The question a consulting engineer has to answer before anything else: on the winter design day, will one packaged unit keep the suite at 70°F, and how much of that heat is coming from the compressor versus the electric heater? Nameplate capacity does not answer it. This is the check, step by step, using the EXINDA Aura HPERV data as the worked example. The method is the same for any all-in-one unit.
1. Start from your design temperature, not from 47°F
Take the 99% or 99.6% winter design dry bulb for the site from the ASHRAE climatic data or the local code. Toronto, Chicago and Minneapolis all sit well below 17°F; coastal cities may sit above it. The 47°F rating that appears on most product pages is a mild-weather number and is only useful for comparing units with each other.
2. Read the capacity at the published point closest to your design temperature
Manufacturers who publish to AHRI conditions give heating capacity at 47°F, 17°F and 5°F. For the HPERV (model ICMPCGFF1AYE):
| Outdoor air | Heat pump capacity | Input power | COP |
|---|---|---|---|
| 47°F DB / 43°F WB | 14,000 Btu/h | 1,687 W | 2.43 |
| 17°F DB / 13°F WB | 7,800 Btu/h | 1,230 W | 1.86 |
| 5°F DB / 3°F WB | 7,165 Btu/h | 1,400 W | 1.50 |
Indoor air 70°F DB / 60°F WB in all three cases. For a design temperature between two points, interpolate in a straight line; it is not exact, but it is closer than using either endpoint. For 10°F, for example: about 7,400 Btu/h.
Two things to be careful with. First, some manufacturers list a "maximum" figure next to "rated"; the maximum is short-term operation and is not a design value. Second, check that the figure is heat pump only, not heat pump plus electric heater. The table above is heat pump only.
3. Add the electric heat according to the staging table
Every unit in this category carries an electric heater, and the control logic decides when it comes on. For the HPERV:
| Outdoor temperature | Compressor | Electric heat | Mode |
|---|---|---|---|
| 41°F and above | On | Off | Heat pump only |
| 23°F to 41°F | On | 1 kW | Heat pump + 1 kW |
| 5°F to 23°F | On | 2 kW | Heat pump + 2 kW |
| Below 5°F | Off | 3 kW | Electric heat only |
1 kW of resistance heat is 3,412 Btu/h. So the total heat available to the suite is:
| Outdoor air | Heat pump | Electric | Total to suite | Total input |
|---|---|---|---|---|
| 30°F (interpolated) | ≈ 10,500 Btu/h | 3,412 Btu/h (1 kW) | ≈ 13,900 Btu/h | ≈ 2.5 kW |
| 17°F | 7,800 Btu/h | 6,824 Btu/h (2 kW) | ≈ 14,600 Btu/h | ≈ 3.2 kW |
| 5°F | 7,165 Btu/h | 6,824 Btu/h (2 kW) | ≈ 14,000 Btu/h | ≈ 3.4 kW |
| 0°F and below | 0 | 10,236 Btu/h (3 kW) | ≈ 10,200 Btu/h | 3.0 kW |
Read the last row carefully. Below the compressor cut-off the unit is a 3 kW electric furnace with ventilation. It keeps the suite warm and keeps supply air at a comfortable temperature, and the unit itself is rated to operate down to −31°F in this mode, but the operating cost at that point is that of resistance heat. How many hours a year the site spends below 5°F decides whether that matters: for most US climate zones 4 and 5 it is a handful of hours; for zone 7 it is not.
4. Compare the total to the suite load
Run the room-by-room heat loss at the design temperature. The number to compare against is the "total to suite" at that temperature, not the 47°F rating. If the load is above the total, the options are a second unit, a larger model, or reducing the load through the envelope. If the load is below the heat-pump-only capacity at design, the electric stages will rarely run, which is the outcome to aim for.
5. Check the fresh-air side separately
The ERV core recovers heat from the exhaust air, but incoming fresh air is still cold after the core when it is −13°F outside. In the HPERV the fresh air passes through the unit and is tempered with the supply air, which is part of the reason the electric stages exist. Two things to confirm on any unit: the ERV's tested efficiency at a low inlet temperature (the HPERV core is tested at −13°F, 40 CFM, 67% sensible recovery), and the defrost method. Exhaust defrost, which the HPERV uses, periodically reduces net fresh air; if the project needs a guaranteed continuous outdoor-air rate, account for that.
6. Size the electrical supply for the whole unit
The heater is on the same circuit as the compressor. For the HPERV the minimum circuit ampacity is 20 A and the maximum overcurrent protection is 30 A, with the 3 kW heater included. Do not size the circuit on the compressor alone.
What to ask any manufacturer for
- Rated heating capacity and input at 47°F, 17°F and 5°F with indoor conditions stated, heat pump only.
- The electric heat staging table and heater sizes.
- Compressor operating range and what the unit does below it.
- ERV efficiency at a cold inlet and the defrost method.
- MCA and MOP with the heater included.
- AHRI reference number if the project requires certified ratings.
The HPERV figures above are from the September 2026 submittal, available on the product page. For how the HPERV and the other packaged units compare at the same conditions, see All-in-one heat pumps with no outdoor unit for apartments: how to choose.
Have a design temperature and a suite load? Send them with the floor plan to info@exindagroup.com and ask for project selection support. You get back the capacity check at your temperature, the staging, and the electrical requirement.
