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Phil, Mechanical Engineer
Category: Home Improvement
Satisfied Customers: 7388
Experience:  Retired contractor, 51 years experience
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How to reduce pressure in a combi boiler?

Resolved Question:

How do I reduce bar pressure of my combi boiler. I switched the valve to increase the pressure but I ended up increasing too much.

Submitted: 4 years ago.
Category: Home Improvement
Expert:  Phil replied 4 years ago.

Phil :

See if you can find a drain in the CH piping at the bottom of the boiler

If not there, you can let water out at the air vents on the radiators..highest points

Let me know how that goes, we continue until you are fixed up.


Customer : There is a rectangular screw that when I turn water comes out. Is it the one? The rectangular screw is on the side of the radiator. Highest point
Phil :

Yes, be careful not to take it out completely, turn it half a turn or so...

Half a cup of water may be all you need to remove.

Customer : The pressure increases the more water that comes out mate.
Not sure why this is happening. I have a Worcester Bosch combi boiler. My friend said there should be a release valve in the boiler itself?
Phil :

Its only possible for the pressure to rise if the feed water valve is still open...

However anytime a boiler warms up, the water expands and runs the pressure up until the boiler cools back down again.

Any time you let water out of a closed circuit, the pressure has to drop..

The boiler has a pressure relief valve fit as well... you can manually trip that, but its not the way to let water out of the boiler. It dumps way too much water way too fast.

There is one other way city water pressure can get into the closed central heat loop on a combi boiler. That happens if there is a leak in the domestic hot water heat exchanger..that allows city water to enter the CH closed loop... depending on the heat exchanger used.

We can check into that later if necessary.

Customer : Hi mate. I am not really good at this I'm sorry. I tried turning the mains off and released water from the radiators.
After 2 buckets of water, the pressure decreased to 2.2 bar. Red starts at 3 so is not very good still. The Worcester Bosch greenstar 24i that I have is only 3 years old. Not sure why this is happening. I paid a lot of money for it
Phil :

Hello again, half a gallon removed should have dropped the boiler pressure...

Its likely the fill valve is leaking, or that water is leaking through the DHW heat exchanger to the CH side of the boiler.

Keep an eye on it. If the pressure goes back up, that will most likely be the case.

Customer : Ok thanks. My friend said that my combi boiler has a lot of safety features. Hence, I am told not to worry about it and try to keep warm especially tonight when it's really chilly. I will keep you posted. Many thanks
Phil :

The fact that the boiler pressure dropped in the first place is interesting.. it has to have a water leak or the boiler pressure relief has to pop off for the boiler to loose pressure...stay in touch on this.

Phil, Mechanical Engineer
Category: Home Improvement
Satisfied Customers: 7388
Experience: Retired contractor, 51 years experience
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