Closed CH system - ? low pressure

1 May 2008
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United Kingdom

We recently had the CH system in our (large-ish) old Victorian house converted from open to closed system.

I have noticed that the pressure gauge (next to small red metal tank which was added as part of change) often falls below red warning needle (set at 1 bar) when heating is off.

When heating is on the gauge reads between 1 and 2 bar.

Is this normal ?

We regularly get air in one particular radiator on ground floor, (immediately above the boilers - these are in cellar), so this needs bleeding quite often. This issue pre-dates the change over to closed system.

Are the 2 issues connected ? If so what can be done to fix this ?

One plumber suggested putting in something to fill leaks and then possibly a power flush to remove corrosion / sludge.

There are no obvious external signs of leak in the CH system.

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For a typical property the pressure should be set to 1 bar with the radiators cold.
The pressure will rise during heating due to water expansion...ideally the pressure should not rise to more than 2 bar when the radiators are all hot...1.5 would be better.

The pressure drop and the continuing trapped air issue indicates a likely leak on the system and needs attention.

I would not suggest a powerflush unless there are known flow are more likely to cause problems (there's always a chance it will create leaks) and it's often suggested due to it being a good earner.
There are chemical leak sealers to inject into the system but they are a last resort...there's no guarantee they will work and they could invalidate your boiler warranty.
Find the leak is the best option...starting with radiator valve spindles etc.

There is some evidence of leak around some of the old radiator valves (green stain) but nothing active I can see. Would this be enough to half fill a radiator every few weeks or so ?

Some of the radiators are pretty old (lovely though) so may be internally created gas/air ? If so how an earth do you track this down to fix it ?
you say that you are reg bleeding some radiators, after you bleed any radiator you have to top the pressure back up, swapping an open vent system to a sealed system always run the risk of small weeps
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Leak under floor maybe?
I've had several customers with one particular rad collecting air and requiring regular's always been down to a leak bizarre as it sounds given it's a presurised system.
How many radiators and what type. If lots and they are the "school" type, they hold a large volume of water. The expansion vessel needs to be sized to hold 4% of the total volume of water in the system without going over pressure. What size expansion vessel (litres) do you have?

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