Mini what?

We had options, but that didn’t make the decision about how to stay cool any easier.

I could eliminate an air-to-water heat pump because of cost. Martin Holladay at said it well: “The cost of these appliances is so high that it’s more economical to buy three separate appliances: an HRV, a water heater, and a ductless minisplit heat-pump.”

An ERV (Energy Recovery Ventilator) with a right sized integrated air-to-air heat pump could work, though.

One should ask for help when it’s needed, so I turned to some green building experts to assist me in the decision making process. The recommendation was to use a minisplit in combination with our ERV to meet our cooling and ventilation needs. And I have come across the same recommendation over and over again, implicitly or explicitly, like in the quote above by Martin Holladay.

What is a minisplit

It is an air-source heat pump that is split into an outdoor condenser and indoor evaporator – thus the split in the minisplit. It is very similar to a conventional AC system, which also has an outdoor condenser and indoor evaporator, but with two important differences.

The minisplit does not require duct work but instead has a wall mounted cassette, which accommodates the indoor evaporator and blower fan.


The minisplit has a expansion and check valve set up that allows for cooling during the summer months. But those valves can be switched, in which case the unit provides heating during the cold season.

How can that be?

The evaporator can be turned into a condenser and vise versa. This requires a reversing valve in the outdoor unit along with the option to switch between a bypass and expansion valve in both the indoor and outdoor units.

During cooling mode, a compressor in the outdoor unit takes the hot, low pressure refrigerant vapor and converts it into hot, high pressure refrigerant vapor. That hot, high pressure vapor is routed to the coil of the outside unit, which acts as a condenser. The coil is cooled by a fan blowing outside air over it. This cooling removes heat from the refrigerant and allows for a phase change – from vapor to liquid. In other words, the hot high pressure vapor condenses into a cooler, high pressure liquid refrigerant, which is now pumped to the indoor unit.

The indoor unit has an expansion valve and heat exchanger coil with a fan. Here the coil acts as a evaporator: the expansion valve allows the cooler, high pressure liquid refrigerant to evaporate into low pressure vapor. This phase change is the reverse from that of the outside condenser. The phase change (from liquid to vapor) requires heat. That heat is extracted from the indoor air, which is blown over the coil (heat exchanger). Once the air passed over the coil, it is cooled and dehumidified, because excess humidity condenses on the cold coil.

The now hot, low pressure refrigerant vapor is traveling back to the compressor in the outdoor unit where the cycle repeats itself.

In heating mode a reversing valve turns the refrigerant flow around. The expansion valve in the outdoor unit is activated, turning the coil into an evaporator, and in the process absorbing heat from the outside air. The hot refrigerant is pumped to the indoor unit, where it bypasses the expansion valve and turns the indoor coil into a condenser, where heat from the phase change is used to heat the building.

Still confused? I don’t blame you! Here is a link to a Youtube video that I found. It shows how a minisplit heat pump works.

Related posts:

Cool ideas?

ERV – keeping the heat

1st floor ventilation planning

Picking an ERV

Summer heat or summer freeze

Heat wave

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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