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 wher 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
Abreviation 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), one for the cold side (the evaporator).
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 (click for more information)
This is where the out-going exhasut 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 effeciency 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.
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 |
4°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 litres/ 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 that many heat pumps require higher temperatures than those above.
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
