Posted by: @rob-of-york

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I plan on changing all the 55-year old radiators, so a flow meter can go in when the system is drained down for that. Then I'll have complete piece of mind about what flow rates can be achieved, before having a heat pump installed.

Great.  You just need to find a way of cranking up your circulator (if necessary) until either the target flow rate is reached or its too noisy, or playing with TRVs/LSVs to force most of the flow down one arm if necessary with the same objective

I don't know if this helps, but I thought I'd add some info on our installation based on a similar (ish) situation.

Unbeknown to us and the installer (which is to be expected) we had quite a lot of radiators running off a single 15mm run. As we found out, and probably due to having a concrete ground floor, most of the downstairs radiators are feed from the 22mm 1st floor pipework . We have 12 radiators in total with 9 of those fed from the upstairs loop.

Once all of the radiators were replaced and the HP was up and running we ran into flow issues, and after investigating further we had in one case 5 radiators being fed from a 15mm run and 3 on another. We decided (well needed) to replace some of the 15mm runs with 22mm pipework, so that only a maximum of 2 radiators ran on a 15mm tee. Running any more than that ended up with a drop off in flow to the final radiator(s) in that run and a cold radiator.

Since going through that additional and painful re-pipe (and with some good balancing), all our radiators get up to heat evenly, which before the re-pipe wasn't possible.

Although painful at the time having to loan the family out for a couple of days to family so that we could lift carpets and floorboards, I am glad we did, as doing this at a later stage would have caused a fair amount of further disruption that undoubtedly would have got me into a whole heap of grief with the other half, especially as I was the one who pushed to install the HP in the first place!

@bash

Thanks, its good to hear about your experience. My concern with the pipework is along very similar lines. There is no way of knowing the actual arrangement of 15mm pipe runs without pulling up lots of floorboards. My best guess is that in the 1970s when the the house was built the pipe runs were done in a way that involved the minimum amount of work and copper pipe. Hence, like your house there may be multiple 15mm pipe runs that visit groups of radiators. The worst could be the three radiators in the living room and the one in the hall, which are all quite large and could well be off the same 15mm pipe given their location and direction with respect to the airing cupboard where the pump is and the central heating pipes effectively originate from.

When we get into the heating season again, I'll take a careful look at the order in which the radiators warm up and see if I can discern anything from that. 

We plan to replace all of the radiators as they are 55 years old. When that happens, I'll get a flow meter put inline, and play with the pump settings to see what flow rates can be supported without causing noise. The pump has 3 fixed speed setting as well as other options, so playing around with that should give me some additional information. Of course the flow could be going down other branches and not enough going to the ones supplying multiple radiators, so I doubt I'll get the full picture.

Once we have the new radiators in place, I'll turn the weather compensation curve on the boiler right down, and see what flow temperatures the system can work at on really cold days. By making the boiler mimic the low temperature heating set up of a heat pump, I am hoping to uncover any pitfalls before committing to getting an ASHP, or worse having one fitted and then running into issues later. I can read flow and return temperatures and radiator temperatures with a thermal camera, so should be able to get good information with my final gas boiler set up before making the switch.

Posted by: @rob-of-york

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Of course the flow could be going down other branches and not enough going to the ones supplying multiple radiators, so I doubt I'll get the full picture.

You can selectively shut down radiators to test flow rates in individual lines or groups of lines.  Obviously you wont expect to get the full flow rate you need for the whole output from the heat pump, but this should enable you to establish if you can get enough down each 'suspect' branch without excessive noise.

@rob-of-york 

These are all sensible pre install things to test. Certainly dropping the flow temp of the gas boiler down as low as you can go, maybe even 30-35c if possible, along with lowering the flow rate down should hopefully give you an indication of what might be ahead of you.

When we ran into flow issues these were most highlighted when the flow temperature and rates were lowered. Running the HP hot with a high flow rate worked, but wasn't the best way to run the system, so we sucked up the extra re-pipe to have a well running system.

I cam to the view that as we'd taken the initial pain of the the install, adding another couple of days to re-pipe was worth it in the long run and I'm glad I did, I wouldn't want to do it again!

When the engineers first mooted the re-pipe work my heart sank. However, even though it was extra disruption, it wasn't as bad as I expected. Being piping specialists they found great ways of getting pipes to places that I thought would be a nightmare, especially the one on the main bathroom as they could fish the pipes through without ripping up the tiled floor!

@jamespa 

This was exactly what I did in the evening after getting the news that the we probably had too many radiators running off one 15mm run.

I went through a process of elimination opening and closing each of the radiators to determine what ones ran off each other.

It immediately highlighted the issue, as I could establish that running more than 2/3 off one 15mm tee starting to become an issue. Any more than 3 and the furthest ones started to go cold.

@bash @jamespa 

Thanks both for the practical advice.

Just to check I'm understanding this right. The game plan would be as follows:

Turn the flow temperature on the boiler down to 30C, the lowest it goes. Turn the circulation pump speed down to the minimum of its three speeds. Turn the heating on and check what happens with the radiators. Some of them may not warm up properly. If so, then one-by-one turn off the radiators that are warm via their TRVs and see if this affects the ones that are cold, if they start to warm up then they are on the same pipe run as the one that was switched off. Some repeats of the process may be needed to find all the pipe runs that have too many radiators on them.

Does that cover it?

Posted by: @rob-of-york

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@bash @jamespa 

Thanks both for the practical advice.

Just to check I'm understanding this right. The game plan would be as follows:

Turn the flow temperature on the boiler down to 30C, the lowest it goes. Turn the circulation pump speed down to the minimum of its three speeds. Turn the heating on and check what happens with the radiators. Some of them may not warm up properly. If so, then one-by-one turn off the radiators that are warm via their TRVs and see if this affects the ones that are cold, if they start to warm up then they are on the same pipe run as the one that was switched off. Some repeats of the process may be needed to find all the pipe runs that have too many radiators on them.

Does that cover it?

TBH I dont understand how that works. Ultimately all the radiators are connected back to the same pipe; I dont understand personally how turning some off and discovering that some others heat up tells you where the split is.  However it was @bash that suggested it (I think) so if he has tested it then perhaps he can clarify the sequencing/process/rationale, because if it does work its a very useful technique!

I don't understand either.

But you can find out which pipe feeds which radiator by closing them all except one. The pipe with water circulating will be hot over its entire length: this is the one going to the rad.

@jamespa 

How it worked for me is that when I had all radiators fully open, that appeared to be on part of a long run of 15mm pipework, at least one wouldn't warm up.

By completely closing what I had established was the first radiator in the run, the furthest would then heat up.

When all were open the furthest wouldn't heat up.

At this point I wasn't able to establish how many were on the run, just that it indicated there were more than what was ideal.

The installers had already worked out this was probably the case and that they ideally wouldn't want more than 2 on a 15mm.

Once the floorboards were lifted they were able to confirm how many were on each run.

Post by: @bash 

How it worked for me is that when I had all radiators fully open, that appeared to be on part of a long run of 15mm pipework, at least one wouldn't warm up.

By completely closing what I had established was the first radiator in the run, the furthest would then heat up.

When all were open the furthest wouldn't heat up.

Thanks, I can at least try this once my new radiators are in place, best case they all heat up. I'm hoping that running my gas boiler with the flow temperature turned down to less than 40C and experimenting with the circulation pump speed, I can get a good impression of what would the system would be like with a heat pump. The boiler modulates down to 4.8kW, so should be able to run for a reasonable amount of time putting out a steady amount of heat at low flow temperatures. (At present, with much smaller radiators, the weather compensation already has it running in the high 30s or low 40s in autumn and spring, and that seems to work just fine).  

Volumisers and Buffers

My updated and approved (by my wife!) radiator plan has a total radiator volume of 128.8 litres plus roughly 15-18 litres for pipes, say radiators and pipes 140 litres in total to be conservative. Also, if I were to retain the current method of operation, all the living room and hall radiators would be open all the time, giving a minimum volume that is definitely not less than 47.5 litres. The other radiators would operate TRVs and shut off if the rooms exceeded my intended room temperature by more than 1-2 degrees.

Assuming a 7kW heat pump, for example the Vaillant Aerotherm, then 140 litres total volume gives 20 litres / kW in full operation, which conveniently meets the low temperature target that RHH suggests: https://renewableheatinghub.co.uk/does-your-heat-pump-have-enough-water-volume-to-run-efficiently/

Further, the Vaillant Aerotherm 7kW, appears to have a "minimum open water volume" of 20 litres ( https://professional.vaillant.co.uk/downloads/aproducts/renewables-1/arotherm-plus/arotherm-plus-tech-sheet-nov-2024-2965654.pdf ) so a minimum of 47.5 litres comfortably exceeds this.

Does this set up mean that I should reject having a volumiser or a buffer in the heat pump system design, since the water volumes already in the radiators make them unnecessary, or is there some other valid purpose for having these present?