28mm run from cold storage tank to hot water cylinder

[Christian McArdle]

This confirms nothing. The vent pipe WILL suck in any installation. It only matters if the pressure difference between the cylinder and vent pipe outlet is enough to suck so far that the water column entirely disappears so that air enters the system. I see nothing to suggest that this has happened. The symptom you are looking for isn't reduced pressure and a bit of sucking at the vent (which will happen in any system, including a healthy one), but funny noises and entrained air in the hot water stream.

Christian.

[John Rumm]

When we moved in here, the hot water system was very similar to what you have described here, and yes, it did suck gob loads of air into the system!

One simple fix you could try would be to arrange the hot out from the cylinder in the way some shower pump manufacturers suggest for setups that are not using dedicated flanges etc:

|V| | | \ \ \ \ / \ / /\ \ / / \ \ /H/ \ \ __| |__ -- -- - - | | | | | | | | | | | | | | |___________|

Hot water feed come striaght up from top connector and then makes a 45 degree bend. You then have a couple of feet of pipe at this angle before it bends straight again and goes off to do the normal vent thing.

The hot water supply to the taps (H) is then teed off the underside of the slanted section.

[grahamrichardnorth]

[grahamrichardnorth]

Christian, I did wonder if other people's vent pipe sucked in water - so thanks as you seem to confirm this.

actually my first attempt at fixing this was to increase the amount of water in the vent pipe (it was only about 12-14" before) and I did this by running the vent pipe at a slight include along the roof line, and then back again, again at a slight upwards incline, and then over the cold water header. this meant I had a pipe of vent pipe of around 8ft of water in it, albeit on its side. I confirmed how much water was in this by holding the pipe, it felt hot for about 8ft.

however, yes I am definitely getting air sucked in to the system as I have an air vent at a point downstair (between the cylinder and the bath tap) and this continually has to be drained of air.

Christian McArdle wrote:

[grahamrichardnorth]

it seems to me as though I need to update the run from the cold header to the bottom of the cylinder in 28mm (and I know not everyone is in agreement on this)

I've been told by a plumbers merchant that the bore size of a 28mm->1" brass cylinder connector is 28mm, so, if my calculations are correct, if I change all of this to 28mm (as I've been told to do in the newsgroup, thanks, and by others) then it should double the flow to the bottom of the cylinder.

the reason for me saying that the bore is 28mm is that my fear was that the bore of the 28mm->1" cylinder connector and the bore of a 22mm->1" cylinder connector would be the same, so moving to 28mm would still be restricted when it entered the cylinder. but my fears were not correct.

I'm g> Christian, I did wonder if other people's vent pipe sucked in water -

[Christian McArdle]

OK.

The basic way this thing works is that when you turn a tap on, the pressure in the hot water cylinder drops, in proportion to the speed of the water flow.

Your choices are:

1. Slow the speed of water (probably not a popular choice).
2. Increase the supply of cold water to reduce the pressure drop.
3. Increase the height of the vent pipe, so that a higher pressure can be present without sucking dry.

Sounds like you are going with option 2, which is sensible. If this doesn't work, try option 3, by running the vent higher. If the vent goes only a foot or so before bending over the cistern, it won't cope with a reasonable pressure differential.

Christian.

[Doctor Drivel]

Yep. If the uprating of the cold feed does not do it a £15 Surrey flange will. If a 1" tapping on the cylidner draw-off then get a 1" Surrey Flange.

[manatbandq]

That's to prevent air bubbles collecting in any horizontal pipe runs and being sucked through the pump. I doubt it wil help in this situation.

MBQ

[Doctor Drivel]

Get the tee away from the cylinder. Come off the cylinder with an elbow. Horizontal to the point where the draw-off drops to the rooms below. Bend the pipe here down to the room below. Just before the downwards bend insert the tee. The vent must go right up to the roof and down to the cold tank.

[grahamrichardnorth]

are you saying that extending the height of the vent pipe before it bends down above the cold header will make a difference?

someone else suggested this, but it didn't seem to make sense since even if I extend the vent pipe the water level in the vent pipe will remain at the same level (the level of the water in the cold header, right?)

Christian McArdle wrote:

[Christian McArdle]

You're right. I had a brain fart. It helps in a different situation not related to your problem.

Basically, that option turns into raising the cold water cistern at high as possible, so that it can cope with a higher pressure differential.

Christian.

[grahamrichardnorth]

I have 'extended' the vent pipe by running it for a long length at a slight incline, and then back again, still at a slight include, back to the cold header. so there is more water in the pipe, even though its not a vertical pipe.

however, its not just the > > someone else suggested this, but it didn't seem to make sense since

[Christian McArdle]

Indeed. The height of the water column determines how much pressure it can resist (1m height resists about 0.1 bar of pressure differential). Making the pipe larger so it contains more water won't really help but will only delay the onset slightly.

Christian.

[Dave Liquorice]

Ah but is that air actually being sucked in down the vent or is it the air relased from the water when it is heated?

The vent should rise vertically from the top of the HW cylinder for a short distance and feeds T'd from this vertical section. The released air will then rise up into the bottom of the vent pipe. If the top connection to the tank goes to an elbow and short horizontal section with the HW feed and vent T's to the end the released air will collect in the horizontal section and be drawn by the water flow into the system and out of the taps.

[Doctor Drivel]

I used Unibond bathroom and shower sealant from the local shed. Expensive at 8 quid a go but it's so bloody waterproof it's v.difficult to smooth it after application 'cos it sticks to

*everything*. Flexible too, the only thing that broke it in our case was for some reason the shower tray managed to drop a few mm (don't ask why, I dunno yet!) and the stuff stretched and broke the grout on the surrounding tiles resulting in much leakage.....

The moral to this tale is if yer tray doesn't drop this stuff is good! IMO obviously.

cheers

witchy/binarydinosaurs

[Roger]

The message from "Dave Liquorice" contains these words:

I am convinced that the air just has to be sucked in down the vent pipe. There isn't sufficient head (short by perhaps as much as 6 foot) to drive water through the tank fast enough to satisfy the outlet at full flow. Once the reservoir in the vent pipe is exhausted (by the time the tap is turned full on) air is bound to get into the main run.

[meow2222]

FWIW... There are hw tanks with the cold header tank sat on top of them, and it can be made to work. Routing the vent pipe thru the header tank would maintain the ability to blow off expansion but eliminate air sucking. Whether this is in line with todays regs I dont know though, maybe not. Having no HW tank vent was how I saw it work, but I'm sure that wouldnt go down so well today. Whatever you do, if theres any possibility of hot in the header, the header must be able to handle the heat safely, as you dont want a blockage to cause a softened plastic header tankful of boiling water to drop on someone.

NT

[dom]

Hey-ho, this seems to be a long running thread. Maybe an extra take on it will help.

1. Are you absolutely sure the cold water feed to the hot tank is totally unobstructed?

Assuming you had reused components from the old location, or even just banging stuff about, could have dislodged/moved/introduced an obstruction.

Assuming you have a gate valve on this pipe, you could disconnect it and do a "discharge into a bucket" test of flow rate.

1. How about blocking the vent pipe with a hand/thumb over the end whilst someone else runs the bath tap full blast. This will at least prove it (or not) as the source of drawn-in air.

[Doctor Drivel]

They come as a prefab unit.

If I understand you this means thee will be a circulation path through the cold tank. Sp, no.

The cold feed has to be restriction free and a larger bore than open vent. And the take the open vent back to the tank following the path of the cold feed,. Do not have the tee to an open vent and draw-off taps at the cylinder. This also prevents heat rising up the open vent pipe too.

A Surrey flange will work too. Approach is, 28mm cold feed with not restrictions. If still a problem use a Surrey flange.

[Doctor Drivel]

Roger, be convinced all you like. There are packaged tank cylinder setups with this sort of head. If what you sad was right they would all suck in air.