Losing Pressure cause?

[minermick]

Hello to all
I`m a newie to the forum and after my friend spoke highly of the help he has had from here i`m hoping I can get help and advice.
My posting may be a bit on the long side so my apologies in advance.

System is a Worcester Danesmoor 12-14 oil fired boiler that heats a 7 rad sealed c/h expansion vessel system and also dhw that is fed from an open vented tank in the loft.

The pressure on the sealed system has been dropping daily for a few weeks and I don’t know what the cause is or what to do about it.
The guage normally sits around 0.9-1.0 bar when not running and rises to around 1.5 in use. The system is off overnight and on checking each morning for the last few weeks the pressure has gone down a bit further each day and is now around 0.2 bar.
When in use last evening the guage was reading 1.4.

When the pressure first dropped I bled the rads thinking there was air in the system, which brought the pressure down quite a bit more so I let more water in via the filling loop to bring it up to 1.0 bar.

I cant find any water leaks and when depressing the valve on top of the expansion vessel air escapes but no water. (I picked up these possible causes from searching other posts)
The pressure safety valve seems ok as the outlet pipe is dry.

I`d be glad of any assistance please?

Thanks
Mick

 

[MANDATE]

From the pressures quoted, not going beyond 1.5bar suggest the expansion vessel is operating as it should. The PRV may have been opened manually at some point and its possible there is a leak there even if it's not obvious.
If it's hot water leaking then it could evaporate.
For starter it might be worth securing plastic bag on PRV discharge pipe and see what it collects over night.
The fact is there is a leak. If the boiler can be isolated from the heating system by closing boiler inlet and outlet valves for over night period and observing the pressure gauge. This will indicate which side of the valves the problem lays.

 

[minermick]

Mandate
Thanks for the reply.

I picked up your reply late last night and taking your idea i taped a bag over the prv outlet outside and this morning it was dry. The boiler was not running during the night, would this matter?
The prv has not been touched since installation 4 years ago, its out of harms way at the back of the boiler cupboard and has got the dust on it to show.
What might happen if the pressure continues dropping down to zero etc? As things are now the system seems to run ok but there must be summat wrong as you suggest.
What might happen if i put more water in via filling loop to bring the pressure up? Is this just displacing air and if so where could the air go?
I`ve put a soapy bubble over the air valve and there appears to be no leak there.
Sorry to ask all these questions but i`v nowhere else to go.
Any advice would be welcome to give me a chance to sort out this problem before i have to call out a heating engineer at £££ ph.

Thanks

Mike

 

[MANDATE]

If the pressure drops to zero, it may prevent the boiler from lighting because there may be a pressure switch which needs pressure for it to operate. So you would need to increase the pressure.
Adding more water is not a problem, it pushes against the flexible diaphragm in the expansion vessel. This compresses the air and the increase in pressure can be seen on the water pressure gauge.
I can't see how any air will be in the water side, but even if there was some it will be discharged through the Automatic Air Valve. (AAV).
The AAV If faulty, I suppose is a possible leak source if it does not seal properly.
If you can, try isolating the boiler from the heating system, by closing flow and return valves and leaving over night. If pressure still drops then you know the leak is not pipe work and/or radiators.

 

[moggridgeplumbing]

Apart from checking all the radiator valves and visible pipe joints also check any drain off points in case washer perished. Very small leaks can go unoticed. If you have done all this and the above and you cannot see any staining on your ceilings then the leak most probably is under your ground floor.

 

[minermick]

Just to update those who have given me help & advice.
I havent replied for a while as the problem seemed to be going away but i`m not so sure.
I`ve been topping up the water in the system when cold every couple of days to bring it up to 0.8 bar where it used to sit on average.
No leaks evident anywhere but i`m coming to the conclusion that there must be a leak under the ground floor.

 

[MANDATE]

I still would not bank on you having a leak just yet!. Well not a water one.
My line of thought is.
If the schrader valve on the expansion vessel is faulty and leaking air, the initial air pressure will drop a little each time heating is used.
Example initial air pressure set to 0.8, water pressure also set to 0.8.
The expansion vessel is basically empty.
Say Schrader valve allows air to escape.
Heating is switched on, water pressure rises from 0.8 to 1.8 due to water expansion and compression of air (normal).
The rate of leakage of air will increase because the pressure is now 1.8 bar.
The result from this is the initial air pressure reduces, lets say to 0.4bar.
It clear to see with a slight drop in air pressure, the diaphragm does not go back quite as far as it did.
The result is the water pressure drops slightly and there is a need to top up.
So where does the excess water go? Well it is now taking up space in the vessel which was reserved for air.

Of course it can only do this until it reaches a point where the volume of air is so reduced the increase in water pressure becomes so great it opens the PRV.
Perhaps it's worth replacing the schrader valve!
It also might be worth fitting a metal dust cap over the valve

 

[dcawkwell]

Check for leaks everywhere.
Could be pressure relief valve is leaking to the outside so check there as well.
Possibly leak under the floor or possibly a leak in the water jacket of
the boiler. The water will be burnt off buy the boiler when it is operating.
Open combustion chamber hatch and have a look inside.

 

[ironman1]

You have a water leak somewhere , 'topping up' the system every couple of days will have the radiators looking like tea bags in a short space of time.

 

[mysteryman]

Check the bag on the pressure relief valve outlet daily. If there's anything wrong with the expansion vessel, the prv will have to discharge fairly soon. Check the pressure with the heating fully hot, it should not be more than 2 bar.

Check also all your radiator valve spindles, and around all rad fittings for leaks. It can evaporate as fast as it leaks, so look very carefully. I think this is the most likely cause.

 

[Agile]

You have a leak but to properly evaluate it you need to repressurise the EXV according to the instructions in the FAQ.

Its important to do it properly.

Can you isolate the boiler flow and return valves? That would demonstrate if the problem was in the boiler or the system.

Tony