Heat Pump Sizing Calculator

A heat pump is one machine that both cools and heats, so it has to satisfy two loads. This tool sizes the unit on the cooling load — rounded to the nearest half ton the way equipment is sold — then checks that figure against the design-day heating load. If you only need the cooling number, the HVAC tonnage calculator covers it directly.

Deciding between a heat pump and a fossil-fuel system first? Compare the trade-offs in heat pump vs furnace, and see worked sizing examples in what size heat pump you need.

These rows show how the same house lands on different sizes as the climate shifts. Notice that in cold zones the heating load can outrun the cooling-sized capacity — the trigger for a cold-climate unit or backup heat. Values assume average insulation and an 8 ft ceiling.

• Sizing on heating load in a hot climate — oversizes cooling, kills dehumidification, and short-cycles the compressor.
• Trusting nameplate (47°F) heating capacity in a cold climate — real output can be 30–50% lower at 5°F.
• Skipping the balance-point analysis, so backup heat is undersized or runs far more than expected on the power bill.
• Pairing a single-stage heat pump with a cold zone — a variable-speed cold-climate (ccASHP) unit holds capacity far better.
• Ignoring defrost cycles, which briefly cut heating output and call on backup heat several times an hour in damp cold.
• Forgetting that the air handler and ductwork must match the airflow (≈400 CFM/ton) for the unit to hit its rated capacity.

For a deeper room-by-room heating breakdown that informs the backup sizing, run the heat load calculator.

Sizing tells you how much capacity you need; the rating labels tell you how efficiently a candidate unit delivers it. Since the 2023 federal minimums took effect, residential heat pumps are rated under the M1 test procedure as SEER2 for cooling and HSPF2 for heating, with EER2 reporting full-load efficiency at a fixed outdoor condition. SEER2 and HSPF2 run roughly 4–5% lower than the old SEER and HSPF numbers because the test uses higher external static pressure that better reflects real ductwork, so do not compare a new SEER2 figure against an older brochure's SEER. The U.S. Department of Energy sets the minimums by region, and EPA ENERGY STAR sets the higher voluntary tier many utility rebates require.

Efficiency and capacity are not the same thing, and a high HSPF2 does not guarantee enough output on the coldest night. The single most useful document for a sizing decision is the unit's AHRI certificate and the manufacturer's extended-capacity table, both tied to an AHRI reference number for a specific outdoor unit, indoor coil, and air handler combination. Those tables list heating output and input power at 47°F, 17°F, and 5°F, and increasingly at −5°F or lower for cold-climate models. Dividing output by input (in consistent units) gives the coefficient of performance (COP) at each point — a COP of 3.0 means three units of heat delivered per unit of electricity consumed, dropping toward 1.5–2.0 as the outdoor air cools. Plot the rated output column against your design heating load and the balance point reveals itself.

One detail the rating sheet hides: defrost. When the outdoor coil frosts in damp cold near freezing, the unit briefly reverses to melt it, subtracting from net heating output and often energizing auxiliary heat for a few minutes. Extended tables already net out average defrost losses, which is another reason to trust the table over a flat per-ton assumption.

Once the heat pump is sized on cooling and checked against the heating load, the remaining decision is how to cover the gap below the balance point. The simplest answer is electric resistance — supplemental heat strips in the air handler, typically 5–20 kW, where 1 kW contributes about 3,412 BTU/hr at a COP of 1.0. Strips are cheap to install and reliable, but they are the most expensive way to make heat, so size them to the difference between your design heating load and the heat pump's rated output at the design temperature rather than oversizing "to be safe." Many controls stage strips in increments and lock them out above the balance point to keep the operating cost down.

A dual-fuel (hybrid) system instead pairs the heat pump with a gas or propane furnace, letting the heat pump run while it is efficient and handing off to combustion when outdoor temperatures fall past an economic switchover point. This avoids resistance-heat bills in cold zones and is a common retrofit when an existing furnace is sound. If you are weighing that path, the trade-offs are laid out in heat pump vs furnace.

• Confirm the indoor coil and air handler are the AHRI-matched pair for the outdoor unit, or the rated SEER2/HSPF2 and capacity no longer apply.
• Deliver about 400 CFM/ton of airflow; restrictive ducts starve the coil and drop both capacity and efficiency below the certificate values.
• In IECC zones 5–8, prefer a cold-climate (ccASHP) inverter unit so the heating load is met with minimal resistance heat.
• Set the backup-heat lockout near the measured balance point so strips fire only when the heat pump genuinely cannot keep up.
• Verify electrical service and breaker sizing before adding large heat strips, which can draw 20–80 amps on their own.

A worked walkthrough of this whole sequence — cooling size, cold heating check, balance point, and backup — is in what size heat pump you need, and if you only need the cooling tonnage as a starting point, the HVAC tonnage calculator gives it directly.