What Size Heat Pump Do I Need? Tons, Load and Backup Heat

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the sources at the end

What Size Heat Pump Do I Need? Tons, Load and Backup Heat

Short answer

Ask for a Manual J, which ENERGY STAR says your contractor should use. Square foot rules give odd answers for heat pumps: Bryant's own page puts a 2,000 square foot house at 36,000 to 48,000 BTU in one place and 60,000 to 100,000 in another, and calls 5 tons, 60,000 BTU, the usual top size for a home. Backup heat covers only what the right size cannot make on the coldest days.

What size heat pump do I need? The quick answer the makers give is square footage times a number of BTU, and for a heat pump it can go badly wrong. Bryant’s own sizing page gives the same 2,000 square foot house two answers that do not overlap.

Part of the trouble is that a heat pump’s heating output falls as the weather gets colder, so its tons do not tell you what it makes on a cold day. This page explains what to ask for instead.

Before you start

What to carry: a torch for the label on the outdoor unit, and the size of the house in square feet. Fifteen minutes.

Gas or electric backup? Trane says almost all dealers fit electric heat strips in the indoor unit as backup. Some heat pumps are paired with a gas furnace instead, which ENERGY STAR calls dual fuel. Whether you plan on the backup doing real work changes how big the heat pump itself needs to be, as NEEP’s guide explains below.

Ducted or ductless? This page is about central, ducted heat pumps, the kind with an outdoor unit and ducts to every room.

Two things the sizing sources warn about. Bigger is not safer. Bryant says an oversized heat pump short cycles, heats and cools unevenly and wears faster. A contractor quoted on Bryant’s page calls oversizing a rampant problem, because systems are sized for the worst case.

A size with no load calculation behind it. ENERGY STAR says your contractor should verify the size with Manual J, a calculation of how much heating and cooling your home needs. A quote that only asks the square footage has skipped it.

Why the rules give two answers

The same 2,000 square foot house on one maker's page. Two of the answers start at 5 tons, which Bryant calls the usual top size for a home, and run well past it.

The square foot formula. Bryant says to multiply the floor area by 30 to 60 BTU per square foot: about 30 to 35 in mild climates, 40 to 50 in moderate ones and 50 to 60 or more in cold ones. For 2,000 square feet that works out at 60,000 to 120,000 BTU, and the top of Bryant’s page says 60,000 to 100,000.

The sizes heat pumps come in. The same page says most home heat pumps run from 18,000 to 60,000 BTU, 1.5 to 5 tons, and calls 5 tons the typical residential maximum. Its questions section puts a 2,000 square foot house at 36,000 to 48,000 BTU, 3 to 4 tons. Its opening line, though, says common sizes run from 40,000 to 120,000 or more.

Why they disagree. Bryant does not say. The 30 to 60 BTU formula is the same one Bryant gives for sizing a furnace, while the tons are the sizes heat pumps are sold in. A ton is 12,000 BTU an hour, Bryant and Trane both say, and NEEP says the rated heating capacity is measured at 47°F. Which size is right for your house, and how much backup heat should carry, is what a Manual J and the steps below settle.

What the right size depends on

Check 1Often free through a utility energy audit, Trane says

Get a Manual J with both loads on it

Both loads. A Manual J gives the heating load and the cooling load. Trane lists what it counts: square footage, climate zone, ductwork, windows, shade and sun, insulation, the number of people and heat producing appliances.

The design temperature. NEEP, the Northeast energy efficiency group, uses the 99 percent heating design temperature from the ASHRAE handbook: 99 percent of the region’s hours are warmer than it. The heating load is the heat the house needs at that temperature to stay at 70°F indoors.

Where to get one. Trane says many utility companies do a free energy audit that gives you one, and an energy auditor or a dealer can do it too.

Check 2

Decide what the backup is for

Backup only as a safety net. NEEP’s guide says that if the heat pump is to heat without backup, its maximum output at the design temperature should cover 90 to 120 percent of the heating load, and to avoid oversizing. Trane still says every heat pump should have an auxiliary heat source as a safety net.

Backup for the coldest days. NEEP says to recognise that supplemental heat covers the difference, and to know what that heat is. ENERGY STAR says a cold climate heat pump keeps working below 5°F, but pairing it with a backup heats the home most efficiently in the deepest cold. NEEP adds that an undersized heat pump leans on the backup, which cuts the savings.

A hot, humid summer too. NEEP’s tool is built for heating dominated climates, where it sizes for heating. Where summers are also hot and humid, NEEP says to compare the cooling load too and pick the system for the higher of the two, adding dehumidification or heat if needed. If cooling or humidity would need a heat pump over 140 percent of the heating load, NEEP says consider other products or extra equipment.

How the backup decides when to come on: aux heat on the thermostat.

Check 3Free, in the maker's data

Read the cold weather output, not just the tons

More than one heating number. NEEP explains that the rated heating capacity is measured at 47°F so every heat pump can be compared, and makers also list it at 17°F and 5°F. What matters in a cold climate is the maximum output at your own design temperature, which NEEP works out from those figures.

Cold climate models. ENERGY STAR says its cold climate certification needs third party verified performance tested down to 5°F. Ask the contractor for the output at your design temperature, not only the tonnage.

Trane’s rule of thumb. Trane says no more than 25 percent over the load for a heat pump, against 15 percent for cooling and 40 percent for a furnace, but it does not say which load. For heating in a cold climate, NEEP’s test is the clearer one: the output at your design temperature should be 90 to 120 percent of the heating load.

More on how heat pumps cope with the cold: how cold is too cold for a heat pump.

The balance point

A sketch of a heat pump sized for part of the load. One sized for the whole house, 90 to 120 percent in NEEP's guide, meets the load line near the design temperature.

NEEP defines the capacity balance point as the temperature at which the heat pump’s output equals the house’s heating load. Above it, the heat pump can carry the house on its own, though Trane says the backup still comes on during defrost and when you raise the thermostat more than 3 or 4 degrees. Below it, the backup heat makes up the difference.

A bigger heat pump pushes the balance point colder, and a smaller one leaves more work for the backup. Trane says on a correctly sized heat pump the heat strips may seldom run. If they run a lot outside defrost, the size, the settings or a fault is worth a look: a heat pump running constantly.

How a wrong size shows up

Too big. Bryant lists short cycling, uneven temperatures, more wear and lower efficiency. A contractor on Bryant’s page says oversizing also brings humidity control problems: a heat pump short cycling.

Too small. Bryant lists struggling to hold the temperature, running constantly, higher energy use and poor performance in extreme cold or heat. NEEP says an undersized one sends you to the backup heat on the coldest days.

Reading the size you have. The model number usually carries the capacity: how to read the label. The cooling half of the same question is here: what size AC unit do I need.

What each one costs

The step	Cost
Reading the label	Nothing
Manual J through a utility energy audit	Often free, Trane says
Checking the backup heat size	Part of the same load calculation

Questions people ask

What size heat pump for a 2,000 square foot house?

Bryant’s page gives two answers: 36,000 to 48,000 BTU, 3 to 4 tons, in its questions section, and 60,000 to 100,000 at the top. Only a Manual J with both loads settles it for your house.

How many tons is 36,000 BTU?

Three. Bryant and Trane both put a ton at 12,000 BTU an hour.

Is it better to oversize or undersize a heat pump?

Neither, Bryant says. Oversized short cycles and undersized runs constantly. For heating, NEEP’s guide aims for 90 to 120 percent of the load at your design temperature.

Can a heat pump heat a house with no backup?

If it can make 90 to 120 percent of the heating load at your design temperature, NEEP’s guide treats it as the whole heating system. Trane still says every heat pump should have backup heat as a safety net.

Should a heat pump be sized for heating or cooling?

In a cold climate NEEP sizes for heating. Where summers are also hot and humid, NEEP says compare the cooling load too and pick for the higher of the two, adding dehumidification or extra heat if needed. A Manual J gives both loads.

Is a heat pump cheaper to run than a furnace?

That depends on your fuel prices and climate: heat pump or furnace.

Sources: Bryant "What Size Heat Pump Do I Need?" and "What Size Furnace Do I Need?", ENERGY STAR "Air Source Heat Pumps" (proper sizing, Manual J, cold climate testing to 5°F, backup heat), Trane "What Size HVAC Unit Do I Need?" and "What Is Auxiliary Heat in a Heat Pump System?", and NEEP "Cold Climate Heat Pump Sizing Support Tools User Guide" (February 2024) and "Just Right Heat Pumps: Why Sizing Matters in Cold Climates"