air in water lines

Fontana

What causes air to keep returning to the water pipes for baseboard heat? When air enters heating is lost, returns when air is bled out..Oil fired furnace..I am at a loss..Thanks, Seb.

STEVEusaPA

You could be losing water somewhere in the system, and bringing in fresh water brings in air.
You could have the wrong type of air elimination that is sucking in air as opposed to removing it.
There are many things, but easiest if someone was on site.
I assume you mean an oil fired boiler.
Post a picture of the boiler and the associated piping, and expansion tank.

Fontana

I will check for water leaks; it is the wife's rental house and will quiz her on any leaks..I assume it would have to be a noticeable leak for the system to go down every couple weeks..She has a service contract but they can bleed the system to get it working but are dumb as to why its happening wanting to sell her a new oil fired boiler [I had 50% to be right] and don't know if the system was ever right as house was dormant for s few years...Thanks, Seb..

Solid_Fuel_Man

Pressure too low, circulator on return and auto vents on high point before circulator causing negative pressure and air to be drawn in. Waterlogged compression/expansion tank with circulator on return.

Taylor