Air Source Heat Pumps
***This page has been updated***
See the new page on ASHP here
These operate in a similar fashion to ground source heat pumps but use the ambient air temperature to generate heat within the home. Unlike ground energy systems the air temperature input for air source systems can vary greatly both seasonally and daily and the systems are not suited to cold winters.
These systems can be highly efficient delivering up to 250% seasonal efficiency. However, as the external temperature falls the operating efficiency falls too.
The significant drawback with ASHP is that you must consider what fuel it is replacing. If it is in place of a conventional gas system it could actually be worse since the change from gas to electricity as a primary fuel source means an increase of CO2 concentration per kWh. Gas consumption emits 0.194kg of CO2 for every kWh used, whereas electricity generation emits 0.43kg per kWh, a whopping 222% more! The system improvements have to counter this before any savings can be made.
-- links to info on ASHP's -- - Efficiency, Cost & Carbon Comparison - Homemicro article on operating cost and carbon emissions
- BSEE magazine article on ASHP - ASHP's compared to condensing boilers
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