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Heatpumps are clean, simple, low initial cost and popular for air to air models. They promise something for nothing – more heat out than the energy you put in.
However there are a few downsides and they’re important:
Background noise - Heat pumps advertisements claim their units are quiet – but, they are not silent. In fact the outside unit is anything but quiet!
Drafts - Think about the health impacts of dry, re-circulated air, the cooling effect of moving air and visual appearance.
Low temperature operation – Yes they can work at below freezing but their advantage of out-performing an ordinary electric heater disappears. The claimed performance is eroded when conditions are cold.
What is the co-efficient of Performance (or COP)?
Heat pumps performance is indicated by its co-efficient of performance or COP. For example a COP of 3.5 means 3.5kW of energy output for every one kW of electricity consumed. Now this performance is only attainable under optimum conditions.
The formula for maximum theoretical efficiency is expressed like this:
So if the air coming out of the heat pump is 35° to achieve a COP of 3.5 the outside temperature needs to be 25° when a heat pump is unlikely to be needed!
Note that this is a theoretical maximum, there are inefficiencies with heat pumps particularly the need to de-ice the evaporator when operating at lower temperatures.
The lower the outside (cold) temperature the lower the COP and when this gets to 1 is when you need them most. A COP of 1 equals an ordinary electric heater!
These source their heat from the earth with either ground coils or “probes” going down 30+ meters. Ground coils need to cover a significant area. The ground offers a higher and more consistent temperature than air so the performance of these systems can be far better than air to air models but they are not cheap!
This is a specialised, emerging industry. Experience in an installer is vital.
Ground to water geothermal heat pumps give a lower grade heat than a boiler (typically 35-45°) so are best suited to underfloor heating.
Radiant heating systems are known to decrease the dust mite population in homes by 50-80%.