CH can't raise pressure above 1 bar when cold, okay when warm

16 Nov 2013
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United Kingdom
I think I got a leak somewhere in the CH system that I can't locate. It seems to be leaking when the pipework is cold, not when sufficiently warm.

This morning the combi boiler, Baxi 830 installed in February, did not come on due to pressure drop, to about 0.5 bar. I tried to top up, initially it rose during filling but it didn't go higher than one bar. So I stopped filling.
The heating came on nonetheless but the pressure didn't rise as much as usual but stayed around 1 bar. Later I tried topping up and it worked, i.e 1.7 bar when warm.

I am waiting for a call from the boiler installer but do you guys have any thoughts for where to look? Could it be within the boiler?
As it seems to be leaking only when cold, I doubled check the conservatory where pipes are surface run. How quickly would I see such a leak if from upstairs on the downstairs' ceiling?

(The electric shower packed up last week too and still is out of order, the previous boiler gave up in cold February, I thought I had exhausted my bad luck for this year. And most of the house got new laminate flooring I layed)
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The boiler is pressurised by the pressure of the incoming mains water supply. If the pressure of the incoming water main is only 1 bar then that is the highest pressure you can put into the boiler.
When the radiators are cold a pressure of 1 bar is perfectly fine.
The only time it should be higher than that is if the boiler were say in a basement and it was several storeys high or there was a fault with the pressure sensor (if fitted).
When the radiators are hot the pressure should normally rise by a 1/2 to 1 bar.
Thanks, yes. But in the past I could raise higher suggesting that typically we got higher mains pressure than this. In principle it seems highly unlikely that our CH pressure dropped just during the night after which the mains pressure dropped. So I still think it's more likely that the CH can't hold pressure above 1 bar, unless I pipes expand with some heat and seal a leak.

That said, our electric shower behaved oddly two weeks ago around the same time in the day and people here said it sounded like water starvation. (Another five days later the shower stopped completely though). Is there such a thing as 7am mains pressure drop when everyone takes a shower?
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Something must have caused the pressure to drop though, and I can hear air bubbling too.
The picture showing ideal pressure for the boiler also shows a higher value than what I could attain this morning.

By how much does pressure in a typical 4 bedroom house drop when the ambient temperature drops from say 20 to 5 degrees at night? (Only the conservatory would have dropped so much though) Could that account to go from 1.2 to 0.5 and the boiler cutting out? I would not have thought so but I am no plumber.
Your boiler pressure this morning was 0.5 bar,what was it yesterday when cold ,do you know ?
Have you checked the PRV Pipework outside the property to see if water has been dripping from it ?
I don't know the pressure from last night but normally when cold it's 1.2 bar. I did look at the PRV and could not see any evidence of any drips but it's high up and it was foggy and dim this morning, so I can't be 100% certain.
So you don't really know when the pressure was lost ,between " usual" pressure and this morning ,could it have been over several days ? Water lost from a sealed system is either from the boiler ,or the systems pipework / rads etc. Process of elimination really ,PRV pipework, and all visible rads and Pipework for starters.
True, it could have been lost over a few days but no more than that as I check frequently. The pipework that is accessible I checked already. Shall I remove TRV heads and lockshield caps, i.e. could it leak along the pins?
And is it more likely to leak when the pipework is cold, or am I reading to much into the overnight drop?
What does the pressure rise to when central heating has been on for a few hours ?
It doesn't take much water loss for system pressure to drop a little over a couple of days. A slightly weeping joint is often enough. Dampness should be visible without removing TRV heads etc. Try wiping with kitchen towel around radiator joints.
A slight leak will evaporate away when the pipes are warm and may not leave any residue.

One way to detect that sort of leak is to draw lines on tissue paper with water soluble ink and wrap the tissue round the suspect pipe/joints, If there is a leak then the ink will run and smudge and stay that way after the water has evaporated.
I think I found two candidates, though currently neither is weeping.

1. The tall radiator in the hall is rusty at the top, see first photo. It's on the flow side and the bleed valve on the other side but without any air, so maybe it's not that.
2. A lockshield valve at a downstairs radiator (that looks like a rough install prior to our ownership of the house). The copper pipework is (poorly) painted but there seems to be some corrosion, 2nd and 3rd photo.

How likely are these the problem, by themselves or jointly? And if I get the second radiator changed, should I as well change the first one?


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Obviously been leaking at some point ,but could be historic. Are either positions damp ?
No, everything seems dry, both before putting the heating on and now that is running. I will keep an eye on them. If it is not one of these, then I really don't know.

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