Is this a reverse cycle unit? Or, do you have a heating element?
If it is reverse cycle, there is a reverse cycle valve that can stick (I hope you problem is this easy). The valve makes the coolant flow the opposite direction and the unit will heat instead of cooling. If the heat has not been run for quite some time this valve can stick or fail to stay open. Most often tapping the valve with a pair of pliers can resolve the issue.
When you clean your air filters on the AC units, run the heat for a little while. This will prevent the reverse cycle valve from sticking.
With best regards,
Sailing Vessel - Cream Puff - SM2K - #275
Currently cruising - Martinique
We started the aft cabin Climma heater today and it's not working.
It starts to heat, based on my hand on the air outlet, but after 10 seconds or so blows only cold air. So the heating element (we have the EH model) turns off after a few seconds. I'll change the capacitors tomorrow, but can anyone add to the things for me to check?