Air-Source Heat Pumps (ASHP) and radiator replacement: How to work out the optimum radiator size for maximum efficiency?

@Jess_OVO happy to oblige!

We had three radiators upsized: 1. Breakfast room, 2. hallway downstairs, 3. en-suite. 

These rooms were underprovided for or the rads of a type where the output could not be determined (in the case of the en-suite).  I dont think I have any photos of the before situation sadly. 

For the majority of other rooms our current radiators turned out to be oversized for a conventional gas boiler.  For example our 5.7 x 3.7m living room has two large double  rads giving what I worked out to be 5kw at boiler design temps, with a room heat loss of around 1kw.  Given the typical 1:3 upsizing rule of thumb for heat pumps running c.45oC we are well catered for. I think our year 2000 heating system was over specced……

No experience on how it will work as installed in May this year, however in preparation for the ASHP I did turn my old gas boiler down to 45oC last winter and it performed fine assuming it was on for longer durations. Interestingly it seemed to use less gas when running that test, and even though I think the circ pump was running too quick so dT was 5oC when it was meant to be 20oC.

Great to get the ball rolling on the radiator guide - @jason.lewis!

Interesting to hear about the pre-heat pump test you carried out to see how the radiators would handle the change

Anyone else  got any insight to share which might help potential heat pump owners anticipate any radiator/underfloor heating changes that might be needed….

@juliamc, @hydrosam, @nealmurphy, @sylm_2000, @Gingernut49, @James_N, @jenthomson, @NinjaGeek, @mrmojorisin04, @RAJ, @JMB, @Wookie, @Amarritt, @briannolan 

Hi Jess, I’m afraid I don’t know how someone considering a heat pump would figure out if radiators need to be upsized. For our house, someone else figured it out as part of the technical pre-installation survey and heat loss calculation.
We got 3 radiators replaced upstairs. They were very old and they were replaced with almost the same width x height, but greater depth and a more modern construction. They proposed to replace the downstairs radiators also, but we plan to replace them soon with wet underfloor, so I said it didn’t make sense to get new radiators and then throw them out later.
Similar to Jason, I can’t really tell you how well they work because the installation was at the start of April. There were a couple of chilly days in April and the radiators seemed to work fine. I’ll be a lot wiser in 6 months 

Hi Jess,

We had the ASHP put in in March, and had no changes to the existing radiators. The installers OHM Energy investigated this as part of the installation spec. They measured all the rooms, the type of insulation, existing radiator sizes etc and worked out the requirements on a room by room basis. Our existing radiators we relatively modern and well sized, and I had already upgraded a lot of the insulation (cavity wall, loft, new windows and doors). There was one that was borderline, but I just decided to make no changes, get the ASHP in, and see what was what.

Like Jason above, having not been through a winter yet I cant say if we have got away with it. Time will tell.

I’m no expert, but I think bigger radiators actually help with efficiency, even if not essential. However it obviously adds a lot to the cost, time and faff of the installation.

We didn’t have radiators at all, as we had a warm air heating system, so we had 7 radiators fitted according to the installers schedule. All are double, except the bathroom which already had an electrically heated towel rail. All are what I would consider ‘normal’ sizes except our sitting/dining room one, which is huge: 2.4 metres long, the room is approx 5 m x 7 m.

Straight away I can see big variance in requirements and approach. It’s so valuable having you trialists here to ask and compare!

This seems to be a pretty simple summary of the requirements, @jason.lewis! So I can be sure that I grasp it, let me reword and put it back to you:

Radiators powered by a gas boiler (which I’m told is about 60-90% efficient) can be smaller, heat the water on demand and can reach temperatures of 60-80 degrees centigrade for short periods of time. 

Radiators powered by an air source heat pump (estimated efficiency is 250-350%) need to be cooler and larger. The hot water comes from storage and the running temperature is between 35-55 degrees centigrade 

You’ve called out a 1:3 upsizing rule. Could you explain how this translates into a radiator size? Not 3:1? Although that seems too big of an upgrade to be a general rule of thumb. @sylm_2000 I seem to remember you having significant work done to radiators. Anything picked up on the equation? Surely @James_N’s radiators weren’t 3 times too big when they were using a gas powered boiler, and are now perfectly sized? And to add to the mix @juliamc calls out radiators being normal size when fitted with the ASHP except the huge sitting/dining room beast. Is it that large rooms need exponentially bigger radiators than normal or small sized rooms?

Will most people fall into the same camp as @briannolan, keeping it simple and getting a survey diagnosis? With the lack of consistency already on show with your home’s radiators, maybe a case by case basis is the easiest?

Keen to dig a bit deeper into this… @hydrosam I can’t remember if radiators were included in your extensive refitting work this summer? 

Pipes it seems, are just pipes. And can remain, please correct me if I’m wrong...

@Tim_OVO sorry I think 1:3 / 3:1 could be too simplistic as quite often gas etc. radiator systems are poorly designed and more often than not significantly oversized. 

The correct way is to get hold of the data sheets for the radiator you are going to use, which now include  output levels (in kW)  for both 70oC and 50oC, the latter covering heat pumps at -2oC external temp.  

The overengineered radiators in our living room meant that we didnt need any upgrades.

@Tim_OVO  interesting to see that Stelrad add 20% to radiator sizes to account for intermittent use of i assume gas systems: 

@Tim_OVO what I meant by ‘normal’ is that the radiators in other rooms are visually dominating ! eg bedroom ones are 700 x 700, 800 x 600 mm. They were all sized correctly by Elite based on the volume and heat loss of each room (eg window size/wall construction etc).

I missed the discussion last week as I was on leave which meant away from internet as much as possible :)

Yes we did had four replacement rads, cost for which was paid to Elite directly as part of the ASHP installation. Each of the rads were bigger (oversized) to the ones we had with the combi boiler. The measurement was not shared with me however the BTU required was measured using the room size and ASHP efficiency requirements as mentioned above. 

Given our installation was April/May we are yet to test for the winter season ahead if they work to provide the heat as required.

Not directly linked but I have left the heating on (with ASHP) for the last couple of months with (tado) smart controls on each rads and didn’t see any uptake on the energy bills. I believe that’s considered as an optimal way to manage central heating with heat pump.