Combi Boiler that tripped out due to Low Pressure > HELP

[stunning]

My combi boiler tripped out this morning. The pressure guage on the front which at the weekend was reading just over 1 bar, was almost at zero. Any ideas to what has caused this sudden loss of pressure?

I have increased the pressure back to 1 bar to get it going this morning, but obviuosly something cant be right for it to have dropped off overnight so quick. Any ideas for things to check will be appreciated.

 

[kevplumb]

check the prv pipe
the one that goes outside

 

[stunning]

Whats the prv pipe? Is that like an overflow pipe? Check it for what?

 

[kevplumb]

see if it's passing water