Vent Pipe Central Heating

[btctworld]

this is propably a really stupid question but here goes

what is the purpose of a vent pipe on the central heating system?

what if you do not have a vent pipe in your header tank?

cheers

 

[Gasguru]

Allows the air to vent from the system( and steam to vent off under fault conditions).

If you don't have a vent pipe you probably have a combined feed and vent.

This is only permited if stated in the boiler manual (boiler normally has to have an additional overheat cutout)

 

[btctworld]

how would i know if i have a combined feed and vent what would i be looking for on my system

cheers brett

 

[kevplumb]

conventional system gives you a starting point

 

[Gasguru]

Generally a combined feed and vent has a single pipe from the base of the feed and expansion cistern to the heating system.

Sometimes the pipe tees just below the expansion cistern to form a vent.

This is very rare to find and was considered unsafe (certainly is if the feed pipe blocks)

The only systems I can think of would be Servowarms and more recently Glowworm Microns

 

[jobloggs]

Combined feed and vent should be in 22mm pipework.

Gasguru wrote

Sometimes the pipe tees just below the expansion cistern to form a vent.

Should there not always be a vent teed off the combined F & V pipe and arching up over the tank?

 

[Softus]

Should there not always be a vent teed off the combined F & V pipe and arching up over the tank?

Yes. I don't think that Gasguru meant to imply the absence of the arched vent pipe, merely that a system with a combined h&v would not have an independent vent pipe rising from the primary circuit.

 

[ChrisR]

jobloggs wrote:

Should there not always be a vent teed off the combined F & V pipe and arching up over the tank?

Yes.

I know you do see various configurations stipulated by manufacturers but I've never seen the point of a separate bit of pipe for the last foot or so.
I have a suspicion that this is one of those cases where it intuitively looks better so everyone copies what they saw somewhere else, it but actually makes no difference. Water going down can pass air going up, in 22mm pipe.

 

[Softus]

...I've never seen the point of a separate bit of pipe for the last foot or so.

I agree that a relative trickle of air, or expanding water, can easily pass up the feed pipe (as long as it's at least 22mm). However, in the case of a 'burp' of rapidly travelling air/water/steam, it would be better to direct downwards into the F&E whatever is being ejected, rather than risk splurging some F&E contents upwards, possibly lifting the lid (where there is one!), and possibly spilling the contents over the edge of the cistern

Addendum: Water Regulation G20.1 gives a clear requirement for the size of air gap if the vent pipe it is over the F&E cistern, but is ambiguous about the need for the vent to be over the cistern.

However, I would suggest that a vent pipe that terminates underwater is not actually a vent pipe in the generally accepted sense of the word, and that the WRs have adopted the generally accepted sense of the word.

 

[btctworld]

the reason i ask because i have a problem with my boiler when it fires up it has a banging noise when the pilot light comes on i can hear the water in the pipes moving along the pipes and sounds as if there is air in the pipes would this be caused by not having the vent pipe we had a plumber around and he thought it was the pump so he changed it and it didnt cure the problem we have put in de sludger and de scaler this hasnt cured the problem i have reduced the water level in the header tank by bending the arm as the level was too high this hasnt made any difference(i read a post about water re-oxygenating) what do you think?

cheers brett

 

[Softus]

Often, a faulty/worn pump is the cause of the symptoms that you describe. However, it's not the only possible cause.

So either your 'plumber' has changed something without knowing what the fault was, or there's more than one fault.

If your system was being re-oxygenated, then bending the arm up now is like closing the stable door after the horse has bolted. You would also need to drain and flush out the system to remove the sludge that forms as a result of corrosion in the radiators.

However, a system that is perfectly plumbed will still corrode if it isn't protected - do you know if your system has had any chemical inhibitor in it?

 

[btctworld]

yes it has had sentinel x 100 in it at all times

 

[ChrisR]

Brett are you describing "explosive ignition"? This is a bang from the boiler and nothing to do with the pipes.

BUt your boiler could well be making bumpy noises as air/steam collects for some reason. Have you looked for a separate vent pipe - what configuration have you got, and what 's your boiler and its age?


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Softus - Does that water reg apply to heating systems - ie no mains water to protect? I'm too lazy to look it up but it doesn't mention combined feeds and vent pipes ?

I rather doubt that there's a rule which is designed to reduce splashing but, well maybe. I've never seen a stipulation which says that the vertical part of the combined F & V must carry on up, with the feed branch off to one side.

I looked at The Keston C25 instructions - (they do show evidence of analytical thought on the whole(!)). They DO say to use 28mm combined pipe. Fair enough I thought, perhaps they've done tests. But they not only show the pipe going straight into the bottom of the f/e tank in their sketch (sideways vent branch), but then go and call the vent pipe "Expansion" which of course is bo11ox!

"a vent pipe that terminates underwater is not actually a vent " - hmm well what does that mean - in either case there's a pipe full of water with air above... If the air had managed to get up the metres of vertical feed pipe, it would surely manage the last few cm?

Can be hard; the quest for the truth on things like this is fraught with dragons saying "so you think you know better" (which you're not), and others saying that because XYZ say something it must be right. Then the crowd (usually including me) chimes in with arguments chosen because they appear to support the accepted view which probably HAS some truth behind it - rather like backing the Favourite nag. But we can all cite codswallop which has passed through dogma into Regulations.
All I'm asking is "why"!

 

[kevplumb]

ive come across a few systems without a vent it just backs up the feed

never had any problems

i suppose some shiny arse has come up with a reg that says its dangerous tho

 

[Gasguru]

The combined feed and vent is described in BS5449. As I'm not willing to be ripped off and pay for it I have know idea what it says. Did see it once on an assessment though.

The Glowworm Micron manual shows a single 22mm pipe and reference to BS5449.

I think we've all seen various configurations but probably don't know the definitive answer.