Daikin Altherma. Help please!

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.

The AA fault could also refer to the high temp status on the DHW cylinder. Under normal conditions, 240v is sent from terminal 13 on the hydro box and if all is normal, the 240v returns onto terminal 14. If the electric immersion heater has caused the cylinder water temp to exceed the set high temp limit, the stat goes open circuit and the fault code is generated, stopping the system.

The fault could be caused by a dislodged cylinder sensor, faulty cylinder sensor, the set point being set too low, or a welded heater contactor.

To reset the fault, the small red 'pop out' trip on the cylinder connection block must be pushed in until it clicks.

Thanks Frank. It has been diagnosed as the 6kw back up heater. Daikin technical tell me that the thermal fuse is non replaceable and I have to buy a new unit at £900 :eek::eek::eek:

If anyone has a second hand one it would be much appreciated!!! :p