Pressure drop

3 Nov 2006
Reaction score
United Kingdom
We have a Worcester-Bosch Greenstar Combi boiler that is 4 years old, including the central heating system. A year ago we noticed that the pressure in the system would drop by about 1 bar every couple of days.

During the warmer months, when the heating was off, the pressure loss was minimal – about 1 bar every 6-8 weeks.

When the cold weather set in a few weeks ago, the central heating was turned up, and the rapid pressure loss returned. We have had 3 visits this week from Worcester-Bosch engineers, one of whom fitted a new gas valve, but we now have a system that loses pressure at the rate of 1 bar every 2 hours. Each night, we top up the pressure to 2 bar, but each morning the pressure is at or near zero, and several of the radiators now make a continuous rushing sound. The radiators have all been bled as necessary.

The engineers have indicated that there may be a leak in the central heating pipework but are not 100% convinced themselves.

Could there still be a fault within the boiler, and how can we isolate the problem?
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Thats a few litres of water been lost!

It is unlikely to be within the boiler but could be the pressure release valve vent.

It never ceases to amaze me of the lack of diagnostic skills of many of the people working in the industry.

Just today I encountered TWO seperate CORGI engineers who had both failed to bring a boiler onto heating just because they could not set the controls correctly! I told them what to do on the phone!

One fitted a boiler and had commissioned it and left it working but At Risk because he had fitted the flue incorrectly causing the flame to be vitiated and to lift off.

The rate of pressure loss before the engineers came, was about 1 bar every 2 days (like last year). Only since the engineers has the pressure loss been 1 bar every 2 hours. This is why we suspect the boiler despite all advice to the contrary. Thanks
Yes, but rather than just talking about it, have you checked the pressure release valve vent outside?

If its not actually leaking when you look at it then tie a plastic bag over it and see if it fills up overnight!

Does the pressure gauge ever go much above the set pressure?

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how strange - I have a Greenstar HE35 and it is doing exactly the same.

I have checked all around the house for leaks, but nothing. I keep filling the system up to 1 bar and then find it lower in the morning. Obviously it could be leaking downstairs under the floor where I would not see it, but I don't really want to lift a few hundred pounds worth of laminate flooring to find it is not...

what I do find strange is the system shut down at about 11 last night and we havn't used it since for heating or hot water. I have filled the system back up to 1 bar and can hear a drip drip drip which seems to be coming from the condensate pipe ? I wonder if these go faulty ??

the system again is about 2 years old

The only coincidences are likely to be that you both have either;

- leaking radiator distribution pipework
- a leaking PRV due to a flat expansion vessel (unlikely at this age)
- or your installer fitted too many rads (too high water volume) for the size of internal expansion vessel

In all these cases the water loss would be higher when you are running the central heating due to water expansion.

You could isolate the boiler at the CH flow and return valves for a day or so but you wouldn't be able to run the central heating while the test was underway. If the boiler pressure was lost when you reopened the CH flow and return valves it would be simple, you are losing water through your system and owe an apology to Worcester.

Those of us in the heating trade hear customers talking about coincidences regularly - but only where their money might be involved.
Start with boiler cold
Set boiler pressure to 2 bar
Close boiler isolation valves
don't use CH OR HW
Leave as long as possible - a few hours might well be enough.
Read pressure gauge
open iso valve whicle watching pressure gauge.

Come back.
Can anyone help. I have a worcester bosch greenstar 24i. The pressure has dropped to 0 and I found the charging key but dont know how to use it to fill the system..
thankyou guys for the advice - I will try definately try this to find out whether it is leaking from the boiler or pipework - unfortunately it is the weekend so the Mrs is doing the washing and wants hot water so I'll have to wait...

i filled the system yesterday to 1 bar at 7am in the morning and used the heating and hotwater on and off during the day.. (8-10am and 4pm to 10:30) It was on .5 bar at the same time this morning so its not leaking a lot, but exactly how much I'm not sure - maybe a mug full ?

if it is the pipework, is any of the Fernox leak sealer stuff anygood ?

If the heat exchanger is leaking internally on a condensing boiler you will never realise as the water goes down the condensate!!
I have had many boilers over the years do just this. First one was the good old Ideal Turbo 2 and Response. Laterly have had a few Worcesters aswell.

1 Start with boiler cold - ok
2 Set boiler pressure to 2 bar - did this at 11:am
3 Close boiler isolation valves - also did this at 11:am
don't use CH OR HW
4 Leave as long as possible - a few hours might well be enough.
5 Read pressure gauge - at 3:30pm the pressure had dropped to less that .5 bar!
6 Open iso valves while watching pressure gauge - no change in pressure gauge.

Then I bled the radiators, and none had air.

We have constantly checked the pressure release vent outside, and there is no evidence that it has ever discharged water.

Also we can hear regular discharges (every 10 mins) into the Condensate Discharge Pipe when the boiler is on, rather more than we expected.

During our experiment with the boiler off and the CH isolated, we collected over 100ml of water from the downpipe the CDP runs into, inside 10 minutes.

Thanks to you all for your very useful input.
sounds like Stan may have nailed it...

prdriskell please keep us updated to whether the Worcester Engineers replace the Heat Exchanger and it solves the problem..

I wonder what the warranty is on these if they start leaking ?


Thanks to the test done on the advice from ChrisR :D , (and from Worcester-Bosch engineer no.3), today's engineer (no. 4) went straight to the Heat Exchanger and agreed the fault is there. A new fan is also required, incedently - it packed up on Monday.

Only trouble was, they don't carry spare heat exchangers as they are too big, and he had no fan on board today, so engineer (no. 5) will be here in 2 days. Perfect weather for not having C/H or hot water!

Big thanks to PanierStan :D

Watch this space.

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