Vent cockup

[PaulAH]

Starting a new thread as this had veered off-topic.

Do the feed pipe in 22mm, and block off the vent.

Are you serious? Where would expansion be taken up - in the feed tank? What happens if the system boils?

What we have (starting from boiler flow) is boiler - pump - auto bypass - MV - vent - HW cylinder - feed - boiler. Note vent is on flow side of cylinder, feed on return, and both of them connect close to cylinder. Predictably, pumping-over only occurs when HW is called. When HW valve is closed it effectively shields pump from vent.

Rads are on a parallel unvented circuit.

Pipework to cylinder is mostly 28mm (hangover from its gravity fed days) with some 22mm. One straight leg of 15mm pipe about 10ft long connects cylinder to F&E tank in loft directly above it. This would be relatively easy to replace. But could we cheat further and use the existing 22mm vent pipe as a feed, simply moving it to bottom of expansion tank and eliminating the current 15mm feed? This would have it feeding to the flow side of the cylinder with 28/22mm all the way.

Why does it need to be 22mm anyway?

 

[keego]

Do the feed pipe in 22mm, and block off the vent.

Combined cold feed, being 22mm it will take up the expansion through the pipework to the F&E tank.

 

[chrishutt]

Just to clarify, a combined feed and vent pipe is essentially the normal vent pipe with a feed pipe running from Feed & Expansion (F&E) tank to join the vent pipe instead of running separately down to join the flow or return pipe. As such the combined feed & vent will continue to have the inverted U bend over the F&E tank (which is not shown in the diagram above).

This arrangement has the advantages of eliminating pumping over problems, reducing water movements to and from the F&E tank and reducing the pipework required. It's only disadvantage is that the conflict between venting air and water flowing down to replace the air, making filling the system slightly slower.

I should add that both the diagrams above show various valves in between the boiler and vent pipe, which is of course not acceptable. the pump and MV should be relocated downstream of the vent connection.

 

[PaulAH]

I should add that both the diagrams above show various valves in between the boiler and vent pipe, which is of course not acceptable. the pump and MV should be relocated downstream of the vent connection.

And herein lies the problem. Some major re-engineering would be needed to make things "normal".

A couple of questions if I may.

If a combined F&V eliminates pumping over, why does it have to be downstream of the pump?

Problem with downstream location of MV is more obvious, but if this is opened regularly (which it is), any trapped air will have the opportunity to escape upwards. Water expansion will take place via the return when it's shut.

Why does Oilman say I can do away with the vent altogether?

With a combined F&V, how far down the feed can/should the vent pipe be connected, or is it OK just to continue straight up and then loop over?

Incidentally, I can't see anything in the building regs or water regulations that forbids this arrengement. They simply state the water must have somewhere to expand (unless I've missed something).

 

[gasgeezer]

The pump is normally donstream so it pulls the cold feed and not air from the vent it is in 22mm min as this deemed to not block as easy as 15mm pumping over is as bad as pulling air because the water movement is oxygenating the syst leading to a shorter life due to corrossion. the pipe can be carried straight up and over the expnsion will be as normal if the boiler o/heats thats when the vent comes into its own incidentally i think you can only have this arrangement if the boiler has a manual resetable o/heat stat.

 

[chrishutt]

If a combined F&V eliminates pumping over, why does it have to be downstream of the pump?

On the contrary, it should be upstream of the pump (assuming the pump is on the flow) to avoid the pump (and its isolating valves) potentially cutting off the clear pipe run from boiler flow connection to open vent.

Why does Oilman say I can do away with the vent altogether?

Does he? I think you're misinterpreting what he says.

With a combined F&V, how far down the feed can/should the vent pipe be connected, or is it OK just to continue straight up and then loop over?

Think of it as keeping the vent pipe as normal but teeing in the 15mm feed (and expansion) pipe from the tank. The connection can be anywhere convenient. The level of the tank base or just above the vent pipe connection to the flow pipe are equally acceptable (strictly speaking the latter is marginally preferable since it minimises the length of shared pipe).

 

[oilman]

You can fix things relatively easily. Remove the existing vent pipe and put in a new vent pipe from the spare spigot on the boiler.

 

[compheat]

Why dont you convert it to a sealed system

 

[PaulAH]

On the contrary, it should be upstream of the pump (assuming the pump is on the flow) to avoid the pump (and its isolating valves) potentially cutting off the clear pipe run from boiler flow connection to open vent.

Sorry, confused by lingo. By upstream you mean "before", as it were. That makes sense. I have relatives in Australia.

Oilman has just posted. Thanks, old bean.

You can fix things relatively easily. Remove the existing vent pipe and put in a new vent pipe from the spare spigot on the boiler.

EASILY? OK in principle but this is exactly what I'm trying to avoid. It's a long and tortuous route from boiler to F&V tank.

I wonder if you could comment on the proposition posing as a question:
"Problem with downstream location of MV is more obvious, but if this is opened regularly (which it is), any trapped air will have the opportunity to escape upwards. Water expansion will take place via the return when it's shut."

It comes down to this: The proposed arrangement (with Chris's up-and-over vent added) may not be textbook but what, in real life, is likely to be the problem? It's been running fine for 18 months except for the pumping-over nuisance.

Thanks for your patience.

Edit: Compheat - thanks for the suggestion but we'd probably end up with a fountain in every room!

 

[oilman]

As you have two thermostats on the boiler, you could connect a PRV direct to the boiler and out through the wall. Then you don't need a vent at all.

 

[chrishutt]

The problems arise if the boiler thermostat fails and it starts boiling. If MV to vent connection is closed the boiler explodes - or something nasty. Probably won't happen, but.......

At the least you should put a PRV and discharge pipe on the flow near the boiler. This will give steam a means of escape. Ideally the boiler should be fitted with an overheat cutout - does it have one?

 

[PaulAH]

You guys are so kind to take an interest in this self-inflicted problem.

So am I correct in thinking the proposed setup (plus gooseneck vent yes/no?) is OK as long as a PRV is fitted? Yet does it even need one? Extract from Boulter manual...

"The Boiler is fitted with a safety overheat/limit thermostat. This will interrupt the power supply to the Boiler and shut it down completely in the unlikely event of overheating..."

I love this. We're bucking the system with an elegant alternative. Up yours JP.

 

[gasgeezer]

if you fit a vent no need for p,r,v but the vent must be 22mm as mentioned by chris you can tee off 15mm to tank but to the flow pipe 22mm

 

[chrishutt]

So am I correct in thinking the proposed setup (plus gooseneck vent yes/no?) is OK as long as a PRV is fitted?

You should know the answer by now! The installation must comply with the Manufacturer's Instructions. Failing specific instruction from the manufacturer, the relevant regulations and British Standards apply.

 

[PaulAH]

Oooh Chris, much as I love you, you can be exacting at times.

Boulter manual says nothing about combined vent/feed so presumably it's acceptable.

If MV to vent connection is closed the boiler explodes - or something nasty.

Wouldn't this kind of drastic pressure escape via the (unrestricted) return?

Gasgeezer stresses the need for a 15mm feed and 22mm vent. So is it just a case of cross-linking the two pipes in figure 1 beneath the tank (with what - 22mm or 15mm?), or do we also disconnect the feed (or the vent) as well as cross-linking?