Heat Pump Supply Air Temperature in Heating Mode

By our estimate from Goodman's heating data, a 3-ton GSZ16 heat pump with its listed ASPT37C14A coil and TXV warms the air about 27 to 31°F at 47°F outdoor and 17 to 19°F at 17°F. That's roughly 97 to 101°F and 87 to 89°F supply air from 70°F return. The math uses Goodman's 35,000 BTU/h at 47°F and 22,000 BTU/h at 17°F for that match, at 1,060 to 1,200 CFM, two airflows from Goodman's cooling data. On the GSZ16, output falls as it gets colder outside, so at a fixed airflow the rise across the coil shrinks.

So the expected vent temperature depends on two things, the unit's heating output at today's outdoor temperature and the airflow it moves. Our AC temperature split calculator works the cooling side of that math from readings or from SHR and CFM per ton. It doesn't run heating mode, so this guide shows the heating check by hand with the same sensible heat equation.

What supply air temperature should a heat pump deliver in heat mode?

Return temperature plus heating output over 1.08 times the CFM. By our estimate, at each size's rated cooling airflow and the middle row of its expanded cooling tables, Goodman's GSZ16 outputs give 26 to 33°F of rise at 47°F outdoor, 14 to 20°F at 17°F, and 9 to 15°F at 5°F for every size from 1.5 to 5 tons. From 70°F return, that's about 96 to 103°F, 84 to 90°F and 79 to 85°F supply.

The outputs come from the heating data in the GSZ16 spec sheet, pages 18 and 19, each outdoor unit with its listed coil and TXV. Goodman says the heating data is for "nominal CFM" but doesn't print that airflow. So we ran two airflows from the same brochure: the one each match is rated at in the cooling performance data on page 20, and the middle airflow row of its expanded cooling tables. The rises are our estimate; where the two airflows differ, the table shows the range.

Outputs are BTU/h. Goodman's note under the heating tables says the data is "for nominal CFM and 70 degree indoor dry bulb," so 70°F return is the baseline for the supply temperatures above. If you go by a flat 15 to 25°F rise, you'll miss the GSZ16 estimates at 47°F and at 5°F. They run above 25°F at 47°F and below 15°F for most sizes at 5°F.

How do you calculate the expected rise by hand?

Divide the heating output in BTU/h by 1.08 x CFM. The 3-ton GSZ16 with its listed coil makes 25,600 BTU/h at 30°F outdoor, so at the 1,060 CFM it's rated at in cooling the rise is 25,600 / (1.08 x 1,060) = 22.4°F and the supply air about 92°F from 70°F return. The colder it gets outside, the less the GSZ16 puts out, and at a fixed blower airflow the rise drops with it.

It's the sensible heat equation contractors use for cooling splits and furnace rises.

ACCA HVAC Blog, The Physics of Heat and Air Flow § Sensible heat equation

"BTU/hr = 1.08 x cfm x ΔT" - Allison Bailes, hvac-blog.acca.org

Bailes explains the 1.08 as 0.075 lb per cubic foot times 0.24 BTU per pound per °F times 60 minutes, and warns that "You can't use the equation below up high in the mountains or at temperatures far from the air you're breathing right now." Above a few thousand feet the air is thinner than the 0.075 lb per cubic foot behind the 1.08, so the constant drops and the same output gives a bigger rise. The FAQ on our HVAC CFM calculator works the 5,000 ft case. Solved for the rise, the equation reads rise = BTU/h / (1.08 x CFM). Here's the check on the Goodman 3-ton:

1. Read the heating output at the outdoor temperature from the spec sheet: 35,000 BTU/h at 47°F, 25,600 at 30°F, 22,000 at 17°F.
2. Get the airflow the blower is actually moving. Without a measurement, use the 1,060 CFM rating point and the 1,200 CFM middle row from Goodman's cooling data as a working range.
3. Divide the output by 1.08 x CFM, which gives 25,600 / (1.08 x 1,060) = 22.4°F at 30°F outdoor.
4. Add the return temperature to get the supply air, 70 + 22.4 = 92.4°F.

Goodman also prints a T/R row in the heating tables, which reads as temperature rise. By our back-solve, it equals output divided by 1.08 x 1,000 CFM for every size at 47°F and below, within about 1%. On the 3-ton at 17°F, for example, 22,000 / (1.08 x 20.4) = 999 CFM. Above 47°F it drifts to about 1,040 CFM. So read it as a 1,000 CFM normalization and work out the rise for each install from its own airflow. A 5-ton moving 1,850 CFM runs about 30°F at 47°F, 46% under the 55.6°F its T/R row prints, and a 1.5-ton on 600 CFM runs well above its row.

Use the output for the coil match on the job. Each GSZ16 heating table names its outdoor unit, indoor coil and TXV, and its numbers belong to that match.

How much does airflow change the supply temperature?

The rise is inversely proportional to CFM for a given output. Take the same 3-ton heat pump from the 1,060 CFM it's rated at in cooling up to 1,200 CFM, the middle row of Goodman's cooling tables and 400 CFM per ton, and the rise at 30°F outdoor drops from 22.4°F to about 19.8°F and supply air from 92.4°F to about 89.8°F, assuming the output stays near 25,600 BTU/h. Low airflow does the opposite.

That's why two identical heat pumps can read more than 6°F apart at the same outdoor temperature, one at 900 CFM and one at 1,200. The arithmetic for 30°F outdoor and 70°F return:

Holding output fixed is our simplification. A real heat pump's output also shifts with airflow, and Goodman's heating table doesn't show by how much, so a manufacturer table at the actual CFM is the better source when one exists. To confirm what the blower is moving, measure total external static pressure. The static pressure calculator checks the measured TESP against the rating, and the blower table gives CFM at that TESP.

Is a 12°F rise at 45°F outdoor normal?

Not for any GSZ16 size at the two cooling airflows in the table above. At 45°F the estimates run from 25.5°F to 31.4°F across 1.5 to 5 tons, so a 12.4°F rise is about 40% to 49% of what the table output supports. For a 3-ton on 1,060 CFM it works out to about 14,200 BTU/h of heat against Goodman's listed 33,800 BTU/h, about 42%.

The reading comes from a homeowner on r/hvacadvice: 45°F outside in Florida, 69.3°F at the filter slot, 81.7°F in the supply plenum just past the coil, after the house lost 3.5°F over two hours of runtime. We don't know the brand or size, so the Goodman outputs serve only as a yardstick for this unit.

1. The rise is 81.7 - 69.3 = 12.4°F.
2. At an assumed 1,060 CFM, the heat delivered is 1.08 x 1,060 x 12.4 = 14,196 BTU/h.
3. Goodman lists 33,800 BTU/h for the 3-ton at 45°F, which at 1,060 CFM is a 29.5°F rise. The reading is 42% of that.
4. Even if the system moved 2,000 CFM, which would be 667 CFM per ton on a 3-ton, the heat would be 26,784 BTU/h, still 21% short.

A reading that low, with the house losing 3.5°F, sits below what Goodman's data supports for any GSZ16 size. The arithmetic can't say what is wrong. That takes a steady-state reading away from defrost, an airflow check and the manufacturer's charging procedure.

Why does the air get hotter when aux heat comes on?

Electric strips add their full output to the same airstream. A 10 kW strip adds about 34,121 BTU/h, which at 1,060 CFM is another 29.8°F of rise. On the Goodman 3-ton at 17°F, heat pump plus 10 kW comes to about 56,121 BTU/h and a 49°F rise, which takes supply air from about 89°F to near 119°F.

The conversion uses the NIST SP 811 factor of 0.2930711 W per BTU/h, so 1 kW is 3,412 BTU/h. Rise math uses the same sensible heat equation:

The strip numbers assume full rated kW at the rated voltage and the same 1,060 CFM. With strips on, the blower may run a different airflow, so check the air handler's manual for the CFM it runs on electric heat. A resistance strip's output scales with voltage squared, so a strip rated at 240 V makes about 75% of its rating on 208 V service, since (208 / 240) squared is 0.75. Homeowners who say the air only feels hot after aux heat comes on are noticing the jump from 89°F to 119°F. At 17°F and 1,060 CFM, Goodman's output works out to an 89°F supply from the heat pump alone.

What makes a good heat pump read like a bad one?

Five things skew a heating-mode check: a reading during or right after defrost, a probe that sees the coil or a strip, return air that isn't at 70°F, Goodman's T/R row read as the expected rise, and furnace expectations. Goodman lists instantaneous capacity, while integrated capacity, in AAON's words, is "a time-averaged capacity that includes defrost," so check in steady heating.

Our HVAC CFM calculator uses the same 1.08 constant to turn a furnace output and rise into airflow, and its FAQ explains where the constant comes from.

Checking cooling and heating splits with BuildSolver

For cooling, enter return and supply dry bulb and wet bulb with the airflow in the AC temperature split calculator, and it returns total, sensible and latent capacity, or the expected split from SHR and CFM per ton. For heating, divide the manufacturer's output at today's outdoor temperature by 1.08 x CFM, as shown above. The heat pump sizing calculator shows what a size delivers at your design temperature.

Both the split calculator and the heat pump sizing page are free without signup. The split calculator is cooling only by design, and its FAQ says the heating-mode rise depends on outdoor temperature and the manufacturer's heating data, which is what the tables in this guide use. Don't take the expected output for a rise check from the sizing page: its standard split system table uses a BuildSolver average of 0.72 of the 47°F rating at 17°F, while the GSZ16 tables run 0.52 to 0.63 across sizes, so the rise check needs the manufacturer's own table. Picking the unit against the house load at the balance point is ACCA Manual S work.

1. Note outdoor temperature, return dry bulb and the supply reading in steady heating, away from a defrost.
2. Pull the heating data for the outdoor unit and coil match, and read the output at that outdoor temperature.
3. Measure or estimate the CFM and compute the expected rise as output / (1.08 x CFM).
4. If the measured rise falls short of it, check airflow first, since an airflow error moves the expected number. Run the measured static pressure through the static pressure calculator and read the CFM from the blower table, then check charge by the manufacturer's procedure.

Sources

• Goodman, GSZ16 Split System Heat Pump spec sheet SS-GSZ16 (8/20), Expanded Cooling Data pages 4-17, Heating Data pages 18-19, Performance Data page 20: goodmanmfg.com
• Allison Bailes, ACCA HVAC Blog, The Physics of Heat and Air Flow, May 2, 2018: hvac-blog.acca.org
• AAON, What Is Integrated Heat Capacity in Air-Source Heat Pumps: aaon.com
• NIST Special Publication 811, Appendix B.8, factor for the BTU per hour: nist.gov

For estimation purposes only. Diagnose and charge to the equipment manufacturer's procedure.