Cold tank overflowing, water running from vent pipe

[John Stumbles]

Curious one: had a call to an overflow from a cold water storage tank. The float valve was OK but there was warm water trickling out of the vent pipe into the tank. Fairy Nuff, obviously expansion of HW back into the tank coupled with the vent pipe having a low clearance from the high-water mark, in a tall house with the HW cylinder in the basement: about 10 metres from water level to base of cylinder. I adjusted the float valve to lower the level in the tank and extended the vent pipe to give about 500mm clearance over the water level and thought I'd fixed it but apparently not: just had customer call back to say the still overflow runs intermittently: for a few minutes at a time then stops again.

"Hot and Cold water supply" gives the height of the open safety vent as

40mm per metre of system height plus 150mm (for luck?) so my 500mm is a bit short for 10m. However that should at worst result in some water running out of the vent pipe when the system is at maximum temperature difference between the cold feed and vent pipes. For the tank to fill up (whether from the vent pipe or back up the feed pipe) and overflow the water must have expanded considerably since the level in the tank is set to about 100mm below the overflow. The tank is about 600mm x 500mm so that's a volume of 60 x 50 x 10 = 30 litres. The HW cylinder is a largish domestic type - say 140 litres - so if the coefficient of expansion of water is about 0.0005/C then I reckon if its entire contents were raised by 50C that would only cause an expansion of 0.0005 x 50 x 140 = 3.5 litres. Are my calculations correct? There seems to be an order-of-magnitude discrepancy between what ought to be happening and what seems to be happening.

The other possibilities I've considered are:

- leakage from the primary to the secondary. However the primary F+E tank is lower than the CW tank so any leakage would result in the F+E overflowing.

- leakage from mains to the HW through a mixer tap somewhere. I have yet to check this.

Any thoughts?

[Andy Hall]

This sounds like fun.

Could it simply be that there is a very good convection happening as a result of the height?

This is the classical "change one variable at a time" puzzle.

How about dropping the temperture of the DHW and see if it reduces.

If it doesn't, then there is a leak of some kind perhaps. However, that could be masked if it gets worse with temperature.

You could arrange to collect the water from the vent pipe. Then see whether it is being replaced from somewhere.

Is it a spinster in Caversham Heights? Has she offered you afternoon tea? Be very careful if so. Rumour has it that they are not picky when it comes to toy boys.

[Aidan]

1) Big house, any secondary circulating pumps hiding anywhere? However, that wouldn't really explain the "......overflow runs intermittently: for a few minutes at a time then stops again. " 2) Cold water main connected to a mixing valve somewhere? That wouldn't explain the "......overflow runs intermittently: for a few minutes at a time then stops again " either. 3) (I like this one, 'cos it's so daft it's plausible). There is a slight inverted 'U' in the open vent pipe. Air collects in the inverted 'U' and pushes the water level down as it accumulates or expands. The water is displaced up the open vent and up the cold feed. The water levels in the tank and the overflow aren't the same because of the air-lock. When the air reaches the bottom of the 'U', some air escapes up the open vent and water levels drop. Process then repeats. There may be some momentary water hammer evident when you open a tap as air escapes.

That does explain the "......overflow runs intermittently: for a few minutes at a time then stops again " bit, but I'm at a loss to explain where the air is coming from. I can only think there's some big-time electrolysis happening with a defective immersion heater. Old consumer unit (no RCCBs)?

[deano]

Not a professional myself, but with much experience and knowledge gleaned from troubleshooting my own CH & HW system, the following springs to mind (please correct me if I am way off the mark)...

1. Has the property owner noticed anything abnormal with his/her CH & HW prior to spotting the discharge from the overflow or does the rest of the system work as it should?

1. Have they had any additions/changes made to the system recently? For instance: my dad recently fitted both cold AND HOT outside taps to his property so that he could wash his dogs after a muddy walk. He attached a cheap 'mixer' hose with shower head to the taps and as a result, the pressure of the cold feed sent water back up the HW system and led to discharge from the storage tank overflow.

2. Are any of the radiators blocked, causing the CH water to circulate much quicker, without raising the room temperatures and therefore activating any roomstats to switch the system off?

1. Are any MCV's malfunctioning?

2. Is the cylinder stat faulty?

1. Is the immersion heater faulty?

Overall, are you able to confirm that the HW stops being heated when told to do so, by other components within the system?

Let me know if any of the above is a potential culprit.

HTH,

deano.

[John Stumbles]

Thanks for all so far. Thinking about it, there is a mixer tap in the basement kitchen but I can't see mains water pushing back the HW against the head it has down there. I can't remember if the bath on the ground floor has a mixer but it's certainly not a bath/shower mixer. The shower itself is pumped off the storage tank.

The flow from the vent pipe is a continuous slow run - not a trickle but not a gush, certainly not full-bore.

I'm wondering whether I've got my calculations right: anyone like to look them over?

[Aidan]

If the cold feed is at 10degC & 10 m , I make it that the height of the water column in the open vent would be 10.167m at 60 degC. You couldn't exceed 10.5m if the temperature was below boiling.

TMVs are a prime suspect for backfeed> The water levels in the tank and the overflow aren't the same because > of the air-lock.

That bit is rubbish. That could only happen if there was a non-return valve or stop-c*ck on the cold feed.

[John]

SMIP

If the MCWS head is adequate to push up to the storage tank above its enough to create your problem John Have you tried turning off the cold supply and seeing if it continues to present the problem?

[mcbrien410]

I stumbled across this while looking for the coefficient of expansion of water...

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "John" saying something like:

I had exactly this backfeed problem when fitting a mixer tap (French). It mixed in the body and not at the exit. It didn't get to the stage of overflowing the storage tank, I sussed it out immediately when the flow of hot was freezing cold immediately after turning off the cold tap, as the introduced cold water in the hot pipe made its way back down.

[John Stumbles]

What's MCWS?

But with the open spout adjacent the head of hot water would have to be extremely low to allow the cold to push up into the storage tank.

However the problem did turn out to be a monobloc mixer tap. It must have a flaw in the casting because cold water was getting through to the HW side with both hot and cold taps turned hard off (and washers and seats in good condition). Actually you could hear the water hushing through when you were close to the tap. Up in the storage tank you could see a little sort of plume of turbulence in the water where warm water was getting pushed back into the tank and mixing with the colder water.

Glad I got to the bottom of it: I was tearing my hair out, reputation-on-the-line time!

[Andy Hall]

Muslim Community of the Western Suburbs

or

Mains Cold Water Service

if you prefer.

Not one you see every day though, is it? :-)

[John Stumbles]

If that's what he means then if the "MCWS head" is not enough to push up to the storage tank then you'd certainly have a problem!

[Aidan]

It's odd that it was coming out of the open vent 500mm above the water level in the tank; must have been something like a NRV in the cold feed pipe.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember John Stumbles saying something like:

Pump fed mains, about 4bar, storage tank about 13ft above kitchen mixer.

Wasn't French, was it?

Heh.

[John Stumbles]

'Heritage'

[John Stumbles]

It wasn't coming out of the vent pipe after I'd raised it and lowered the water level in the tank. It was coming back up the feed pipe, hence the little plume of turbulence by which you could see it.