Mitsubishi Electric [Outdoor unit failure]

This is the one published grouping rule in the whole category, and it is worth more than most individual code meanings. The last row of the FTC self-check table reads, verbatim, "U*, F*, A* — Outdoor unit failure — Refer to outdoor unit service manual". It is printed in the FTC5 and FTC6 installation manuals and again as the last row of Table 4 in the FTC7 cylinder unit service manual; the older FTC4 table has the same row as "U*, F*".

In one line Mitsubishi tell you that any code beginning with those letters points outside the house, at the compressor, the inverter, the fan, the pressure switches and the refrigerant circuit. That matters because it draws the qualification boundary for you, and the boundary depends on what is in the pipes. PUHZ-W and PUHZ-HW outdoor units hold R410A and PUZ-WM and PUZ-HWM hold R32; both are fluorinated gases, and work on a circuit containing them requires F-Gas certification in the UK, a statutory requirement rather than a manufacturer preference.

The PUZ-WZ Ecodan R290 range holds R290, a hydrocarbon that is not an F-gas; there the law does not demand the certificate, but Mitsubishi describe R290 as a highly flammable refrigerant and the outdoor unit is for trained installers, not a general handyman. Either way, a U, F or A code is not something to hand to somebody without the right qualification for your unit. It is also the group where the machine is most likely to have protected itself rather than broken: the high pressure switch and the inverter protections trip for reasons that include a blocked air path, a water flow problem indoors and unusually hot or cold weather, and an engineer may find nothing failed at all. What you can usefully do before the visit is rule out the outside causes: snow, leaves, garden waste, a fence panel or a wheelie bin blocking the airflow, all of which make an outdoor unit work far harder than it should.

1. 1

   Go and look at the outdoor unit. Clear snow, leaves and garden debris away from the grille and from the space around it.

2. 2

   Check nothing has been put against it — a bin, a fence panel, a stack of pots, a garden chair. It needs a clear path for a lot of air.

3. 3

   Do not pick ice off the fins or pour anything on the unit, even if it is iced up.

   The fins are thin aluminium with refrigerant pipework directly behind them. Puncturing that releases a mildly flammable gas on an R32 unit and a highly flammable one on the R290 range, and turns a service call into an incident.
4. 4

   Note the exact code and whether the system is still producing any heat. Both are useful and neither costs anything.

5. 5

   Read the refrigerant off the outdoor unit’s nameplate, then book accordingly. R410A or R32: ask specifically whether the engineer holds F-Gas certification, because for those gases it is a legal requirement. R290: ask for an installer trained on the Ecodan R290 range.