Hot water tank overflows into cold water storage tank

[itambrose]

I've got a very basic plumbing set-up in our house: old non-combi boiler (Potterton Flamingo 50...snazzy, eh!) with a 2-slide wheel for water+heating plus these three:

a) hot water tank located on first floor of house
b) old water storage tank located in the loft
c) heating header tank located in the loft too

In short, the hot water tank continuously overflows up into the cold water tank while the hot water is being heated. This happens to the point that if we're heating the water for a while, it will take the cold water tank up to the overflow level and then starts to drip outside. I know hot water expands, but don't believe it is intentional for this to happen all the time. We switched the heating and water off for a few days and the cold water tank just fills up to where it should and stops, no overflowing - great. Fire the hot water back up again, and starts to overflow into that tank and overflows outside. Short-term answer is only to fire the water up when needed, but in the winter we need the heating on for much longer than an hour-ish and with the old boilers, you must also heat the water when the central heating is on. Only got worse recently. Can't be draining a few bowls of water out of the hot tank just to prevent this, waste of water.

Any ideas? Thanks for any help you can offer. Want to avoid a wet winter in- and outside

[plumbbob]

This is such an obscure problem, I can't actually think what the cause may be.

The overflow is certainly not caused by expanding water. That would only amount to a cup full or so, certainly not enough to fill the tank.

Can I confirm a few points before my brain fries because what you appear to be describing, I can't think of a single instance this can occur. Something doesn't seem right.

So, you have two header tanks in the loft? One is larger than the other? The expansion pipe over the larger header tank has water flowing from it when the boiler is heating the hot water? The large tank then fills to its overflow and runs outside?

Now thinking about the header tank heights, is the top of the larger tank higher up than the smaller tank? Specifically, if you joined the two tanks together, which way would the water flow? From the larger to the smaller or the other way round?

[itambrose]

Hi,

Thanks for your reply! Some answers to your questions:

So, you have two header tanks in the loft? One is larger than the other?

Yes

The expansion pipe over the larger header tank has water flowing from it when the boiler is heating the hot water

Yes

The large tank then fills to its overflow and runs outside?

Yes

Now thinking about the header tank heights, is the top of the larger tank higher up than the smaller tank? Specifically, if you joined the two tanks together, which way would the water flow? From the larger to the smaller or the other way round?

They're both really on the same level. If anything, the smaller central heating head tank is going to be the lower as the larger tank is on a small plinth.

It's bizarre because this has only happened recently, and also we only noticed how much it was because the larger cold water storage had a failing seal on the connector taking overflow pipe outside, hence water coming through the ceiling.

One thing I didn't mention is that the boiler has 6 hot water temp settings: Min, 1, 2, 3, 4, 5 and Max. It's been on 1 since we moved in 8 years ago and has generated scorching hot water without any problems. Turning it down to Min has reduced the temperature slightly, but still boiling hot, and doesn't seem to have alleviated the problem. We haven't had the heating on since March, we're only heating the water. And I think it has to be related to the water heating as it doesn't happen when the boiler is Off. I've watched family members completely re-plumb their entire houses so am very familiar with how it all connects and serves other units....and this has just stumped me.

don't let it fry your head too much! thank you

[Perry525]

I think the expansion pipe on the central heating has been fitted in the wrong position and as the pump runs the pressure is pushing the hot water up the pipe into the header tank.
The expansion pipe should be fitted just before the return pipe goes into the pump, this way the pump is pulling the water from the expansion pipe.

[plumbbob]

I think the expansion pipe on the central heating has been fitted in the wrong position and as the pump runs the pressure is pushing the hot water up the pipe into the header tank.
The expansion pipe should be fitted just before the return pipe goes into the pump, this way the pump is pulling the water from the expansion pipe.

But Perry, that would only happen to the small central heating tank not the larger hot water header tank. Over pumping is a common fault, and this fault is something different because the hot water is not pumped.

The only thing I can think of is the coil in the hot water cylinder has a leak allowing the primary and secondary waters to mix. Usually though when this happens, the smaller tank overflows because the water level is lower than in the larger hot water tank. Maybe the pressure from the pump when the boiler is on is forcing the water the other way into the hot water side.

Do a couple of tests. With the heating off, leave everything to settle overnight and see if the level in the smaller tank rises on it's own. Next day, wedge the ball valve shut in the small tank then run the heating. As the bigger tank fills does the smaller empty? ENSURE THE TANK DOES NOT EMPTY!

One final thing, confirm the heating expansion pipe is over the smaller tank and the hot water cylinder expansion is over the larger tank. It is unlikely, but they could be mixed up.