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Heating, Home Heating Systems (31117 other questions)
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low water pressure in worcester Bosch combi boiler?

i keep having to add water via loop to my w-b 350 combi boiler with increasing frequency.
the pressure falls from 1.5 to 2 bar to below 0.5 bar on a daily basis and there does not seem to be an obvious leak-checked under downstairs floorboards- and pressure relief valve pipe and system drain pipe appear to be dry.
any suggestions?

tim may
January 2007

There is more help available. Mend - low, water, pressure, worcester, bosch, combi, boiler
Need to mend your Bosch Worcester home heating system?

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Answers - mend, repair, fix

Problem could be your expansion vessel. Remove cap from underside vessel and insert narrow blade screwdriver. If water sprays out the vessel diaphragm is leaking and the vessel must be replaced. This happened to me and after replacing vessel problem resolved.

The vessel has two compartments separated by a diaphragm. One side contains air pressured to approx. 1.5 bar (22psi) the other side contains water. When the pump starts the expansion vessel acts as a pressure dampener.

David Jones
January 2015

pump runs but does not ignite gas

November 2013

ok, i may be dumb but how do i top up the pressure in a 24i rsf combi?.
big writing version please.

January 2012

Please could someone tell me how to reset the pressure relief valve on a worcester bosch 42cdi?

Many Thanks.

December 2010

have had this on new heat cells for worcester condensing boilers. to check. turn appliance off and cut plastic condensate pipe outside of the boiler, if water contines to flow through this then the heat exhanger is faulty.
richard heating engineer oxford

richard cottrell. safe cert ltd
April 2009

I have had a similar slow leak from day 1, for the last 5 years. I have come to the same conclusion that it is in the boiler, but the trouble is it takes about 6 months to loose pressure and your solution although it is very elegant, it will not work on my installation. But it was good to read and know that it isnt just me in this world that is suffering.

Keith Spencer
July 2008

I too have a Worcester Bosch HE40 Plus Combi Boiler. For the last 3 months it has been constantly losing pressure and I have had to fill it up using the filling loop. Various plumbers have been called to see it and up until now they have all reckoned it was a 'microleak' in my heating pipes or radiators. I looked under the ground floor and at the ceilings looking for signs of a leak, but could find nothing. I thought I would just have to wait until all the water I was topping the system up with showed itself somehow. One plumber advised me to inject some Fernox leak sealer into the system to seal the microleak. I did this and for a day or so it seemed to stem the need to top up the pressure. I thought I had cracked it, then it started losing pressure again. I thought that it might need a higher concentration of the leak sealer, as it had worked for a while, but after adding two more units to the system it was still as bad. I trawled the internet for answers, it does seem that alot of people have this problem but no-one has posted an answer. I checked the pressure in the expansion vessel, via the shrader valve, it was fine. I tied a polythene sandwich bag over the pressure relief valve outlet to see if the water was leaking through the relief valve, but it was bone dry. I came to the conclusion that the water must be leaking from inside the boiler, but how to prove it? A post on the internet gave me a clue. After topping up the pressur to 1.5 bar, I closed the flow and return valves insiside the boiler to the radiators, effectively isolating the boiler and the heating system, and left it overnight. When I looked at the pressure the next morning it had dropped to zero. When I opened the valves up again the pressure went up to 1.5 bar again exactly where I pressurised it to. I tried the same exercise again during the day and found the pressure dropped to zero within an hour. I called out the Worcester engineer as my boiler is still within the two year warranty period and was able to categorically say to him that it was the boiler that was at fault not my heating system. He immediately replaced the heating cell/ combustion chamber which he said must be leaking water into the combustion chamber which was why there was no sign of water elsewhere. He also noticed that where the condensate drain ended there was algae growing under the drip of the pipe. He said that as the pH of the condensate was acidic algae should not be growing under the drip, which was another little clue as to the cause of the problem. In fact the condensate drain should not constantly drip at all, it should release the 150ml of condensate that has built up and then not drip at all until the next 150ml of condensate is released. If there had been a leak in the secondary heat exchanger which heats the hot water for the taps the pressure on the pressure gauge would rise because water at a higher pressure from the mains would enter the heating system. The only other clue that I had was that I got an error code of EA which meant flame not detected, I gather that too much water may have leaked into the combustion chamber and the gas could not be lit.

All this goes to show that the unthinkable can and does happen. Who would think that the casing of the combustion chamber/heating cell could leak in a virtually new boiler.

I hope that my findings help someone else identify their problem of having to constantly top up the system yet finding no leaks in the heating set up.

June 2007

At the lower right hand side of the boiler is a pressure relief valve which allows water to escape via a drainpipe to the building exterior. It has a red test knob which may be used to manually release pressure from the system when it is partially rotated. My system was losing pressure due to the deterioration of the seal which was pitted. A replacement cured the problem.
Before doing this, it might be worthwhile checking the pressure in the big red expansion cylinder which should be about 0.5 bar or 7.5 p.s.i. If the bladder inside the cylinder is deflated then it cannot absorb the water expansion in the radiator system when it is hot, therefore water is expelled from the system. Now when the system cools the pressure will be low , maybe to the extent that the low pressure switch disables the whole system. To check this you should turn off the mains supply to the boiler, turn the red test knob a quarter turn so as to reduce the system pressure to zero on the gauge. Now check the expansion cylinder pressure on the schrader valve using a tyre pressure gauge and add pressure, if necessary, using a footpump to achieve about 0.5 bar. Finally, top up the system from the water main to get a pressure of about 1 bar on the system gauge and retest the system. NB. The lower box assy. and various inner panels must be removed for access so always switch off the mains to the boiler. Also take care with the pressure gauge feed tube which may kink and break if forced against sharp edges etc. Best of luck.

Bob Phillips
January 2007

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