AS Heat Pump in Old Farmhouse - Radiator Size?

Hi we are on oil, no gas and our farmhouse has rubble walls and solid ground floors. I get that everyone says 'you'll need bigger radiators' but I can't get a sense of how big that means in practice. 10% bigger, 90% bigger? For context, roof insulation could be improved but probably not by much and windows are DG and doors pretty well draft proofed. I'm not going to insulate the walls, either outside or inside. 


  • matt_drummer
    matt_drummer Forumite Posts: 1,157
    1,000 Posts Name Dropper
    You may not need bigger radiators at all.

    Before installing an air source heat pump you should carry out a survey to work out the heat loss of each room in your home.

    The heat loss of each room is calculated with reference to the lowest outside temperature that you will usually experience and the temperature each room is desired to be.

    The heat loss of your home is the same whatever your heating source.

    An ASHP is most efficient at low flow temperatures, usually lower than those with a gas or oil boiler.

    That lower flow temperature may require bigger radiators to provide the required heat than those that worked with your oil boiler.

    But they may not, I didn't need any radiator changes when I replaced my gas boiler for an ASHP.
  • matelodave
    matelodave Forumite Posts: 8,396
    Part of the Furniture 1,000 Posts Name Dropper Photogenic
    As Matt say, the radiator size is related to the amount of heat that your room requires based on its heat loss calculated from the building construction, the inside temperature required v the outside temperatures. There are heat loss calculators on t'interweb which help you determine how much heat the room requires.

    Likewise the heat delivered to the room is the product of the radiator dissipation between it and the room it's heating, based on the size and construction of the rad (single panel, double panel, finned etc) the size of the radiator and the temperature difference between the room and radiator. Again their are calculators on the web, which help you determine the heat output of the radiator.

    The heat output from a radiator is specified at what's called DeltaT, which is usually around 50degrees - meaning that if you want the room at 20 degrees the water through the rad has to be around 70 degrees and most existing rads would be sized for high flow temps.

    As the efficiency of a heatpump is much higher at lower flow temperatures then you need a rad that will provide the same heat output but with a much lower temp of say 40-50 degrees, giving a Delta T of 20-30 degrees. Again there are radiator output specifications and derating tables on the web which allow you to calculate the heat output of radiators at various Delta T.

    For instance a radiator which outputs 1000 watts at a delta T of 50 will only output 510watts at a deltaT of 30 and 300 watts with a deltaT of 20 degrees.

    To do it properly you have to do the calculations and not just a wet finger in the air guess. Some rooms the rads may well have been oversized and are perfectly capable of delivering the heat you require at lower flow temps, but some may not

    Have shufti at this -
    Never under estimate the power of stupid people in large numbers
  • Reed_Richards
    Reed_Richards Forumite Posts: 3,367
    Part of the Furniture 1,000 Posts Name Dropper Combo Breaker
    When I got a heat pump I replaced virtually all of my radiators but my new ones all had internal fins to give a larger surface area.  The old radiators were all basic single or double panel types, quite a few of the new ones actually occupy less wall area than the old ones did.
  • cannugec5
    cannugec5 Forumite Posts: 339
    Part of the Furniture 100 Posts Name Dropper Combo Breaker
    All  of our radiators were also replaced. 

    In the kitchen I previously had a very small radiator, the replacement is the same width but reaches to the ceiling! Most of the others are marginally bigger, just a few cm. But in the hall it is smaller than before. 

    In the bathrooms we have towel radiators and the also extend higher than previously. 

    We have one extra radiator that is in the upstairs hall, where none previously existed. 

    We have a sunroom with five exterior walls.  Initially it was suggested that it required 2 radiators. However I objected and they agreed to leave it with just one as it has a door separating it from our living accommodation. It gets very hot in there and I’ve never needed the radiator on! 
  • markin
    markin Forumite Posts: 3,353
    1,000 Posts Fourth Anniversary Name Dropper Photogenic
    Are the floors insulated, If not you may as well rip them up and do underfloor heating.
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