Posted by: @chandykris

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The radiators were bled and balanced as part of the installation. They installed 6 new radiators (including this one in the open plan area) and reused four radiators we had downstairs for the upstairs bedrooms. All in all, there are 14 radiators and two towel rails, and all but one seem to be doing the job. Any ideas on what could be the issue, or whether I can try something to narrow down what the problem could be. Appreciate your help.

If the rads are sized correctly then I would normally say that it could (most probably, but not certainly, is) be an emitter balancing issue.  However you say the rads were balanced.

Can you measure the inlet and outlet temperature of the radiator in the cold room, how do these compare to other rads?.  Are there any TRVs in use, if so where?  Is the lockshield valve on the rad in the cold room fully open.  What is the diameter of the main pipework to the radiators?   You may have to turn all the other rads down a bit to force more water through the one in question, but without a bit more diagnostic info its difficult to say.

Thanks @jamespa for the pointers. There are TRVs on all the rads, but they all are fully open. We have 10 mm microbore to the radiators. Let me check the lockshield valve, hopefully that solves the problem. If not, will check with the installers after a few days.

@chandyKris We had to turn one of the bedroom radiators to nearly off to force the water to flow to the lounge two. At first the installer did the balancing but frankly the day was too hot for success even with all the windows open. So he had another go when we returned from holiday and we fiddled further a week afterwards.

It’s very fiddly when the lock shield is nearly closed so you might get further control if the trvs do the fine adjustment at the end. But using the lock shields is best then it’s permanent. But you’ll get some pointers about which other radiators to close up by using the trvs as a temporary measure. Then open the trvs again and close more on the lock shield on that radiator. It’s a slow job!

Thanks @judith. Definitely, the upstairs bedrooms get a lot warmer, they are all exceeding 22 degrees and staying at that temperature steadily. I have turned down the TRVs in the bedrooms temporarily and check whether that increases the temperature in this downstairs room. I have also raised a support request with Aira, so hopefully they should call and investigate the issue.

Posted by: @chandykris

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We have 10 mm microbore to the radiators.

Why?

That limits the amount of energy which can be delivered.
So large emitters are more likely to struggle.

Posted by: @chandykris

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I have turned down the TRVs in the bedrooms temporarily

I appreciate why you have used the TRVs at this stage.

But the final solution will need to be using the lockshield valves to restrict flow to each emitter.

Posted by: @chandykris

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Definitely, the upstairs bedrooms get a lot warmer, they are all exceeding 22 degrees and staying at that temperature steadily.

It's likely that those upstairs bedrooms are being warmed by heat rising from the room(s) below.

I wonder if that was properly taken into account within the initial energy survey.

You can test this further by simply switching off those radiators completely.
Then check what temperature you get in the bedrooms.

It's difficult to sleep in a room that's above 14°C at night

@transparent As it was a retrofit, changing 10 mm microbore to bigger pipes was out of question. Changing pipework for a three storey house would have been prohibitively expensive. Having said that, the maximum heat that can flow to this radiator on 10 mm microbore (as per HeatGeek calculations) is still way higher than the heat loss of the room. If I remember, 10 mm microbore at DT7 is about 1.6 kW and the heat loss of this room is about 1.2 kW. So, don't think microbore is the problem. Unless, I have understood it incorrectly.

My DIY skills are rather limited, so if turning down TRVs upstairs helps with the downstairs room, I will ask the installer to balance properly with the lockshield valves.

I did raise the point about bedroom design temperature being high at 21 degrees, but then the design engineer said this is MCS standard. Having said that, it does feel really toasty in the bedrooms now, so maybe turn down a notch. The dog will appreciate that definitely!!

Posted by: @chandykris

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My DIY skills are rather limited, so if turning down TRVs upstairs helps with the downstairs room, I will ask the installer to balance properly with the lockshield valves.

I did raise the point about bedroom design temperature being high at 21 degrees, but then the design engineer said this is MCS standard. Having said that, it does feel really toasty in the bedrooms now, so maybe turn down a notch. The dog will appreciate that definitely!!

MCS spec for bedrooms is actually 18C.

Its quite difficult for an installer to balance radiators properly because it takes a long time.  If you post a picture of one of your lockshield valves we can tell you what to do, you will most likely need a 7 or 8mm spanner (or an adjustable spanner.  Normally its as simple as popping off the plastic cap then using the spanner on the spindle projecting out of the top.  Often the easiest way to start if dirst to turn it off fully, then to turn it up a turn, and work from there making small changes and leaving for a few hours each time.

@jamespa Few others on the forum mentioned about the 18 degrees too. But, the design engineer didn't budge. I wonder whether that's to keep the existing towel rail radiators in the bathrooms as-is.

I have done exactly like you have mentioned, pop the white cap off and turned the valve anticlockwise until it's fully open.The temperature has gone up slightly around 18 degrees now. I assume it will take a few hours to settle, as I have turned down the TRVs in the bedrooms and my study to four from five. It has dropped a little bit down upstairs by 0.5 degrees and is steady now. I will speak to Aira on Monday and see whether they can do anything remotely like the flow rate before making further changes.

Posted by: @chandykris

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Few others on the forum mentioned about the 18 degrees too. But, the design engineer didn't budge.

designer is talking rubbish.  See page 14 of this document 

Posted by: @chandykris

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I have done exactly like you have mentioned, pop the white cap off and turned the valve anticlockwise until it's fully open.The temperature has gone up slightly around 18 degrees now. I assume it will take a few hours to settle, as I have turned down the TRVs in the bedrooms and my study to four from five. It has dropped a little bit down upstairs by 0.5 degrees and is steady now. I will speak to Aira on Monday and see whether they can do anything remotely like the flow rate before making further changes.

It sounds like its moving in the right direction.  TRVs may not have the same effect as lockshields since they will switch on and off rather than presenting a static turned down position.  4 out of 5 is really still high so my guess is you will either need to turn it down quite a lot more to get from 21 to 18 (if thats your desired temp) or resort to the LSVs.  As an alternative if you have got the static 'decorators caps' for the TRVs, unscrewing the head and replacing it with one of these is an option giving you fine control but acting like an LSV.

Unfortunately trial and error is the name of the game here, until flow balancing valves designed for heat pumps become commonplace, which currently they are not.

Looks like this issue is now resolved. Aira installed a K3 radiator, with 20% more output, and now the temperature is holding steady around 19.5 degrees in the open plan living kitchen dining room. Given the decent solar gain in that room and heat from cooking, this room is now perfect. The cold snap next week will be a good test, but pretty confident that it should hold up even in sub zero temperatures.