feed & expansion locations with low head

[kev999]

hi all,

I'm currently following up some suggestions about stopping air getting into my vented system in another thread. Hopefully these will solve things, however, I wanted to check what I 'think' I know about the feed and vent positions and their effect on suction is correct, and more importantly what you think of the setup in practice:

The setup is as follows: The first things in the system after the boiler outlet (hot side) are the vent, feed and pump - in that order. All are very close together: the (22mm) vent is about 60mm before the (15mm) feed, and this in turn is about 50mm before the pump inlet (its a typical 5m head pump).

My understanding of the theory is that the feed is the neutral point, therefore everything in my system 'should be' in positive dynamic pressure except for the 50mm between the feed and pump?

However, I'm wondering if, in practice, it could not always be like that - i.e. is it likely that the neutral point doesn't actually stay neutral enough, and air could get get sucked down the vent pipe occasionally?

The reason I ask is that the pump is very high up, so the head of the system is only 1m. I know that I'm not getting 'pumping over', so the vent pipe being in the positive side of things is fine. Its more if things can go the other way - is this head too small and so could the negative pressure extend back to the vent pipe sometimes?

Cheers
kev

 

[Mike Jackson]

Check your boiler instructions, some manufacturers will allow a combined feed and vent configuration which will eliminate all possibility of pumping over or dragging air in.

 

[kev999]

Check your boiler instructions, some manufacturers will allow a combined feed and vent configuration which will eliminate all possibility of pumping over or dragging air in.

Hi, Thanks for that. I had a look at the manual - I can't see it specifically banned, but its definately not one of the configurations they say are permitted - all of them have separate pipes. In case it means anything to anyone its an old Glow-Worm Fuelsaver (MkII), but still going strong after a new gas valve a few years back.

I'd wondered about this before actually, but am a bit cautious in case removing the vent pipe affects the safety of the thing - when I first moved in to the house I found (after bleeding the rads) that the feed was completely blocked so presumably the vent was all that stopped 'bad things' happening. If I tried this, is there a safety valve I can cap the old vent with in case the combined feed/vent gets blocked?

I think back when I first began looking at this, someone suggested an oversized vent pipe as an alternative to losing it. I opted instead for raising the header tank at the time, but could still be worth a try?

Thanks again,
Kev

 

[Bernie2]

Are you saying the boiler has two vents, one on the boiler and the other close coupled with the cold feed?

Yes you can put a safety valve on, but I would not cap any vents if I where you, until I was 100% sure at least one was free flowing and able to vent into the header cistern.

Seems to me to have been an old system, with one of the boilers top flow tapping's used as a vent or possibly a Primary flow with a vent off and the other top flow tapping used to feed a pump and rad circuit. And somebody has adapted it with a close coupled feed and vent and left the old vent connected.

Don't know though! Just a guess!

 

[kev999]

Hi,

my wording may have been misleading, there is only one vent. By 'the old vent' I meant what would become the old/unused vent if i adopted the single/combined feed/vent approach. Now i come to think about it though, thats an assumption on my part; perhaps its the current vent that should become the combined pipe, rather than the current feed?

Thanks for the response though - the caution about needing to be certain that at least one pipe is flowing freely seems very important. It is now and I'm quite clued up on inhibitor and flushing/maintenance etc. but it hasn't always been like that with past owners and the feed pipe was definately blocked when I moved in. I guess it may not be for future owners either, so i don't really want to leave a time-bomb!

I had a thought last night though (probably too much alcohol!). No one has reacted in horror at the 1m head so I guess it may not even be the vent pipe thats responsible for the air getting in. I wonder if I could/should cap the vent pipe temporarily for a few days to see if it makes a difference - a temporary push-fit stop end on its outlet or something. Although the feed pipe is currently only 15mm I know that its clear and free flowing at the moment so perhaps this wouldn't be too risky for a few days?

Cheers
kev