How to Set Up and Test Heat Pump Defrost on an Install

Why this matters

A new heat pump installed in October will not see a real frost load until the first wet night near freezing, usually after the crew has moved on. If the defrost interval is wrong for the climate or the coil sensor is clipped in the wrong place, the first sign is a solid block of ice on the outdoor coil, a no-heat call, and a customer who has been running on strips for a week. Defrost is the one heat pump function you cannot observe on a mild install day unless you force it. This card sets the board up for the site and forces one complete cycle before you leave.

Before you touch the outdoor board

The outdoor unit is 240 V and the board sits next to the contactor and the fan capacitor. Setting the interval pins happens with the unit dead: open the disconnect, lock and tag it, and verify absence of voltage at the contactor line side with a tester proven on a known live source before and after (NFPA 70E Article 120, current edition; 29 CFR 1910.333(b)(2)). Discharge the run capacitor with an insulated resistor tool before reaching past it.

Forcing defrost happens with the unit live, which is the part of this job that is actually hazardous. You will reach to the test pins on an energized board, so use an insulated jumper or a jumper with insulated alligator clips placed with the unit off and then powered, keep the other hand out of the cabinet, and stay clear of the fan guard: the outdoor fan stops at the start of defrost and restarts without warning at the end. Forcing defrost also drives the system to a state it would not choose, so watch the head pressure through the cycle (step 5).

What you need: the outdoor unit's installation instructions and defrost board sequence, gauges or a pressure probe on the liquid line (it carries head pressure in both modes) and, if you want both sides, the vapor line, a clamp meter for the indoor strip heat circuit, and a supply-air thermometer.

The board

 OUTDOOR DEFROST BOARD (typical layout)

  [30][60][90]  <- interval pins, one jumper
     |
  [TEST] [TEST] <- short to force/speed timer
  [DF1]  [DF2]  <- coil sensor plug
  [O] [W2] [Y] [R] [C]  <- 24 V terminals

 OUTDOOR COIL, sensor on bottom circuit:

   top    =========================
          =========================
          ==========  coil  =======
   bottom ==[S]====================
             ^
             sensor clipped to feeder tube
             or return bend, coldest circuit

Pin labels, the interval values offered and the sensor plug name vary by maker; read the board's own legend, not this one.

The steps

1. Set the defrost interval for the site. Time-temperature boards start a defrost only when two things are true: the coil sensor reads cold enough to allow frost, and the compressor has run the selected number of minutes in heating since the last defrost. The interval pin sets that run time, commonly 30, 60 or 90 minutes, with some boards offering 120. Read the maker's guidance for the factory setting and when to change it. The usual logic: humid climates with long stretches just above and below freezing frost quickly and want a shorter interval; dry cold climates frost slowly and run more efficiently on a longer one.

On the example: a 3-ton heat pump on a coastal site that spends much of the winter between 30 and 40 F with high humidity. The board shipped at 90 minutes. The maker's instructions recommend a shorter interval for high-humidity regions, so the jumper moves to 60. The ticket records the change and the reason, because the next tech who sees 60 will otherwise "fix" it back.

What breaks if skipped: at too long an interval in a wet climate, the coil frosts solid between defrosts, the last defrost cannot clear it, and ice builds from the bottom up across several cycles. At too short an interval in a dry climate, the unit defrosts a coil that has no frost, wasting the strip heat that tempers every defrost.

2. Confirm the coil sensor position and contact. The sensor tells the board the coil is cold enough to need defrost and warm enough to stop. It belongs where the maker puts it, which is typically on a feeder tube or a return bend on the lowest circuit, because the bottom of the coil frosts first and clears last. Check it is clipped tight to bare copper, the clip is the maker's, and the sensor lead is routed away from the compressor and discharge line.

On the example: the sensor was found zip-tied to the vapor header halfway up the coil, moved there in shipping or by a previous handler. Back to the bottom feeder tube shown in the maker's diagram, with its clip.

What breaks if skipped: a sensor mid-coil reads a warmer tube than the bottom circuit. It terminates defrost when the middle is clear and the bottom inches are still ice, and that ice grows every cycle. This is the single most common cause of a heat pump that "defrosts fine" and still ices up from the bottom.

3. Run heating to a steady state. Restore power, call for heat with the auxiliary strips available, and run at least 10 minutes. Note the head pressure at the liquid line, outdoor ambient and supply-air temperature. You are establishing the baseline the defrost cycle will interrupt.

On the example: 41 F outdoor, light drizzle, head pressure steady, supply air 94 F with return at 68 F.

4. Force a defrost with the test pins. On most boards, shorting the test pins compresses the interval timer so the board enters defrost within seconds rather than minutes. Whether it will enter with a warm coil depends on the board: some force a defrost regardless, some require the sensor to read cold and will otherwise run a short forced cycle and terminate as soon as the jumper comes off. Follow the maker's test procedure for that board, including whether the jumper stays on through the cycle or comes off after initiation. Never jumper the high-pressure switch or any safety to make the test run; if a safety opens during the test, the test has found something.

5. Watch the sequence. In defrost the board does four things in a set order, and you should see each one:

• Reversing valve shifts to cooling-mode position. You hear the whoosh, and the vapor line drops to suction pressure and goes cold, because it is now the suction line. The outdoor coil becomes the condenser: head pressure at the liquid line sits low against the frosted coil at first and climbs as the ice clears.
• Outdoor fan stops, so the heat goes into the coil and not into the air.
• Auxiliary heat comes on through the board's W2 output to the indoor unit, so the house does not get a blast of cold air from an indoor coil now acting as an evaporator. Clamp the strip circuit to confirm current.
• Termination: when the sensor warms past the board's termination temperature or the board's maximum defrost time runs out (commonly about 10 minutes, on the board legend), the valve shifts back, strips drop, and the fan restarts, usually after a short delay so the coil can drain.

Watch the head pressure at the liquid line while the fan is off. With the outdoor coil as condenser and no fan, head pressure rises faster than in normal cooling. Stop the test by removing the jumper and opening the disconnect if it approaches the unit's high-pressure cutout setting from the nameplate or manual.

On the example: the valve shifted within 6 seconds of the short, the vapor line fell to suction, head pressure climbed steadily, the fan stopped, and the strip circuit read 19.8 A, which on this 10 kW kit is one of its two 5 kW elements, the first step its wiring diagram brings on in defrost (5,000 W divided by 240 V is 20.8 A at rated voltage, and a little less at the 230 V measured on site). Supply air dipped from 94 F to 86 F and held. With the sensor reading a coil at 41 F ambient, the board terminated on temperature after 2 minutes 40 seconds, valve back, strips off, fan restarting 30 seconds later. Steam rolled off the coil as it started.

6. Remove the jumper and confirm normal operation. Take the test jumper off and confirm the board is back to its normal interval, either by its status light pattern or by its documented behavior. Run heating 10 more minutes and confirm head pressure and supply air return to the baseline from step 3.

What breaks if skipped: on many boards a test jumper left on keeps the timer compressed, so the unit defrosts every few minutes for the life of the jumper. The customer sees a cloud of steam and hears the valve shift all evening, and the strip heat runs during every one of them.

7. Restore and close. Open the disconnect, verify dead again, close the control panel with all screws, then restore power and confirm the unit restarts in heating. Record on the ticket: interval setting and reason, sensor location, time to initiate, time in defrost, how it terminated, strip amps during defrost.

Finish test

The install passes when all five are observed and recorded: the valve shifted into defrost, the outdoor fan stopped, auxiliary heat was proven by current on the strip circuit, the cycle terminated on its own (temperature or maximum time, and you know which), and heating resumed at the baseline supply temperature with the test jumper removed. A cycle you forced but did not watch to the end has not been tested, because termination is the half that fails.

What changes the procedure

Demand defrost boards. Some boards decide when to defrost from coil temperature against ambient, or from changes in the system's own readings, rather than from a fixed interval. There are no interval pins to set; the setup is sensor placement plus the forced test, and the maker's procedure governs the test.

Inverter heat pumps. Defrost is run by the outdoor inverter board's own logic, usually with the compressor slowed rather than cycled, and is typically forced through a service mode or button rather than pins. The observations in step 5 still apply. See related: 'A2L Refrigerant Handling (R-454B and R-32)' where the unit is on an A2L refrigerant, for the maker's service-mode cautions.

No strip heat installed. Strip-less installs either temper defrost with a furnace or not at all. On a dual-fuel system the furnace usually tempers defrost; confirm from the control's instructions how it hands back at termination, because a heat pump returning to heating while the furnace still fires upstream of the indoor coil drives head pressure up. Watch that hand-back in step 5 the same way you would watch the strips drop.

When it does not work

If the board will not enter defrost, terminates instantly, or ices up later in the season, the diagnosis is a separate job: see related 'Defrost Control Diagnosis on a Heat Pump'. Do not re-pin the interval or move the sensor again as a fix without that diagnosis.

References

• Outdoor unit installation instructions and defrost board legend (interval options, test procedure, sensor location, termination temperature and maximum time).
• See related: 'Defrost Control Diagnosis on a Heat Pump'; 'How to Check Heat Pump Heating Performance in Cold Weather'; 'Heat Strip Kit Sizing and Circuit Field Card'.
• NFPA 70E Article 120 (current edition) and 29 CFR 1910.333(b)(2) for proving the outdoor unit dead before setting pins.