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


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We’ve been keeping an eye out for some commonly asked heat pump questions we could look to answer. As we’ve pretty thoroughly covered pipe insulation, now it’s the turn of the radiators.

 

How many of you were able to keep the existing radiators when your heat pumps were fitted, and if these needed replacing how were the new radiator sizes worked out?

 

Do any of you have heat pumps powering underfloor heating in any part of your home and was this existing or has it been added along with the heat pump? How does @Transparent’s comprehensive underfloor heating guide series relate to a heat pump set up?

 

How is someone that’s considering a heat pump able to review their current radiators to know if they’re suitable or will need replacing? Tips and advice needed here!

 

Be great if anyone’s happy to share any before/after photos of any changes made to their radiator/underfloor heating set-up too.

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Best answer by Tim_OVO 2 September 2021, 12:47

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@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.

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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 

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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 :grin:

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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.

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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.

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

 

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……

 

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...

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@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.

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

 

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@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).

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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.

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@Tim_OVO I think you’ve got the jist of it though hot water to the central heating circuit is not supplied from storage in our cases (that would be a better solution as described by transparent but requires space for a thermal store), the heat comes straight from the pump via some valves. 
 

I had four rads changed as part of my work, though not the ones suggested by Northern Gas, others were changed based on my calculations because the NG ones made no sense (the utility room would have had the biggest rad in the house if they had their way!! 😂). There are radiator size calculators online but they tend to be quite basic. You need to put in dimensions of the room, the wall, ceiling and floor materials/u-values (how well insulated they are), in which part of the house the room is and which way the face. Both NG and Reina group did these calcs and I’ve now seen the paperwork, they were actually very similar so something was lost between the designer of my system and the person who bought all the kit. The rads changed were all changed from single emitter to doubles. 

The radiators are then sized to produce an output for the room. For Gas systems they typically work at 60-70C+ so they can use a smaller output rad to achieve the same level of heat input to the room. To work out radiator sizing you need to understand the deltaT which is the difference in temperature between the required room temp and the system output temp. So 20C in room, 70C from the boiler is a deltaT of 50C. Heat pumps work at lower temperatures so a lower deltaT is used. From my paperwork it looks like a design flow temperature of 50C has been used, a deltaT of 30C. 

Its a fairly involved set of calculations to do for each room which is why the installer would carry out a survey. On traditional systems plumbers have got used to what size works and experienced plumbers would be able to estimate what’s needed or just oversize the rad and let the thermostatic valves do the work because boilers are designed to stop/start regularly, whereas heat pumps work best when they can run continuously in tick over so sizing the rads correctly for a heat pump will bring more benefits. 
Ive bounced around a bit here but hopefully that makes sense. 

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Some really great input in terms of what is factored in when calculating required radiator sizing. 

 

From room size, to levels of insulation and even room facing (I’m guessing southerly facing rooms will generally require less heating/smaller radiators as brilliantly demonstrated by @Transparent’s heating capturing southerly facing glass roof.

 

@hydrosam makes some really Important points on the difference between these calculations for heat pumps as opposed to gas boilers. Seems like the importance of getting them right increases with a heat pump, to allow the correct temp to be supplied without needing to power on and off like a gas boiler might do.

 

 I’m guessing underfloor heating would be even more effective in allowing for a more constant power demand. Anyone got underfloor heating already installed or considered getting this fitted post-install?

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I would love to get under floor heating and am looking at retrofit panels which can be laid on top of an existing concrete floor. Much disturbance though so will give radiators a good try first.

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@Jess_OVOwrote:

Seems like the importance of getting them right increases with a heat pump

Ah… I wonder if that’s why the radiator manufacturers are wary of publishing online calculators for houses with Heat pumps?

There are plenty of radiator-sizing calculators available for houses with a standard boiler pushing out 70°C, such as this one from Stelrad. But it’s noticeable that they don’t allow you to change the water temperature, only the characteristics of the room.

That suggests to me there less margins for error with the lower temperatures supplied from a Heat pump.

 

Choosing to install underfloor heating removes these constraints of course. The radiating surface is massive in comparison to a wall-hung radiator.

 

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Some great insight into radiator changes from heat pumps: summarises things nicely:

 

There are radiator size calculators online but they tend to be quite basic. You need to put in dimensions of the room, the wall, ceiling and floor materials/u-values (how well insulated they are), in which part of the house the room is and which way the face…. the rads changed were all changed from single emitter to doubles. 

The radiators are then sized to produce an output for the room. For Gas systems they typically work at 60-70C+ so they can use a smaller output rad to achieve the same level of heat input to the room. To work out radiator sizing you need to understand the deltaT which is the difference in temperature between the required room temp and the system output temp. So 20C in room, 70C from the boiler is a deltaT of 50C. Heat pumps work at lower temperatures so a lower deltaT is used. From my paperwork it looks like a design flow temperature of 50C has been used, a deltaT of 30C. 

Its a fairly involved set of calculations to do for each room which is why the installer would carry out a survey. On traditional systems plumbers have got used to what size works and experienced plumbers would be able to estimate what’s needed or just oversize the rad and let the thermostatic valves do the work because boilers are designed to stop/start regularly, whereas heat pumps work best when they can run continuously in tick over so sizing the rads correctly for a heat pump will bring more benefits. 
 

 

A good tip on managing heating with a heat pump:

 

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.

 

Some call outs here to underfloor heating as a good option with heat pumps:

 

I would love to get under floor heating and am looking at retrofit panels which can be laid on top of an existing concrete floor. Much disturbance though so will give radiators a good try first.

 

There are plenty of radiator-sizing calculators available for houses with a standard boiler pushing out 70°C, such as this one from Stelrad. But it’s noticeable that they don’t allow you to change the water temperature, only the characteristics of the room.

That suggests to me there less margins for error with the lower temperatures supplied from a Heat pump.

Choosing to install underfloor heating removes these constraints of course. The radiating surface is massive in comparison to a wall-hung radiator.

 

And a top tip online calculator which can help work things out:

 

For those of you who want to try the heat calculations for yourself, the MCS Heat Loss Calculator is available on the mcscertified website, along with a user guide. There are also a couple of webinars which explain how to use the various sections to produce radiator sizing for a heat pump.

In the Design Details tab you can specify the construction of the floor, windows, door, external and internal walls, ceiling and roof for each floor of the property. You then input details for each room specifying which floor it is located on, its dimensions and area of external wall and windows etc. along with the size of internal walls and the adjoining room temperatures.

The Heat Pump Rad sizing tab then lists each room in turn and gives the calculated heat loss in Watts. You can enter the size of existing radiators and it will give the output of the radiator at a delta T of 50 degrees C (for gas boilers) and also for the chosen delta T for your heat pump. It uses correction factors supplied by Stelrad:

So if you wanted a room temperature of 21 degrees C and the mean water temp was 51 degrees C, the radiators would have to be twice as powerful as those for a gas boiler.

 

 

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Non-heat pump owner here, so this is more like a comment/question than an answer. Worse than that, it’s a comment from an amateur/armchair heating engineer rather than a practising one.

(And a request to be educated by those who know better!)

I looked at the heat being put out by our radiators last winter, and decided that they only ever come close to being used at their rated output when a room has been allowed to get cold and needs unexpectedly to be brought to a comfortable temperature, and sooner rather than later.

Provided that situation can be avoided I estimate that, even on the day when we burned the most gas last winter, the radiators themselves only needed to be at around 40 - 45 degC, to maintain a comfortable temperature (around 20 degC). (That wasn’t on quite the coldest day, but there was a strong wind and, in our house, that makes an important difference.)

That’s a deltaT around 25 (rather than the 50 used in standard tables), and radiator output is reduced to about 40% of nominal (output is roughly proportional to the 1.3 power of deltaT, and (25/50)^1.3 ~ 0.4).

This is not inconsistent with the fact that our rather old boiler probably puts out 70% of the 36 kW rate at which it burns gas (when it’s running) as useful heat, i.e. 25 kW: the sum of the nominal outputs of all the radiators on the heating circuit is around 22 kW. This is not very different from 25 kW, and I am encouraged that the system has been reasonably well designed.

On average, on that day when we burned the most gas (300 kWh in 24 hours is an average rate of 12.5 kW), and assuming a 70% efficiency, our demand for useful heat would have been 70% of 12.5 kW, i.e. just under 9 kW. I believe (based on pipe sizes and how long it takes heat to travel from the boiler to the furthest point in the circuit) that, when the CH pump is running, water flows in the heating circuit at a rate of 0.3 kg/s. If the useful heat is 9 kW, that would correspond to a DeltaT at the boiler (this is the amount by which the return flow is warmed up by passing once through the heat exchanger in the boiler, and completely unrelated to the other deltaT, introduced above) which is close to 7.2 degC.

This is a lot less than the “conventional” difference between flow and return temperatures, that one can read about, for a gas boiler, which is 20 degC. I am sure that what happens in practice is the boiler only burns gas for about 30% of the time, but the pump continues to circulate water around the heating circuit all the time. This is effectively what is called pulse width modulation. It does modulate its output after all, albeit in an unintelligent way.

My impression is that heat pump owners must modify their expectations. No longer can they “turn the heating up” and expect an immediate warming. Getting more heat energy in the same time is an increase in demand for heating power and, with a heat pump, that limit is much closer to normal operation than with a gas boiler. Instead, one has to travel back in time and turn the heating on earlier, so that the heating power can remain the same but the heat energy is increased by having that power in effect for longer.

Control systems can do this kind of thing, but instead of travelling back in time they use forecast weather conditions to change the way they operate. I have read that at least some heat pumps can modulate their output, but I suppose that modulation isn’t as easy as in a gas boiler.

I would love to know whether heat pump owners are happy: does the control system do what they need?

Simon

PS Interested though I am in the possibility of replacing that ancient gas boiler with a heat pump, the scale and effectiveness of possible improvements in insulation, reduction in air changes per hour, etc is far from clear. So, in the meantime, later this month the horrendously inefficient gas boiler will be replaced by a more efficient one. It’s a cop out, I know. But in a few years time? Maybe by then I’ll know enough and have done enough to understand whether a heat pump can do the job.

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For those of you who want to try the heat calculations for yourself, the MCS Heat Loss Calculator is available on the mcscertified website, along with a user guide. There are also a couple of webinars which explain how to use the various sections to produce radiator sizing for a heat pump.

In the Design Details tab you can specify the construction of the floor, windows, door, external and internal walls, ceiling and roof for each floor of the property. You then input details for each room specifying which floor it is located on, its dimensions and area of external wall and windows etc. along with the size of internal walls and the adjoining room temperatures.

The Heat Pump Rad sizing tab then lists each room in turn and gives the calculated heat loss in Watts. You can enter the size of existing radiators and it will give the output of the radiator at a delta T of 50 degrees C (for gas boilers) and also for the chosen delta T for your heat pump. It uses correction factors supplied by Stelrad:

So if you wanted a room temperature of 21 degrees C and the mean water temp was 51 degrees C, the radiators would have to be twice as powerful as those for a gas boiler.

 

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This is interesting:

Available here:  Stelrad ENU Techdoc - 1st DRAFT for comment.pdf

So if you are looking at getting the most out of your ASHP by running it at lower flow temps than originally envisaged, you can cross ref your room by room heat loss calcs to see if you radiators are up to the job of delivering the desired room temp.

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This IS interesting, another self serving resource. Albeit potentially a bit tricky to get one’s head around the numbers. But maybe that’s me, I’m better at letters than numbers. 

 

These methods we’ve got on this thread, at the very least will be helpful for someone to cross reference with what the installer has recommended, in case there’s any doubts! 

 

Have any trialists ran these calculations at home to see if what they’ve got fitted are within the right range? 

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@Tim_OVO i did this weekend, and this confirmed that because we run ourhome at 19 degrees and not 21 I can turn down the heat pump temperature to 45oC or less and should still be comfortable on the coldest of days. The lower the pump temp the better the COP. I will certainly be giving it a try.

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