CH boiler installation - isolation valves query

[Rob Graham]

This is a follow on from a previous post.

I've got to the point of following the boiler re-build instructions and have come across a warning note saying that I must not fit isolation valves in an open vented system.

Now I can see why, but in my case I have a boiler pump and a CH pump - reason is that I have a wood burner with a back boiler, and the oil boiler and its pump go off when the wood burner is hot. The wood burner piping is open vented without disruption but the boiler pump has isolator valves either side of it for replacement - I had to do that last year and I would have had to drain down most of the system if the valves weren't there.

Is there a solution to this or is this the boiler manufacturer protecting himself?

Rob

[Harry Bloomfield, Esq.]

Rob Graham wrote on 23/08/2020 :

My single pump for HW + CH in an open vented system has isolation valves either side of the pump too.

[Rob Graham]

Thanks for the feedback which sent me off to google the topic and find that inevitably there are diagrams that show the vent before the pump and ones that show it after!!

I suspect that ideally it should be before the pump based on the instructions I'm referring too.

Anyone around to resolve this please !!? Rob

[Fredxx]

I don't see the issue as long as the valves are open when the pump and/or boiler are running. They are only fitted for maintenance.

If you have two sources of heat, the usual technique is to use 2 pumps; each with their own non-return valve to prevent backflow for each system.

You could even use a pipe/cylinder stat to turn off the boiler if the woodburner is providing sufficient flow temp.

[Dave Liquorice]

Is the pump in the flow or the return? Where is the feed attached relative to the vent?

What you are trying to avoid is pump over, be that a constant dribble from the vent when the pump is running or just a little spurt when the pump stops or starts.

IIRC the feed and vent should connect to the primary loop on the same side of the boiler and within a few inches of each other. I'm not sure if the order of vent/feed is governed by being on the suck or blow side of the pump.

[Roger Mills]

There must be a clear path from the boiler the vent pipe, with no possibility of blocking it with any sort of valve. But other than that, it doesn't matter.

My boiler is in the kitchen on the ground floor, and the pump and zone valves are all in the airing cupboard on the first floor, along with the HW cylinder. The fill and expansion tank is in the attic, immediately above the airing cupboard. The fill and vent pipes are both teed into the boiler flow pipe, just before the pump.

There are isolation valves either side of the pump, and also on the CH flow and return pipes - allowing me to isolate the CH if necessary whist still keeping the boiler heating the HW. None of these valves interferes with the clear path between boiler and vent pipe.

[spuorgelgoog]

AIUI the vent should be fitted on the 'push' side of the pump so that air isn't sucked into the system through the vent.

Owain

[Rob Graham]

OK - thanks all for your inputs and questions. I am of the opinion that what must be implied by the instructions is that there must be no electrical valve in the circuit to the vent whcih could fail closed. If you are operating isolator valves on either side of the boiler pump then it has to be assumed you know what you are doing and have electrically isolated the boiler.

And yes the vent is on the 'push side of the boiler pump. The network of pipes all join at a Dunsley Neutraliser which has the vent so it is at atmostpheric pressure.

Again thanks - and regretably I've got another question from the same set of instructions but I'll post that separately. Rob