Hot water tank overflows into cold water storage tank All aspects of plumbing questions and answers, help, tips and information

[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.

[billobach181]

A few years ago a friend had similar problem where the cold water header tank filled with hot water after a few years of working normally. After checking everything a few times we found the problem which was the hot water from the boiler was passing to the cold tank through the shower valve which had lost its sealing capacity. We changed the shower valve unit and problem was solved.
This may not be relavent in this case but it's just as peculiar.
Bill

[lets have a look]

scorching water isn't good or safe and could be causing a problem,the shower,I found the same problem once,does it have a lever control, if so and left in the mid position the cold can go up the hot pipe into the cylinder and up the expansion pipe, fit a check valve on the hot feed below the bath tap.

[rexton]

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

Hi,
Did you find a solution to your problem? I have the same problem which I think has only happened in the past week? Any ideas/ solutions?

[CJDURBS]

After 20 years with no problems I now have the same issue that hot water is overflowing into the cold water tank. My home is a single storey with an electric boiler in the house and the cold water inlet tank in the loft. The hot water only supplies three sinks and two showers. Not hot water is also coming out from the sink and shower cold water outlets. The boiler is only powered at the night by low rate electricity. I have tried shutting of the water to the Aquastream power showers as I wondered if the mixing valve had become defective such as to permit hot water to flow into the cold water pipe but it did not resolve the problem. Could it be that the boiler thermostat has failed, causing the water to overheat? If so how can I check if the thermostat is working correctly or not? Any help would be appreciated. Thanks. cjdurbs

[rexton]

Hi,
Many possible solutions were proposed for my problem from a leaking hot water cylinder to mixer taps. My plumber soon ruled out the cylinder leak as he said the water was been forced up into tank in attic and therefore presumably it could not be from hot water cylinder. After 2 non return valves were fitted to sinks downstairs and the problem continued to happen it turned out to be a faulty or poor quality non return valve that was fitted to downstairs thermostat on a mixer tap. He fitted a spring loaded (think that's what he called it) non return valve and the problem has not recurred since..(approx 2 months now). He said the cheaper non return valves don't last. Mixer taps seem to be a common cause of the problem.

[CJDURBS]

I have subsequently resolved my problem by replacing the immersion heater thermostat, which had stuck in the on position and was causing the boiler to overheat and the hot water to return through the boiler overflow pipe to the cold water tank. The new thermostat cost GBP10.29 from Screwfix and was easy to replace without draining the water or removing the immersion heater from the boiler tank. For safety the mains power to the immersion must be switched off at the mains input switch before removing the lid of the immersion heater to do this job.

[alanr1947]

Not a problem but you may be interested in my deductions from a similar problem. My overflow from loft cold tank yields water every now and again, although has done regularly for the last couple of weeks, and is always overnight, never during the day. This has been intermittent for a couple of years now. AND I find the water in my cold tank is quite warm. Everything else about my simple plumbing and heating circuits work just as expected.

Have done some research regarding possible causes - does water pressure overnight rise above normal and could thus cause a dying ball valve to leak more? My Water Board (Anglian) says no - google results suggest my water board operative does not know what he's talking about. As to warm water and the occasional day we wonder if boiler did heat water before we got up - that could only be hot water from indirect hot tank getting to cold water tank in loft - how, was never going to be simple to determine.

Until that is - I read this post - where there is a very plausible explanation of how - if the end of the expansion pipe ever gets below the water level - a thermal cycle could start whereby warm expanding water could vent all the air and end up delivering hot water into the cold tank (hot water rises) and replace it with cold water from the loft tank. Result is a warm cold water tank and a not so hot hot tank.

So potentially we have high overnight mains water pressure, leaking past a dying ball valve, raising the level in the cold tank, then in early morning - on comes the boiler to heat hot water, expansion occurs, thermal cycle starts and if high pressure sufficient, then we get a wet drive as overflow does what it was designed to do.

Today I have replaced the ball valve, and cut one inch off the expansion pipe as I noticed its tip was covered with scale suggesting contact with tank water. Also noticed (I did not fit this lot) the open end of the expansion pipe was below the level of the exit of the overflow pipe - so confirming that when overflow occurred the expansion pipe was below the water level thus encouraging the probability of thermal cycling. So shortening the expansion pipe should reduce the occurrence of thermal cycling, and a new ball valve should stop high pressure being an issue.

Now got to give it a few weeks to be certain the above detective work is valid - and I will try and remember to post again with conclusions.

[CJDURBS]

I had a similar problem which was caused by a defective permanently on thermostat in the boiler. This caused an excessive water temperature in the boiler such that the hot water overflowed into the main loft tank, heating the loft tank water and causing it to overflow. The problem was resolved by replacing the boiler thermostat.