Main water tank overflow

[NellieF1]

The overflow for my main water tank in the loft is dripping a reasonable amount of water while heating up the water (I have a GloWorm gas boiler, approx 17 year old). In the loft I have the main water tank (the overflowing one) and a header tank for the heating. I also have a water tank in an airing cupboard.
The main water tank seems to overflow once the water has been heating for over about 30 mins.

Once a bath etc has been run the overflow stops dripping and does not resume even when the tank refills (I guess ruling out a ballcock issue?).

Any ideas...?

 

[Softus]

In the loft I have the main water tank (the overflowing one) and a header tank for the heating.

Which one of those is the higher?

Any ideas...?

What's your boiler?

Do you have any shower valves in the house that are fed from unbalanced supplies?

 

[TicklyT]

I wouldn't rule out the ballcock.

The first thing I would check is the water level in the cold water storage cistern. A good time would be just before the water heating is turned on, and then again after hot water has been used. The levels should be the same.

With everything cold the ballcock should shut off completely when the water level is a couple of inches below the overflow outlet.

If the initial water level is too high, water will overflow as it expands on heating, so re-setting the float level may cure it.

If the ballcock doesn't shut off fully the level will creep up slowly, especially overnight, when mains water pressure increases as usage drops, but may only reach the overflow when the water is heated and expands. This may require a replacement of the valve or seat.

If everything is OK with the ballcock, then we're going to have to start looking elsewhere as Softus has said

 

[NellieF1]

Thanks for the replies...

The main water tank is higher, I do not have any shower valves...

The ballcock is definately shutting off properly....I will check the water level when cold to ensure that is is not too high....resetting the float level may work as you say...

 

[richard_1234]

Just a thought but if the problem only happens when the hot water is on. Maybe the water is been heating too high and casuing the pressure to rise and expell back into the tank via the expansion pipe causing the tank to fill up and over flow. Try turning down hot water temperature.

 

[Softus]

Maybe the water is been heating too high and casuing the pressure to rise and expell back into the tank via the expansion pipe causing the tank to fill up and over flow.

There's a cold feed pipe, and a vent pipe, but no expansion pipe on a DHW cylinder.

Try turning down hot water temperature.

On no account should you do this - it's a health risk to store hot water at less than the recommended temperature.

 

[John_D]

how high is the expansion loop above the cold water tank? look for 450mm is it dripping?

next thought I have recently had 2 hot water cylinders leaking internally between the central heating coil and the domestic water. This results in water overflowing from the lower tank warning pipe and being filled from the higher tank ball valve. If this applies your DHW tank needs replacing.