MHI room air conditioners: the published operating range

What this guide covers

• A room has been left unheated over a cold snap and the system will not bring it back.
• Somebody wants to use the unit in a space that runs colder or warmer than a living room.
• Water is forming on the surface of the indoor unit in a humid room.

What changes the answer

• MHI states that operating outside the specified range may result in the protection devices being activated, preventing the unit from working.
• The manual notes that long-term use above approximately 80 percent indoor humidity may result in condensation forming on the surface of the indoor unit.

The indoor limits are the ones people trip over

Everybody quotes outdoor limits, because that is the number in the brochure. MHI publishes indoor limits too, and they explain more complaints. Heating is specified for an inside temperature of approximately 10 to 30 degrees Celsius. A holiday property, an unheated garage conversion, or a room shut up through a cold week can easily be below that when somebody switches the system on and asks it to recover.

Cooling has the mirror problem at the other end, specified for an inside temperature of approximately 18 to 32 degrees. A conservatory or a loft that has been baking all afternoon can be above 32 when the system is asked to start, which places it outside the specified range at exactly the moment it is most wanted.

Neither of these is a fault in the equipment. It is a system being asked to work outside the conditions its manufacturer documented, and MHI is explicit about the consequence: operating outside the range may result in the protection devices being activated, which prevents the unit from working.

Why the heating range narrows at the top

Cooling is specified up to 46 degrees Celsius outside and heating only to 24. That asymmetry is not a limitation on the hardware so much as a statement about what the mode is for. Above 24 outside there is no heating demand worth serving, and a heat pump asked to heat into that condition is working against a temperature difference that makes no sense.

The shared floor of approximately minus 15 is the more interesting end. It is the point below which MHI stops specifying behaviour for this equipment, and it applies to cooling and heating alike on the documented models. Capacity has been falling well before that, which the manual says plainly: as a characteristic of the heat pump system, heating capacity reduces when the outside air temperature gets colder.

The humidity clause explains the drips

MHI specifies indoor humidity below approximately 80 percent and states what happens above it: long-term use may result in condensation forming on the surface of the indoor unit, leading to water drops. That is a different mechanism from a blocked condensate drain, and it produces a different pattern.

Water from a blocked drain comes from the drain pan and appears where the pipe leaves. Water from the condition described here forms on the outside of the cabinet, on the louvres and the front panel, because the surface is below the dew point of the air in the room. If the drips are on the outside of the unit in a humid room, the drain is not the place to start.

What to do with a room outside the range

The manual gives a direct answer for the cold end. Once the outside temperature has fallen far enough, MHI advises that it would be better to use an additional source of heating. Adding heat rather than demanding more from the heat pump is the manufacturer's own recommendation, not a compromise.

For a room starting far below the heating range, the sensible sequence is to raise the space with another source until it is inside the documented range and then let the system take over. For a humid room, addressing the humidity is what removes the condensation, and running the system in dry mode is one route to that rather than a fix for a fault.

Questions people ask about this