Central Heating Question

Diaphragm expansion tank broken on boiler when radiators installed

When my plumber installed new radiators to my heating system, the diaphragm expansion tank on the boiler broke - which has sent the bar pressure on the boiler steaming over 3 and water constantly pouring out of the overflow pipe. I understand I need to get a boiler person in to fix this. But, the plumber claims that it was probably caused by draining down the system, and that because the boiler was getting a bit older the diaphragm broke. Is this just my tough luck, or is this possibly caused by the plumber either not venting the system when he drained it or over-priming the system when he filled it up again, or something else?

PS - is it wise/unwise to continue to operate my boiler while I wait to get it fixed? Any ideas on what to expect for a replacement cost? Thanks.

3 Answers

Best Answer

Hi there. Unfortunately it is probably tough luck! An expansion vessel diaphragm is a moving part and made of rubber which makes them fail often. Just go into a plumbing/heating supplier, the amount of spare expansion vessels they keep. (that being said overcharging an expansion vessel can cause premature failure - if it's older than 10 years it has done well though!)

I would not run your system until it is fixed. As a rough guide of price, the expansion vessel will be £70-£120 and labour £50.

If you need any more help, check my youtube channel 'plumberparts'

Hope that helps.


Answered 13th Nov 2011

J Lawrence Plumbing

Member since 16 Mar 2009

its hard to see that its the plumbers fault all he has done is drain the system.To get the vessel changed could be very expensive,it is not wise to use the boiler for too long without getting it sorted get a heating engineer asap as you should of done to change the radiator.

Answered 13th Nov 2011


Member since 1 Jun 2008

I agree with James, expansion vessels fail every now and then, but over charging the pressure vessel can cause premature failure.

A rough price for an expansion vessel is between £100-£150 dependin if they still have them in stock.
If the boiler is as old as you say it may be discontinued and very costly to replace.

Answered 17th Nov 2011

S.Banks Plumbing, Heating & Eco Energies Ltd

Member since 14 Jul 2009

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