Installing a Mini-Split Heat Pump

Introduction

Heat pumps are one of the most efficient ways to heat a house - since they pump heat, rather than create it, they can paradoxically have an efficiency greater than 100% (a resistive electric heating element has an efficiency of 100%, and a fossil-fuel heating system less than 100% - because some heat is lost up the flue).

Traditionally in North America ducted air heat pumps have been used for cooling, and also for heating in winter. Typically a large unit is installed which serves the entire house through a duct system installed during construction. The warm and cold sides of the pump are contained in a single unit.

In recent years the ductless split heat pump has been introduced. The warm and cold sides of the pump are contained in separate units, connected by refrigerant lines. A mini-split unit can serve a single room, while a dual or triple split unit can serve two or three rooms with separate indoor units connected to a single outdoor unit. The compressor is located in the outdoor unit, while the indoor unit is relatively small and lightweight.

Since the refrigerant lines are of modest diameter, and the indoor units small, these systems can be retrofitted to an existing house relatively easily with minimal disruption.

(these instructions were written for the Senville Aura series; there are a number of similar units by other manufacturers such as Fujitsu and Lennox)

The first step is to determine the correct size of heat pump. For a given location, climate records will give an outdoor temperature range. For instance, on 97% of days the temperature will not fall below -10C or rise above +25C. Then, given a desired indoor temperature range, say +15C to +21C, it is possible to calculate maximum heat loss and gain for a particular room given the envelope area (walls, ceiling etc.) and R-value of each component such as walls, doors, windows. This may have already been done as part of the original building design. Alternatively, one might estimate it based on floor area and local construction practice.

The next step is to determine suitable locations for the components (indoor and outdoor units). For the Aura, the indoor unit weighs about 10kg and is designed to be mounted high on an exterior wall. The outdoor unit weighs about 40kg and can be mounted on a concrete slab, on a bracket hung from a wall, or on a metal stand. Consult the installation manual for recommended clearances. The outdoor unit should not be exposed to severe weather and should not be located where it can suck in debris, dirt, leaves etc. In operation, it will blow warm air (when cooling), or cold air (when heating), so should probably not be sited next to a patio etc. where people sit.

The two units are connected by refrigerant lines, and by an electrical cable, and can be at some distance from one another. For the Aura, the maximum allowed line length is 20m, and the maximum difference in height is 8m. In addition to the lines and cable, both units have a condensate drain, which should be routed outside using pipe with a consistent downward slope.

If no engineering drawings are available for the units, it may be necessary to make measurements yourself to determine the location of mounting brackets and service lines. For instance, the smaller Aura indoor units have brackets on an 18" pitch, while the outdoor units have brackets on a 21" pitch.

The outdoor unit is significantly noisier (54dBA) than the indoor unit (37dBA); it should probably not be near an open window.

The smaller Aura set requires nominal 230V split-phase AC at 9A, on a 15A breaker. Larger sets may require 20A or 25A service. The indoor unit(s) are powered from the outdoor unit. Since regular outlets are probably nominal 115V, and the heatpump should have a dedicated breaker, it is probably necessary to run new cable from a breaker panel to the outdoor unit location.

The Aura is shipped with refrigerant line stubs folded inside the back of the unit. There are cutouts on either side of the case, so the lines may be routed out the left side across the wall, through a hole in the wall behind the unit of the left, or the lines may be bent straight and routed directly through the wall behind the unit on the right. Alternatively they could, with care, be bent to the right. It is necessary after installation to connect the refrigerant lines, so the geometry must allow access (not locate the joins inside a wall, for instance).

If the indoor unit is mounted on wood-frame construction, at least one bracket should be secured to a wall stud. Ideally a backing plate should be used if there are not two studs available at suitable locations, though cavity-wall toggles to hold 5kg should be adequate.

All measurements and planning should be done before drilling a single hole - try not to have the coolant lines emerge behind a downpipe that cannot be moved, or sever a wall stud or water pipe.

You need regular tools to screw things to a wall - drill, screwdriver, wrench etc. for screws and lag bolts, masonry bits for cement or stone. You also need something that will cut a circular hole some 2.5" diameter through an exterior wall - probably a hole saw, or a reciprocating saw and a marker. You need a stud finder to locate studs in a wood-frame wall. You need fish tape to fish electrical wire - at least, it helps.

For cutting and fitting the refrigerant lines, you need a tube cutter. The Aura uses flare nuts, so you need a flaring tool. You can bend copper tube by hand, but for a small radius you need a tube bending tool - the tube can buckle if bend too sharply by hand. The Aura uses 1/4" tube for refrigerant supply, and 3/8" or 1/2" diameter for the return depending on the size of the unit, so you need bending and flaring tools for each pipe size.

To tighten flare nuts, ideally you should use a flare nut wrench of the correct size which is less likely to slip or damage the nut. It is possible to make these from ring spanners by cutting the ring with a carbide disk - the resulting slot fits over the pipe so the wrench can be slid sideways over the nut.

The Aura installation specifies a particular torque for each pipe size. You can obtain a suitable torque wrench, use a socketed torque system with a crows-foot wrench, or use a spring balance with some simple math to account for the length of the flare wrench.

Purging the refrigerant lines requires a vacuum pump and pressure gauge.

Check measurements to ensure that the through holes will be in a good location both inside and outside. You will need to measure carefully from an opening such as a door or window. Remember that floor and ground levels may be different inside and out, while interior walls may not line up between floors.

Secure the mounting plate to the wall, or at least mark the location, then drill the through holes for the refrigerant lines, cable, and condensate drain. These holes should slope slightly downwards towards the outside to discourage water ingress. The Aura kit comes with a plastic liner and trim ring, though this is too short for a thicker wall. ABS plumbing pipe may be used to support or conceal the lines in a longer run.

Route the terminated and insulated refrigerant lines, the drain line and the cable. The Aura requires 4-conductor cable (two supply lines for 230V, a signalling wire and ground wire). Mount the unit on the mounting plate and connect the inter-unit cable and condensate drain. Fasten the flare nuts on the refrigerant lines but leave the fittings exposed for leak testing later.

Locate the outdoor unit in the desired location with a good route for the refrigerant lines, mark the mounting bolt locations, and drill the mounting holes. Fasten the unit in position.

If using wall brackets, make sure they are securely fastened to the wall - with wood-frame construction, secure them to studs with lag bolts. In the picture, the horizontal bar is fastened to studs on 24" centres while the unit feet and brackets are on 21" centres.

Connect the inter-unit cables to the outdoor unit. Route the cables with a downward loop so that water does not track down the cable into the unit.

Cut the refrigerant lines to length, re-fit any insulation if required, thread the flare nuts onto the end of the pipe and flare the ends. Ensure there is enough slack to allow connection to the unit without sharp bends, and preferably enough to cut and re-flare the end if the first attempt is not satisfactory. Connect the lines. Connect the 230V power lines, ensuring that the unit is properly grounded.

Consult the installation instructions.

The Aura is pre-charged with refrigerant, though if refrigerant lines more than 7.5m are required, additional refrigerant must be added.

The refrigerant lines and indoor unit coils must be pumped down and allowed to outgas any moisture. The Aura is equipped with a purge fitting with a Schrader valve to accept, I believe, a 5/16" pipe gauge coupling with Schrader opener such as "Yellowjacket" brand. Pump the lines down for at least 20 minutes, ideally longer, and make sure the pressure does not rise significantly when the pump valve is closed at the end of this period. Open the packed valves to allow refrigerant into the lines, disconnect the coupler (closing the Schrader valve), and leak-test all the joints including those on the indoor units. Close up the indoor units, connect the power, turn the system on and test it.

• While the Aura installation instructions suggest leak-testing using soapy water, this may be illegal in some jurisdictions. Leak testing to a positive pressure with dry nitrogen prior to introducing refrigerant is preferred.
• In some jurisdictions, installation or even purchase must be done by a licenced technician.