0.5 bar/per hour pressure drop on system boiler

21 May 2009
Reaction score
United Kingdom

After partially draining our system to remove a radiator, I refilled and bled the radiators but I've noticed that the pressure gauge keeps dropping to almost zero. I keep using the filling loop to bring the pressure up to 1 bar, but within an hour it will have dropped to 0.5 bar even with the boiler turned off.

I've checked the pipework I was working on and there are no leaks, I've checked the pressure valve on the expansion vessel and this gave out air only. I can see a bit of moisture around the slotted screws under the bulkhead(?) where the CH flow and return pipes join the main chassis, but not enough to even form a drip in an hour (with the boiler turned off and cold).

So after the small essay, my questions are:
Is this moisture likely to be enough to cause this much of a pressure drop?

Any idea what else the problem could be?

Could the pressure have been at virtually zero for while now, even though the CH and DHW seemed fine?

Our set up is a Worcester Bosch Greenstar 30 CDi system boiler with a hot water storage tank/emersion heater.

Thanks for any help, this is driving me nuts.
Sponsored Links
Have you checked the actual pressure reading at your expansion vessel
Or checked your auto air vent valve to see if its passing?
How did you drain the system?? If you used the prv (which you shouldn't)it may not have reseated correctly and be leaking from the blow off pipe........
I've looked at the Auto Air Vent when I top the system up and there was no sound of air escaping or water leaking out.

I haven't tested the pressure on the EV, but could do that when I get home (only with a basic car tyre pressure gauge). Are you thinking that it could just be a faulty gauge?
Sponsored Links
The gauge usually measures the system,your e/v may need pumped up if its lost pressure and taking in more water than it should
The boiler has got a drain point inside it and I used that.

I must admit I keep reading about prv's, but I can't see one in my boiler. There is what looks like some sort of release valve at the entry point to the hot water storage tank, but there's no sign of air or water escaping here.

I've had a good look inside the boiler and it seems bone dry except for a bit of moisture around some screws beneath the flow and return of the CH and also at the condensate removal pipe (but I assume this isn't pressurised).
PRV is behind the pump to the right. Another crap WB design point :rolleyes:
You have a leak, end off. It could be less serious than it seems if the expansion vessel is lacking pressure.
Could you make an educated guess as to how much water you add every time?

You must have a lot of rads to need a 30 kw system boiler.
If you indeed have that many rads, you probably need an supplementary expansion vessel.
bengasman, with the boiler off, pressure sits at about 0.3-0.4 bar.

Getting it up to 1.0 bar is about a 10 second turn on the filling loop. 10 seconds on my kitchen tap gives about 2 pints. I guess the filling loop has more pressure against it, so it's less than 2 pints, but still a good cupful I'd have thought.

I depressurised the expansion vessel a bit when I gave it a quick press to see if water came out, but I must have topped the system up about 6 times over two days by 0.5 bar each time before I touched the e/v.

We've got 12 rads of various sizes - wouldn't a system boiler automatically come with an expansion vessel that would do the job? Sorry if that's a dumb question.

I first noticed the problem when I refilled the system after partially draining it to temporarily remove a radiator (it's currently bypassed with some 15mm copper). I assumed it had happened as a result of what I was doing, but if it's a leak that I can't see anywhere could it have been like that for months without us noticing any heating/hot water problems? The last time I actually checked the pressure was about 6 months ago after a chippie put a screw though a pipe.

Thanks again. I am trying to get the installer out for a service in a few weeks, but I've not been able to get hold of him yet.
As a very rough rule of thumb, the e.v. volume should be 10-20% of the system volume.

If we ignore the cylinder capacity for simplicity, and you take 1 metre of standard rad as 1 kw, you get 15 double panel rads, which with added pipes would add up to 150-200 litres, needing between 15 and 40 litres worth of e. v.
The built in e.v. in cdi combi's is about 10 litres so I guess the system version would not be much different.

If it takes 10 seconds to top up, you would be close to a litre each time; it is unlikely that a leak that size would go unnoticed for long.

I take it that the screw in pipe problem was adequately fixed?
Would that have been the groundfloor by any chance?
The screw through the pipe was on the first floor landing. I've just double checked the repair and there's a nice dry end feed joint on there.

Whatever is leaking must be on the groundfloor for us not to have noticed it, although no obvious damp smells yet. The screw in the pipe incident came through the ceiling in a couple of hours. Having said that, could it be the coil in the hot water tank leaking, we'd never see it there would we?

One last question, which I guess is partly related to my one pipe system question. When the boiler is working well we do get a fair bit of 'steam' coming out of the flue, I don't know if this is just when it fires up. Does this give any indication of how efficiently the boilers working? The room stat is usually only ever set at 18 deg C if that makes any difference.
If you have a vented cylinder, and the leak is in the coil, you would be unlikely to notice from a sealed system; a few litres a day would just disappear.

Steam from the flue is normal for your boiler.

If you want to do a test for that anyway, it is quite easy.
turn boiler off
isolate the flow and return with the valves under the boiler after you have brought the pressue to about 2.
leave boiler off overnight, and check in the morning.

Don't turn the boiler on before you have switched both valves back to open.
It is a vented cylinder so I'm very tempted by that test, although I'll do it tomorrow now.

Do you isolate the flow and return valves with the slotted hex bolts/screws under the 90 degree joint where the 22mm pipes terminate into the chassis? Assuming that's how you do it, do you fully open them afterwards (or count the turns it took to close them)

Thanks again, I'll post in a few days when I've made any progress.
I've now topped up the system to 2 bar and closed the CH flow and return valves. Over about 90 mins it's dropped to 1.0 bar so I guess that means a leaking coil, given that I can see none of the DHW pipes are leaking and the boiler itself is only two years old.

I suppose the next sensible test is to open the CH valves, top back up to 2 bar and then close the DHW valves and cross fingers that the pressure doesn't drop again.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links