How to Wire a Heat Pump With an Air Handler and Heat Strips

Why this matters

A split heat pump with electric strips has three pieces of equipment answering one thermostat, and the low-voltage wiring is where they agree or argue. Wire it wrong and the symptoms are confusing rather than obvious: heat on a cooling call, strips running every hour of the winter, a defrost that blows cold air into the house, or emergency heat that runs the compressor anyway. Most of those faults pass a quick "it cools, it heats" check on install day and surface weeks later as a high electric bill or a no-heat call in the first frost. This article lands the wires and then proves each of the five operating modes separately. Terminal letters and their meanings are in the shelf's reference card (See related: Thermostat Wiring Reference); this is the job of applying them to this equipment combination.

Start state

Safety first. The air handler carries 240 V to the strip heaters and blower, the outdoor unit carries 240 V to the compressor and fan, and they are on separate circuits. Before touching any low-voltage terminal, open both disconnects, lock and tag them, and prove both units dead with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices at 29 CFR 1910.333(b)(2). Low-voltage wiring with power on also costs you a blown control fuse the first time R touches C.

What you need before you start:

• The wiring diagrams on the air handler panel, the outdoor unit panel, and the strip kit, plus the thermostat's installer guide.
• Thermostat cable with enough conductors: commonly 18/8 from thermostat to air handler (seven needed plus a spare) and at least 18/5 from air handler to outdoor unit (five needed; an 18/6 or larger leaves a spare).
• The strip kit installed in the air handler with its own supply circuit already in place and matched to the kit's nameplate. Sizing that circuit is covered in Heat Strip Kit Sizing and Circuit Field Card; running a new 240 V circuit goes to the electrician.
• A meter for 24 V checks, a clamp meter, and supply and return thermometers.

One fact to find before you land a single wire: which way the reversing valve energizes. Most makers energize the valve in cooling (the O terminal); some energize it in heating (B). The outdoor unit's diagram or label says which. Everything else in this job can be corrected in a minute; a thermostat configured for the wrong one produces heat on every cooling call and cold air on every heating call, and on install day in mild weather nobody may notice.

The wiring

 THERMOSTAT       AIR HANDLER        OUTDOOR UNIT
 heat pump mode   terminal strip     defrost board

   R -------------- R -------------------- R
   C -------------- C -------------------- C
   G -------------- G
   Y -------------- Y ...splice........... Y
   O -------------- O ...splice........... O
   AUX ------------ W1 <------------------ W2
   E -------------- W1     defrost heat
                    |      output, lands
                 jumper    on air handler W1
                    |
                    W2   (second strip element)

What each conductor does, on this combination:

• R and C come from the air handler's transformer and go on to the outdoor unit, so the defrost board and contactor run on the same 24 V supply.
• Y starts the compressor through the outdoor contactor. At the air handler it passes through as a splice; many air handlers have no Y function of their own.
• O energizes the reversing valve in cooling on this example's equipment. On a B-type unit, the same conductor lands on B and the thermostat is configured for B.
• G runs the blower on fan and on every heat pump call.
• AUX (labeled W2 or AUX on most heat pump thermostats) calls the strips as a second stage when the compressor cannot keep up.
• E calls emergency heat: strips only, compressor locked out.
• The defrost board's heat output (labeled W2 on this board; boards vary, so read the diagram) lands on the air handler's W1 alongside AUX, so the strips temper the supply air while the indoor coil runs cold during defrost.
• The W1 to W2 jumper makes both strip elements run together on any heat call. Leaving it off and using an outdoor thermostat or a separate thermostat stage to bring in W2 is the alternative when the kit and the climate call for staged strips.

The steps

1. Land the outdoor conductors. R, C, Y, O and the defrost heat output, five conductors on an 18/5. Tighten every screw onto the stripped copper, not onto insulation. Skip the defrost output (the one most often left off because a five-conductor run had no spare) and every defrost cycle blows cold air into the house.

2. Land the air handler conductors. R, C, G, W1 with the jumper to W2 per the kit diagram, and the Y and O splices to the outdoor run. Wire nuts on splices go inside the air handler's control box, not in the return plenum where air movement and vibration work them loose.

3. Land the thermostat. R, C, G, Y, O (or B), AUX and E. Do not jumper AUX to E at the thermostat unless its installer guide calls for it; many thermostats drive both from internal logic.

4. Configure the thermostat before the first call. Equipment type heat pump, reversing valve O or B to match the outdoor unit label you read before starting, auxiliary heat electric, any compressor lockout and auxiliary lockout temperatures the shop uses for this climate, and emergency heat enabled. Skip it and the thermostat runs its factory defaults, which may be gas furnace or the opposite valve.

5. Restore power and test each mode in turn. Close the covers you can, remove your locks and tags, close both disconnects, and stand clear of the outdoor fan guard. Use the thermostat's installer test mode to call one output set at a time, and do not move on until the mode in front of you passes. The tests are in the worked example below.

6. Force one defrost by the method in How to Set Up and Test Heat Pump Defrost on an Install, and confirm the strips come on during it.

Worked example: a 3-ton heat pump with a 10 kW strip kit

A 3-ton split heat pump, an air handler with a 10 kW kit of two 5 kW elements jumpered to run together, on a single 240 V supply circuit. The outdoor unit's label says the valve is energized in cooling, an O-type unit. Outdoor temperature at test time is 58 F.

Cooling test (Y, G, O). Within two minutes the vapor line at the outdoor service valve was warm, not cold, and the supply register was warming. The tech went back to the thermostat: its factory default was B. In B mode the O/B terminal is energized in heating and dead in cooling, so on a cooling call the valve sat de-energized, which on an O-type unit is the heating position. Changed to O. Re-test: 24 V present between O and C at the outdoor board on the cooling call, vapor line cold, return 75 F, supply 57 F, an 18 F split. Pass.

Heating test (Y, G). O de-energized, valve shifted with its whoosh, vapor line hot, return 72 F, supply 96 F. Pass.

Auxiliary test (Y, G, AUX). The clamp was placed around one leg of the strip supply inside the air handler, on a lead feeding the elements rather than the blower, with power locked out, then power restored and the display read from outside the cabinet with the door closed as far as the leads allow. Expected current comes from the kit's resistance, not from its kW alone: at rated 240 V, 10,000 W is 41.7 A, and the elements' resistance is 240 squared divided by 10,000, 5.76 ohms. Measured voltage at the air handler was 236 V, so the expected current is 236 / 5.76, about 41.0 A. The clamp read 40.6 A, about 1 percent under expected, which is inside the elements' manufacturing tolerance. Both elements are running. (One element alone would have read about half.) Pass.

Emergency heat test (E). Strips at 40.6 A, blower running, and the outdoor contactor open with 0 V between Y and C at the outdoor board. That last reading is the point of the test: emergency heat that still runs the compressor is not emergency heat. Pass.

Forced defrost. Valve shifted to the cooling position, outdoor fan stopped, and the clamp on the strip circuit read 40.6 A through the cycle, fed by the defrost board's output into W1. Supply air stayed warm. Cycle terminated on its own, heating resumed. Pass.

Clamp removed with the air handler locked out again, covers on, power restored, thermostat returned to the customer's schedule.

What the cooling test caught. A B setting on an O-type unit is wrong in both modes: the valve sits de-energized on cooling calls and energizes on heating calls, so the unit runs backward either way. It escapes when the only check is a quick heat call with the strips on, because the strips warm the supply air enough to pass a hand at the register while the compressor is actually cooling. Testing each mode with the outputs separated, and reading the vapor line rather than the register, is what catches it.

What changes the job

A communicating system. Matched communicating equipment uses two to four conductors and a data bus; the letter-by-letter wiring above does not apply, and configuration happens at the thermostat or control during commissioning. Follow the maker's commissioning sequence and do not mix in a conventional thermostat.

Dual fuel. With a gas furnace instead of strips, the furnace replaces the strips as auxiliary heat and the thermostat or a fossil fuel kit must lock the compressor out while the furnace fires, so the coil never sits downstream of the heat exchanger at high head pressure. That is a different wiring scheme and a different decision (See related: Heat Strip Vs Dual Fuel Cold Climate Decision Matrix).

Staged strips. On larger kits the shop may want only half the strips in mild weather. Remove the W1 to W2 jumper, land W2 from an outdoor thermostat or a second auxiliary stage, and test each element on its own call; expect about half the full-kit current on the first stage.

How to verify you got it right

The ticket shows a reading for each of these, not a checkmark:

• Reversing valve type found on the outdoor unit label, and thermostat configured to match.
• Cooling: valve energized (24 V O to C on an O-type unit), split recorded.
• Heating: valve de-energized, supply temperature recorded.
• Auxiliary: strip current measured against the current expected at the measured voltage.
• Emergency: strip current present and 0 V at the outdoor Y.
• Defrost: strips proven by current during a forced cycle that terminated on its own.

References

• Air handler, outdoor unit and strip kit wiring diagrams; thermostat installer guide
• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120
• 29 CFR 1910.333 Selection and use of work practices
• See related: Thermostat Wiring Reference; How to Set Up and Test Heat Pump Defrost on an Install
• See related: Heat Strip Kit Sizing and Circuit Field Card; Heat Strip Vs Dual Fuel Cold Climate Decision Matrix