Overflow problem

[Mrspoppins]

What would be causing our overflow to run continuously when we have the central heating on? And what can be done to stop it? Thanks in advance.

[alavahalf]

... sounds like your hot water tank has an internal leak from the coil ... new tank required ...Turn your heating off..!

[The Builder]

I guess you mean the overflow from the boiler Mrs P ............ or do you mean the roof tank overflow (non-combi sytem)?

[Mrspoppins]

The Builder, its the roof tank . Alavahalf, it's only on for hot water at the mo, although here in overcast NI, we'll soon need it again for heat lol!

[boxtops]

Is your float needing attention? the valve arm in our tank had somehow got a little bent with the result that the float wasn't bringing the valve up to shut off the incoming water when it was full, hence overflowing from the high-level outlet under the gutter. can you check? - we just gently bent the arm up a couple of millimetres and the float could then come up to the right level.

[The Builder]

What Boxy has said is often the case.......... but it would do it all the time ......... not just when the heating is on.
What Alava suggests sounds more likely to me.......... pumped water in the CH system finding its way into the HW water and sending it up to the roof tanks .......... a possibility.
Whatever ........ it sounds like it needs looking at Mrs P

[Wak]

Would the water in the CH system expand when heated due to the system being switched on and would this cause the excess water to flow back in to the roof tank sufficiently to cause the over flow due to the fact that the normal water level is too high to start with????? If you see what I mean.
Just a thought!

[AlBags]

^ No

An Open Gravity CH system has own header tank (a small one)
If water is coming from this, then it's from thermal expansion .. Float Ball valve is set too high. Bend arm down a bit or reset float height.
If sealed system .. impossible. Only thing that could happen would be blow-off would crack open and vent at 3bar.
Leakage across failed coil in cylinder is unheard of .. If it WAS ruptured, there could be mixing . which is not good .. You need to know if gravity or sealed system!

[The Builder]

That's an interesting point that Wak has made .................. could be as simple as that ????

[Hopkirk]

Yes but it wouldn't keep happening, unless the tank was getting refilled.

[Wak]

I think it would, Hopkirk. Imagine if the water level when cold was set to a fraction below the overflow level then, when it was heated it would cause an overflow to get rid of the expanded volume of the entire heated water in the system. Upon cooling, the entire water in the system would then contract and drop the ballcock below it's normal full level and thus automatically refill up to the overflow pipe again.
I think it all depends upon where the water level is when cold and therefore, the ballcock arm could need adjusting to lower the water level.
And then again, my friend, it could be something else completely different and too technical for us non-plumbers.

[Mrspoppins]

Thanks for all your replies, we will try all your suggestions, but will probably get a plumber in eventually!