20 yr old system thought I would look in the head tank and found hot water in there, stains down the inside of the tank sugest the hot water is coming in via the vent pipe. The system is indirect and has an oil fired boiler. Last time I looked in the tank about 7 yrs ago everything was ok.
Can anyone suggest what the fault is?

Argus

Hi Scruff

Yes the small CH header tank

Hot water has flowed from the vent pipe into the tank its not continuous but had occured not long before I looked in and did not reoccur while I watched. It has been going for some time because there was a jelly like scum in the tank with a rusty colour and a rusty stain down the side of the tank where water from the vent has run down the side.

The system is working ok but is slow to get going all the radiators are ok.

Yesterday I installed a new circulating pump whist the system was drained down I was connecting a heated towel rail and decided to do the pump as well. I hoovered all the gunge out of the head tank put fresh fernox f1 in refilled and bled all the radiators. system started up ok but when I checked the head tank some reirculation had occured.

May not be able to do more until after the holiday - family pressure

Argus

Double check where the feed and vent are in relation to the pump. I know you said it was working fine in the past but sometimes they work fine even with things in the wrong order...at least until something changes.