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

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,

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.

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

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.

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?

@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.

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.

This is interesting:

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.

@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.

That's very interesting to read, thanks @M.isterW.

In the meantime my old boiler with its timed on/off programmer has gone, replaced by a more efficient boiler, wireless remote controlled TRVs and a fancy time+ temperature+more zoned controller (evohome).

I mention this because, if it's allowed to, the evohome will demand heat from the boiler ahead of time, in order that zones reach set temperatures on time.

It needed a period in which to "learn" how quickly and how strongly zones respond to changes in heat input, but it seems to do a reasonable job. Mostly, it is very good at maintaining set temperatures, even as set values change over the course of a day (the system can schedule up to 6 changes in the set value per day, independently across all the zones).

If I were deliberately to impose (in software, somehow) an artificially low limit on boiler output, I suspect that the evohome would have learned to demand heat even further ahead of schedule, and zones would still have been brought to temperature on time.. I will do this, eventually, because I want to know whether a heat pump could do the job

The reason I'm bothering to describe all this relates to modulation.

Most of the time, the heating power needed is a great deal less than the 32kW that the boiler can deliver. No problem, I thought, the new boiler can modulate down to 10%, i.e. 3.2kW so, much of the time, it should be able to come on and stay on.

It's a good job I was interested in this, and enough to want to observe "with my own eyes" sustained slow burning of gas at a rate of the order 3 or 4 kW…

Because every time I watched, laboriously recording gas meter readings and times, and correlating them with when the boiler fired up and shut down, I worked out that it wouldn't go below 11kW.

Eventually, in a revealing conversation with the manufacturer's technical support, it emerged that the boiler was an old model, and 11kW was indeed the minimum output.

(No, that error would never have come to light without my obsessive reading of smart meters. The wrong boiler will soon be replaced by the right boiler, and at the manufacturer's expense.)

Back to the controller.

Its link to the boiler is crude. The evohome just demands heat or not: on/off. It could say how much (via OpenTherm), but doesn't. Whatever the boiler manufacturer might claim about OpenTherm compatibility, in practice, the installer doesn't try to make it work that way (not with evohome, anyway).

Receiving only simple on/off commands, the boiler is left to work out what output to deliver, what modulation to run at. Maybe it will do a good enough job, I'll have to wait and see. All my experience with control systems (including designing and implementing some) tells me that evohome could do a better job - it has "learned" how zone temperatures respond to heating. The boiler hasn't got that information, and I don't see how it can do as effective a job all on its own.

I have a plan though. Over the years, I've sometimes seen "micromanagement", but fortunately never applied to me. If need be, I'll micromanage the evohome, and do everything I can to help the boiler to modulate its output the way I expect it will need to.

With apologies for straying far from the thread topic.