DIY Doctor

Hot Water and Pressure Drop Issues With 1 Year old Combi Boiler

Postby monet208 » Mon Apr 08, 2019 6:31 am

Hi all
We have retired and downsized and our new house has a year old combi boiler
We are doing a lot of work in the house and 2 days ago early evening a bedroom radiator was lowered (pipes were frozen to do this) and a hall radiator was put back on after being off for a week due to decorating- all by our very good plumber
Next morning I had a shower ok but when my wife got in no hot water!!
Pressure guage had dropped to zero so topped up and hot water again
Pressure dropped marginally by afternoon so topped up
All fine since
Can find no leaks in the system anywhere
Could the initial radiator work I mentioned at the start of this HELP question be the cause
Having never had a combi with wall thermostat any help would be appreciated
David
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Postby htg engineer » Sat Jun 22, 2019 8:05 am

There can be a few reasons why the pressure will drop, one is a leak on the system:

When the boiler is in use, the pressure will rise, but should not reach 3bar.

If the pressure rises above 3 bar, it is likely to be a faulty expansion vessel, which needs recharging or replacing.

Once it goes above 3 bar, water will be lost via the pressure relief valve.

The pressure relief valve may be passing water, check outside for any water or signs of staining.

Water/pressure can also be lost via the condensate pipe if the boiler heatex is leaking, pinholes, corroded etc.

If you're sure there's no leaks on the radiators, pipework etc. Turn the boiler flow and return isolation valves off, pressurise to 1bar and leave for a day/overnight to see if it holds or falls, it may then fall once the valves are opened again, meaning it is on the pipework/radiators/valves somewhere.
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