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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.

If you have a question about anything in the above blog, please ask it in the comments section below.

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3 comments - read them below or add one

Kensa Heat Pumps

Kensa Heat PumpsComment left on: 31 January 2011 at 9:49 am

Hi Malcolm,

Thanks for the comment. 

In my opinion there are a few reasons why the results of the EST trail show heat pumps with a worse performance than our European counterparts and in fact this is touched upon in the report.

 1) The main and largest reason is simply the housing stock. The insulation of properties in the UK are generally less than those in mainland Europe. In the UK we have been use to low energy costs, particular due to North Sea oil and gas. Low insulation results in the heat pump having to work harder and therefore it is less efficient.This is backed up by the fact that some of the models in the EST trail are the same as the ones in the link you posted. Same heat pump, different building, lower performance.

2) Installation - our European friends have been installing heat pumps for a large number of years. In the UK they are a relatively new development. As such our installers are on a learning curve. With experience and now MCS, the installations will get better and hence the performance will increase.

3) Monitoring issues - I'm not sure how the heat pumps in mainland Europe have been monitored. Obviously any differences in how the results are obtained can result in differences in reported performance.

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malcolmcroComment left on: 28 January 2011 at 8:40 pm

My personal view is that the climate in the next 20 years is highly unpredictable and a well specced heat pump may need some help in a bad Winter. However there is always the good old convector heater for emergencies.

More important in the future must be increasing the efficiency of heat pumps to their maximum. I have followed up rumours that continental heat pumps are more efficient and found an interesting web site in which users can record and compare their COPs. Figures above 5 are being achieved by several manufacturers.

Can Mr John Barker-Brown explain why British heat pumps fall so far behind and say when they will catch up? A similar site for British pumps has been proposed but is not encouraged by the Heat Pump Association.

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Kensa Heat Pumps

Kensa Heat PumpsComment left on: 7 December 2010 at 10:33 am

Kensa have been manufacturing heat pumps for over 11 years and have never used an internal immersion heater in a heat pump. Our heat pumps are designed for the UK market and suitable for up to 24kW on single phase supplies which means if correctly sized they do not require immersion heaters. Kensa are also not the only manufacture to take this stance. If the heat pump is sized correctly it simply does not need one.

As to reliability, a heat pump has less maintenance and is generally more reliable than an oil or gas boiler and these don't have immersion heaters as a back up for the space heating so why do heat pumps need them?

I think the main point is that a number of suppliers don't tell clients the unit has an immersion heater, part size the heat pump due to the electrical supply constraints and the first the client realises is when he receives a large electricity bill which is not acceptable.

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