The Heat Pump That Blew Cold Air in Every Defrost

The complaint, as it reached the third tech

A homeowner with a 3-ton heat pump and an air handler installed in October called in January for the second time: "Every hour or so, cold air blows out of the vents for a few minutes. It is freezing in here when it does it." The first callback ticket, from a December visit, read "system heating normally, 24 F rise, customer educated on heat pump supply temperatures. No fault found."

The third tech did not start at the equipment. He started at the paperwork, because a complaint that one competent tech already could not reproduce is usually a complaint that only exists under conditions the first visit did not have. This case is reconstructed from the record the shop already held, and the gaps in that record are as much the finding as the loose wire.

What the record said, read backwards

The December callback. Visit logged at 1:40 pm, outdoor temperature on the ticket 52 F and sunny. The tech measured a 24 F rise in heating, checked the thermostat, and found nothing. Every word of that ticket is true, and it could not have found this fault. A time-temperature defrost board only starts a defrost when its coil sensor reads below its initiation temperature (near freezing) and its timer has run out. At 52 F in sun the outdoor coil never frosts, so the board never called a defrost during the visit. The complaint was not present to be reproduced.

The customer's own words. "Every hour or so" and, on the second call, "mostly on wet, cold mornings." That is a timer interval and a frosting condition, described by someone who does not know either term. It points at defrost, not at the heat pump's normal heating.

The install sheet. The commissioning sheet had lines for charge, static, heating rise, strip heat amps per element, and "defrost board set to 60 min." It had no line for a forced defrost with a supply temperature reading. The sheet recorded what the board was set to and never recorded what the system did when the board acted.

The install photos. The shop's install checklist requires a photo of each low-voltage termination. The outdoor unit photo showed red, blue, yellow and green conductors of an 18/5 cable on R, C, Y and O, and the white conductor folded back along the cable jacket, unconnected. The air handler photo showed the thermostat's white on the strip heat terminal and the outdoor cable's white folded back the same way. Nobody had looked at either photo after it was uploaded.

The mechanism

During defrost the board shifts the reversing valve to cooling and stops the outdoor fan, so hot discharge gas goes to the outdoor coil and melts the frost. The indoor coil becomes the evaporator for those minutes. The indoor blower keeps running, so whatever air crosses that cold coil goes straight to the registers.

That is why every heat pump defrost board has an output, labeled W2 on this board and W on some others, that energizes during defrost and is meant to be wired back to the air handler's strip heat control. It brings on a stage of strip heat to temper the air while the coil is cold. Without it, the house gets cooled for the length of every defrost.

THERMOSTAT       AIR HANDLER          OUTDOOR UNIT
R -------------- R ------------------ R
C -------------- C ------------------ C
Y -------------- Y ------------------ Y
O -------------- O ------------------ O
W2/AUX --------- W1 strip relay
                 W1 ...  X  ......... W2 defrost out
                     white not landed
                     either end
G -------------- G

With W2 landed, the defrost output and the thermostat's auxiliary call both reach W1, the first strip stage. With it folded back, only the thermostat can bring on strips, and the thermostat has no idea a defrost is happening.

Proving it on site, safely

The tech planned the visit for an early morning in the low 30s with high humidity, the condition the customer described.

Isolation. Two circuits feed this system: the outdoor unit's disconnect and the air handler's circuit, which carries the 240 V strip heat. For any work inside a cabinet, both were switched off, locked and tagged, and proven dead with a tester checked on a known live source before and after, per NFPA 70E Article 120 in the current edition and the work practices at 29 CFR 1910.333(b)(2). The strip heat terminals inside an air handler sit inches from the low-voltage board, and a thermostat cable job is a common way to get bitten by a circuit you forgot fed the cabinet.

The before reading. Before any wiring work, the system ran with panels on and the white still folded back, to catch a natural defrost. One came 40 minutes after arrival. Return air read 68 F. Supply air read 52 F within two minutes of the reversing valve shifting, and stayed there for the six minutes the defrost lasted. That is the cold draft, measured: 68 minus 52 is 16 F below room temperature at the registers.

Landing the wire. Both circuits locked out and proven dead as above. The white was stripped and landed on W2 at the outdoor board and on W1 at the air handler, sharing the terminal with the thermostat's auxiliary conductor, matching the maker's wiring diagram for this pairing (see related: How to Wire a Heat Pump With an Air Handler and Heat Strips). Terminal screws checked for bite on the conductor, not the insulation.

Restore. Panels on before power. Air handler circuit first, then the outdoor disconnect, with the tech standing to the side of the outdoor unit rather than in front of the coil, because the fan starts without warning on a power-up with a heating call. The crankcase heater and compressor delay on this unit meant a few minutes before the compressor started, which is normal.

Forced defrost, the finish test

The board has test pins for exactly this. Following the manual, the tech shorted the test pins for the time the manual gave to force a defrost with the system running in heat. Forced defrost is a test the maker built in; it does not involve blocking airflow, jumping a safety or touching a combustion or pressure-relief path, and nothing else should be used to force one.

Readings during the forced defrost:

• Reversing valve shift and outdoor fan stop within seconds of the pins being shorted.
• Strip heat amps: one 5 kW element drawing current on the clamp, confirming W1 energized from the board.
• Supply air: 68 F, against 52 F before the wire was landed.
• Termination: on the board's coil sensor, with the valve shifting back and the outdoor fan restarting.

The 68 F is worth understanding, because the customer will feel it. Indoor airflow on this system measured about 1,000 cfm. A 5 kW element at its 240 V rating adds 5,000 times 3.412, about 17,060 Btu/h, and 17,060 divided by 1.08 times 1,000 is a rise of about 16 F. The air leaves the cold indoor coil near 52 F and picks up about 16 F across the element, landing near 68 F. Defrost tempering is designed to make the supply air neutral, not warm. At a supply voltage under 240 V the element puts out less and the air lands a degree or two cooler. If the customer had expected 90 F air during defrost, the conversation would need to explain that before the next call.

The tech followed the defrost test procedure in How to Set Up and Test Heat Pump Defrost on an Install for the remaining checks: initiation temperature setting, timer interval, and termination, all recorded.

Showing the customer

The customer stood outside with the tech for a second forced defrost. The tech pointed out what they would hear and see from now on: the outdoor fan stopping, a whoosh as the reversing valve shifts, a cloud of steam off the coil that looks alarming the first time, a few minutes of the compressor running with the fan stopped, then a second whoosh and the fan restarting. Inside, the registers would blow air near room temperature for those minutes instead of cold.

That conversation is part of the fix. A customer who sees steam pouring off the outdoor unit at 6 am with nobody having warned them makes an emergency call, and the next tech starts the hunt again.

What the shop changed

Three record gaps let a single folded wire survive two visits.

The commissioning sheet now carries a forced-defrost line: supply air temperature during defrost, strip amps during defrost, and termination observed. A setting written on a sheet is not a test.

Callback visits on a timed or conditional complaint now require the tech to reproduce the condition or say in the ticket why it could not be reproduced. "No fault found" at 52 F in sun on a defrost complaint now reads as "condition not present", which is honest and tells the next tech what to do differently.

Install photos are reviewed by the lead before the job closes, termination by termination. The photo that would have caught this was in the system for three months.

Where the answer flips. Some communicating systems handle defrost tempering through the thermostat or the indoor control rather than a dedicated board output, and on a dual-fuel system the furnace, brought on through the dual-fuel control's own wiring, tempers defrost instead of strip heat. In those cases a folded-back white is not a fault and the manual's wiring diagram is what decides. Here the maker's diagram for this outdoor unit and air handler showed the W2-to-W1 conductor, and its absence was the fault.

References

• See related: How to Set Up and Test Heat Pump Defrost on an Install
• See related: How to Wire a Heat Pump With an Air Handler and Heat Strips
• See related: Thermostat Wiring Reference
• NFPA 70E Article 120, current edition; 29 CFR 1910.333(b)(2)