Percolation?

[Softus]

Hi Guys

Today, I are mostly a punter: I can't figure out the following problem, and am hoping that someone with greater experience will know the answer.

Symptom : water flows from the cold storage cistern warning pipe

system : open vented, fully pumped, S plan
boiler : gas, some kind of Potterton
MZVs : Honeywell 2-port, 2 off
roomstat : on 21 degC
cylinder stat : on 60 degC
programmer : can't remember, but it's a two-channel digital unit
vent : clear

One outlet from the cistern is the cold feed to the cylinder, in 22mm.
Another outlet, slightly higher, is to the cold side of a Stuart Turner twin impeller pump.

The cistern was renewed in July 2005 - was 20g, now 50g.
The symptom appeared in November.

The system is slightly unusual in that all hot water is pumped (I suspect that the wrong kind of mixer taps were chosen, with the bathroom cold being fed from the mains).

My observations are as follows:

1. The water level in the CSS gradually rises, over a few hours (according to the customer), until it overflows down the warning pipe.

2. The water in the CSS is distinctly warm - not measured, but around body temperature.

3. With a strong torch, I can see subtle signs of heated water emerging into the cistern from one of outlets - the one that goes to the cylinder.

4. The F&E cistern contains stagnant water, implying that the primary circuit doesn't leak water to anywhere.

5. The float valve shuts off nicely, and I've seen that the water level still rises.

So, heated water is rising up the cold feed. I don't know why. The cistern water is therefore at an unhealthy temperature. Also, because the water expands, the level rises and runs out of the warning pipe, irritating the customer.

I'm tempted to fit a single check valve, but there's a risk of it blocking which would starve the pump. So I'd like to find the cause and stop it happening, but I'm stuck.

Has anyone seen anything like this?

 

[Nige F]

No but I think I can envisage an explanation-mains pressure on a bathroom mixer crossing via a porous casting into the hot side of the mixer- backflowing to the top of the cylinder and then dropping by thermosyphon in the cylinder- causing de stratification, and thus causing hot water to (again) thermosyphon up the cylinder cold feed . and appear in the feed cistern anyone got a better idea

 

[chrishutt]

I recently came across a similar situation, caused by the failure of the non-return valve on the hot inlet of a shower valve that took its cold water directly from mains.

The hot and cold supplies mixed upstream of the shower flow control (which I think is quite common) so that the hot water from the cylinder was slowly pushed back into the cylinder and up the feed pipe to emerge in the feed tank and cause it to overflow.

Short term solution was to fit a separate check valve on the hot supply to the shower valve, although ideally this sort of valve should not be used with such pressure differentials.

 

[Softus]

Thanks to you both for two valid possilibities - I wouldn't have come up with the porous casting idea but am kicking myself for not checking to see if there's a missing or faulty NRV on the shower hot supply.

I'll let you know what I find.

 

[Softus]

In case anyone is interested, I fitted a single NRV on the output from the hot side of the twin impeller pump.

Guess what - the symptoms continued. I went back to observe, but of course every time I go there the system behaves impeccably.

I'm due to make another visit, and hoping to catch it in the act, but if anyone has any ideas I'd be most grateful.

 

[chrishutt]

Are all the hot water outlets pumped? Perhaps the kitchen hot supply is via a different connection and sink tap is a mixer tap? In that case mains cold might just find some way of escaping back up the hot supply.

 

[Softus]

Yup - when the pump was installed (apparently by the previous owner) the dropper on the hot output from the cylinder was cut and the pump 'inserted' into that water path. The cold side feeds only the bathroom shower.

 

[chrishutt]

So when the pump operates to supply just a hot tap, the cold side of the pump could be pumping against a dead end, resulting in damage to pump and a very high outlet pressure!

A twin-ended pump also serving hot only outlets must have an additional device to avoid the situation above, normally I think a small by-pass between hot and cold outlets. Is there one? Either way, I think this might be worth considering as the possible source of the problem!

 

[HarrogateGas]

It could be the coil in the cylinder that s passing, what level is the ch header tank?

 

[ChrisR]

Can't see the reverse flow being at the shower as they're both pumped.

I have come across a few porous castings but only ever changed the tap, knowing that nrv's reduce the flow to what is already the slower side of the mixer. There are some bath mixers we would all refuse to change, of course..
Nrv's make often make funny noises (vibrating) and I've not found an answer to that. Bigger diameter ones on low flows are worst, I've been advised to put a small one in parallel with a bigger. I have a collection (which I'll not be able to find when I need them) of nrv's taken out of taps, which are very small.

Most of the bigger/better pumps will stand zero flow at one end for some time without doing damage. Checked on the last one put in but don't have figs to hand.

 

[chrishutt]

It could be the coil in the cylinder that s passing,

An obvious suspect, but softus has reported -

4. The F&E cistern contains stagnant water, implying that the primary circuit doesn't leak water to anywhere.

 

[HarrogateGas]

Difficult judement to make, it depends howmuch it is overflowing, id be tieing the ball cock up over nght to see if the level drops.

 

[chrishutt]

I agree. You've got to be absolutely sure that the obvious things aren't the cause of the problem before chasing after obscure phenomena.

Another outlet, slightly higher, is to the cold side of a Stuart Turner twin impeller pump.

The cold outlet should really be the lower of the two, so are you sure you've got them the right way round?

 

[Agile]

If the kitchen tap is a mixer type it could be allowing cold mains water to flow back up the hot supply pipe.

If course it should have had a NRV in the hot but nobody fits them as they should! Even if there is then it could be leaking.

Test would be to isolate the cold supply to kitchen.

Tony

 

[Softus]

So when the pump operates to supply just a hot tap, the cold side of the pump could be pumping against a dead end

Not could be, but actually is. I've advised them about this, and I don't feel that I can force them to let me make changes.

...resulting in damage to pump

Almost certainly!

and a very high outlet pressure!

Um, I don't think this would happen

A twin-ended pump also serving hot only outlets must have an additional device to avoid the situation above, normally I think a small by-pass between hot and cold outlets. Is there one? Either way, I think this might be worth considering as the possible source of the problem!

I haven't quite cottoned on to what you're saying here Chris, but I'm intrigued.

If you're asking whether or not there is a bypass - I believe there isn't.

If you're proposing that there should be one, I don't see how this would work, because I think the hot output would effectively become a mixed output.

To recap the weird symptoms, I've seen with my own eyes a very slow drift of heated water emerge from the outlet of the cold storage cistern, into the cistern, using a strong torch, and the temperature of the water is very warm, confirming that heated water is being fed back, and the water level rises until it reaches the warning pipe.

I wish that when I'd seen it I'd uncoupled the hot outlet from the pump and checked whether or not the feedback was coming that way, but I was short on time and brainpower.

My reasoning has been that the only way hot water could be forced up to the CS cistern would be under a pressure that greater than the head of the water in that cistern. The only thing that fits the bill is mains pressure, and the places that main cold and pumped hot meet is at the three mixer valves in the house - kitchen, bath and shower.

I wondered about the cylinder, but the water in the F&E is stagnant, and the CS cistern is higher than the F&E, not the other way around.

Is this a conundrum or what?