Central Heating Header Tank Overflow

tonylevene

I recently de-scaled, flushed and re-filled my CH system. Now, when the system comes on each morning the header tank overflows. Then it settles down and no further problems occur during the day. In normal operation the level in the header tank is comfortably below the overflow pipe and the water is not hot.
It seems to be an over-pumping problem, caused by a temporary blockage. The CH and HW circuits each have their own control valves. The valves are about 17 years old and it has been suggested that the balls mybe sticking until they get hot. That seems logical, but why did the problem only occur since I flushed the sytem?
I have tried to create the problem during the day by switching off both circuits and letting the sytem cool down, but no success (probably did not get cold enough)
Any ideas please?

missile

I suggest you have a look at the pump, most have adjustment and if you turn down the flow rate that may solve your problem.

owned_by_2_Siamese

By chance is your hot water tank a 'mainsflow' - i.e do you get hot water at mains pressure? These tanks are the reverse of normal cylinders in that cold water passes through a coil inside a tank of hot water heated by the boiler. I had exactly the same problem - the cause was that flushing the system had removed/loosened some scale exposing a hairline crack in the coil. When cold the mains water passing through 'pressurised' the hot water heating system forcing it up to the header tank (where it promptly overflowed) As it heated up and expanded the crack 'closed' so the leak stopped - until the system cooled down. The only fix was a new tank I i(n fact we had two in the end as the pressure expansion vessel was duff but we didnt know) It cost us over £700!!! in the end it happened again so we ripped it out and had a combi.

Fleeting_Glimpse

Tough one...cold be a number of things.

How are your DIY skills?

Buy a 'sealed system' kit from a plumbers merchant...about £70.

http://www.plumbworld.co.uk/584-707

Remove the header tank completly and cap off the vent pipe.

Secure the new expansion vessel to something solid, connect the mains feed to the inlet on expansion vessel and the 15mm feed from old header tank to the expansion vessle outlet. You'll need a warning pipe to outside in 15mm copper...use the old overflow hole?

Turn on mains...pump up pressure to 1.5 bar....all problems solved...about 2 hours work.

Downsides are you'll have to keep checking pressure & could make some old fittings leak.

caleyles

Be Careful, Be very careful,:eek: not all central heating boilers are suitable to convert to a sealed system, i.e. older style gas boilers with cast iron heat exchangers etc, some can be fitted with pump overrun etc, before converting make 100% sure that the boiler can work in a sealed system, most modern boilers can with their built in overheat stats etc but check your boiler data to be safe:)

EliteHeat

caleyles wrote: »

Be Careful, Be very careful,:eek: not all central heating boilers are suitable to convert to a sealed system, i.e. older style gas boilers with cast iron heat exchangers etc, some can be fitted with pump overrun etc, before converting make 100% sure that the boiler can work in a sealed system, most modern boilers can with their built in overheat stats etc but check your boiler data to be safe:)

100% agree with this. Phone the manufacturers to check, but first determine the cause of the overflow - there is absolutely no need to convert to a sealed system - you do need to find the cause of the problem though.

tonylevene

owned_by_2_Siamese wrote: »

By chance is your hot water tank a 'mainsflow' - i.e do you get hot water at mains pressure? These tanks are the reverse of normal cylinders in that cold water passes through a coil inside a tank of hot water heated by the boiler. I had exactly the same problem - the cause was that flushing the system had removed/loosened some scale exposing a hairline crack in the coil. When cold the mains water passing through 'pressurised' the hot water heating system forcing it up to the header tank (where it promptly overflowed) As it heated up and expanded the crack 'closed' so the leak stopped - until the system cooled down. The only fix was a new tank I i(n fact we had two in the end as the pressure expansion vessel was duff but we didnt know) It cost us over £700!!! in the end it happened again so we ripped it out and had a combi.

Thanks for that observation. I don't have hot water at mains pressure, just form the normal cold water storage tank. However I do wonder whether there is a crack in the coil and that water is seeping into the CH system overnight and then the normal expansion when the boiler fires up in the morning causes the overflow. I can test that by closing the feed to HW cylinder last thing at night and see what happens next morning. I'll let you know.