Expansion tank overspill - HELP!

Hi all - I'm hoping someone with superior knowledge to me can help...

I have a gas fired open vented system with an expansion/header tank in the loft (Potterton boiler). My problem is that when the heating is on the expansion pipe intermittantly coughs hot water and steam into the CH tank with such violence that the water is escaping from the tank (and the lid) and thereafter down through my 2nd bedroom ceiling…

I first noticed that the ball valve was ancient, knackered and filling the tank too full, so I replaced it and set the water level to top out around 2 inches below the overflow, leaving around 8 inches of water in the tank. The expansion pipe is about 3 inches above the current water level, but obviously this rises as the pipe coughs more water into the tank, which means that water escapes from the tank as before, and then the level rises until the overflow is called into action, at which point I get a nice outside shower in my garden from the run-off…

Now I thought it might be that the system was overheating, so I turned down the boiler setting from "2" and the thermostat down from 24 celsius (which I didn't think was excessive), but it still does the same thing. As a preventative measure I've tried to silicone seal the tank lid onto the tank so no water can splash over the side in the loft, but this is not exactly a cure...

I'd quite like to stop the coughing, but don't know what's causing it in the first place - any advice? Could it be something as simple as a faulty thermostat that is just letting the heating temp rise and rise, or is there something rather nastier in store for me? All my radiators work fine.

The CH is off until I figure this out. I'm not averse to calling in a pro, but I had the wool pulled over my eyes by my last plumber and would like to be better prepared this time…

Many thanks in advance, and hope I can repay the favour in another area...

BenH

I first noticed that the ball valve was ancient, knackered and filling the tank too full, so I replaced it and set the water level to top out around 2 inches below the overflow

Click to expand...

set it to 2 inches above the outlet COLD

I have a gas fired open vented system with an expansion/header tank in the loft (Potterton boiler).

Click to expand...

which one

the water level to top out around 2 inches below the overflow, leaving around 8 inches of water in the tank.

Click to expand...

Set it much lower to leave room for
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expansion!
You only want about 3-4" of water in the tank. The vent pipe should poke down through the lid, just

When the pump starts or mor importantly stops, depending on the plumbing, the water levels in the tank and vent pipe move a bit, especially if there's any air whatsoever in the system, so start by lbleeding all your rads.
That often causes a spurt of pump-over.
It'll be less at lower pump speeds.
Next contributor is a build-up of scale where the feed (15mm) pipe joins the circuit. That usually needs the pipes cutting, for a scrape-out.
Another thing you can do is extend the upturned U shape of the vent pipe, so the pulse of water would have to go higher to pump-over.

The vent and feed should join the circuit very close, less than 150mm or so apart, but any scale means it behaves like a long resistive piece of pipe between the two.

Very high chance that it is your boiler stat that has gone completely and turning it down to 2 would make no difference .

Wow. That's a lightning response, thank you!

Namsag: the boiler stat may well be faulty - there's almost no friction when you turn it up or down, so I think it's set where it is...

KevPlumb/ChrisR:

Ah...so the water level is way too high as is? I take it it just has to cover the outflow at the bottom of the tank by a couple of inches, then? This would certainly stop the spillage, but eventually, the extra water dumped into the tank by the expansion pipe would see the level rise again, no? I can live with bailing it out every so often - for a while - but long-term it'd be nice to only have to go in the loft to get my suitcases.

Should I turn down my pump speed (and to what level?)?

If there's anything else I need to do short of flushing the whole system out (due this summer when the heating is not so needed), I'd appreciate the advice. If ever you guys need a French/German translator or a discounted Croatian holiday, let me know...!

BenH

BenH said:

This would certainly stop the spillage, but eventually, the extra water dumped into the tank by the expansion pipe would see the level rise again

Click to expand...

The idea is that the water level in the tank rises and falls as the system heats and cools. Water that is purged out of the vent will return to the system at some point and so the level will fall again.

At the moment you don't have room for the expansion and so it overflows. When it cools and the level drops, cold water is admitted by the ballcock to top it back up to the level just under the overflow so when it expands again there's no room for water displaced by the expansion.

Of course. Thanks.

That'll solve the overspilling - so do I need to worry about the coughing expansion pipe at all, then?

You could try some of the suggestions made by ChrisR.

But lowering the level in the tank will also lower the level in the vent pipe to the same level and may solve both problems.

I would check the ballcock to make sure it actually does stop the water comming in and that the level stays low when system is cold. They have a habit of dripping slowly and filling the tank overnight.

Thanks Stem. It seems to me there's a five point check list here - I'm very open to correction...

1. Lower the ballcock and thus water level in the expansion tank to 3 or 4 inches, max, and ensure ballcock does not do the drip feed thing (I bet it is doing, may need a better valve).

2. Bleed all radiators.

3. Ensure CH pump is not on highest setting.

4. Ensure boiler stat is not stuck on high setting (replace/fix if necessary) .

5. If all this is done and the problem still remains, it'll need de-sludging (there's no other evidence it needs it as the system was fernoxed recently, but you never know).

Is there anything else I should check for before first having a go myself and then calling a pro if/when I overreach my abilities?

BenH

OK - update:

After reducing the water level in the expansion tank and bleeding the radiators (the CH pump was already on its lowest setting), my expansion pipe is STILL coughing out hot water quite fiercely.

I discovered that having the lid on the tank actually made things worse - the steam formed condensation on the underneath of the lid and this turned back into water before running off out of the tank. Without the lid on, it splashes out occasionally, but only the odd drop compared to the gushings when the water level was higher and the ballcock bust. The water level is also remaining constant, at the new, lower level.

HOWEVER...

what is causing this coughing fit from the expansion pipe? I'd far rather it didn't spit and blow like that.

I'm unable to test whether the stats work, either room or boiler, but suspect one of them may be dodgy, or that there's sludge in the system.

Any other suggestions? It may be time to call in a professional...

BenH

Some expansion is normal that's what the vent pipe is for, condensation on the lid is also normal every F & E tank I've seen has some, (it normally runs down my sleeve when removing the lid). Insulation of the tank can reduce condensatin and the lid should be tight fitting to prevent water escaping and shaped to encourage the condensation to run back into the tank.

If there is air in the system it may take a while to escape and then settle down, but if it's really severe and you are concerned perhaps it would be best to call an expert to check it over.

Don't leave the lid off the tank for an extended period, it will cause the condensation to form on the roof timbers instead of the tank lid and could cause rot.

Thanks everyone. I feel more at ease now, and may get a "taller" F&E tank to stop water escaping (the lid clearly doesn't fit tightly enough...)

BenH

how high is your u-turn vent pipe above f&e tank?

you know it should be min of 450ml above the cistern on fully pumped system

see here http://img252.imageshack.us/img252/1909/20070211205712rr4.jpg

Hi there

It was about 30mm above the water level when I started this topic, now it's more like 120mm after fitting my second new ball-valve (the first replacement could not fully shut off, which wasn't going to help long-term).

Things are better now, but the odd drop of water still splashes over the side of the F&E tank. I may have to rig up a mini-shower curtain - or get a taller model with a lid that is watertight.

Nice to know that the coughing is normal, though, even if it could be less violent.

Thanks again to all those who posted.

BenH

No, it's the height of the vent pipe where it bends over that matters. How high is the bend, above the water level in the F&E? This is what 62945835 had in mind.

This is the height that water pressure in the boiler will have to push water over, for it to be expelled from the bittom of the vent pipe. Altering the pipe run to make this bend highert will reduce the "coughing."

It is preferable for expansion to travel back up the feed and expansion pipe, the vent should only carry flow if the F&E pipe is for some reason unable to carry the flow (e.g. partial blockage or boiling) or if the layout of the F&E pipe and the Vent pipe is incorrect in relation to the pump.

Please describe or photgraph the point where the F&E pipe and the Vent Pipe join the circulation, and their spacing relative to each other and the pump; and whether they are on the flow or the suction side of the pump.

This pumping over will bring dissolved air into the system and speed up corrosion of the radiators and the boiler. What sort of sediment is there in the water and in the bottom of the F&E tank? What anti-corrosion chemical is used?