A slightly odd and disjointed question, but cant quite get my head around it..

Normal pressurized central heating system (with expansion vessel) set to 2bar....

If parts of the system freeze what pressure would the pressure vesel guage show?





Reason I ask.... my grandmothers heater is complaining that there is now flow through it (or low pressure).... the pressure guage reads as 0-bar... the temp has been below freezing for some time so its possible some of the pipework has/is frozen...

Trying to work out if the pressure will come back when it thaws, or if the pressure loss is likeley to be caused by a broken pipe somewhere...

hmm, if frozen bits that will not reduce the pressure. Of course a leak will.....
Have you topped up the system and tried again. Some need topping up from time to time (but not often, my home one about 2-3years, another rented house about 8 months) so it may have been losing pressure for some time? It will soon show where the leak is too
Best to avoid leaving these things without frostguard settings, just too much could go horribly wrong.....

Usually pressure drop is caused by a small leak in the system - so unlikely to come back if the ambient temp goes up again

i would of thought the pressure would increase when water freezes it expands this is why it splits pipes (not enough to notice on the guage though) the low pressure is more than likely a little leak somewhere check the pressure relief pipe outside isnt dripping too

[Edited on 29/11/10 by adam1985]

If it is the type of pressure vessel with a bladder in it to hold the air then it is possible that the air has escaped and then a small change in the volume of water in the system due to temp etc will cause the pressure to drop to zero.
Paul

Had exactly this problem today, pressure dropped to zero, boiler (condenser type) shut down and displayed a fault
code.

Turned out to be the outlet pipe from the condenser had frozen outside the wall and blocked. Thawed it out and
re-pressurized the system and all is well. No Idea why the pressure dropped though. Had it been an auto dump
I'd have expected to see water somewhere but no.

Hmm.. have suggested she re-pressurises the system and sees what happens...