Monday, September 9, 2019

Mini-split Heat Pump Installation

We live in a home which was built in 1963, older than we are. The structure has some great attributes and some not-so-great attributes. Among the not-so-great is the lack of air conditioning, and a pair of ancient furnaces with very high gas bills. We set out to do something about this.

  • We wanted to add air conditioning.
  • We wanted a much more efficient heating system.
  • The ducts were 50+ years old and clearly leaky, with dirty patches in the insulation at each joint where the duct has been pulling air through for decades. They would not pass current inspections.

As essentially none of the existing HVAC would remain, we could consider options which didn't preserve any of it. We decided to go with a ductless mini-split heat pump system.

A heat pump is an idea which has been around for a while. It operates similarly to an air conditioner in that it repeatedly compresses and expands a refrigerant, circulating it in and out of the house while doing so. The difference is that where an air conditioner always compresses the refrigerant outside of the house to release heat, the heat pump can also reverse the process to release heat inside. A heat pump can either heat or cool based on where it allows the refrigerant to expand.


There are heat pumps which can replace a central furnace and hook up to the existing ducting, but as the ducts were in terrible shape we opted for a mini-split system. There is no central air handling nor air ducts in this system, there are individual units in each room which are connected to a compressor outside.

In each major room a head unit is mounted high on the wall, and contains refrigerant coils and fans. Air is circulated within the room, not drawn from nor exhausted to the outside.


The head unit connects to power and two refrigerant lines. This picture was taken during the installation, with the wall open and the two copper refrigerant lines not yet hooked to the head unit.

Note that there is no air duct: air is not moved through the home with a mini-split, only refrigerant. The head unit can cool or heat air drawn from the room, using the refrigerant to pump heat in or out of the house.

I emphasize the lack of ducts because it was a big mental hurdle for us. In a retrofit the heat pump units can go anywhere, placement is not constrained to where ducts currently go.


The head unit contains a filter in front of the fan, but the activated charcoal portion of the filter covers only a small portion of the area. We have no way to measure the effectiveness of this filter, but we are skeptical as it seems like air can flow around it easily.

The refrigerant connections are quite small, half inch diameter copper pipes plus insulation, so they can run between studs in the walls and under the house. They all eventually lead to an outdoor unit, which contains a fan and radiating fins like an air conditioner outdoor unit would.

The outdoor unit is available in a few capacities, rated in British Thermal Units (BTUs) like 20k - 50k. The head units inside the home are also rated in BTUs, from 9k through 24k, and one adds up the rating of the head units to determine the capacity of outdoor unit required.

Our home needed two outdoor units, a larger 50k BTU unit for the upstairs and smaller 20k unit for the lower level.

With a furnace or central air, a single thermostat controls the HVAC. That thermostat might be very sophisticated with multiple room sensors, but there is a single central point where control can be implemented.

With ductless mini-split systems, there is no single point of control. Each head unit implements its own local control, it can implement its own schedule, etc. The system is supplied with a handheld remote control for each head unit. It appears to be infrared, and it is not strongly paired with a given head unit. If you take it into another room and point it at a head unit in that room, it will control the head unit in the new room.

The remote is quite complicated. It can change the mode from heating to cooling to fan (and others). It can program weekly schedules. For some models of head unit, it can configure an occupancy sensor feature to aim the airflow directly at people in the room and turn off if nobody is present. Etc, etc.

We use the remote controls for all but one of the head units. For the last unit, we had reasons to not want to change how the HVAC system is operated and wanted to retain the existing themostat on the wall exactly as it was. Mitsubishi has an interface to allow this, connecting any 5-wire thermostat to control a single head unit.

A few things we wish we'd known at the start, in case anyone reading this is planning their own heat pump installation:

  • All of the head units attached to a given outdoor unit have to be cooling or heating, not a mixture of both. We got lucky in this: we needed separate outdoor units for each level, and this matches our usage as the lower level doesn't get so warm while the upstairs needs cooling during the summer.
  • The smaller outdoor units can be attached to the side of the house on a bracket. The larger outdoor units require a concrete pad to be poured. Had we known this we might have chosen to go with three smaller outdoor units and had them all mounted to the side of the house.

At the time of this writing we've had the system for five months, through our first summer. It has been great having the option to cool the house on those days which need it. We are just heading into the cooler months, and we're hoping to see a substantial reduction in the energy bill.

We're quite happy with the system. Heat pumps are also an effective means to help with global warming by improving efficiency and reducing use of methane, and are #42 on Project Drawdown's list.

Our system consists of:

  • 3 x MSZ-FH15 15k BTU head units
  • 1 x MSZ-FH12 12k BTU head unit
  • 3 x MSZ-FH06 6k BTU head units
  • 1 x MXZ-8C48 48k BTU outdoor unit
  • 1 x MXZ-2C20 20k BTU outdoor unit
  • 1 x PAC-US444CN thermostat interface
  • electrical panel work to rearrange breakers and install new 40A and 25A circuits
  • permits and fees
  • demo and removal of old ducts and furnace equipment

The total cost was $31,665 for equipment and installation, in the SF Bay Area where the cost of living is high. Our gas bill in the winter with the old furnaces was often $400/month, which should decline substantially with an electric heat pump powered by solar panels on the roof.

The system was provided and installed by Alternative HVAC Solutions in San Carlos, CA, and we were quite pleased with their work.