What a good heat pump installation looks like

The manufacturer's installation manual for the specific model is the governing document. A good installation follows it step by step and leaves a record of the key measurements. The details below come from manufacturer manuals and an industry installation guide; the exact numbers for your system are in its own manual and differ by model.

Installation quality has a measurable effect. ENERGY STAR says "improper installation can reduce system efficiency by up to 30 percent."^{[Source 6]} NIST's lab and modeling study of installation faults found that duct leakage, refrigerant undercharge, oversizing, low indoor airflow and overcharge "have the most potential for causing significant performance degradation."^{[Source 7]}

Line set

The line set is the pair of copper refrigerant lines between the outdoor and indoor units (Line set).

• Length and height limits. Each model has a maximum line length and maximum height difference. Examples: Mitsubishi's MUZ-FS06/09/12 allow 65 feet and 40 feet of height difference, while the FS15/18 allow 100 feet and 50 feet.^{[Source 4]} Daikin's split-system manual sends installers to separate long-line guidelines for runs over 79 feet or rises over 50 feet.^{[Source 2]}
• Additional charge. Units ship charged for a stated line length. Daikin's factory charge covers 15 feet, with ".6 ounces per foot of liquid line" added beyond that.^{[Source 2]} The Mitsubishi MUZ-FS requires added charge for piping beyond 25 feet.^{[Source 4]} NEEP's guide says: "Always use a scale when adding/removing refrigerant."^{[Source 1]}
• Insulation and support. A DOE and New Buildings Institute inspection checklist calls for refrigerant line insulation of R-3 or better, support within 6 feet of the unit, and UV protection outdoors.^{[Source 5]}

Brazing with nitrogen

When copper joints are brazed, dry nitrogen should flow through the pipe so the inside does not oxidize. NEEP: "Any brazed connections shall be completed with dry nitrogen to prevent oxidization."^{[Source 1]} Daikin: "'Sweep' the refrigerant line with nitrogen or inert gas during brazing."^{[Source 2]}

Flare connections

Ductless systems usually use flare fittings. They are tightened to a specified torque. Mitsubishi's MXZ multi-zone manual lists, for example, 10 to 13 ft-lb for 1/4 inch and 25 to 30 ft-lb for 3/8 inch flares.^{[Source 3]} NEEP says to apply refrigerant oil to the flare and tighten to the manufacturer's torque, and to not reuse factory-supplied flares and fittings.^{[Source 1]}

Pressure test

Before evacuation, the system is pressurized with dry nitrogen and watched for pressure loss. Daikin's manual calls for pressurizing "to 450 PSIG" and holding "for 15 minutes (minimum)."^{[Source 2]} NEEP notes the test pressure must stay below the service valve rating, "typically 300-350 PSI" for the systems it covers.^{[Source 1]} Use the number in your model's manual.

Evacuation and vacuum targets

A vacuum pump removes air and moisture from the line set and indoor coil. The depth is read in microns on a micron gauge, and the gauge is then watched with the pump isolated to see whether the vacuum holds.

Source	Target	Hold test
NEEP cold-climate install guide	500 microns or less, triple evacuation	Held for at least 15 minutes in each of three cycles, valved off to check for change[Source 1]
Daikin IOD-4002G (R-410A split)	Less than 500 microns	Isolate and hold 10 minutes or more; a reading that stays below 1,000 microns passes as having no leak; a rise above 2,000 means "a leak is present"[Source 2]
Mitsubishi MXZ-C (2014 manual)	No micron number given; "Vacuumize for more than 15 minutes"	Hold 1 to 2 minutes and confirm the compound gauge reads -14.7 psi[Source 3]

The sources do not agree on a single method. The 500-micron target with a decay test is the more common requirement in the documents reviewed here. Follow the manual for your model.

Refrigerant work, including evacuation and charging, must be done by a technician certified under EPA Section 608.^{[Source 9]}

Charge verification

After the system runs, the charge is checked by the method the manufacturer specifies in the installation manual for that model. NIST found a 30% undercharge could raise annual energy use by as much as 17% to 23%, and a 30% overcharge by 10% to 16%, in its modeling.^{[Source 7]} See Refrigerant charge.

Condensate

Heat pumps produce water in both modes: the indoor coil in cooling, and the outdoor coil during heating and Defrost. Indoors, the DOE/NBI checklist calls for a minimum 1% slope (1/8 inch per foot) on condensate drains, and a secondary drain, auxiliary pan or float shutoff where an overflow would cause damage.^{[Source 5]} NEEP: "Drain shall slope downhill."^{[Source 1]} Outdoors, meltwater from Defrost cycles drains from the base of the unit, which is one reason for the elevation requirements below.

Clearances

Manuals give minimum clearances on every side for airflow and service. Daikin's split manual requires 60 inches above the unit to any overhead obstruction.^{[Source 2]} The DOE/NBI checklist adds a 30 by 30 inch working space on the service side.^{[Source 5]} For ductless wall units, NEEP suggests at least 6 inches from the ceiling as good practice.^{[Source 1]}

Elevation above snow

In snowy climates the outdoor unit must sit above the snow so the coil can breathe and defrost water can drain. The DOE/NBI checklist: "In locations susceptible to snow, outdoor units must be installed on stands above the snow line."^{[Source 5]} NEEP calls for "adequate clearance above historical average maximum snow depth."^{[Source 1]} Daikin's suggested minimum elevation is tied to design temperature:^{[Source 2]}

Design temperature	Daikin suggested minimum elevation
+15°F and above	2.5 inches
-5°F to +14°F	8 inches
Below -5°F	12 inches, or above the annual snowfall level

Mitsubishi's multi-zone manual says: "In areas with heavy snow, please install a canopy, a pedestal and/or some baffle boards."^{[Source 3]} Local snow depth, drifting and roof runoff all matter, so stand height depends on the site.

Airflow, ducts and commissioning

For ducted systems, airflow across the indoor coil and duct leakage affect performance as much as the outdoor unit does.^{[Source 7]} ANSI/ACCA 5 QI, the quality installation standard, covers design, equipment installation, distribution (ducts and airflow) and system documentation and owner education.^{[Source 8]} Ask for the measured airflow and Static pressure readings along with the refrigerant readings.

What you should get at the end

• Pressure test and final vacuum readings, with hold time
• Line set length and any refrigerant added, by weight
• Charge verification readings
• Static pressure and airflow for ducted systems
• Model and serial numbers, the AHRI certificate, warranty registration confirmation and the permit
• A walk-through of the thermostat settings, including backup heat lockouts

Electrical work is covered in Heat pump electrical requirements and permits in Permits and inspections for heat pumps.