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A brief overview of Heat pumps

August 16, 2021

Heat pump cop and scop - seasonal coefficient of performance  

The COP of a heat pump at the most basic level is a representation of the energy out (kw heat) compared to the energy in (kw of electricity).  A COP of 4 may be given which would indicate that for every 1kw of electricity the heat pump uses to run it will output 4kw of heat to the home or hot water store. 


The SCOP is referring to the seasonal average, this is a more important figure as the cop may only state the best case scenario.  For instance an air source heat pump running on a mild day and outputting a low heat to a well insulated home would hit the maximum cop of say 5 or more but in the depths of winter it would only manage 2.5, the average is calculated and each appliance is rated. 


SCOP and SEER are new ratings driven by the E.U ErP directive in order to make a clearer and more fair comparison for homeowners, previously when just the COP was given it was easy for heat pumps to be mis-sold.  The cost difference between electricity prices and (for instance) gas prices have meant that often it is not cost effective to install a heat pump in place of a more traditional heat source but when salespeople could state the maximum efficiency rather than average it was confusing for the end user. 


SEER is the seasonal energy efficiency rating in cooling which we won't go into here, many heat pumps are capable of cooling a space as well as heating.  SEER this measures that efficiency and gives it a rating in the same way as the SCOP rates heating.  We will be mainly focussing on heating spaces and water within this article. 

Air source vs Ground source heat pumps 

Most heat pump installations are air source, they are easier to install and quite a bit cheaper, air source uses the latent heat in the atmospheric air around the home to heat the inside.   Ground source works in the same way but instead of air being sucked in it has a fluid running around pipes that are buried in the ground, the pipes extract warmth that sits in the earth all year round. 


Although ground source costs a fair bit more than air source it is cheaper to run as more energy can be taken and the ground stores more heat throughout the year. 


How does a heat pump work? 

Although heat pumps seem very futuristic and confusing they are surprisingly simple machines, you may have heard that "they are just a fridge in reverse" and actually that's pretty spot on.  A fridge works by extracting the heat from the inside and pumping it away, this is why the back of your fridge is always warm and especially in small kitchens you rarely need a radiator. 


Science lesson - Heat likes to move from warmer areas to colder ones, this is the second law of thermodynamics.  The opposite does not occur naturally and is harder to do artificially but with a little energy from an outside source we can utilise that natural force to our advantage. 


Heat pump components:



The Evaporator is a type of heat exchanger that receives the low-level heat from outside, this could be simply air or via fluid from ground loops or even bodies of water like a lake. 


This low level of heat in the internal refrigerant causes the gasses to evaporate, at low pressure and low temperature these gasses can absorb energy even at -20°c.  In fact, air at -18°c carries 85% of the energy (heat) as it does at +21°c. 


Once the gas has evaporated it is compressed in order to drive the temperature higher, heat pumps exploit the gasses boiling point which is altered by pressure.  


The gasses have evaporated then been compressed in order to become liquid again, this occurs in the condenser which is a second heat exchanger.  At this point the heat is transferred from the refrigerant gasses to the system water (in case of air-water systems) and can be output to radiators or underfloor and the coil of a hot water cylinder. 

Expansion valve 

Once the heat has been exchanged to the system and starts being used in the property the gasses then pass through an expansion valve, this decreases the pressure back to near original levels in order to go back to the evaporator so that the process can start again. 

Sounds simple?   

Well ok maybe we won't be building one ourselves but the basic fundamentals are condensation and evaporation which we see evidence of naturally most days.  Utilising these natural affects by just controlling pressure allows latent heat to be controlled and used in the home. 


Heat pump cost 

It is likely that all heat pumps will cost more to install than a traditional gas boiler or simple electric heating, an entry level air source heat pump cost could be as little as £6,000 fully installed.  Some care should be taken to avoid cheap products and installers, an investment of this level should last a long time.  A well installed system should last twenty years but there are a lot of horror stories where either equipment wasn't reliable or the installers didn't know what they were doing. 


A ground source heat pump cost will be higher, the loops will need to be dug into trenches or laid in a deep borehole.  Boreholes are the most expensive option and usually only required when space is not available for horizontal laid ground loops, they can be up to 150m deep but typically range from 15m to 50m.  Installation costs for a ground source heat pump range from £1,500-£2,000 per kw of a horizontal system, boreholes will be bespoke. 


Is a heat pump right for me? 

This really depends on the property and your usage, as mentioned above heat pumps really suit newer or very well insulated homes.  Much of the existing housing stock in England is unsuitable without significant insulation upgrades, there are many companies who will argue that a heat pump is always better for the environment or your pocket but it's simply not the case.  For modern or relatively efficient homes a heat pump can be an excellent addition, if you want to find out if your home would benefit then feel free to contact one of our engineers for a no obligation survey and estimate. 


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