Mystery Object - AAV?

[Axel]

Most modern boilers are fitted to a sealed system to the PRV is vital.

On an open vented it is not so important, UNLESS the pipe(s) connecting to the Feed & Expansion tank are blocked.

Does anyone know what would blow first if there was no PRV and the above pipes were blocked??

 

[45yearsagasman]

May I ask one question, do modern boilers have a PRV function included?
If you fit a boiler to an open vented system then normally PRV's are not required unless called for in MI's.Sealed systems have to have a PRV and these are normally fitted in the boiler or as a separate install near the expansion vessel.

 

[Hugh Jaleak]

If the cold feed and vent are blocked then IIRC from college its the boiler that blows first. Goes off like a bomb Did actually happen round here, a BBU exploded, taking out the chimney breast with it....

 

[Onetap]

The standard remedy for old leaking PRV's on open vented systems that is to plug them off.
END OF

By that, Mr. Gasman, do you mean the standard remedy within British Gas? If so, that is no recommendation.

I'm sure you've seen a lot of these that have been dripping. Usually, they would be encrusted with limescale, where the water has evaporated from the hot metal, leaving the dissolved solids on the surface. Isn't that fairly usual?

This one looks very clean; why do you think that might be?

Maybe Brian de-scaled it before taking a photograph or maybe he lives in an area with very soft water. Any other possibilities that you can think of?

 

[Onetap]

Does anyone know what would blow first if there was no PRV and the above pipes were blocked??

You can't predict what might fail, because the rest of the system isn't designed to yield at any specific pressure. I'd expect the expansion of the water might split a pipe or push a compression joint off. It would get dangerous if the thermostat also failed 'ON' ( which they do, quite regularly, by the contacts welding themselves together) causing the water to boil.

The back boiler mentioned by HJ killed an elderly lady; I believe it was redundant and the boiler had been capped off, with water in it, by a housing association's plumbers.The fireplace was still in use.
http://www.hse.gov.uk/press/2009/coisw61109.htm

There was also an incident last winter in which an immersion heater thermostat stuck on and the CF & OV pipes had frozen in an unoccupied flat. The house was nearly demolished.

 

[Norcon]

Any other possibilities that you can think of?

Slight touch of steam cleaning me thinks.

 

[Hugh Jaleak]

That wasn't actually the incident I had in mind, i'm sure there was an incident of a BBU exploding in Kettering, Northants a few years back, reported in the local press. Cant find any reference to it now though, although I think fortunately no-one was home when the explosion occurred.

 

[Onetap]

Any other possibilities that you can think of?

Slight touch of steam cleaning me thinks.

Possibly there is no accumulation of limescale because the PRV only started dripping recently.

The cold feed was blocked. If the open vent had also been blocked or frozen, the PRV would have operated to relieve the pressure caused by the normal expansion of the water. They often do not re-seat properly once they have opened.

If the PRV had been removed by someone applying the 'standard remedy' that would probably have required a new boiler. If the thermostat had also failed On, it would have required a new house.