Combi system.One radiator getting air in

[whatsthenews]

New Combi system (boiler and microbore re-pipe)about a year old. All radiators have TRV's apart from one of the bathrooms
Started noticing a single loud bang,possibly from the hall radiator shortly after radiators have gone off.
I say possibly because the radiator in the room above the hall keeps getting air in it and needs bleeding, so it could be coming from this one, but it doesn't sound like it.
Difficult to know how long it's taking for air to stop it working properly because it's a room we only use occasionally and it's not until one of us goes in in and realises it's a bit nippy that we check and bleed it, plus have hardly had heating on over the last couple of months.
Whilst we can bleed the radiator and solve the immediate issue, is there anything else we can do to try and work out why/where air is getting into the system? The boiler pressure isn't dropping except slightly after we bleed the radiator and no signs of any leaking radiators.
We had the downstairs rooms replastered after the system was fitted and the plasterer took the rad's off the walls and laid them on a variety of tool boxes etc while he was plastering. Is it worth checking that the valves are all tight?
Thanks in advance

 

[John.g]

I have all mine on the return because it is the coldest end of the rad and IMO gives a better room temperature but don't know if this gives any real benefits or not.

 

[Ben-gee]

It would only be a relative thing anyway, so maybe set to 3 3/4 instead of 4.

 

[whatsthenews]

Another arrow points away from the radiator (look under the 15). It's bi-directional. Thus this TRV can have the flow running either way without issue.

Yes bud see it now , I would still swop it though probably change it to be honest I like em on the flow and balance on the lockshield but we're all different 😁👍 kop

I like them on the flow too, on principle. That said, in my own house I've put at least one on the return, simply because if I had put it on the flow, it would have been shoved in a corner where it would have been hard to adjust and unable to sense the room temperature properly. Does it even make any difference, from a technical perspective, do you think?

Yes, I saw the other arrow when I looked for it! Would it be correct to say that having the TRV on either end or the other (or hanging from the light fitting) wouldn't cause the radiator to keep filling with air/hydrogen ? That was the original issue. Thanks again for all the help.

 

[SJB060685]

Yes, I saw the other arrow when I looked for it! Would it be correct to say that having the TRV on either end or the other (or hanging from the light fitting) wouldn't cause the radiator to keep filling with air/hydrogen ? That was the original issue. Thanks again for all the help.

TRV's and any other valve can draw air into the system if on a negative pressure side of the system and something is lose, even when I say negative pressure, its still a positive absolute pressure, just less than atmospheric. As I said before your system should be under positive pressure, so i would expect to see leaks.

 

[Ric2013]

TRV's and any other valve can draw air into the system if on a negative pressure side of the system and something is lose, even when I say negative pressure, its still a positive absolute pressure, just less than atmospheric. As I said before your system should be under positive pressure, so i would expect to see leaks.

Agreed. If the TRV has some kind of weep and it is under negative pressure then it could draw air in. As it is under positive pressure, you would expect to see water leaking out rather than air going in, if it has a weep. I suppose (clutching at straws now) the cavitation resultant from the water flowing through a partially closed valve (whether TRV or lockshield) could result in a pocket of negative pressure which could draw in air at that point of the system.

The OP has suggested that what side the TRV is on may have nothing to do with the air in the system. I think he has a point and that we're off-topic.

 

[John.g]

Assuming combi has the circ pump on the flow and its E.vessel before the pump suction then the pump suction will be at the EV pressure (say 1.0 bar min) and the pump discharge at the EV pressure+the pump head, say (1.0+0.5), 1.5 bar so unlikely to pull in any air. If the EV has been installed on the pump discharge then the pump discharge will be 1.0 bar and the pump suction (1-0.5) 0.5 bar, still "positive" but depending on Hx/pipe (esp. micro bore) losses just may be negative back along the system. Any one who is familiar with combi boilers would know where the pump and EV are generally positioned.

 

[whatsthenews]

Assuming combi has the circ pump on the flow and its E.vessel before the pump suction then the pump suction will be at the EV pressure (say 1.0 bar min) and the pump discharge at the EV pressure+the pump head, say (1.0+0.5), 1.5 bar so unlikely to pull in any air. If the EV has been installed on the pump discharge then the pump discharge will be 1.0 bar and the pump suction (1-0.5) 0.5 bar, still "positive" but depending on Hx/pipe (esp. micro bore) losses just may be negative back along the system. Any one who is familiar with combi boilers would know where the pump and EV are generally positioned.

so not very likely?

 

[Chuck]

so not very likely?

I suspect the TRVs are a red herring.

IME, it's normal for a re-piped system to evolve gas for several (typically six) months after the work has been completed. This is due to a 'passivating layer' forming on the inside of the fresh pipework. Inhibitors slow this process but don't stop it. If the amount of gas you are having to vent is gradually reducing each time then this is probably what's happening.

The banging noise that brought you here is probably unrelated to the above process. My guess is that pipe-clips that were nice and tight at the time of installation have now had time to relax. What you are hearing is slip-stick friction occuring when the pipework contracts as it cools. Somewhere a pipe will be passing over/through a somewhat tight gap that needs opening up a bit and a piece of hemp lagging used to let the pipe slide rather than stick.

Edit: I haven't read all 30 posts on this topic very carefully so my apologies if I'm repeating stuff that others have already said.

 

[SJB060685]

I suppose (clutching at straws now) the cavitation resultant from the water flowing through a partially closed valve (whether TRV or lockshield) could result in a pocket of negative pressure which could draw in air at that point of the system.

I know you're clutching at straws and I could be wrong myself but I'm not convinced that the above you mentioned would occur. If it did we would see a lit more air ingress than we do. The only other thing I thought of and again I could be wrong is air being drawn in at the highest point of the system where the static head is at a minimal?

 

[John.g]

Maybe crack open each/every rad vent with boiler/circ pump on and check that water is issuing.

 

[whatsthenews]

Maybe crack open each/every rad vent with boiler/circ pump on and check that water is issuing.

so just like you'd do to bleed them?

I suspect the TRVs are a red herring.

IME, it's normal for a re-piped system to evolve gas for several (typically six) months after the work has been completed. This is due to a 'passivating layer' forming on the inside of the fresh pipework. Inhibitors slow this process but don't stop it. If the amount of gas you are having to vent is gradually reducing each time then this is probably what's happening.

The banging noise that brought you here is probably unrelated to the above process. My guess is that pipe-clips that were nice and tight at the time of installation have now had time to relax. What you are hearing is slip-stick friction occuring when the pipework contracts as it cools. Somewhere a pipe will be passing over/through a somewhat tight gap that needs opening up a bit and a piece of hemp lagging used to let the pipe slide rather than stick.

Edit: I haven't read all 30 posts on this topic very carefully so my apologies if I'm repeating stuff that others have already said.

No problem and thanks. It was actually the issue with the upstairs rad' being cold and requiring bleeding that brought me here, but I thought it was worth mentioning the banging sound. It's difficult to know if it's requiring bleeding less frequently or whether there's less air emitting each time as it's in the spare bedroom. Will eventually be a home office/spare bedroom but other than rewiring when we moved in and just done the bathroom and created an ensuite, we haven't touched upstairs yet. I'll make a point of putting the heating on once a week and checking it.

I know you're clutching at straws and I could be wrong myself but I'm not convinced that the above you mentioned would occur. If it did we would see a lit more air ingress than we do. The only other thing I thought of and again I could be wrong is air being drawn in at the highest point of the system where the static head is at a minimal?

I also wondered if it might be the highest point in the system and if that might explain anything.

 

[king of pipes]

Assuming combi has the circ pump on the flow and its E.vessel before the pump suction then the pump suction will be at the EV pressure (say 1.0 bar min) and the pump discharge at the EV pressure+the pump head, say (1.0+0.5), 1.5 bar so unlikely to pull in any air. If the EV has been installed on the pump discharge then the pump discharge will be 1.0 bar and the pump suction (1-0.5) 0.5 bar, still "positive" but depending on Hx/pipe (esp. micro bore) losses just may be negative back along the system. Any one who is familiar with combi boilers would know where the pump and EV are generally positioned.

Hang on I need to sit down 😂😂.

 

[whatsthenews]

I'm going to update here in case there's any connection with the radiator issue, although I think that's good now.
For a month or so I've been suspicious that our Baxi combi (a year old) might be acting strangely (or what I thought was strangely). A few times when I've been passing through the utility room I heard the boiler (in the garage) running when I didn't think it should be.

This morning I was in the garage for wet cig' break and the green light came on and ?pump started running. The temp was showing 24c. After about 10 seconds , the burners fired up and the temperature rose to 45c in less than 20 seconds, and then the burners switched off and the light went off.
Nobody else in the house, so it wasn't anyone running hot water.
The thermostat was on manual. Thermostat (POS) is in the hallway and obviously temp might have dropped, hence boiler firing up, and when I went back in the house one of the radiators in the kitchen (first in the line) felt warmer at the top, but the radiator in the hall was barely lukewarm.
About 17.30 tonight we were working in the utility room and the temp in the house had fallen (I checked the thermostat, set at 20.5 and actual temp was 20 and it was showing that the boiler was on), but the boiler seemed to be firing up then switching off , then about 5 minutes later the same thing. Radiators were all hot when I went back in the house. Maybe that's normal if there's not much of a difference between the actual and set temp'?
On the Salus thermostat there's a setting called TPI (Time Proprtional and Integral) and the options are RAD,UFH or ELEC. Obviously we have it set on RAD. It says on RAD setting the CPH (cycles per hour?) is 6. Is that normal? Do boilers have a CPH setting?
Bloke is coming to service it at the weekend anyway, but as you know I'm nosey (inquisitive?) and like to be at least partly clued up.
Thanks for any

 

[John.g]

Can you see the boiler water setpoint (SP) when the boiler fires up?

 

[whatsthenews]

Can you see the boiler water setpoint (SP) when the boiler fires up?

No. The only temperature display is the ? actual boiler water temp' that was at 24c ,then rose to 45c.
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No. The only temperature display is the ? actual boiler water temp' that was at 24c ,then rose to 45c.

There are 2 dials. One for radiators and one for hot water.
When I turn the hot water dial, the temp display changes ,presumably to the hot water set temp'. It's set at about 45c funnily enough??
The radiator temp' is set at about 70c.

 

[John.g]

Can you set the HW SP temp to its max or ~ 60C and open a hot tap only and see if the boiler runs "normally" at this temp for 5 minutes or so, then change back to CH but leave the HW SP at 60C and CH SP at 70C and watch the temperature rise again before the boiler cuts out. The first test IMO should prove that the circ pump is OK and if the boiler continues to cut out after 20 secs or so on CH then it may point to some problem with the circulation around the radiator system or a boiler problem.

 

[whatsthenews]

Can you set the HW SP temp to its max or ~ 60C and open a hot tap only and see if the boiler runs "normally" at this temp for 5 minutes or so, then change back to CH but leave the HW SP at 60C and CH SP at 70C and watch the temperature rise again before the boiler cuts out. The first test IMO should prove that the circ pump is OK and if the boiler continues to cut out after 20 secs or so on CH then it may point to some problem with the circulation around the radiator system or a boiler problem.

Hi, sorry for late reply. Thanks will do so. Just to say- I turned the HW SP up to about 56 yesterday.
It was playing silly buggers earlier today. The "heating" light (as opposed to HW light) is lit. Again, the rad in hall was barely hot, but took the TRV off and it was jammed.
Sorted that and turned the stat up to 22c (actual room temp' was 20c) heating came on and rad' was red hot within 4-5 mins.
Thought I'd solved the issue but it's still doing it, but boiler water temp now goes up to 56c (the HW SP) then cuts off.
Does the boiler "act" differently depending on the difference between the actual and set room temp?
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Hi, sorry for late reply. Thanks will do so. Just to say- I turned the HW SP up to about 56 yesterday.
It was playing silly buggers earlier today. The "heating" light (as opposed to HW light) is lit. Again, the rad in hall was barely hot, but took the TRV off and it was jammed.
Sorted that and turned the stat up to 22c (actual room temp' was 20c) heating came on and rad' was red hot within 4-5 mins.
Thought I'd solved the issue but it's still doing it, but boiler water temp now goes up to 56c (the HW SP) then cuts off.
Does the boiler "act" differently depending on the difference between the actual and set room temp?

Just wanted to say that the hall radiator was hot first thing this am when heating came on on auto, but maybe it was jammed on 3? Does that make sense?
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Hi, sorry for late reply. Thanks will do so. Just to say- I turned the HW SP up to about 56 yesterday.
It was playing silly buggers earlier today. The "heating" light (as opposed to HW light) is lit. Again, the rad in hall was barely hot, but took the TRV off and it was jammed.
Sorted that and turned the stat up to 22c (actual room temp' was 20c) heating came on and rad' was red hot within 4-5 mins.
Thought I'd solved the issue but it's still doing it, but boiler water temp now goes up to 56c (the HW SP) then cuts off.
Does the boiler "act" differently depending on the difference between the actual and set room temp?
[automerge]1601065843[/automerge]

Just wanted to say that the hall radiator was hot first thing this am when heating came on on auto, but maybe it was jammed on 3? Does that make sense?

Ran HW with HW SP at 60c and the boiler temp reaches 60c and stays there. Then (with room stat turned up to 22c, actual room temp 20.5) boiler temp didn't go above 60c, BUT it didn't cut out when it reached 60c either.

 

[John.g]

"Ran HW with HW SP at 60c and the boiler temp reaches 60c and stays there. Then (with room stat turned up to 22c, actual room temp 20.5) boiler temp didn't go above 60c, BUT it didn't cut out when it reached 60c either."

Is the CH SP still set to its original 70C?, if so, then when room stat requests heat the boiler should ramp up to 70C and once it reaches this SP temperature the boiler will simply modulate down to maintain this 70C until the room stat is satisfied which will then cut out the boiler, if its maintaining the HW SP of 60C, then this is incorrect IMO. While its running on CH at 60C can you reduce the HW SP from 60C to say 45/50C and see if the boiler cuts out or reduces to this temperature, this would confirm that it is using the HW SP for both hot water and heating as appears to be the case.

 

[whatsthenews]

"Ran HW with HW SP at 60c and the boiler temp reaches 60c and stays there. Then (with room stat turned up to 22c, actual room temp 20.5) boiler temp didn't go above 60c, BUT it didn't cut out when it reached 60c either."

Is the CH SP still set to its original 70C?, if so, then when room stat requests heat the boiler should ramp up to 70C and once it reaches this SP temperature the boiler will simply modulate down to maintain this 70C until the room stat is satisfied which will then cut out the boiler, if its maintaining the HW SP of 60C, then this is incorrect IMO. While its running on CH at 60C can you reduce the HW SP from 60C to say 45/50C and see if the boiler cuts out or reduces to this temperature, this would confirm that it is using the HW SP for both hot water and heating as appears to be the case.

It didn't yesterday with CH SP set at 70c, but this evening it's going up to 60c pretty rapidly (with HW SP at 60c) ,then gradually going up to about 65c. If I turn the HW SP down to 40c with heating running the boiler temp doesn't drop to 40c. Maybe it's just modulating or it's got something to do with it being a condensing boiler?
Anyway, engineer seemed to think it was fine. Only a few iron filings in the magnetic filter which he said is normal for the system being about a year old.