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What the domestic RHI means for air source heat pumps

Posted by Chris Davis on 11 October 2012 at 9:40 am

The long awaited details of how the Government will support householders who install renewable heating systems have been announced at last. The RHI (renewable heat incentive) is due to be introduced in the summer of 2013, and one of the answers most keenly awaited has been whether or not air source heat pumps would be included.

Well, the answer is very clearly yes, and air source heat pumps are expected to be one of the most popular technologies installed under the scheme.

The proposals issued by Government last week are at “consultation” stage, so things could still change. It is also quite complex with some areas of the scheme still needing a lot more work, but here are the most relevant issues concerning air source heat pumps:

Eligibility for RHI

The RHI will be available to any householder wishing to replace their existing system with a renewable heat technology. The list of technologies includes air to water heat pumps, but excludes air to air heat pumps and exhaust air heat pumps.

The installation must also comply with MCS (both installer and product must be accredited under the Microgeneration Certification Scheme) and suitable energy efficiency measures, identified as green ticks in a Green Deal assessment, must also be met. 

The homeowner or the landlord will receive the RHI, and second homes won’t be eligible.

Heat pump efficiency levels

DECC is keen to ensure that heat pump installations funded under the RHI perform to an acceptable standard of efficiency. The proposal is that all systems must be able to demonstrate a minimum Seasonal Performance Factor of 2.5 or higher.  There may be some enhanced incentives for systems which provide monitoring information to the user or can demonstrate much higher levels of efficiency.

RHI tariff level and scheme duration

The RHI will offer tariff payments over 7 years which will be made on the basis of “deemed” heat usage (a calculation for the expected heat usage for the property).  The calculation method for this has still to be agreed.

The domestic RHI is intended to be a “boiler replacement scheme”, so proposals on tariffs have been designed to compensate for the additional upfront costs of renewable heat compared to the fossil fuel alternative, as well as other non-financial barriers including the perceived “hassle factor” of changing to a new technology. My previous blog on this gives an outline of what these costs might look like. The tariffs have been structured to provide a better return for home owners who are off the gas grid.

A tariff of between 6.7p and 11.5p per kWh of renewable heat generated is being considered for air source heat pumps. The consultation is asking for more evidence of installation costs in certain scenarios to ensure the final tariffs are correct.

As an example of what this means for a typical UK home and assuming a tariff towards the higher end of the proposal, this would provide an RHI payment of around ÂŁ2,000 per year, plus of course any additional energy bill savings.  So on the face of it, the scheme could be very attractive for air source heat pumps.

Boiler replacements

The scheme assumes that in most cases the heat pump will be installed to replace an existing boiler and in order to receive the tariff you will be expected to remove your existing boiler (to prevent you using your fossil fuel boiler and still claiming the RHI). However, it will be possible to install an air source heat pump in conjunction with an existing boiler to work in “bivalent” mode, as long as you install a meter to measure exactly how much heat the heat pump is producing.

Will existing air source heat pump installations be eligible for RHI?

If you’ve already got an air source heat pump installed, then the good news is that you could be eligible to receive the RHI payments too. If your system was installed after July 2009, is MCS accredited and was installed by an MCS installer you can claim the RHI. This includes those funded under the RHPP, You will also have to meet the energy efficiency criteria. Heat pumps installed in a “new build” property are not eligible.

What next?

The consultation closes in early December and we expect to get formal announcement of the RHI scheme – including confirmation of the tariff rates - before the end of March next year.  

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

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6 comments - read them below or add one


UrbanEnergyComment left on: 12 March 2013 at 1:13 pm

Could also have batteries installed for the solar so that will then run the ashp in the evenings

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ChrisDavis from Comment left on: 7 March 2013 at 10:33 am

Solar Wind, thanks for your comment.  The answer is probably quite difficult to give succinctly on here, however a few points to consider:

Firstly, your total heating and hot water load for a 4 bed house seems quite low.  Is it a new build?

Normally, the back up heater for hot water would only be used for sterilisation, which can be programmed within a heat pump controller. Our recommendation would be to do only once per month, or once per week at the most.  This boosts the tank temperature from around 50C to 60C, so the cost of doing this is fairly minimal, around £5 per year if you do it once per month (or even if you did this weekly it would only cost around £35 per year).

The new heat pump design regulations (MIS3005) mean that the back up heater should not be used for space heating above a design temperature of around -2C and any usage should be included in running cost assumptions that an installer is obliged to provide you with.

You asked about diverting PV energy in to your hot water tank with a heat pump system.  This is still a very good idea, not least because producing hot water is the least efficient part of a heat pump system. 

At Dimplex we are actually already looking at how we can use PV and heat pumps together in an even more intelligent way, eg not just to drive an immersion in the tank ,but to actually start the heat pump compressor and provide water heating this way (with free electricity) and even to help with the heating in the spring and autumn.  I'd be happy to talk to you about this in more detail if you are interested, you have an ideal situation with a large pv system and what would appear to be a requirement for quite a small heat pump.

There is of course no reason why you can't use an off peak tariff to heat the hot water tank at night, either directly if you wish at 6p/kW or (more usually) to run the heat pump at this time at 2-3p/kWh (remembering of course if its air source then the air temp will be lower and therefore not as efficient as if this was done during the day).  What you also have remember though is that you will pay more for your day rate electricity when you have an off peak tariff and our advice generally is that it is more cost effective not to use an off peak tariff, as a significant proportion of the time your heat pump will run for space heating is outside of the off peak hours.

Based on your figures of 13,000kWh per year (which do seem a bit low to me) an ashp system running at a seasonal efficiency of 3 would give you a running cost of around £540/year, so around a 30% saving.  On top of that there is of course the RHI to add in which would give you (I estimate) around £800 per year income, so you are going to be in the region of £1000 per year better off with a heat pump.

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Solar Wind

Solar WindComment left on: 1 March 2013 at 5:55 pm


Can someone be specific about the Immersion heater used for sterilisation and back up heating contained within the ASHP systems please?

I already have a 4kWh solar PV system on a 4 bedroom detached house which is heated with night storage heaters. The last two years my average electricity usage for heating and hot water was 13,100kWh per annum at 5.83p a kWh off peak tariff costing £780.00.

However, last year I also built my own solar diverter which delivered 1,018kWh of surplus solar PV into my immersion tank. This year I hope to do better with an improved design.

My question is this, can I still divert any surplus solar PV into a heat pump storage tank, bearing in mind that I will have less solar surplus as the heat pump will be using some of the power during the day.

And is there any reason why I can't heat the heat pump water tank at night while the off peak supply is available at 6p a kWh?

I had planned to go over to a heat pump system for heating and hot water but can't see that I would be any better off, in terms of system running costs.


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neilwilliamsComment left on: 22 January 2013 at 11:37 am

One thing that concerns me is assumptions made about energy use, when calculating RHI payments. DECC's factsheet Domestic energy consumption in the UK 2011 implies typical annual space heating  of 12800kWh and hot water 3800kWh, assuming 27m households. If heat pumps require better than average insulation, we could be looking at around 10000kWh space heating, which might be reduced even further to around 9000kWh if common sense measures like setting the thermostat at 18 deg.C are adopted. Adding in hot water, which itself could be reduced by better insulation and thermostats, total use might only be 12000kWh. DECC have also confirmed that the heat generated by electricity will have to be excluded, which at COP 3 is 25%, giving an RHI eligible figure of maybe 9000kWh. This gives a  subsidy of £600-£1000pa for 7 years under the tariff proposed. Given likely installation costs of £10k+ for ASHP, this doesn't seem very cost effective, except for off grid properties using very expensive fuel.

By the way, I am a big fan of heat pumps and believe they are the solution in the long term.

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Cathy Debenham

Cathy DebenhamComment left on: 7 January 2013 at 10:57 am

All heat pumps take electricity to run, so if you have solar PV, then the electricity used while they are generating will be free of charge. However, when the heating is needed most, the sun is least likely to be out, so it will not reduce your bills completely!

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banjaxComment left on: 22 December 2012 at 1:54 pm

If a householder has PV installed, how could the PV be utilised to complement and improve the efficiency of the Air Source heat pump ?

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