I have a baseboard hot water furnace system, Repco brand, installed about 35 years ago. It uses an older style expansion tank mounted in the joists above the furnace. The furnace has a circulator pump.
Almost once a month it seems i loose heat in the system. The cause is that there is air in the system, so the water doesn't circulate. I have run water through the system to remove the air, and that corrects the issue for a while, but then the problem reoccurs in a month or so. Each time the expansion tank seems to be full and i drain about half of it. An auto-air bleeder was installed on one radiator upstairs, but that doesn't seem to help. Does anyone have any ideas what the issue here is ? I kind of suspect the expansion tank, because i think it shouldn't be filling up and maybe has a leak, and has no pressure. I thought the tank should be pressured.

I also should mention, this furnace heats the 2nd floor of a duplex, The HW leaves the furnace travels up to a baseboard inthe apartment, then down to the first floor to a baseboad in the hall/entrance, back up to the apartment for the remaining baseboards, and finally back down to the furnace in the basement. Some one else said this wasn't the correct way to install this, but this furnace worked fine the first 30 years, it's just the past 5 that it seems to be acting up, and now recently it has been once a month almost.

Any Ideas would be helpful.
thanks John.