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Can you use radiators with an air source heat pump?

Posted by John Lightfoot on 24 January 2011 at 9:47 am

The simple answer to the question can you use radiators with an air source heat pump is yes!

Radiators need to be sized for the flow temperatures your heat pump will be set to. The lower this set point, the more efficient your heat pump will be, and therefore the cheaper to run. The disadvantage of lower flow temperatures is, however, that the lower the water temperature, the more radiating surface area you need to emit the same amount of heat.

So if you currently have a oil fired boiler powering your heating system, with flow temperatures around 80C, and you exchange that boiler for a heat pump with flow temperatures of,  say 50C, you will need radiators double the size, or double panel radiators where you currently have single panel radiators, or two radiators in a room where you currently have one. If you set your heat pump to have even lower flow temperatures the radiator size will increase accordingly.

You will hear many people referring to over-sizing the radiators for heat pumps. I prefer to use the more appropriate term, right-sizing the radiators. All your heating engineer, or radiator supplier, will need to know, is the heat output you require from the radiator and the flow and return temperature to determine the size of radiator(s) you need to provide the amount of heat output you require.

If the radiator is sized correctly it will not matter what type it is, panel, decorative or skirting etc. they will provide the heat you require.

You will often hear that underfloor heating should be the only type of heat emitter you consider when using heat pumps. The fact that underfloor system can work very well with flow temperatures as low as 35C do make them an ideal partner for heat pumps. If, however, you have an old property it may be impractical to install an underfloor heating system, and in many cases correctly sized radiators are the only practical option. 

The other question people often ask if swapping out an oil boiler for a heat pump is – “Can I use my existing radiators”? This is not such an easy question to answer!

Your existing radiators can be used, but as explained above they might be too small, in which case you might need to add additional radiators into the system. Your heating engineer will also need to ensure that your system has been thoroughly cleaned, to make sure that suspended particles in the system do not clog up the filter they will install; to protect the heat exchanger in your heat pump from getting blocked (a very expensive item to unblock).

Of course every installation is different and my advice above is somewhat generic. The best advice of all is, call in an expert to survey your installation and they will let you know the best options open to you.

Photo by Ruthanne Reid

More information about air source heat pumps

Using an air source heat pump with an existing heating system: 5 key things to consider

Heat pump information page

An introduction to air source heat pumps

Air source heat pumps keep guests warm, happy and intrigued

10 questions to ask when choosing your air source heat pump installer

Are heat pumps cost-effective?

Find a heat pump installer


About the author: John Lightfoot is director at Thermal Energy Ventures Ltd.

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

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8 comments - read them below or add one


periodhousestoreComment left on: 8 January 2019 at 6:02 am

You can make your cast iron radiators look attractive with the trending radiator covers and simultaneously safeguard your kids from the accidental touch with the radiator surface. Find the best radiator cover that matches your surroundings and fits in any interior.

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geyserComment left on: 12 April 2017 at 11:08 am

Interesting article. But I am not sure it points out the most relevant points. The size of the radiator for example has little to do with the efficiency in a domestic setting. Indoor air source heat pumps are relatively new to the market. They are positioned in the loft or kitchen cieling to collect excess hot air which rises to the top of the building. The heat pump will work at any temperature and the efficiency is increased if there is more excess heat to capture. Sure a larger radiator would help distribute more heat but considering that the hot water in the radiators is recirculated then there isn't much heat loss from having small radiators and there is rarely any need to go above a comfortable temperature.

The way to capture heat from the radiator is therefore to set it to the comfortable temperature and any heat which makes it's way up to the loft area can be redistributed into the system. These heat pumps keep the temperature of the water at up to 60 degrees. A better way to improve efficiency of the heat pump is by reducing the temperature of the water in the cylinder. Rarely do people need water over 60 degrees so by maintaining that temperature, no added energy is needed beyond what the highly efficient heat pump can produce. These are the indoor heat pumps I am referring to -

Underfloor heating costs can be reduced in a similar way although they are better sourced from a ground source heat pump.

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Sara Lou

Sara LouComment left on: 18 August 2016 at 12:17 am

We have some friends in the countryside who recently installed such a heat pump and i know they're really pleased with how it turned out. They even managed to reuse some of their original radiators in the new setup so they saved some money there too, always a nice surpise!

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JohnWLComment left on: 28 January 2011 at 10:02 am

Hi Ian, Thanks for taking the time to comment on my blog. The point you raise is a point well made regarding the energy efficiencies measures one hopefully takes prior to introducing any renewable into their property. The situation you mention together with the possibility that the radiators you have were originally oversized, which is not a uncommon situation, make it all the more important to make sure that heat loss calculations are carried out by a professional and that way you will not have to waste your money buying additional radiators if they are not required. Thanks for highlighting this Ian.

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JohnWLComment left on: 28 January 2011 at 9:55 am

Bob, thanks for taking the time to comment. I do understand where you are coming from with regards to Natural refrigerants vs "traditional" refrigerants. I would however point out that most high quality monoblock ASHPs are fully tested in the factory to make sure they are not leaking. Given that refrigerants are not a problem unless they are released to the atmosphere the emission issue is one confined to the exception rather than the rule. One thing one also needs to consider is the efficiency of the ASHP at the conditions that it will be consuming most energy and this is, in my view, a more important criteria / consideration than the type of refrigerant the product has within it's circuit. We are currently looking at natural refrigerants, but will only introduce them when we can achieve higher efficiencies than those that can be achieved with “traditional” refrigerants.

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Cathy Debenham

Cathy DebenhamComment left on: 27 January 2011 at 5:18 pm

Thank you Bob and Ian for the useful comments.

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Ian Smith

Ian SmithComment left on: 26 January 2011 at 3:13 pm

One thing to bear in mind here is that radiators tend not to get downsized when retrofit efficiency measures are installed such as better glazing, loft and cavity wall insulation so they may already be oversized and no longer need a 60C flow temperature.

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Bob Irving

Bob IrvingComment left on: 26 January 2011 at 2:40 pm

If you are going to buy an air source heat pump, please make sure it is one of the most modern ones which uses carbon dioxide as a refrigerant. A major part of the emissions from heat pumps comes from leakage of refrigerants, which are stronger greenhouse gases than CO2.

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