Boiler pressure dropping when heating is on

[boilerwoes]

Hi all,

I have a Vaillant ecoTec plus 831 boiler, just over 3 years old now. The problem I have is that when we use the heating, the boiler pressure drops - and the amount it drops is proportional to how long the boiler is on - the longer the heating is on, the more it drops. So it drops from 1.1bar to 0.6bar in about 4-5 days when the heating is on about 6-8 hours a day.

It stands to reason there is a leak somewhere. But I noticed that if the heating isn't on and we just use the hot water, then there isn't a drop in pressure (or if there is, it isn't noticeable).

So it is either a leak in the system, which gets worse when the hot water is pumped through, or there is an issue with the boiler such that when the heating is on, the pressure reduces?

I checked the boiler overflow pipe when the heating was on but didn't detect any moisture in the pipe.

What do people think? Leak in the system that gets worse with hot water, or boiler issue? If it is a leak in the system, I am surprised that it gets worse when there is hot water in it, as I though that for any joins etc, hot water would "seal" weeps shut due to expansion of the copper pipe.

Cheers!

 

[GQuigley67]

there is a leak somewhere in the heating circuit, your heating circuit and domestic hot water are seperate and the pressure only refers to the heating circuit. The leak will be more noticeable when heating is on with a combination of the hot water and the pump circulating the water

 

[Shaunthegas]

I would expect a leak on a compression joint somewhere on the heating circuit, underfloor maybe. A leak on a radiator valve ( check the gland nut also ), remember as the water heats, causing expansion, the pressure increases.
Are there AAVs on the circuit? These are prone to weep occasionly.
There'll be one on the pump manifold in the boiler itself and possibly the main heat ex. But if your not "Gas safe" then best not to fiddle there.
That'll be the obvious places to start. But say if you can't look under your floors to check, you can get a company to thermal image the floors, ceilings and walls and see heat loss on pipes. Expensive, so check what you can first.

 

[warrington]

Have you checked the expansion vessel. If your expansion vessel in the boiler you may loose pressure. If water comes out when you check the vessel,so you know it is vessel.

 

[Jonwells]

With a ecotec it will be expansion vessel, as mentioned before, check the scroder valve on the expansion vessel, if water is present it's the expansion vessel, this isn't always correct though, as some scroder valves are on top of expansion vessel, so air under pressure will be expelled in some cases.
Best thing to do is, top boiler up to 1bar cold, turn heating on, and all stats to max, watch boiler pressure rise, at 3 bar max, water will be expelled out the prv pipework, then see if boiler is up to max temp, this can be 90 degrees or more.
Note, as soon as the pressure stablelises, up to max 3 bar, this means either boiler is up to max temp or if less than 3bar; then prv is weak.
Which ever the case on a standard 7 rad system, water will be expelled out the prv pipework.
Let the system cool, and as soon as it Rose before, the pressure will again drop. But this time to below 1bar when cold.
There are simpler ways to check the expansion vessel, such as pressure of 0.8 bar in the nitrogen (air side) of the expansion vessel scroder valve, (car tyre valve), when there is no pressure in the heating side of the boiler( depressurised to atmosphere).
Be aware, that i have seen valiant prvs that have stuck, with pressures up to 4 bar, a very dangerous situation.
If all this fails, it's a leak on system pipework, but a 1 in 10 chance. Jon