Worcester 350 Combi-expansion vessel

25 Feb 2006
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United Kingdom
I have a Worcester/Bosch 350 Combi. The CH 'Low Pressure' light is illuminated. Having followed the troubleshooting tips from other posts on this site I have checked the expansion vessel air pressure. I have drained the CH system water via a radiator. I have no air pressure in the expansion vessel and after removing the pressure gauge, water is dripping from the valve.
I believe this means that the expansion vessel membrane is ruptured. Please can you confirm my diagnosis, and if correct, how difficult is it to change the expansion vessel. Is it a job for a pro ?

Is it possible to repressurize the system enough to get the hot water back on pending the expansion vessel change ? We can live without the heating for a while but we are beginning to smell a bit !!! Thanks
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You could drain a radiator, preferably the closest one to the boiler on the CH circuit, leaving about 1/3 water in it. It will then do as a temporary measure. Don't be tempted to just leave it this way and not change the vessel though.

The vessel is a job for a CORGI engineer though as if memory serves it requires removal of the gas valve. Might be wrong about that, but if the gas valve is in front of it then call someone in, if the diverter is in front it might be best ringing someone also as they can be a sod to get back in!
Thanks ACOperson, I'll try the rad tip just to get the DHW on. I have re-pressurised the CH but the Low Pressure light is still on even though the direct reading gauge on the front panel indicates 1.2bar, this is taken off the CH return just below the burner. If I have lost the water from the burner and have an airlock at the point where the takeoff is, how to I bleed it ?

The expansion vessel sits up front beneath the burner, the Gas Valve is off to the left out of the way. I am more concerned with what is connected behind the Expansion Vessel as I cannot see this bit. Is it likely to be the diverter, or does the expansion vessel connect direct into the CH flow return behinf the backplate ?

thanks again
First off your diagnosis is right the expansion vess will have to be replaced, its one of the easy ones to do .Drain down the boiler from the brass drain point beside the by pass screw underneath then undo the philips screw holdindg a sliding plate to the left and below the vessel ,remove screw and push plate up,about 30-40mm then undo the 8mm bolt at the top right of the vess and pull the vess forward.have cloths over the electrics and a bowl below as about a pint of water will follow slowly. fit new o ring and replace tricky bit is to line up the plate slide before you re fit.
Most of the expansion takes place in ch mode so in the mean time you should be ok just on hot water if you can get the low pressure switch to free ,they have a habit of sticking if you cant free it by raising and lowering the pressure quickly you can swap the green and white wire over till you can get a new switch,it still has two overheat stats and the pressure switch was not even fitted to some of the later models.
The new tank comes pre charged and usually with too manu o rings and a couple of useless clips.
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Thanks COG, I'll give the low pressure switch trick a try and have a wash!!

I'll let you know how I get on with the expansion vessel.

thanks again.
COG, your tip with the low pressure switch worked a treat, I now have hot water again. A bit of grumbling from the boiler at first and the overheat switch on the top of the boiler popped. I removed the panel on the front of the boiler, the one with the two windows in for monitoring the burner, and opened the little plastic screw at the high point of the boiler, this let out some trapped air. I replaced the panel and fired it up again, it ran a little longer this time so I repeated the process until it now stays on.

Just one follow up question if I may, when I bleed down the boiler per your instructions, how do I refill it post expansion vessel replacement ?

many thanks for the help. I now have clean kids again, much to their disappointment !!
Had same problem - made great discovery. Swapped green and white wires, fitted new drain cock (they grandly call them "pressure relief valve"!). After while Low Pressure Light came on again so swapped wires back to original place - and boiler fixed with no need for new switch ! Sensor had got knickers in twist but solving the slow drainaway solved the problem - all for £13.
firstly, i doubt that anyone cares seeing as the OP is 3 years old. :rolleyes:

secondly, your WPS is probably blocked either in the WPS itself or the orifice to the return pipe that it sits in.

thirdly, it actually is a pressure relief valve and not a "drain cock". :LOL:

secondly, your WPS is probably blocked either in the WPS itself or the orifice to the return pipe that it sits in.

thirdly, it actually is a pressure relief valve and not a "drain cock". SURE - WHATEVER.
What you did was not a safe practice that any competent boiler engineer would be expected to do. The correct procedure is to remove the LPS and unblock the inlet. I did one on a 350 earlier this week!

There is no benefit to post as you have when all that you have done is given inappropriate and potentially dangerous advice!


I have one of these boilers (350 RSF) and have been told the expansion vessel has gone and needs replacing (it keeps losing pressure and needs to be topped up regularly)

The Worcester part number is 87161425060 which is obsolete and not in stock anywhere any more

Part number 87161425000 which seems to be a similar spec and for slightly newer (if still old) Worcester boiler models is still available from a number of eBay sellers

Is this part likely to be compatible / close enough or not?

Or should I just give up and get a new boiler?
1. Don't know about the part you quoted.
2. If you have room, you could fit an external expansion vessel instead.

I’m trying to get this sorted remotely and can’t get a plumber to do an ad-hoc repair.

Since the identical spare part seems impossible to source I’m probably looking at getting a new boiler, it’s had a decent 23 year run

Would a Worcester Bosch Greenstar i 30kW be a suitable new replacement for this boiler?

The spec on my 1997 vintage 350 RSF is 35 kW hot water output, 25 central heating whereas the Greenstar i 30 is only 30 kW for hot water but it’s only a 2 bed 2 bath apartment with 1 shower and 2 bathtub that would rarely be used at the same time so if anything the old boiler may have been over specced?

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