28mm run from cold storage tank to hot water cylinder

I've just moved my hot water cylinder to the loft. The cold header tank is located above the top of the hot water cylinder tank, and the connection from the lower part of the cold header tank to the cylinder uses 22mm all the way, although has 3 90degree elbows.

I have a problem with air being sucked in the vent pipe to the hotwater when the bath tap is fully open. I confirmed this by hold the end of the vent pipe in a glass of water; all the water was eventually sucked up when the hot bath tap was fully open

After reviewing several posts to this news group I read that the most obvious problem is the poor flow from the cold header to the cylinder. Most posts seem to suggest upgrading this pipework to 28mm, reducing the number of elbows with bends, etc...

I have a couple of questions I hope someone can help with:

1: I didn't have this problem before when the cold header was in the loft, and the hot cylinder was on the landing. am I right to think that the flow from the cold header to the cylinder was under much higher pressure because of the head of water. now both cold header and hot cylinder are in the loft the head of water has been reduced significantly from what is was before.

2: even if I upgrade the pipe to 28mm, I'm not aware of a fitting that connects 28mm pipe to the 1inch cylinder connector. would I have to convert the 28mm down to 22mm to connect it to the cylinder, and therefore wouldn't this still be the bottleneck?

thanks for any advice or links to sites that sell the appropriate connectors

Reply to
grahamrichardnorth
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Replace with 28mm and keep elbows to a minium. Bend if you can. 28mm connectors are standard.

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may want to fit a 28mm open vent too. This means more water in the pipe to be dragged through the draw-off before reaching any air, then the cold feed may have settled out. Also use full-bore isolation valves on the 28mm pipe.

Reply to
Doctor Drivel

just as a test before replacing all with 28mm I connected up the cold water feed from the cold header to the bottom of the cylinder also. (I have two tank connectors on the cold header, 1 for hot and 1 for cold). so essentially now both are feeding the bottom of the cylinder.

however, the vent pipe still easily sucks up water from a glass when the bath hot tap is on full.

surely 2x22mm are better than 1x28mm so I'm stuck again since I'm not convinved that 28mm pipe will make the situation any better.

Doctor Drivel wrote:

Reply to
grahamrichardnorth

does it matter if at the top of the hot water cylinder the 22mm pipe that exists the cylinder goes in to a tee whose other 2 connections run left and right (i.e. the pipe carrying the hot water to downstairs runs horizontal until it bends down 90degrees and goes through the roof. the other part of the tee again runs horizontal (well slight upwards really, until it bends upwards and then over and cold header tank as normal.

by theory is that since most diagrams and installations I've ever seen have a tee, but is is positioned such that the vent pipe comes out of the top of the tee, and the hot water exists the tee sideways, perhaps there is a problem with my arrangement.

could the fact that both the hot water to downstairs and the initial part of the vent part exist the tee horizontally mean that when the bath hot tap is on full there is a sucking force applied to the water in the vent pipe ?

I'm runn> just as a test before replacing all with 28mm I connected up the cold

Reply to
grahamrichardnorth

The message from snipped-for-privacy@yahoo.com contains these words:

That is your basic problem.

The vent pipe on the hot outlet means that the run from cold tank to hot tap is effectively 2 independent parts and if the lower run can pass water faster than the upper run then air will be drawn in. The pressure head driving the cold water into the tank can be no more than the difference in height between the water surface in the cold tank and the vent junction. The pressure driving the water into the bath is the height difference between the water level in the vent pipe (maximum when the level drops to the vent junction) and the bath tap and parity is reached when the water level in the vent pipe (by then actually in the hot tap supply pipe) has reduced the head sufficiently for the flows to match.

You are stuck with the connections you have but every little improvement will help the flow into the tank. Removing elbows and increasing to 28mm will both help. There are tables around that will tell you what flow you should get with various lengths of pipe at various heads and the equivalent length to add on for each fitting but it seems to me that the head disparity is so great that you will have to put some restriction in the feed to the hot tap to avoid drawing in air. A gate valve would do. Experiment till air is not drawn in and then take off and hide the handle.

You may be getting the same problem on some or all of your other hot taps but have not noticed it as full on is less likely on those taps.

One final point. Have you asked yourself does it actually matter if air is drawn into the hot supply in that manner. I am not entirely sure it does but it might depend on what is likely to be sucked in with it.

Don't listen to Dribble. He is permanently in my killfile but I couldn't help noticing he advised you to increase the diameter of the vent pipe. You won't notice the fraction of a second difference the increased reservoir in the vent pipe delivers before it starts to feed air.

Reply to
Roger

Roger, you haven't half a clue what you are on about. You are also a big guesser too.

Reply to
Doctor Drivel

thanks Roger, appreciate the long detailed response.

just to be clear (as clearly I'm no expert plumber:-) ) the problem is essentially caused because the head of water between the top of the cold header and the top of the hot cylinder has not been greatly reduced because I moved the hot water cylinder in to the loft along with the cold header.

before I moved the hot cylinder to the loft I had about 6-7 feet head of water from the top of cold header to the top of the cylinder, and the head of water to the bath tap would havev been, say, 8-9 feet - i.e. not much difference.

have I got that all correct?

I did try previously only partially opening the bath hot tap and it did not seem to have an air in it, so you comment about essentially restricting the flow to the hot table seems to marry up with that test I did.

why did I decide to move it to the loft.......

Roger wrote:

Reply to
grahamrichardnorth

From the cylinder draw-off connection have a bend, then a length of pipe then a tee. from the tee go to the taps. The other part of tee is the open vent. Don't go up and over the cylinder from the draw-off on the cylinder and have a tee on this just above the cylinder. You may want to have the draw-off from the cylinder in 28mm then a 22mm vent pipe at the tee.

Two 22mm pipes still are entering the cylinder via one 22mm connection of the cylinder. It may be worth your while having a length of 28mm plastic pipe from the tank to cylinder to avoid elbows, but only use compression joints, not push-fits and use a full-bore 28mm isolation valve.

Reply to
Doctor Drivel

Yes, one based on no experince.

From the cold tank to the hot tap you have a pipe. In between this pipe you have a fat pipe, called a cylinder. This fat pipe can be heated up. The less restrictions you have to flow in the complete run of pipe, inc' cylidner, to the tap, the problem will disappear.

Reply to
Doctor Drivel

so you're saying that because the flow from the cold header to the tank is not large enough, when the hot tap is turned fully on it is dragging the water (and then air) from the vent pipe? I'm not certain I understand this? the pipes from the loft to the hot water taps are all

22mm also.

so this big pipe as you describe it is all 22mm (in fact from header to cylinder its two 22mm, although I'd concede that since they join in a

22mm tee before entering the bottom of the cylinder the flow is still limited to 22mm) then why is additional water (and air) sucked from the vent pipe?

thanks for your replies also Doctor Drivel, any/all help ideas are appreciated > > thanks Roger, appreciate the long detailed response.

Reply to
grahamrichardnorth

Reply to
grahamrichardnorth

The message from snipped-for-privacy@yahoo.com contains these words:

It is an unusual installation anyway if the cold water tank was not in the loft to start with but if I understand you correctly what you have done is to increase the head to your bath tap by a full storey while maintaining the height difference between hot tank and cold tank. Given the figures you quote that is probably somewhere around 10 feet so instead of having 6 foot of head driving the cold water into the hot tank and 1 - 2 foot of head sucking the hot water out you have 6 foot of head driving the water in and 11 - 12 foot of head sucking it out. If you have plumbed your bath hot tap in 22 mm pipe then I can't see how you can resolve your problem without restricting the flow to the tap but that is just a gut feeling. I don't have the tables to resolve the issue.

Reply to
Roger

just to clarify, all I've done is move the hot water cylinder from the landing to the loft. in the loft I have built a wooden platform and sat the cold header tank (that was always in the loft) on to the platform. the hot water cylinder sits on the joists (on a think piece of wood to spread the load over 6 joists) and the bottom of the cold header is now just above the top of the hot water cylinder.

the head of water above the top of the hot water cylinder must be around18 inches (essentially this is the height of the cold water tank since the water in the cold header is only a few inches from the top when full). the head of water for the hot bath tap must now be something like 8 feet (to the top of the water in the cold header)

I'm still unsure if this vast difference in the head of water is the cause of the problem or not. all the plumbing is 22mm, right the way to the bath tap.

Roger wrote:

Reply to
grahamrichardnorth

The message from snipped-for-privacy@yahoo.com contains these words:

As I said before don't listen to Dribble. Unless you know the subject well you can't tell whether anything he is saying has any value, and if you do know the subject you don't need to bother reading the rubbish Dribble routinely puts out. The most that can be said for Dribble is that he is very good at cutting and pasting but when it comes to thinking problems through you can almost guarantee he will get it wrong.

So according to Dribble if you upgraded the pipe to the hot tap from

22mm to 28mm your problem would at least be reduced. The plain truth is that that would just make matters worse.
Reply to
Roger

Roger, you are pure amateur and an idiot to boot. I am no amateur. To the OP, don't take too much of what he says seriously at all. He is a know loon.

Reply to
Doctor Drivel

The message from snipped-for-privacy@yahoo.com contains these words:

Which is what I had thought you had done in the first place before you quoted figures that appeared to say different.

It is. What you have to remember is that the vent pipe splits the run from header tank to tap so that if the resistence to flow is greater in the first part of the run the vent pipe allows the water in the second part of the run to escape quicker than it is replenished. The crucial point is not actually the top of the cylinder but the junction with the vent pipe. I think the bath tap, even when fully open, will restrict flow to a certain extent but, other things being equal, you need the same head between cold water level and junction as between junction and tap.

So you have got at the most 18 inches of head driving the water into the tank and perhaps 9 foot of head sucking the water out. If your header tank previously sat on the joints you should at least be getting a greater flow of water at the hot tap than you had before.

Do you have room in the loft to raise the header tank further? That too will improve matters.

Reply to
Roger

does this mean that moving a hot water cylinder to the loft is fundementally flawed then? noone's loft is going to allow the cold header to have a huge head over the cylinder in the loft.

Roger wrote:

Reply to
grahamrichardnorth

The message from snipped-for-privacy@yahoo.com contains these words:

No, but you have to cut your cloth to suit yur coat. As I said before you will have a greater flow at the hot tap now than you would have with your previous setup and that would continue to be the case if you put a gate valve in the lower run and throttled the flow down till you got no air admitted.

Some lofts at least would have more headroom than yours and thus more scope.

Reply to
Roger

At last you got it Roger. 10/10. V good. That is why the cold feed should be a large bore unrestricted pipe to give "flow" to compensate for the low head.

Reply to
Doctor Drivel

In article , snipped-for-privacy@yahoo.com writes

Just so I understand: have you extended the vent pipe to the new cold header tank position?

Reply to
Mike Tomlinson

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