Air in rads, leak somewhere?

Spiral Notebook

Hi. I have a traditional oil burner heating system. 2 story house, expansion tank in attic, hot water cylinder upstairs, rads upstairs and downstairs, towel rails in bathrooms. Cylinder is a dual cylinder as have oil burner and a stove with a back boiler.

1 towel rail and 1 radiator down stairs (more or less beside each other) need to be bled every 7 days or so (top 1/4 is cold). Water level in expansion tank in attic doesn’t drop over the 7 days (tied up the ball valve to check this), but once I bleed the radiator and towel rail it drops 15-20mm. No water is coming back up the expansion pipes (I put a 1/2 cut plastic bottle under the expansion pipes to check this).

Suppose my questions are:

1 - Does this sound like I have a leak somewhere - I have checked and these is no visible signs, so if it is the case then I can only assume its under the downstairs concrete floor.

2 - If i have a leak, why is air replacing the water thats leaking somewhere, and not water from the expansion tank in the attic?

Just to note, if I turn off the flow and return to the rad and towel rail in question, I have another radiator filling up with air (it’s fairly close to the original 2 No.).

I’ve used a lighter when releasing the air and it doesn’t ignite.

Just looking for opinions before I get the plumber to have a look.

I’m not experienced in plumbing, so I’ll try and answer any questions anyone may have that might help me understand what’s happening.

Thanks

jimf

get a plumber that is experienced in dual systems to check it over for you

once you have no blockage from the header tank and the level is not dropping when tied up it doesnt sound like a leak

i wonder just a guess is it possible the pump is drawing air from the expansion pipe

btw im no plumber but just to reply with my tuppence worth