Sizing a Replacement Heat Pump in a House With Marginal Ducts
A 2,400 ft² two-story house built in 1998 has a calculated heat loss of 48,000 Btu/h at a 24°F outdoor design temperature and a calculated cooling load of 31,000 Btu/h total, including 8,200 Btu/h latent. The existing system is a 3-ton, 10 SEER air conditioner paired with an 80% gas furnace. The duct system is rated at 1,200 cfm and measures 0.72 in. w.c. external static at that airflow. The homeowner wants a heat pump and has been quoted a 4-ton variable-speed cold-climate unit with a 10 kW electric strip in the air handler. The contractor argues the extra tonnage is needed because the existing system “runs all the time” on hot days. The electrical service is 100 A. The climate has a 99% heating design of 24°F and about 1,800 heating degree days. The homeowner wants to keep the gas furnace only as a backup if it makes sense.
1. Is the proposed 4-ton cooling capacity appropriate? Compare it with the load and explain the operating consequences.
Read the worked answer
Start with the loads, not the old equipment's run time. A nominal 4-ton unit is about 48,000 Btu/h, and at 95°F it may deliver roughly 45,000–47,000 Btu/h total, with sensible capacity near 34,000 Btu/h. The design load is 31,000 Btu/h total and only 22,800 Btu/h sensible, so the proposed unit is about 45–50% oversized on total capacity.
Consequences:
- Short cycling and poor latent removal; the house may feel cold and clammy at part load.
- More off-cycle losses and defrost penalty in heating.
- Required airflow near 1,600 cfm, while the existing duct is rated at 1,200 cfm and already at 0.72 in. w.c. The blower would be pushed far above its static limit, reducing airflow and capacity.
A 3-ton cold-climate unit is a better match for the 31,000 Btu/h cooling load. Verify its expanded sensible capacity at the design indoor wet bulb, and do not size from the old system's runtime; that complaint is more likely an airflow, charge, or duct problem.
2. If the selected 3-ton cold-climate unit produces 32,000 Btu/h at 24°F, how should backup heat be handled and what size strip is justified?
Read the worked answer
At 24°F the deficit is 48,000 − 32,000 = 16,000 Btu/h. A 5 kW strip produces about 17,100 Btu/h at 240 V, which matches the deficit. A 10 kW strip produces about 34,100 Btu/h, more than double the steady-state need, and it creates a large electrical load on a 100 A service.
Next decide between strip and gas backup. Keeping the existing 80% furnace as dual fuel avoids the large electric load and gives reliable capacity below the balance point. Set a compressor lockout near the balance point from the expanded performance table, roughly 35–40°F, and a fossil-fuel lockout above it. If the furnace is removed, use staged 5 kW strip with outdoor lockout, but confirm the service and air-handler circuit can carry it.
Size the strip for defrost and pickup as well as the steady-state deficit. A strip that exactly matches the 24°F deficit may be marginal during defrost, so check the manufacturer's recommended backup sizing and the actual heat-loss calculation.
3. What site measurements and documents should be checked before signing the equipment selection?
Read the worked answer
Before signing, collect:
- The full Manual J and Manual S outputs, not just a block load, plus room-by-room loads if ducts are marginal.
- Measured external static and actual airflow at the proposed blower tap, not the nominal 1,200 cfm rating.
- Line-set length, diameter, and vertical separation; existing line-set condition and size.
- Electrical panel load calculation, available breaker spaces, and conductor routing.
- Outdoor unit clearances, snow depth, defrost drainage, and noise path.
- Thermostat wire count and whether a C wire is available.
Check the manufacturer's expanded heating table at 24°F and 17°F, not just the nominal rating, and confirm the unit's balance point. Verify the air handler can be set to the required cfm at or below 0.80 in. w.c., or include duct corrections in the quote. Finally, confirm permit and code requirements, because changing from gas to all-electric may trigger electrical and mechanical permit reviews.