A bit of a combi mystery (to me anyway)

[swest4]

Hi Folks,

I'm new to this forum and am looking for some help with my combi. I'm no expert by the way.

I moved into my flat a year ago and to be honest took no notice of the combi until around a month ago when i stopped getting hot water. For about a week a monitored the pressure and it didn't budge from 0.5 when cold.

Anyway, the plumber came and topped it up to 1.3 and fitted a diaphragm, which fixed the hot water issue. The morning after his visit however, the pressure had dropped to 1. Since then the drop has slowed down (0.2 over 3 days). I've checked the radiators and there's no sign of a leak. I also took the front off the boiler and there's no sign of damp or wet inside.

Is it possible that topping up a system for the first time in at least a year could induce some pressure loss through a sort of settling down period?

The only other thing i noticed is a bit of green (only very little) around the return pipe under the boiler.

I hope someone can help. I just hope it's nothing sinister under the floor (the downstairs neighbour has never complained of a leaky ceiling though).

Cheers

 

[MANDATE]

One of the most essential parts of a combi boiler is the expansion vessel.
On a vented system, the water expands when heated and the extra volume is pushed back to the header tank.
On a sealed system, there is no header tank so the extra water has to catered for by the expansion vessel.
The vessel is divided by a diaphragm to keep the water separated from the air.
Initially the 'air' side is pumped up to about 0.8 bar before any water pressure is applied. So with diaphragm fully biased towards the water side the full volume of the vessel contains air. This air will get compressed when the heating is on and the air pressure will rise and so will the water pressure.
If the initial air pressure is allowed to leak over time like a car tyre the water pressure pushes the diaphragm towards the water side.
This would explain where a small amount of water has gone and because it would lose some pressure it would have to be topped again. secondly the 'air' volume would be reduced and the effect of compressing a reduced volume increases the pressure beyond the design parameters.
Now a built in safety device is the Pressure Relief Valve, which opens at 3 bar pressure. Once some water has been discharged the PRV closes and the pressure will have dropped to something like zero or maybe 0.5bar.
So topping up is required.
The PRV once opened often does not close properly due to dirt on the seating face. So quite often the problem starts with the expansion vessel and after correction still leaves you with further problem.
This further problem would still be a 'leak' which would occur even with the heating off and more so with it on.
When heating is switched on make a note of the pressure before and after expansion, it should rise around 1 to 1.5 bar and it should not get close to the 3 bar to open the PRV.
Don't know if you can see the discharge pipe from the PRV. that is the most likely place to see if the PRV is the cause.
It is possible a small leak may be due to the valve spindles on radiators.
There is a way of isolating the boiler from the heating circuit and seeing if the pressure holds or still leaks.
Don't forget if you bleed a radiator the pressure will drop and topping will be needed.
Also there will be a AAV automatic air valve so it should get rid of air that is circulating around.

 

[swest4]

Thanks a lot mandate.
I should have said that the diaphragm was fitted to the divertor valve rather than the expansion vessel.
I've looked at the pressure cold and then hot and it rises by about 0.4-0.5 bar when switched on.
I wonder, however, if there has always been a leaking PRV without me knowing (i can't get at where the outlet pipe is on the outer wall). Is there any other way to know whether the PRV is leaking without being able to see the exit pipe (i can't reach it to put a bag over it as i've seen suggested on other threads)?
As i say before the plumber came, the boiler had been rock solid at 0.5 bar for a week or two. It might very well have sat at around 0.5bar for 12 months before the plumber came and topped it up as I hadn't looked at it before.

I'm guessing that if there was a leak under the floor, the pressure would have gradually gone to zero over a 12 month period rather than being sat at 0.5 (or maybe i'm being optimistic).
The radiator valves seem fine. There isn't even any rust to be seen or brown patches on the inside of the caps. I should also add that the boiler is 10-12 yrs old.

 

[mcmoby69]

fill the boiler to 1.5 bar. turn the heating on, when it is hot,locate the blow off pipe under the boiler, feel if it is hot where it goes thru the wall. look outside too , is it dripping?

look at the pressure gauge, is it over 2.5 bar when the system is hot?

if the answer is yes to these questions then the EV either needs recharging or replacing or is not big enough.

 

[swest4]

Thanks. I'll hunt out where the pipe goes through the wall. The boiler pressure goes up from 0.8 to 1.3-1.4 when hot - never over 2. I guess this means that the EV is fine?

I'm heading off to my wifes family over christmas so i hope the system will be OK switched off for a few days from tomorrow.

 

[swest4]

OK,

Although I guess that there is a discharge pipe going to the outside of my flat, i can't see it. The wall is stone and not smooth (it's a tenement in scotland) and so it might be between a couple of stones. I also can't see where it exits the house as the boiler is flush to the wall (it's not obvious when i look inside the lower front cover). I'm thinking of isolating the boiler from the other pipework by closing off the flow and return pipes. There are screw valves on these so i think i can do it. Does this sound like a good idea?

 

[MANDATE]

It does appear the vessel is functioning OK. pressure is not reaching the 3 bar, so the PRV is not being opened due to high pressure.
This to me leaves either a leak in the heating system or the PRV may have been opened and closed manually at some time and not resealed.
You could as suggested isolate the boiler from the heating to prove which side the leak is on.
Before you do that it might be worth opening and closing the PRV to see if any sediment can be cleared.
Or 2 other options
(a) Replace the PRV with a new one.
or (b) Clean up the existing PRV. This is easier said than done,
I had to make a 'C' type spanner to fit under the knob when the valve is in the open position. I did all this with old valve removed, but I'm fairly sure I could remove the new valve head without removing valve body.