Glowworm 30sx losing pressure

[HarryGato]

I've read through existing posts but need some clarity before I go insane!

Pressure overflow was leaking for a few days and so had an engineer come out at the weekend. He has replaced a couple of boiler parts but now the system is losing pressure and needs topping up constantly.

This problem was not there previously but the engineer isolated the boiler to "prove" that the pressure loss was within the system and not the boiler. No signs of leaks (although the ground floor pipes are cast in concrete).

So... the scenario is:

When the CH is on the pressure is maintained
When water is on pressure drops
When boiler is off pressure drops


Question is, what does this mean? If this is a leak then is it on the CH loop or the water loop or both???

All help/(non-abusive) comments appreciated.

 

[John506]

Have you checked the pressure in you expansion vessel? You can use one of those £5 argos tyre pressure readers that connects to the schrader valve, might be worth pressing it with your finger first, if water comes out, the diaphragm inside has gone.

Should say in the MIs what the pressure should be, if not it will be stamped on the vessel itself, check it with system cold.

The pressure is only to do with the central heating side, it will not go down if you have a leak on the cold/hot water.

 

[HarryGato]

Thanks for the reply. I've checked the EV and that is fine - blowing air. When the boiler is isolated it is holding pressure which would indicate problems within the system. Which is the bit I'm struggling with!

Trying to isolate the fault to the water or heating loops.

 

[MANDATE]

The drop in pressure on the water gauge indicates a leak.
In most cases, failing to check the air charge in the expansion vessel leads to the vessel not operating as it should. The water pressure rise too high and when it reaches 3bar it opens the pressure relief valve.
When this opens the pressure drops quite low and action to re pressurize is needed.
It is quite common for a PRV once opened not to re seat properly, so you can get a small leak, so the pressure may well drop overnight.
With the boiler/PRV/expansion vessel/ isolated and gauge now holding pressure proves the leak is beyond the isolation valves in the heating system.
More than likely at joints/valves at radiators.
The 'hot water' is not related. In a combi hot water is the cold water going through a heat exchanger so it comes out hot. It is at mains pressure and not related to your system pressure.

Don't forget a small leak may be hidden under the lockshield valve covers and it may well evaperate fairly quick. The packing gland on the lockshield valves would be the most likely place for a small leak