worcester bosch 240 combi boiler loseing pressure

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Dumfriesshire
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Hi, I have a worcester 240 combi boiler that is loseing pressure whilst boiler not running. To start the tale it developed a leak from the 3 way valve last week, I took the covers of and realised where water was coming from, ordered part and fitted it myself. I re-pressurised the system etc. got it up and running and thought that was the end of my problems, turned the thermostat down when going to bed as usual and next morning pressure on gauge had dropped to zero!! Repressurised the system turned thermostat back up and heating and water perfect again, thought maybe air in system until I realised every time thermostat turned down and boiler not running for any longer than about an hour it losses all pressure. When i pressurise system I put it to 1 bar, when boiler running it raises to approx 2 - 2.5 bar and as i say when switched off it losses all pressure between half hour til an hour. I did see the outside drain dripping one morning a couple of days ago when boiler was running but just been outside and checked it and not dripping now and boiler been running about 2 and a half hours. After fitting 3 way valve is ther any special way of bleeding system? as all radiators hot and doesnt appear to have air in them. Any help or suggestions would be appreciated.
Thanks in advance, Ashley
 
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Your expansion vessel needs checking, it's probably not got any air pressure in
 
twgas is correct it will be your expansion vessel, you should not just fill the system when pressure drops as this is usually down to expansion vessel losing its pressure not the water vanishing into a black hole as most people seem to think :)
 
Your expansion vessel needs checking, it's probably not got any air pressure in

Thanks guys, how do you check your expansion vessel, is it something i can do myself? and can it be re-pressurised with air?
 
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yes and yes depends on you though, drain boiler only remove black dust cap from valve and use car type pencil tyre gauge, it is the same as a car valve, should be around 0.75 bar - 1.0 bar, use foot pump to re charge and then refill boiler, very easy
 
yes and yes depends on you though, drain boiler only remove black dust cap from valve and use car type pencil tyre gauge, it is the same as a car valve, should be around 0.75 bar - 1.0 bar, use foot pump to re charge and then refill boiler, very easy

Thanks again, will give this a try tomorrow, when you say to drain boiler, what is the best way to do this? do i have to drain all radiators etc?
 
no just the boiler, from the drain off tap on the boiler, your instruction manual should tell you how to do this, It'll only take a short time
 
Thanks very much guys for all your help, I checked expansion vessel this morning to find it had water in air side of tank. I managed to get a new vessel from local plumbing supplier & fitted it by lunchtime. Boiler now doesnt appear to be increasing pressure excessively & doesnt drop below 1 bar when turned down for any length of time so thanks again, I would never have done it without your help!!!
 

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