HVAC return line keeps going cold
Hello everyone, I'm a maintenance tech at a hotel and am having an issue with a heater that I can't seem to nail down. Admittedly I'm not an expert in the heating department but I know enough to normally keep them running, and this might be a simple answer. We've got a real cold spell coming so I'd like to get it straightened out.
The HVAC has hydronic heat and the coil and both pipes stay hot when in the 'Off' position (zone valve open), but within a minute of turning the heat on, the return pipe starts going cold while the supply stays hot.
There is a room above and below this particular unit and both of them are tied into the same lines and neither one of them is having any trouble. The coild doesn't seem to be frozen, the t-stat seems to be operating just fine, as does the zone valve. When I bleed it I seem to get a tiny bit of air but no blasts like from an HVAC that is airbound. There also don't seem to be any leaks anywhere.
Any ideas on what this issue could be? Any help appreciated.