Problem: (system particulars below) In zoned system, first floor is heated w/in-slab tubes, upstairs by baseboard radiators (normally not run as we don't use the second floor for much). Unless air is periodically removed from system by running upstairs baseboard radiators, low water cutoff shows a yellow warning light and cuts blower off while tank gets up to temperature. It will restart quickly, run a few seconds, then get cut off again, and repeats this until the tank is up to temp.
The problem started after the system had been in place for several years. Low water cut-off is mounted high so light cannot be seen when standing, unless you're elevated or unless you put a finger behind the light and get the reflected glow. First two techs replaced the Aquastat which seemed to work for a while, though at great expense.
Third tech was about to replace the Aquastat and happened to see the Low Water Cut-off light. He suggested bleeding the air from the system by running the upstairs zones (normally closed) and that worked. He had no suggestions for why this needed to be done or why the air was accumulating in the closed zones, or how the problem could be prevented.
At this point, we have to run the upstairs zones for an hour or so every 2-3 weeks (when we notice the system clicking on/off).
Anyone know of a firm fix for this issue? Is something waterlogged (e.g. the expansion tank)? System particulars below. Any info appreciated.
System (11 years old):
Buderus Boiler G115 w/Becker blower, on top of integrated Buderus hot water heater
Amtrol Extrol Expansion Tank for Hydronic Heat Systems, Model 30 (pre-charge 12 psi, max 100 psi, temp max 240)
Honeywell Aquasat (3-4 years old)