Ground source heat pumps don't need immersion heaters
Posted by John Barker-Brown on 6 December 2010 at 11:52 am
Would you install an 11kW boiler into a building with a 20kW load? No. The same should be true of ground source heat pumps.
Due to the constraints of the UK National Grid there are a number of large properties with single phase electricity, where the upgrade cost to three phase supply can be extortionate. Conversely, three phase electricity is widely and cheaply available on the Continent, therefore any ground source heat pump manufacturer importing to the UK has had no need to develop large single phase heat pumps. This is why most current Continental suppliers can only provide single phase units up to 11-12kW.
The danger with undersized heat pumps is, under peak conditions they rely on 3kW, 6kW or even 9kW worth of direct immersion to supplement the peak heating load. In turn, this means when energy needs are the highest, the unit is at its least efficient, significantly adding to the running cost and carbon emissions for the client. To make matters worse, many manufacturers fail to inform their clients that the unit actually contains an immersion heater. The first the client knows about this is when they receive their electricity bill!
In housing developments, the additional electricity demand of a large number of ground source heat pumps with in-built immersion heaters can even lead to the requirement of additional electricity sub stations being built!
Ground source heat pumps should not require any direct immersion element as the peak heating load should be provided by the heat pump ensuring the maximum cost saving for the client. If the heat pump is sized correctly, back up immersions should not be necessary. This stance is backed by the UK Heat Pump Association.
Further problems arise if the ground array is undersized for the load of the building, the design of this element is critical to a successful ground source heat pump installation. Whether using slinkies, straight pipe or boreholes, if there is not enough pipe, adequately spaced, in enough ground, at best the heat pump COP will fall with the reduction of flow temperature to the heat pump. At worst, this can force the heat pump to switch to immersion back up and in extreme cause issues with the ground.
In fact in-built immersion heaters are often used by some suppliers as a 'get out of jail card’. They are relied upon as the ground arrays have been deliberately undersized allowing the system to be fitted in properties with insufficient land. In the recent Energy Savings Trust report it is suspected that in-built immersion heaters are one of the reasons that a number of the sites reported lower than expected efficiencies. Phase 2 of this trail will look further into this.
Heat pumps don’t need immersion heaters, so why fit them?
Illustration by Jeremy Atkinson
About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.
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