a scheme very much like the feed in tariff for solar pv; for each kw of energy you generate you are paid for. This means you get paid for keeping your house warm! The scheme is already well underway for commercial ground source heat pumps but it is being extended to air source heat pumps at commercial and domestic level as well. Good tariff levels are promised; it will begin in 2013 and will make installing a heat pump very financially attractive.

Q. What is COP and what does it mean?
COP or Coefficient of Performance is the means of stating the ratio of the heat output by the heat pump relative to the amount of energy it uses to operate. The higher the COP the better, for example:
A COP of 4.2 would be; for every 1kw of electrical energy in, we would receive 4.2kw of heat energy out this is sometimes expressed as 1:4.2 or 420% efficient.

Q. What is SPF and why is it important?
SPF or seasonal performance factor is average COP over the course of a year. Remember that COP will increase in warmer weather (summer) and decrease in colder weather (winter.) SPF gives us an average, year round figure. It is a good indicator of a heat pump’s overall performance. Performance in heat pumps varies from manufacturer to manufacturer. SPF is a good way to compare heat pumps efficiency’s when looking at different models. However, with lots of different variables and each installation differing to the next in design and location, it is not an absolute given that ‘this is what your heat pump will do’ but it is a good indication of what to expect.

Glossary Of Terms

Efficiency
The word "Efficiency" is defined as the ratio of useful heat output to energy input. e.g. if an open fireplace loses half its energy up the chimney it is said to be 50% efficient.

Efficiency is commonly used to describe how effective something is. On this website the term efficiency relates to energy efficiency.

COP
The COP or 'Coefficient of performance' is found by dividing the useful heat output by the energy input. e.g. a heat pump that produces 4 kWatts of heat for 1 kWatt of input power has a COP of 4. The open fireplace example with 50% efficiency would have a COP of 0.5. (1/2)

SPF
Seasonal Performance Factor is similar to COP, but is an average figure taken over the year. It is usually lower than quoted COP figures, especially if back-up electric heaters are used.

Source
This is wherever the heat is being extracted from. eg. the outside air, river or ground. Sometimes referred to as an ambient source.

Spring
This is where water comes directly from the ground.

Stream
a small river.

Sink
This is the name given to the part where the heat is usefully dissipated, such as radiators in the room, underfloor heating etc.

Emitters
Another term used to describe radiators or underfloor heating. This is the component that emits the heat into the building.

Open Loop
This is the type of source where river or ground water is pumped through a heat pump then expelled to the environment a few degrees colder.

Closed loop
This is where a sealed plastic ground pipes are used which usually contains a glycol antifreeze. i.e. the most common trench or borehole system.

DX system
Abbreviation for 'direct expansion'. This is where the refrigerant flows directly within the ground pipes. This system is less common, and may have some disadvantages, however, it can promise higher efficiencies since there is one less pump and one less heat-exchanger.

Slinky
The name sometimes used to describe the type of ground collector pipes which are coiled before burying in a trench.

Horizontal collector
This can be either coiled 'Slinky' or straight pipes that are buried up to 2m deep in open ground (your garden). The pipe is usually plastic and contains a Glycol antifreeze solution.

Borehole
This is simply a vertical hole drilled in the ground. A ground source collector pipe can be installed in this.

Antifreeze
This is simply an additive that gives water a lower freezing point. Ethylene or Propylene Glycol is most commonly used in heat pump systems.

Brine
Brine is normally defined as 'salt water'. However, this term seems to be have adopted to describe any antifreeze mixture. A brine-water heat pump usually means one having glycol antifreeze on the 'cold' side and water on the 'hot' side.

Refrigerant
This is the working fluid within the heat pump. It evaporates in one part and condenses in another. By doing so, heat is transferred from cold to hot. This fluid is sealed in and will not degrade within the life of the heat pump.

Heat Exchanger
This is a simple component that transfers heat from one fluid to another. It could be liquid to liquid, liquid to air, air to air. Two heat exchangers are housed within the heat pump, one for the hot side (the condenser), the other for the cold side (the evaporator).

De-superheater. This is a small heat exchanger fitted to the compressor discharge that can produce a small amount of heat at a higher temperature.

Passive Cooling.
Passive cooling is where the ground water is simply pumped around underfloor heating. This gives limited amount of free cooling. It will only work with boreholes or large trenches in very wet ground. We repeat:- a limited effect, but its free!

Passive heat recovery ventilation
This is where the out-going exhaust air passes its heat to the incoming fresh air with only the use of a simple heat-exchager. It uses no heat pump.

Geothermal
This is defined as 'heat from the ground'. Proper geothermal is heat from the earth's core extracted from very deep in the ground, as in the steam that powers the whole of Iceland. The term seems to have been adopted to describe heat pumps. We prefer the use of the term 'Ground source heat pump' .

Buffer tank
This is simply a large water cylinder that is used to improve the efficiency and durability of a system. It reduces the number of stop/starts that the compressor makes, and ensures a high flowrate through the heat pump.

Heat Pump Rating.
A heat pump is given a kW heat output rating. This value will vary depending on the working temperatures.The electrical power input will be between a 1/2 and a 1/4 of the heat output.

Inverter.
This is a sophisticated electrical device that can vary the capacity of a heat pump. It therefore can vary the heat output to match the heat demand. (An inverter is also a soft-start)

Soft Start.
This is an electrical device that reduces the start surge that is taken by a conventional compressor. It does not save energy, but stops lights flickering, and may reduce wear & tear.

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Some useful figures and conversions.

1 kW (kilowatt) is a unit of Power or a rate of energy. (A 1 bar fire consumes 1 kW)
There are 3,411 Btu's in 1 kWatt. i.e.10kWatts = 31,400 Btu/hr.
There are 860 kcal/hr in 1 kW
A normal immersion heater uses 3kW when heating
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1 kWh. (kilowatt hour) is a quantity of energy
( A 1 kW heater would use 24kWhr per day )
I kWatt Hr. = 1 unit of electricity = 1 bar fire used for one hour.
Note gas bills now use kWhr. instead of the old confusing units. Therms etc.
1 kJoule x 3,600 = 1 kWhr.
Note heat pumps are usually rated by their heat output, not their electricity input.
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If 10Kwatts were extracted from water having a flow rate of 0.8Lit/sec then the temperature would drop by 3°C (3K).

A heat pump with a heat output of 10kW and a COP of 4 can be represented by the following equations:-
COP = heat output / electricity consumption
4 = 10kW / 2.5kW

the heat extracted from the ground = heat delivered – electricity consumption = 10kW – 2.5kW = 7.5kW
0°C = 32°F (freezing point of water)
10°C = 50°F
20°C = 68°F (room temperature)
100°C = 212°F (boiling point of water)
or, if you have a calculator, °F-32,/9,x5=°C, °Cx9,/5,+32=°F
1 lit/sec = 3.6m³/Hr. = 13.19 Galls(UK)/min.

This chart gives an indication of flow rates for a 10kw (heat output) heat pump taking heat from a river or spring source.
(the extracted heat has been assumed to be 7.5kW)

Source inlet temperature	Source outlet (return) temperature	Flow rate Lit/sec.	Flow rate M³/h.	Flow rate Gallons/ min.
6°C	3°C	0.6	2	28
10°C	7°C	0.6	2	28
10°C	4°C	0.3	1	14

For a river source system the flow rate would need to be at least 0.6 liters/ second to avoid freezing in the evaporator.

For a spring source, the flow rate would ideally be that same, but if the supply was limited, then half that rate may suffice.

Note, these are example values. Manufacturers data should be available for specific equipment.

Spring water source purity figures.

Water purity for normal copper-brazed stainless heat-exchangers as used in almost all heat pumps.
The following list will give some idea of the requirements.

Check with the heat pump manufacturer to get specific data relating individual heat pumps.
Sulphate < 100 mg/l

Free chlorine < 0.5 mg/l
Chloride < 300 mg/l
Nitrate < 100 mg/l
pH value 6.5 - 9
Electr. conductivity 50 - 1000 µS/cm
Oxygen < 2 mg/l