pressure dropping with combi boiler

[BrianRHobbs]

Hi Guys,

I'm hoping someone can give me some advice regarding my combi boiler. Recently it's been losing pressure (dropping down to zero). When this happens, I top it up to about 1-1.5 bar but over the course of 1-2 weeks, it drops back down to zero.

I've searched the forums before regarding this & have seen 2 possible causes for this:

1. a leak in a pipe
2. a dodgy pressure release valve

without taking the carpet up, it's hard to check the pipes, so I've been keeping an eye on the outlet pipe. Here's what I've seen:

at about 2 bar, there are occasional drips at about 3 bar, there are constant drips at just over 3 bar, there's a constant trickle

I've got 2 questions I thought I'd ask before getting a plumber out:

1. are these pressures normal/typical?
2. should I be getting the drips / trickles at those pressures?

thanks in advance, Brian

[fullflow plumbing]

Your pressure relief valve shouldn`t be letting any water past it until it gets to 3 bar, so there`s something amiss there .

most combi`s are pressured upto around 1.0 bar when cold, when operating they typically rise to approx 2.0 - 2.5 bar when hot, any more then the pressure relief valve is there to stop it getting past 3.0. The PRV should open quickly and discharge water out quickly till pressure drops back to safe levels upon which it will close again. Only time they trickle is if the water in the central heating system is manky and a bit of sludge is stopping the valve fully close again.

Sounds to me like you`ve got a leak, although quite a minor one if its taking two weeks to let the water out. Try checking all the visible joints first - rad valves are a good starting point as they often get wacked by kids/vacuum`s etc, air vents next then the pipework unfortuantly.

[Andrew Gabriel]

No. Pressure shouldn't be going that high. You should be filling to about 1 bar when system is cold. (Might need to be higher pressure in a 3-storey house if the pressure guage is on the bottom floor.) It will increase when hot, but probably not to more than 2 bar. PRV should not let anything out until it gets to 3 bar. If it goes to 3 bar when hot, then the expansion vessel is probably not working. It might just need pumping up, or if the diaphram is broken, it needs replacing (or a new one fitting somewhere else and just abandon the old one, which is often a good option for those at the back of the boiler casing which aren't easily replaced).

Yes, if pressure reaches 3 bar, the PRV should discharge.