DIY Doctor UK DIY Forums

I have an Indirect central heating system.The problem started when the overflow pipe from the ch system started dripping outside. checking the expansion tank in the loft, the ch vent pipe was trickling hot water into the expansion tank. Since then, I have had a CH engineer replace the pump, the indirect cylinder tank, thermostats on most radiators,boiler serviced and the whole system power flushed, costing £710, and the problem still exists. There is a valve next to the pump and hot water tank, that works when the system is set to hot water, hot water and ch, or ch only, the hot water comes out of the vent pipe into the expansion tank when the system is set to hot water, either hot water on its own or with the ch as well, appears ok when on ch only.
Any ideas on this problem would be welcome, as I cant afford to keep spending and just hoping the problem is curred.
Thanks in advance for any advice, Mark.

hello.
check if the cold feed pipe from the tank to the heating system is warm when the boiler is on. its normally in 15mm and comes out low level of the tank into the pipes in the airing cupboard. if it is warming up then the coil in your cylinder has probably split which means a new one is required as your domestic hot water is entering the heating sysyem.

Hi Dave, Thanks for your reply. What you have suggested below is the thing I originally thought it was, so a new tank has been fitted. Since fitting the new tank, have also fitted thermostats on radiators, had the whole system power flushed and a new pump fitted as the old one when it was taken off to fit the power flusher was seriously blocked up.
The last thing that has been done is the vent pipe in the loft has been raised, only by a short amount, this seems with the other items above to have done the trick, or as best can be done. When the pump goes off (when it has been on for hot water) it dumps a load of water through the vent pipe into the expansion tank in the loft, but when the pump kicks in again it draws the water from the expansion tank taking it down to its original level again. I think this is normal, but propbably not the amount it does in my system. My plumber thinks we may have to live with this until we can afford to change the boiler, he thinks even after power flushing there could be a large lime scale build up in it causing this problem, as it still kettles when hot, but not to the extent that it did before the power flush.
Thanks again for your reply.