Header tank (expansion) overflowing hot water (Thorn M44/54)

[G major]

Whenever boiler fires up, hot water or heating, overflow occurs. Not just a drip, more like a running tap!
I have replaced ball cock and checked that the valve does not leak. The hot water thermostat appears to be working (when knob is turned it clicks at various places on dial, depending on temp. and boiler fires up!) Heating is OK all rads hot (bleeding not required) so I assume pump is ok.
What else can I check/test?
Any sugesstions gratefully received.

 

[weareleeds]

Is it a fully pumped system if so switch on & check to see if it is pumping over (into exp tank). Usual cause is a blockage in the "h" if you have this arrangement. Difficult to help without much info regarding your system

 

[G major]

Pump is only operative when radiator heating is required, not for hot water. (if this helps!)

I have just switched on for hot water. Have watched expansion tank input pipe that hangs above it - no water input from there. However, it looks as if there is water coming in from what I would think is the return pipe at the base of the expansion tank. (I can see a gentle swirl (distubance) of the water at the input ) This pipe would appear to come from/go to the hot water tank on the floor below. The water in the expansion tank is warm. Hope this helps a bit more!

 

[Gasguru]

The gentle swirl is normal - this is expanded water entering the feed tank.

When the system is cold the water level in the feed tank should only be about 25 - 50mm above the feed outlet. If neccesary bend down the float arm and bail out water to set the correct level.

 

[G major]

I have just bailed out excess water down to point where ball cock valve shuts off. This is about 2 inches above the 'feed outlet' and leaves about 3 inches above water surface to overflow.
I will monitor how long it takes for the water to reach the overflow, as I'm quite sure it will!
System is currently calling for and providing both hot water and radiator heating - It's cold in Dorset!
I will report back when system starts to overflow again!

 

[Soggy_weetabix]

change ball vlv on header tank (i'm not going to exlaim why as Its a 5 min and cheap job for a DIY'er) but basically it may be letting by when system is cold enough to fill the tank to a level which will overflow then system is hot.

David

AND...ignore what ive said as Ive just read your first post.

I would go for pumping over, even on a gravity h/w system. I only say this from experience.

 

[G major]

It's overflowing again!! Just 3 hours since I bailed out expansion tank and started boiler. Source is still the 'feed outlet' at base of expansion tank. Hot water rising into expansion tank.
???????

 

[Gasguru]

If the feed tank base is at the same level or lower than the cold water storage cistern the problem is likely to be a perforated coil within the hot water cylinder.

Reduce feed tank water level. Tie up or isolate the cold tank float valve and drain tank down to the same water level as feed tank.

Put the boiler back on and see if the water level now overflows.

If it just rises a small amount this confirms a leaking cylinder coil.

 

[G major]

Followed your instructions - and water levels behaved as you described.
So I need a new water cylinder. Perhaps I had better search for threads on diy buying and replacement!

Many thanks for sharing your expertise.

 

[doitall]

Whenever boiler fires up, hot water or heating, overflow occurs.

For want of a better explaination it probably is the cylinder although this doesn't explain why it only overflows when the boiler is on.

Two questions.

Has any work been done on the heating.

You don't have a primatic cylinder by any chance.
no you cant have because you have an header tank so one question.

I would turn the storage tank ballvalve off and open the hot taps, if the coil has gone the header tank will empty as well (boiler turned off for this please)

 

[G major]

Having checked this afternoon it is apparent the overflow is running contiuously whether or not the boiler is operative.

In answer to your question - no work has been carried out recently.

My tank is just 30 years old!

 

[doitall]

Thats good news then

New Cylinder required.