vent pipe goes down then up????

[jobloggs]

All the pipes on my oil fired boiler go down into the floor. The system is vented by a branch from the flow side of my HW cylinder 2 floors up.

Am I right in thinking the vent pipe is there in case the boiler overheats and boils the water? In that event does it matter that the pipe goes below the level of the boiler before rising? There appears to be a PR valve on the top of the boiler (little device with a fairly hefty spring visible in the top).

Thanks

 

[chrishutt]

Sounds like quite a serious fault - at least if the boiler stat fails it will be.

 

[cozzmic]

How old is the system, I remember seeing this type of set up on systems that are about twenty five + years old. It seemed to be the standard thing to do in the old days. The only thing is if the safety valve doesn't lift if there is a fault, most likely the excess pressure will force its way up the return. But in a perfect world you should be vented off of the boiler or if your system allows convert it to a sealed one, or how about fitting an expansion vessel near the boiler just incase while leaving the system as is. Chris Hutt what do you think of that idea it just came to me.

 

[chrishutt]

convert it to a sealed one, or how about fitting an expansion vessel near the boiler just in case while leaving the system as is.

Interesting idea, but a few problems.

Firstly the boiler manufacturer would have to say that the boiler was suitable for a sealed system, which I doubt. If it was a gas boiler it would have to have an overheat cut-out to be suitable. I would expect the same to apply for oil.

Secondly, the expansion isn't the problem. That is accommodated by the feed and expansion tank so fitting an expansion vessel doesn't add anything. It's the escape of steam that is the problem. One option would be to fit an additional PRV on the pipework with a proper discharge pipe, but the first problem remains.

 

[jobloggs]

I'm in this house 25 years and the pipework was in about five years before that.

I had the boiler replaced about 12 years ago and three extra radiators put in a first floor extension. Plumber never said at that time that there was a problem. Prior to the extension there was not really a continuous upward route from top of boiler to the F&E tank but now there is. Looks like that is the way to go. It will also solve a problem I would likely have with pumping over if I put a stat on the HW cylinder and 2 port valve on the return pipe from the HW cylinder.

Cozzmic mentioned converting to a closed system. What happens to one of those if the boiler stat fails and the pressure valve sticks closed. Is that not a much worse case as steam has then no escape at all?

 

[cozzmic]

Cheers Chris, just one of those thoughts, sensible answer tho,
Jobloggs as Chris says you would have to fit secondary stat protection to make it a closed system as if the boiler when to meltdown and got above 100'C the water would flash to steam and make a new view out the side of your house. Also the boiler may or may not be suitable for a sealed convertion but the manufacturers instructions could tell you.