Surging Water from Vent Pipe

[Gelf]

Hello,

I have an open vented central heating system with an un-vented hot water cylinder.

It is laid out in standard S-plan with 2 zone valves, one for hot water, one for Central heating. Hot water is 15mm from the zone valve central heating is 22mm from the zone valve, but 15mm to each radiator, I assume that it Ts off the 22mm flow return near each rad (but I a havent raised any floor boards to find out)

Vent is 22mm and Cold feed is 15mm, both teed off between boiler and pump and within 150mm of each other.

There is a bypass valve for pump overrun. Boiler is a greenstar 40cdi

In my system the vent pipe only clears 150-200mm above the water level of the tank (at the top of the upsidedown u bend) However there is no room to increase this height to the recommended 450/500mm, it is also not possible to drop the tank to get the required height.

The issue I have is that when the central heating turns off there is a small amount of water discharged (~100ml) from the vent pipe into the f+e tank. I think this is either because of the momentum of the flowing water when the zone valve closes has to go somewhere so it goes up the vent pipe and over, or there is a build up of pressure until the bypass valve opens and this causes water to be pushed up the vent pipe.

I also have a feeling the cold feed may be partially blocked (slight sticking of a magnet), but the cold feed is not replaceable due to access problems (I might be able to feed plastic through it though). However I did flush the cold feed pipe with mains water and the flow rate was good, so I think the problem is mainly due to the lack of height of the vent pipe.

Interestingly this only happens with the c-h and not the hot water.

I'm not sure when the problem started but I noticed it due to orange water coming out of the overflow pipe, this didn't happen until a BG engineer came round to service the boiler and decided that the pump would be better at setting #3 instead of #1 because one of the radiators was cold (I have since bled the radiator and turned the pump down to #1). Other causes may have been having the h/w cylinder changed to an unvented (not BG) or when the boiler got changed before I bought the place (maybe to one with pump overrun).

I have seen this thread :
//www.diynot.com/diy/threads/ch-sytem-vent-pipe-diameter.158037/

And the problem seems similar, in the third diagram down there is a surge arrester which I think would do the job of absorbing the extra volume of water in the vent pipe without compromising its safety? However I can't see these for sale anywhere so I assume you have to make one?

The other solutions that may be possible are:

1) To route the vent pipe in a zig-zag (or coil) so it can accomodate a greater volume of water (in the normally air filled bit) before it goes over the top of the vent.

2) To join the feed and vent pipes together just under the tank and hope that this will provide and alternate path for the surge.

Any suggestions or help welcome?

 

[jackthom]

Is there any scope for lowering the level of the water in the F&E tank? Otherwise as you say providing more volume for sudden "expansion" eg by using 28mm pipe (and maybe zigzag a little) above water level should help.

I like the surge arresting tank idea but ATM can't think what could be scavenged to make one.

 

[Gelf]

I replaced the old pt1 ball valve with a pt2, but it's already at it's lowest setting.

Although I think I could eke out another few cms, if I spun the ball so it wasn't under the arm.
How much water is required in the tank when cold?

 

[jackthom]

Depends on actual volume of water really but around 4" depth seems to be common.

edit: There's someone on Ebay selling larger diameter copper pipes up to 54mm dia. if you want to get creative with soldering.

http://www.ebay.co.uk/itm/copper-pi...-28mm-35mm-42mm-54m-diy-gas-new-/281293879976

Another possibility is to make up a couple of manifolds with T connectors and have a number of parallel vent feeds going upwards above the water level.

 

[Gelf]

I was thinking that the manifold idea, might be a good option.

Edit: Dammit forgot about the pi(e), long day...

Quick bit of maths (updated) :

22mm -> 3.1 ml/cm
42mm -> 12.5 ml/cm
54mm -> 21 ml/cm

so a 4 x manifold in 22mm by 15cm should be plenty

 

[jackthom]

My maths is getting very rusty but 22mm is 20mm internal, so 1cm internal radius gives pi x 1 x 1, or around 3.1ml per cm.

By my calculations a 4x manifold with in excess of 60cm of pipework should give you more than 180ml to play with.

 

[Gelf]

well this is embarrassing,
added some pi and updated the maths.

 

[Agile]

But what is the actual depth in the F&E tank???

A simpler idea would be to put a link between vent and feed pipe as that would reduce any pressure difference.

Tony

 

[St0rmer66]

Tee the cold feed into the open vent pipe (obviously capping the stub of cold feed remaining on the system side). It will stop this problem entirely.

 

[Gelf]

Depth in f+e tank is 15cm

If I was to link the feed and Vent, where would I do it, as close as possible to the tank?

Are there any issues with doing this?

I guess the feed and vent pipe were separate for a reason, is this so you can put a valve on the feed or because the gunk is likely to block the feed pipe so keeping the vent separate ensures it doesn't get blocked?

Thank you all for your help BTW

 

[DP]

In my system the vent pipe only clears 150-200mm above the water level of the tank (at the top of the upsidedown u bend) However there is no room to increase this height to the recommended 450/500mm, it is also not possible to drop the tank to get the required height.

No need to alter anything. Get your heating man to look at the two pipes and tees where they connect before the pump. Bet your system was not pumping over when it was newly installed

 

[Agile]

When cold, the depth in the tank should only be 100 mm.

Bend your ball valve to give this.

Sounds as if you have encouraged the pumping over by DIY changing the ball valve.

Plumbers do have some knowledge in this respect!

Tony

 

[Gelf]

The pt 1 ball valve had the water level set so it was dribbling out of the overflow

 

[Agile]

Yes, but if it was not leaking when closed, the shaft should have been bent to adjust it to 100 mm cold water depth.

Tony

 

[Gelf]

So pumping over happened before ball valve change,
In an effort to reduce this I changed the ball valve to lower the water level in the tank.
Looks like I haven't lowered it enough.

I think the pumping over probably started when the boiler was changed (before I bought the place) to one with pump overrun.
However I have no idea, as it was only noticed once the water was dribbling out of the overflow.

So to address the problem:

1) lower the cold water level in the tank to 100mm.
if that doesn't work

2) T the feed and vent pipes together
If that doesn't work

3) call in a plumber