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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Plumbing Questions >> Vent pipe siziing https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1315827596 Message started by brainstrain on Sep 12^th, 2011, 12:39pm

Title: Vent pipe siziing
Post by brainstrain on Sep 12^th, 2011, 12:39pm

When I installed my central heating I unwittingly made the vent pipe to the header tank in 15mm instead of 22mm. This, as you will all know, results in the water coming out of the overflow if the pump is set to too high a speed. I currently run with the pump on its lowest speed setting which is OK but makes the system slow to heat up. I am loathe to replace the whole pipe run with 22mm as at least half of it is behind a recently tiled box. My question is this: Can I just replace the pipework in the loft with 22m or 28mm? Will this solve the problem (or at least help)? The pipework in the loft consists of about 5ft horizontal run and 4ft vertical. The amount of pipe above the header tank water level is around 2ft.
Thanks
Jim

Title: Re: Vent pipe siziing
Post by thescruff on Sep 12^th, 2011, 2:45pm

Had you connected the cold feed and vent correctly you wouldn't be asking the question.

Post a pic or two showing how/where the cold feed and vent is connected and I'll tell you what you've done wrong.

Title: Re: Vent pipe siziing
Post by CWatters on Sep 12^th, 2011, 11:13pm

In other words it might not be the size of pipe that's the problem.

What the-scruff needs is a photo and or drawings showing where the feed and vent connect to the system in relation to the pump.

Title: Re: Vent pipe siziing
Post by thescruff on Sep 12^th, 2011, 11:57pm

Correct CW, if it's pumping over then there is most likely a blockage in the cold feed area, or a major installation error.

Title: Re: Vent pipe siziing
Post by brainstrain on Sep 14^th, 2011, 1:26pm

Can't get my photo to upload so I'll have to describe the relevant parts of the system. Apart from the vent pipe and feed pipe the whole system is in 22mm. Between the boiler flow pipe and the pump is a feed from the header tank. The pump direction is from the flow pipe towards the diverter valve and off to the rads/tank etc. The vent/overflow pipe is tee'd off the return pipe and this is the last return tee before arriving back at the boiler return inlet. Distance from vent tee to boiler about 7ft. Distance from feed pipe to boiler about 7ft. Distance from feed pipe to pump about 1 ft. Total run of the 15mm vent pipe is: 12" vertical, 10 ft horizontal, 8 ft vertical, 4ft horizontal and 3ft vertical to the overflow bend. Height of the overflow bend is about 18" about header tank water level. Of this the last 7 ft is accessible. The system was designed around the info which came with the boiler and, except for the vent pipe diameter, follows the standard layout which can be found in many places. The pump has just been changed (by a real plumber!) and the system drained, refilled and checked for correct operation. There are no blockages. The system works well at the slow pump speed, albeit a little slow to warm the rads. The system bleeds easily. If nobody can make sense of my original question which was if I replace the pipework leading to the header tank with 22m or 28 mm, will this solve or improve the situation, I will leave it alone and put up with a slightly inefficient heating system. Thanks to all who have responded. I thought this may be a well known plumbing problem with well documented solutions. Looks like I'm wrong.
Jim

Title: Re: Vent pipe siziing
Post by thescruff on Sep 14^th, 2011, 3:07pm

It is a well know plumbing problem Jim.

It's piped up wrong. ::)

Makes models and type of system would help.

Lets assume it's a fully pumped Y plan.

The order should be.

Boiler.
Vent.
Cold feed.
Pump.

All in the flow, and the cold feed/vent should be within 150mm of each other.

Title: Re: Vent pipe siziing
Post by CWatters on Sep 14^th, 2011, 6:34pm

Google found a drawing..

Title: Re: Vent pipe siziing
Post by brainstrain on Sep 15^th, 2011, 3:16pm

I think I'm getting somewhere at last! I skilfully managed to swap the feed and vent connections so that I have the feed between the boiler and the pump and the vent into the return. The diagram in the boiler installation instructions shows the feed should go to the return and the vent between the boiler and pump. This also seems to be the way shown in many web pages. Has this method been superceded by connecting both feed and vent between the boiler and pump as shown by CWatters? I'll have to get a plumber in to correct the problem so should I get him to swap over the pipes (easy) or connect both pipes as shown in CWatters diagram from Google (not so easy)? Any thoughts gratefully received.
Jim

Title: Re: Vent pipe siziing
Post by thescruff on Sep 15^th, 2011, 4:32pm

The plumber should know how it goes, otherwise refer to my post or CWatters pic.

Anything else is wrong.

Title: Re: Vent pipe siziing
Post by CWatters on Sep 15^th, 2011, 4:41pm

The idea behind connecting them to the system close together (within 150mm) is so that the pressure in each is the same. That way there is no tendency for water to circulate via the expansion tank.

With any other layout there are two possible problems depending on which is at the higher pressure and how much difference there is..

1) Pumping over via the vent pipe (the problem you had) or
2) Drawing air in via the vent pipe and water going the wrong way up the feed (and possibly out of the tank overflow).

If the vent is connected to the return that might be at a relatively low pressure (because the pressure created by the pump has been lost by the time the water gets through the rads and back to the return where the vent is connected). In other words you might get problem 2) with the system drawing in air via the vent.

If you swap the pipes and get neither of these problems you got lucky. However you might ge problem 2) if the pressure changes in the heating system. For example when the TRV shut down and restrict the flow or the system gets a bit silted up.

I've sometimes wondered if using one pipe up into the loft would work. eg something like this. Perhaps the_scruff can tell us if there is a regulation or other problem with this approach (I'm not a plumber!). The pipe would need to be 20mm (I think that's a regulation for vent pipes?)

[gallery fullsize]CWatters/1316101065.gif[/gallery]

Title: Re: Vent pipe siziing
Post by thescruff on Sep 15^th, 2011, 5:00pm

Yes you can have a combined feed and vet, not the best plan though

Title: Re: Vent pipe siziing
Post by brainstrain on Sep 16^th, 2011, 8:50pm

Thanks everyone who replied. It's clear now what is the problem. I'll get my local plumber in to rearrange the pipework. I'm hoping to try to get both feed and vent on the boiler to pump pipe as recommended. It'll be interesting to hear what my plumber thinks is the best arrangement!
Thanks again.
Jim

Title: Re: Vent pipe siziing
Post by thescruff on Sep 16^th, 2011, 9:44pm

CWatters first pic is how it should be done, and if your plumber doesn't agree, get another one that knows what he's doing.