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Discuss WTF happened here? Viesmann went bang in the Central Heating Forum area at PlumbersForums.net

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Two viessman boilers that work in tandem. Noticed upstairs radiators weren't getting hot because of air.

Turned off boilers, bled the radiators. There was a lot of air trapped, so had to continuously repressure the system was I was bleeding.

About 90 mins later (I was doing other stuff in-between), ready to start boilers again.

Pressurised to 1bar, switched them on. Everything seems ok.

All of a sudden pressure rises sharply on both boilers, into the danger zone and bang! one of boiler goes. I switched power off and called boiler technician out.

Looks like a pipe exploded because of the pressure. I'm trying to figure out how that happened? Definitely put it to 1 bar, and switched on when the system was cold.

Any ideas? Need to get the boiler repair but don't want this happening again
 

gmartine

Gas Engineer
Messages
1,706
Two cacaded boilers in a large house or commercial property, no idea but pressure relief valve(s) should've easily let go before any pipes went bang as you describe.


Didn't take previous advice did you?


Not for you, get it properly seen to repaired and boilers serviced.
 
Messages
5
Two cacaded boilers in a large house or commercial property, no idea but pressure relief valve(s) should've easily let go before any pipes went bang as you describe.


Didn't take previous advice did you?


Not for you, get it properly seen to repaired and boilers serviced.

That was the other boiler which was fixed.
 
Messages
2,159
Any ideas? Need to get the boiler repair but don't want this happening again
Could be something wrong with the expansion vessel(s) but there must also be something else wrong with the configuration as the pressure relief valve(s) should have prevented anything going 'bang'. Could be something wrong with the way the boilers have been paired/cascaded because such arrangements and controls should 'fail safe'.

System needs to be looked at by someone who really understands multi-boiler systems, these are normally the domain of heating engineers with 'commercial' tickets. (Experienced domestic engineers may also have relevant experience/expertise but don't assume without checking.)
 
Last edited:

Last Plumber

Esteemed
Plumber
Gas Engineer
Messages
4,314
Two viessman boilers that work in tandem. Noticed upstairs radiators weren't getting hot because of air.

Turned off boilers, bled the radiators. There was a lot of air trapped, so had to continuously repressure the system was I was bleeding.

About 90 mins later (I was doing other stuff in-between), ready to start boilers again.

Pressurised to 1bar, switched them on. Everything seems ok.

All of a sudden pressure rises sharply on both boilers, into the danger zone and bang! one of boiler goes. I switched power off and called boiler technician out.

Looks like a pipe exploded because of the pressure. I'm trying to figure out how that happened? Definitely put it to 1 bar, and switched on when the system was cold.

Any ideas? Need to get the boiler repair but don't want this happening again
The engineer who attends should be able to diagnose the cause.

Any photo's
 

Last Plumber

Esteemed
Plumber
Gas Engineer
Messages
4,314
I've just read that other post re-air vent internally.
Did you close it to stop it leaking?

If you have air in the system when the boilers are running and that enters the pump, the circulation will stop or reduce to too little flow. The water in the heat exchanger (due to amount of heat and lack of flow) will reach boiling point before the safety side can react quite often and boil the water. You can have water above 100 C at a higher pressure than atmospheric but once the pressure is released, the water turns to steam instantly (laws of physics). That sounds like what has happened to me but I haven't seen it. Very roughly 1 x cubic inch of water will turn into 1 x cubic foot of steam.

The air tap on the internal shunt/pump should never be closed in my opinion. If it is leaking, it needs to be replaced.

Let us know what the explanation was once you've had the engineer in.
 

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