Almost certainly one for @ashp-bobba to put his Ecodan error codes to the test 😉 

Posted by: @hjd

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Just wondering if anyone has experienced an F8 error code with the Mitsubishi Ecodan PUZ series? A neighbour is getting it every time the outdoor temperature drops to below c. 5 degrees and the unit switches off.

Has your neighbour tried googling it to find out what the code means (other search engines also available)?

Yes - its just that the F8 code doesn’t actually appear much in Google searches!

Well the Mitsi error code lookup says F8 means "No input detected at outdoor unit" and that the cause may be "Outdoor board defective". It suggests "Check mains power is ok" as the checking procedure.

That's now the full extent of my ability to add any value here, so over to @ashp-bobba or other more qualified person than me.

@hjd It means the outdoor unit hasnt detected an input or the pcb is defective. Needs looking at by engineer or at worst an electrician.

@hjd F8 is uncommon and could indicate incorrect heat exchange due to a lack of flow or a faulty sensor, if you think this is just happening when it gets colder I would suggest all the normal things you chan check for yourself like the filter and valves, if you are not getting an L9 fault then it is probably moving the water above it minimum but when it needs to kick in and exchange a decent amount of heat then it might meet the threshold where it fails.

As this fault could indicate a sensor, flow or heat exchange problem, if you don't see anything obvious in the filters and all valves are open it may be worth getting an engineer to take a look to check if the system is silted up.

Are you able to provide the flow rate for the water in heating? This is a good 1st step.

Thanks for everyone’s input. That helps me and my neighbour to do some more investigation.

@ashp-bobba, do you have a list of Ecodan error codes? If you do, please can you send them to me so I can train Wattson them?

@editor Pages 59-74 for all Error codes and Request codes in the attached link, F8 is not showing as I think it is a shadow code related to P9. We have a log in code for a installers portal that gives us a different kind of access which is on a cloud system so live rather than downloadable but its the same as the manuals.

I think it is worth speaking to tech support as F8 is uncommon and points toward sensors or flow (heat exchange) 

https://library.mitsubishielectric.co.uk/pdf/book/PUZ-WZ_50-60-80_VAA_Service_Manual_OCB816#page-1

Posted by: @editor

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@ashp-bobba, do you have a list of Ecodan error codes? If you do, please can you send them to me so I can train Wattson them?

It’s errormentary, my dear Wattson.

Sorry, I’ll get my coat….

Just to add to the F8 non existing code, in the book is all of the blinking codes and could be worth double checking it not trying to display a P8 which could look like a F8