Bosch 28CDi Boiler - Pressure drop

16 Feb 2009
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United Kingdom
Hello All,

Recently i have noticed thatt the pressure gauge on the boiler has been dropping down from its normal 1.5bar to around 0.5 to 0.2bar. In order to get the pressure back i am having to insert the Tee key and add water to the system (not sure how much i am adding but the tap is being fully opened and it takes about 15seconds to reach 1.5bar pressure)

I realise this sounds like there is a leak somewhere in the system but i have checked all accessible piping and not found any problems additionally there is no visible sign of any leak anywhere in the house.

This issue is occuring every 48 hours or so but only when the boiler has been turned off, when its left running (Central heating on) the pressure remains relatively constant at 1.5bar.

I am confused, could this be some other boiler issue (a valve or something) or is it more than likely a leak i have not located yet?


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I think your boiler has an automatic air vent ( AAV ) inside the combustion chamber and this is probably leaking.

As only gas qualified people should open the combustion chamber you should seek a competent CORGI registered engineer.

In our local area we would charge about £84 plus £23 to supply and fit the part although that would include a service.

On your model its a good idea to change the AAV on a service every five years or so.

Thanks for the reply,

Just one question though i am having to add water to get pressure back in the system so if the AAV is leaking is some of the water in the system being leaked out or evaporating in the combustion chamber due to the leak?
Yes, thats what I expect is happening.

Its a common cause of a lot of DIY searching for leaks which a competent CORGi would identify in 10 minutes!

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Your expansion vessel needs pumping up. It's the red cylindrical thing behind the gas valve and pump. Drop the sytem pressure (via the prv) and get yourself a footpump. there's a dustcap (just like your car tyres).
Stick about 0.5 to 1bar in there and you'll be sorted
I agree with Stevies62 Pump up your expansion vessel to about one bar. But make sure there is no pressure in the boiler or you will get a wrong reading.
Thanks for the replies.

The boiler actually stopped a couple of days ago due to low pressure so my wife got a guy out to look at it as part of our insurance policy. He repressurised the system and has said that the problem is due to 'sludge' in the system and that we need a power flush undertaking in orderto sort it out, he quoted £550 for this!

Since then when the heating is off thepressure drops to 0.3bar in about 12 hours, when the heating is left on low the pressure drops to 0.3bar in about 24 hours.

Given that the pressure is dropping when the system is off or on would the expansion vessel still be the the issue or are there other possibilities?
when the expansion vessel is not working the excess pressure is forced out via the pressure relief valve. When the system cools there is less water in the system hence less pressure. Pump the vessel back up before you pay for anything else. :LOL:
Thanks for the info.

How can i identify and activate the PRV in order to ensure the boiler pressure is at 0bar before pressurising the vessel?
best not to use the PRV. They can let by if you use them. But if you must it will be located at or above the boiler and has a red knob that you turn a quarter turn. best to use a drain off cock (DOC) should be one on the boiler but if not any one on the system will do. It is brass with a spout that you push a hose on. Don't undo the square bit too far or it will come out. Remember to vent the system when you fill it back up and open the big chrome screw on the pump to let the air out. Have fun
Thanks for the info.

How can i identify and activate the PRV in order to ensure the boiler pressure is at 0bar before pressurising the vessel?

28 cdi prv is on the bottom right side inside the boiler behind the drop down pcb panel (2 screws) or it can be operated by removing the undertray (pull), dont use it unless you want to change it.

there are two draincocks on the boiler underneath, white spouts on either plastic manifold. they may not work if they are blocked.
Thanks for the advise guys.

I will give it a go and let you know how i got on.

I droped the system pressure and recharged the vessel to 1.0bar on Saturday (checked pressure with gauge after pumping up, the gauge on the boiler also went to to 1.0bar during pumping). The pressure has dropped to about 0.3bar again in the last 24hours (heating has been on for around 3 hours during this time) I had to recharge the system again in order to get the boiler producing hot water this morning.

Any other ideas as to what could be the problem as the system is still not holding pressure. The tendancy is that if i recharge then have the heating on the pressure in the system remains for longer than if i recharge and do not put the heating on afterwards, not sure if this fact will give any pointers?
When you were pumping up the system, if the boiler gauge went up at the same time, you were getting a false reading on the pressure vessel. Try again, but this time the boiler gauge must read 0 and the vessel 1.0 bar. If the boiler gauge goes up in pressure let more water out of the heating system. It will work. When you restart the heating put a cup under the PRV outlet outside. Don't block the pipe. If you still have a problem, see if the water has filled the cup.
You have a false reading on the pressure vessel. Try again but make sure that the boiler gauge reads 0 when the pressure vessel reads 1.0bar.
It will work.

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