Mini Split

A mini split is a heat pump in two parts: a compressor standing outside and one or more small indoor units, called heads, hung on a wall or ceiling and joined to it by thin pipes. It makes no heat of its own. A fluid in those pipes collects heat from the outdoor air, carries it inside and releases it, and in summer the machine runs the other way to carry heat out. Moving heat is much cheaper than making it, costing roughly half the electricity an electric heater would use for the same warmth.

How well it works is mostly decided before it is bought. Its size should come from a heat-loss calculation made on measurements of the actual building rather than from a figure per unit of floor area; the common rule of one ton of capacity to every 46 square metres (500 square feet) is only a rough check on a quote. It should also come after sealing air leaks, insulating and improving windows, since that work shrinks the load and so shrinks the machine needed. Undersized, it runs constantly and never reaches temperature. Oversized, it cools a room so fast that it stops before wringing any moisture out of the air, leaving it cool and clammy - a fault that looks like success while people lower the thermostat at three to five percent more cooling cost per degree.

Heating output falls as the outdoor air gets colder, which is when it is most wanted, and below a certain temperature the machine falls back on electric resistance heat, the very cost it was chosen to avoid. Upkeep is light but frequent: a filter checked monthly through the busy season and the indoor coil cleaned yearly, which is harder than it sounds on a head hung high on a wall. Water condensed out of the air drains by gravity through the same hole the pipes use, so a blocked drain appears as water running down the inside of the wall rather than as an error message, and a slow leak in the pipes appears only as the machine running longer each season. Opening those pipes is work for a certified technician in most places. A head recirculates room air and brings none in, so a house sealed tight for this machine still needs ventilation put back.