Feed and Vent fun :DDDDD

[fixitflav]

Virtually all open vent systems (except gravity to cylinder) I know of in my area are done with the vent off the flow at cylinder and feed off the cyl return.

I would guess they are old systems. I bought 1st house in 1972 which was piped like that, but it was going out of favour vs boiler - open vent - cold feed - pump then or shortly after, due to the issue of seesawing causing aerated water entry. I modified it, and piping was simpler as the cold feed point was then directly below the F/E tank.

 

[ShaunCorbs]

That wasn't one of your original options, but that would be OK.
Should then be open vent - cold feed - pump - boiler. You definitely don't want the cold feed on the discharge side of the pump or there's a good chance there will be negative pressure somewhere, if pump DP is 8m.
As I'm sure you know, the open vent and cold feed should be no more than 150mm apart to avoid risk of over-pumping, but where they are in the circuit relative to other kit is irrelevant.

No it wasn't as it would be under neg pressure

Can't get the vent and feed before pump unless it's on the return as it's an ab roof, which isn't the best as it won't vent if the boiler boils

It's a bloody catch 22

 

[fixitflav]

as it would be under neg pressure

Not sure I understand that. It won't be under negative pressure if the cold feed is on the pump suction pipe.

Can't get the vent and feed before pump unless it's on the return as it's an ab roof, which isn't the best as it won't vent if the boiler boils

Hard to say without knowing the details. You've clearly looked at lots of options so it's unlikely I can add anything, but I'm game to try if you want, no doubt others here will too.
The bit about the boiler boiling - is it solid fuel? I didn't think boiling was an issue with gas or oil, as there are control and limit-stats.

 

[ShaunCorbs]

Not sure I understand that. It won't be under negative pressure if the cold feed is on the pump suction pipe.

Hard to say without knowing the details. You've clearly looked at lots of options so it's unlikely I can add anything, but I'm game to try if you want, no doubt others here will too.
The bit about the boiler boiling - is it solid fuel? I didn't think boiling was an issue with gas or oil, as there are control and limit-stats.

it would go

pump on return pushing into boiler
feed on flow
vent on flow

there is an always risk in certain circumstances eg if a stat goes bad etc

 

[fixitflav]

pump on return pushing into boiler
feed on flow
vent on flow

I'd be a bit nervous with that arrangement. I did sketch below based on some assumptions:-
Pump DP 8m, total system loss 8, 3m across boiler, 5m across rads and pipework
TWL in F/E tank 3m above fill point on pipe
Boiler, pump and rads are all at same level
Shows pressure -2m after the rads. The assumptions won't be spot on and it could be refined to allow for different levels, so it might be OK in practice, specially if it's a tall building so plenty of head from the F/E tank. On the other hand, it might be worse.

there is an always risk in certain circumstances eg if a stat goes bad etc

What precautions are normally taken which you can't do on this job? I'm curious because it might apply to my house!

PS I hope you're charging the customer double time!

 

[ShaunCorbs]

tbh these days seal the system unless its ropy like the one im doing

and some boilers have flow switches in dont fire unless theres flow

 

[Vee]

Why not seal without pressurising. Use the header as top up, with gate and non return fitted. Sealed system controls as required with heat only or system boiler ( Worcester Cdi used to not need minimum pressure to operate, but check with them).

 

[ShaunCorbs]

Why not seal without pressurising. Use the header as top up, with gate and non return fitted. Sealed system controls as required with heat only or system boiler ( Worcester Cdi used to not need minimum pressure to operate, but check with them).

thats not a bad shout would the non return open tho also would need a prv

 

[Vee]

Non return is to stop the water backing up into the header. Therefore system controls required, i.e PRV and expansion vessel required either external or internal to boiler (system). Doesn’t need filling loop as header acts as top up vessel. Works a treat without stressing existing pipe work. As the system is sealed and there is no open vent non return will stay shut as for air to get in water would need to get out.

 

[ShaunCorbs]

Non return is to stop the water backing up into the header. Therefore system controls required, i.e PRV and expansion vessel required either external or internal to boiler (system). Doesn’t need filling loop as header acts as top up vessel. Works a treat without stressing existing pipe work.

yea but would it open under gravity and not stick