Is an air release valve a pressure release valve?


Postby mortod » Wed Dec 10, 2008 3:03 pm

Had a (wood chip) boiler installed at our local school, and the manufacturers instructions state that a safety valve should be fitted to the boiler. But the device at the specified position appears to me to be an air release valve (a small brass cylinder with a bike tyre type valve on top, with the word 'COMAP' emossed on it together with some temperature and PSI details).

Can an air release valve double as a pressure release valve, or am I correct in my belief that the plumber has fitted the wrong device?
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Postby stoneyboy » Wed Dec 10, 2008 7:07 pm

mortod,
You are right a automatic vent is not a pressure relief valve.
end
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Postby htg engineer » Wed Dec 10, 2008 7:22 pm

An auto-airvent is not a pressure relief valve.

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Postby Steve the gas » Thu Dec 11, 2008 5:30 am

Hi,

You are describing the expansion vessel.
There should be a blow off vent/pipe leading to the outside, from a PRV, normally with a red knob on it.
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Postby mortod » Fri Dec 12, 2008 9:35 am

[quote="Steve the gas"]You are describing the expansion vessel.
There should be a blow off vent/pipe leading to the outside, from a PRV, normally with a red knob on it.[/quote]

Thanks for the replies. No expansion vessel - it is not a sealed system (ie there is a header/expansion tank). But the boiler manual is quite clear that there should be both an air release valve and PRV. Is a PRV still required for a vented system (ie with a header tank)?
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Postby stoneyboy » Fri Dec 12, 2008 12:22 pm

The only problem which may occur is that the water in the pipes in the area of the expansion tank freeze and then there is no safety device for expansion. Because the woodchip boiler does not have any inherent means of instant shutdown in an over-temperature situation a pressure relief valve should be fitted.
end
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Postby mortod » Sat Dec 13, 2008 11:52 am

Thanks - that makes particular sense in our case, as the boiler has been installed with no pump overrun, so with a highish jacket temperature, the boiler does indeed overheat, cutting out at about 88C due to the overheat stat. If that was ever to fail then I guess it would start producing steam which is where the PRV comes in. Until I can convince the plumber to rectify his installation we are keeping the jacket temp low.
mortod
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