Hello all.

I am hoping someone can help me out with a problem I am having with my Altherma air to heat pump.

I have the code AA showing on the indoor interface. This reads as Indoor unit LT: Thermal protector in back up heater open.

I have reset the thermal switch on the back up heater but it has not helped. When reading the electrical drawings I can see there are what look like 2 switches in series which take the coil voltage to the contactor. (Q1L and something else, possibly F1T)

The thermal switch contact is now closed, but the signal then runs through something else, but I cannot work out what it is. I know the problem is here as it is open circuit. The cable runs into a small hole in the end of the back up heater, but it is a sealed unit so I cannot investigate. It is the left hand side of the backup heater as you look at the unit.

Could anyone please point me in the right direction, or tell me what is in the backup heater causing the problem? I really really don't want to replace the heater for what might be a switch of some sort.

Many thanks in advance.