Daikin Altherma 3M heat pump no longer starts the compressor

[AL Ken]

We have 2 Altherma systems, the first (11kW) has been running for 3 years driving UFH perfectly well with over 20,000 hrs of operation. Recently I added another circuit and had to repeat the air purge and flow commissioning tests which passed successfully.  The problem is shown in the 2 photos attached where the system just sits there (12 hours) without starting the compressor. The settings are the same as they were before I added the extra circuit (setpoint fixed). In order to test the system I turned off the new circuit I added. The initial flow is 22.1 LPM but drops to the 11 LPM as shown in the photo after a while without the compressor ever starting. 

The 2nd Altherma is running perfectly with the same settings. I have compared all the settings by exporting them and comparing the .INI files but cannot see any differences. I am happy to share this setting file (.INI file) if there is a Daikin specialist capable of looking at it and spotting the issue. 

Can anyone advise where I should target my efforts please? 

 

[JohnMo]

Are you sure you don't have an air lock?

 

Do you have a call for heat? Is this from anything external?

[AL Ken]

Hi JohnMo

The system passes the flow rate test with 22 lpm and I have run repeatedly the air purge cycle. So there is enough water flowing but the compressor refuses to start. I attach more photos in case someone sees a screwup. 

Best Regards, AlKen

Edited October 30 by AL Ken

[JohnMo]

Is the menu saying you have external thermostat(s), if so are there calling for heat?

[Dillsue]

In your screen shots it says the flow switch is OFF. I don't know which way round Daikin set things up but if flow is detected I'd expect the flow switch status to be ON???

 

If you're using an external thermostat what are the heating minimum and maximum setpoints doing? I'm assuming those are air temperature limits rather than water flow temp??

Edited October 30 by Dillsue

[AL Ken]

Hi DillSue

No I do not have an external thermostat so I do not understand why this choice is enabled but when you select it the choices are 1 contact or 2 contacts. The 2 contacts is selected (blue) but cannot be changed. In the Installer sensor section I have External Thermostat set to None so I do not understand why the Thermostat is shown (Note on the other nearly identical Altehrma system which is a bit newer the Thermostat choice is greyed out).  

[Dillsue]

No idea if it's the same on your HP but on our Therma V whether you have an external thermostat or not is set with a DIP switch on the controllers PCB. If ours is set to be using an external thermostat it would need a shorting link(or closed contact from the thermostat) fitting on the thermostat terminals to give a call for heat and get the HP to run. Might be worth checking how your thermostat interface is configured to make sure you've not inadvertently selected it to run under thermostat control when there isn't one. Or just drop a link on the thermostat input.

 

Still puzzling why your flow switch is showing OFF

[AL Ken]

Hi DillSue

Thanks for the reply. I opened the unit and checked the jumper was still in place across the thermostat connection - it was. 

The Flow OFF is intriguing as I can find no setting or description of it. We have a second Altherma 3M but 2 years newer that has exactly the same settings, shows Flow switch OFF as well but runs perfectly.  

I would contact Daikin but I need to put a maintenance contract in place before they will talk to me. 

Anyone heard any issues with the firmware versions? I have not updated it in either unit as they were both running well. 

 

 

[JohnMo]

If it ran before it should run now.

1. Walk system have you left any valves closed?

2. Did you change any settings prior to or during new parts being added.

3. 

On 29/10/2025 at 15:36, AL Ken said:

setpoint fixed

Don't see on any of you screen shots, what the set point is - if you have a thermostat jumper in place, the compressor is waiting for the return temperature to drop low enough to get a start permissive.  Depending on the target set-point and system dT this may not ever be achieved.  Have you altered the target - what is set too? Increase it does compressor start?

[AL Ken]

Hi JohnMo 

Thanks for the reply. The set point is shown on the image showing a house and was set at 42 degC at that time - I have tried from 30 to 42 but the compressor still refuses to start. The return temperature as shown in the second photo is 19 degC which is a long way under the target temperature of 42 degC. This return temperature cannot go much lower as it is already picking up heat from the underfloor heating tubes. The dT is set to 8 degC but since the compressor is not running the outlet temperature never goes up above the 19 degC and hence the dT can never be reached.  

I dont get it, the system is exactly as it was for the last 3 years during which it ran for 20000 hrs, now it wont run the compressor nor report any error codes. 

[JohnMo]

Do you have the control settings, set correctly?

 

Leaving water, external thermostat and room thermostat looks for different things. If you have selected room thermostat and don't have the required interface, it's looking for a signal it will never get.

 

Get the manual out or download and go through everything in the setup wizard, the only stopping it starting is a setting.

 

[AL Ken]

Thanks again for the reply. I downloaded the INI settings files from the Altherma that is not working and the one that is - there are 370  parameters and they are identical in the 2 systems so its not a setting issue. I downloaded a copy of the settings description list and put them all together in an Excel spreadsheet to highlight differences - there were none. 

 

Eventually I got a repairman to come and after wasting 2 hours repeating what I had done he telephoned the Daikin hotline. The Daikin hotline asked for the serial number and when he told them they said "oh yes there is a fault known in the compressor drive filter board that was part of a recall".

Daikin are sending a new one for free since they accept it is their fault - unfortunately I will have to pay to have it fitted which I find a little annoying since it is Daikins fault. 

 

So anyone with an Altherma 3 model (probably other models may be affected also) older than 2022 they should make sure your service agent is aware of this and any other recall. 

 

So Problem Solved and thanks again to those who tried to help.