Loss of pressure in C/H system

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says-les

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I've had at least four visits over two years from Gas Board to a Potterton Boiler which has to be re-pressurised at least once per week.  Wonder if it can ever be fixed.  I was away for four days and the system pressure dropped to zero.  Pipes below floor show no sign of leaks.  Is it possible to have an air leak without water showing?  There are six radiators divided over an upstairs and downstairs terrace house.

Engineer replaced the expansion tank within the boiler three visits ago.

What can be done?  New system?  System is eight years old.

Says-Les

 
the water must be going somewhere. maybe slowly leaking under a floor somewhere or possibly through blow off. new system wont fix it unless you replace all pipes and everything

 
"new system" will only fix it if every pipe, every radiator and hot water tank is also replaced.

It won't be an air leak, it will be water leaking somewhere. if it only leaks when it is hot it might just be evaporating so not visible as a drip.

Are you sure the expansion vessel WAS replaced and the pre charge air pressure set correctly?  Check the drain from the over pressure relief valve for drips when the boiler is running.

 
I'd say water loss somewhere .     Under floor ?    Do you have to recharge the system  from the main water fee ?  

I have a standard combi  boiler , charges with that jumper flexi onto the mains .    A very slight drip  on the bathroom radiator .. which evaporated quickly but left a mark ...was enough to shut down the boiler once a week  as the pressure dropped below  1.0 bar  .

 
Leak  sealer can be added to the system. Its like the radweld stuff used on car cooling systems, but first have a good look at all your radiator valves, connections and glands.

It takes very little leakage to drop the system pressure so you aren't necessarily going to see water, maybe just a build up of scale like lettuced around a leak source.

Also check the over pressure relief valve. If that passes at all it's probably unseen as it will drain outside.

First though, as someone else said, check your pressure vessel has been properly pressurised. If the system pressure rises quickly when the boiler is started from cold its a sure sign the pv needs re-pressurising.

 
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