How to Check Heat Pump Heating Performance in Cold Weather

Why this matters

"The heat pump is blowing lukewarm air" is the most common winter call on a heat pump, and most of the time the unit is doing exactly what its maker says it should at that outdoor temperature. The rest of the time it is short of charge, short of airflow, or iced, and the strips are covering for it on the electric bill. The only way to tell the two apart is to take the strips out of the picture, read the heat pump alone, and compare what it delivered against the maker's heating table at the same outdoor temperature. This card is that test, laid out as the reading sheet you fill in, line by line, with the reason each line exists.

Lock out the strips first, and know how you will put them back

The strip heat is a 240 V circuit carrying tens of amps, and it is the thing you are about to disable. Use the thermostat's auxiliary heat lockout or emergency setting if it can hold strips off for the test without other changes. Where it cannot, open the strip-heat breaker at the air handler or panel, choosing one that feeds only strips: on many air handlers the first breaker also feeds the blower, and opening it ends the test. Either way, clamp the strip circuit during a heat call and confirm 0 A before you take a reading; a thermostat that still energizes one stage of strips on a temperature drop will add several degrees of rise and make a sick heat pump look healthy.

If you open the air handler to reach a breaker or clamp a conductor, the panel exposes energized 240 V terminals: keep the clamp jaws on insulated conductors, do not reach past terminals, and close the panel before the blower runs. Work near those exposed terminals falls under the shop's electrical safety program per NFPA 70E. The restore step at the end of the sheet is mandatory, and it is a test, not a switch: the customer whose strips you leave off finds out at 3 a.m. at 15 F.

The reading sheet

 HEAT PUMP HEATING CHECK         date/time ______
 Unit: 3-ton, R-410A      strips: OFF (0.0 A)
 ----------------------------------------------
 A outdoor air at coil           30 F
 B coil condition                light frost
 C minutes since defrost end     14 min
 D minutes steady in heating     20 min
 E vapor line (suction)          64 psig  11 F
 F liquid/discharge (head)       300 psig 96 F
 G return air dry bulb           68 F
 H supply air dry bulb           87 F
 I rise (H - G)                  19 F
 J total external static         0.50 in w.c.
 K airflow from fan table        1,150 CFM
 L delivered 1.08 x K x I        23,600 Btu/h
 M maker table at A, 70 F in     24,600 Btu/h
 N delivered / table (L / M)     96 percent
 O compressor amps / RLA         11.2 / 16.0 A
 P strips restored, amps         20.4 A on call

The worked values are one real-shaped visit, carried through below. The pressure-to-temperature conversions are read off the gauge's R-410A scale.

Filling it in, and why each line is there

A. Outdoor air at the coil. Read it in the shade at the coil inlet, not at the thermostat's outdoor sensor and not from a weather app. The maker's heating table is indexed on outdoor dry bulb, and capacity falls steeply with it: a reading taken 5 F off puts you on the wrong row of a table that may drop several percent per row. On this visit, 30 F.

B and C. Coil condition and time since defrost. A coil under heavy frost moves less heat, and the minutes right after defrost termination run cool while the indoor coil recovers. Wait at least 10 minutes after a defrost ends, and if the coil is heavily frosted, let a defrost happen first and start the clock from its end. Line C exists so that anyone reading the sheet can see the reading was not taken in a transient. Here, 14 minutes since termination, light frost only.

D. Steady run time. At least 15 minutes of continuous heating with strips off, longer on a system that just started from cold. Pressures and supply air should hold within a pound or two and a degree for the last five minutes. This visit, 20 minutes.

E and F. Suction and head. In heating, the vapor line at the outdoor unit is the suction and the indoor coil is the condenser. Read both and convert to saturation. The useful checks are relationships, not absolute numbers: suction saturation sits well below outdoor air (here 11 F against 30 F, a 19 F split), and head saturation sits several degrees above the supply air leaving the indoor coil (96 F against 87 F). What is normal for both at a given outdoor temperature is in 'Heat Pump Cold-Weather Operation Reference'; the maker's heating table often lists expected pressures too.

G, H and I. Air temperatures and rise. Return in the return drop before the filter, supply in the trunk past the coil and out of line of sight of any strip element. With strips locked out, the whole rise belongs to the heat pump. 87 minus 68 is 19 F.

J and K. Static and airflow. Measure total external static per 'How to Measure Total External Static Pressure on a Furnace' (the same ports work on an air handler) and read the airflow for that static and speed from the maker's fan table, per 'How to Estimate Blower Airflow From Static and the Fan Table'. At 0.50 in w.c. on this speed, the table gives 1,150 CFM. Treat that as an estimate good to roughly 10 percent; it is the weakest number on the sheet.

L. Delivered capacity. Sensible heat equals 1.08 times CFM times rise, where 1.08 holds for standard air near sea level; the derivation and the altitude correction live in 'Temperature Rise and the Sensible Heat Airflow Formula'. Here: 1.08 times 1,150 is 1,242, times 19 is 23,598, rounded to 23,600 Btu/h. Check from the other direction: 23,600 divided by 1,242 gives back 19.0 F.

M. The maker's number. From the expanded heating performance table for this outdoor model and indoor match, at 30 F outdoor and 70 F indoor, at the table's rated airflow: 24,600 Btu/h. Note the two small mismatches rather than hiding them. The table is at 70 F return and you measured 68 F, which slightly favors the measured unit, since a cooler indoor coil condenses at a lower pressure. The table is at 1,200 CFM and you have 1,150, which slightly disfavors it. Both are inside the noise of line K.

N. The ratio. 23,600 divided by 24,600 is 0.96, so 96 percent of the table. The shop default: within 10 percent of the table passes; 10 to 15 percent low gets a second reading 15 minutes later; more than 15 percent low on two readings is a performance problem to diagnose. The 10 percent band is not generosity. It is the measurement: a one-degree error on a 19 F rise is about 5 percent, and the fan-table airflow carries about 10 percent on its own.

O. Amps. Compressor amps against nameplate RLA (11.2 A against 16.0 A here). Where the maker's table lists input power, a volts-times-amps estimate is only a rough comparison because the power factor is unknown; use it to catch a compressor drawing far more or far less than the table implies, not to compute efficiency.

P. Restore. Close the strip breaker or clear the thermostat lockout, call for auxiliary heat, and clamp the strip circuit: 20.4 A on this kit's first step, with the supply air climbing past 87 F within a minute. A strip circuit you did not see draw current has not been restored, it has been switched. Then return the thermostat to the customer's normal settings and write the strip amps on the ticket.

What this visit concluded

At 30 F, this heat pump delivered 96 percent of its published capacity, and that published capacity is about two thirds of its 36,000 Btu/h rating at 47 F (24,600 divided by 36,000 is 0.68). The customer's "lukewarm" 87 F supply air is what this unit makes at this temperature, with the strips designed to carry the rest of the load below the house's balance point. That is a conversation, not a repair: show the customer the table row and the measured row side by side. See related: 'Heat Pump Cold-Weather Operation Reference' for how to explain supply temperatures and balance point to a homeowner.

What a failing sheet looks like

Same house a year later, after a line set was disturbed by a deck build. Same 30 F, same 20 minutes steady, strips off at 0.0 A.

• E: 48 psig, about 0 F saturation, a 30 F split against outdoor air instead of 19 F.
• F: 255 psig, about 86 F saturation.
• H and I: supply 80 F, rise 12 F.
• L: 1.08 times 1,150 times 12 is 14,904, about 14,900 Btu/h.
• N: 14,900 divided by 24,600 is 61 percent. Second reading 15 minutes later, 62 percent.

Two readings more than 15 percent low, so it goes to diagnosis. Read the directions before guessing. Low suction, low head and low rise together point at too little refrigerant moving, which is undercharge or a restriction. Low indoor airflow would push head up in heating, not down, because the indoor coil is the condenser and less air across it raises condensing pressure; so this sheet argues against an airflow fault even before the static is rechecked. The diagnosis itself, including where the deck crew pinched the line, is not this card's job; the sheet's job was to prove the heat pump was the problem and to point the direction.

What changes the test

Variable-speed and inverter heat pumps. Capacity at a given outdoor temperature depends on what speed the compressor chose, and a lightly loaded house may run it at a fraction of maximum. Use the maker's service mode to hold the compressor at the speed its table is published for, or the comparison in line N means nothing.

Very cold weather near or below the unit's low-ambient limit. Some units lock out the compressor below a set outdoor temperature by design. A cold-climate unit's table may continue well below that. Check the table covers line A before comparing.

No published heating table for the match. Then line M does not exist, and the test falls back to relationships: suction-to-outdoor split, head-to-supply split and rise against what 'Heat Pump Cold-Weather Operation Reference' gives as normal. Say on the ticket that no capacity comparison was possible.

References

• Maker's expanded heating performance data for the outdoor model and indoor match; installation instructions for the strip kit.
• See related: 'Heat Pump Cold-Weather Operation Reference'; 'Temperature Rise and the Sensible Heat Airflow Formula'; 'How to Estimate Blower Airflow From Static and the Fan Table'.
• See related: 'How to Set Up and Test Heat Pump Defrost on an Install'; 'HVAC Superheat and Subcooling Method Reference'.
• NFPA 70E (current edition), Article 130, for work near the energized strip-heat terminals.