having a dispute with another installer

[Ricky]

This is from some very old FAS (Irish) training course "Trade of Plumbing" but mentions a BS standard.

Open Vent Pipes and Safety Valves
The open vent pipe of non-ferrous material and minimum diameter of 22mm should be sited
so that it can discharge into the F&E cistern above the overflow level. The pipe should rise
continuously and avoid any horizontal runs. Any heated water which may be discharged from
the boiler must have a clear unobstructed path via the open vent pipe to the F&E cistern.
No valves or components, other than full bore pipe fittings, must therefore be fitted at a point
between the boiler and the discharge point of the system.
In addition to the open vent pipe, a safety valve should be fitted on the boiler or on the flow
pipe as near to the boiler as practicable. The location should enable visual indication if the
valve has lifted, but should not be in a position that could be a hazard to any person or
electrical
components in the event of discharging. Safety valves should conform toB.S. 759
In our previous diagrams we have shown the ideal locations for the cold feed and open vent,
in practice however you may find that they are actually part of the pipework circuit from the
boiler, show in the diagram below.

Says it all, thank you, trouble is plumbing and heating safe working practises for open vented systems are largely forgotten as there are so few of them around now.

 

[Ric2013]

I must say the gas boiler (installed in 1978) in the house I grew up in had no overheat stat and it did have a safety valve. Not sure the MI's actually called for one, however. Given the lack of overheat device, it does make sense.

I don't see the problem in this case however. IF the boiler is plumbed with an unobstructed primary flow with the cold feed and vent coming off that primary flow, with the pump (and pump valves) AFTER the F&E close-coupling, having an extra set of valves on the return will make no difference whatsoever as the return can already be isolated from the open vent via the pump valves. The important thing is not to obstruct the flow. The heated water will still have 'a clear unobstructed path' to the open vent even if it will struggle to cold feed and vent at the same time, but you'd hope the modern overheat thermostat would have tripped well before this led to damage.

To be fair, I do see where people are coming from in that it seems inherently safer to plumb boilers in the old-school method where the boiler is in the neutral point between the cold feed on the return and the vent on the flow, but this simply wouldn't work with the restrictive waterways and low water contents in a modern boiler.

 

[Ric2013]

A valve should absolutely NOT be on the flow between the vent and boiler.

I'd accept in my own house, but I'd want the valve on the flow locked open and a safety label of some kind. If nothing else, I would imagine this could cause serious damage to the heat exchanger in the event the system were brought up to temperature and then the valves shut. I think the contraction of water as it cooled might cause significant damage.

Apologies for not noticing what the fuss was about - I hadn't seen there was an isolator on the FLOW as well as on the return.

 

[John.g]

The quote in post#15 puts it well IMO as it states, "should be fitted".

 

[Ric2013]

The quote in post#15 puts it well IMO as it states, "should be fitted".

If you are referring to the safety valves, I agree, and it may have been the law back in those days even in England. This is clearly a modern boiler, however. Surely you aren't suggesting that virtually all modern RGIs (even Shaun!) don't know the rules? I suspect no-one is fitting PRVs or other safety valves as a supplement to open vents because boilers now have built in safety features that can deal with no-flow situations quite effectively (even an 8kW Thorn Apollo from the 1980s will shut down in seconds if an airlock develops in the flow pipe), and because (I assume) the current rules have been relaxed to take this into account.

Obviously if the text you quote is for a semi-gravity system, then it makes even more sense as there will be no pump on the gravity circuit and it is therefore perfectly possible to have this circuit without valves and with no need for valves either.

If I'm right in my above (first paragraph) assumption, then the only issue we should be dealing with is the isolator on the flow. Obviously we can always go one better than the legal requirements, but IF today's law doesn't require a safety valve, then I wouldn't even want to suggest it is 'good practice' on a modern installation.

 

[John.g]

Don't think It was a legal requirement back then either as it would have stated "must be fitted"

 

[Ric2013]

And if sealed system and prv after pump like it normally is

Is that correct, or just the rubbish you end up coming across? I'd be more inclined to want the PRV to be upstream of the EV and thus between the pump and the boiler, though I'm ready to be told I'm wrong.

Don't think It was a legal requirement back then either as it would have stated "must be fitted"

I think 'should be fitted' means 'and jolly well shall be fitted', though, being course material, it isn't 100% clear what was a legal requirement and what was just the author trying to influence his students. I note the only 'must's in that entire extract refer to the same thing: not blocking the path between the boiler and the open vent i.e. ON THE FLOW (even though the wording is clumsy and you need to take the two 'musts' together for it to be clear that this is what it means), so I think your interpretation that 'must' refers to a legal requirement is probably correct.