How do I know what size heat pump I need?

Start from a room-by-room load calculation at your local design temperatures. Then compare candidates' real heating and cooling capacity at those temperatures, from the manufacturer's extended performance data, with the loads. Square footage and the old unit's size are not reliable guides.

Should a heat pump be sized for heating or cooling?

It depends on the climate and the equipment. In cooling-dominated climates the cooling load usually sets the size, and backup heat covers the coldest hours. In cold climates a variable-speed heat pump can often be sized closer to the heating load, as long as its lowest output still lets it run properly in cooling.

What happens if a heat pump is oversized?

In cooling it short-cycles, removes less moisture and lets temperatures swing. A variable-speed unit tolerates some oversizing better, because it can slow down, but only down to its minimum capacity.

What happens if a heat pump is undersized for heating?

Below its balance point it can't keep up on its own, so backup heat runs more often. That is expected on the coldest days; if it happens for much of the winter, running costs rise and comfort suffers.

Heat Pump Sizing, Step by Step

How to size a heat pump properly: start from a Manual J load, use design temperatures, read extended performance data, find the balance point, size backup heat and keep cooling from being oversized.

calc.blue teamOctober 8, 20265 min read

Sizing a heat pump is harder than sizing an air conditioner or a furnace, because its heating capacity falls just as the house needs more heat. Get it right and the heat pump carries most of the winter on its own, with backup heat for the coldest hours. Get it wrong and you get either a clammy, short-cycling house in summer or an expensive backup system doing much of the heating in winter.

Here is the process, step by step.

1. Start from a real load calculation

Everything starts from the house's heating and cooling loads at its design temperatures: a room-by-room Manual J calculation. Square-foot rules and "replace like for like" skip the house: its insulation, windows, air leakage and orientation. Many existing systems were oversized to begin with, and the house may have had new windows or insulation since.

If you only have a whole-house number for now, a heat loss calculation gets you the heating side, and a quick load estimate gives a cooling ballpark. Both are estimates, not a Manual J.

2. Use the right design temperatures

Loads and capacities are compared at the design conditions: usually the 99% heating dry bulb (colder than that for only about 1% of the year's hours) and the 1% cooling dry bulb, from ASHRAE data for the nearest representative weather station. Some building departments publish their own. The free tools can look these up by city.

3. Read the extended performance data

A heat pump's model number tells you its nominal size, not what it delivers on your design day. Use the manufacturer's extended (expanded) performance data, or a cold-climate heat pump listing, to get:

Heating: maximum capacity at 47, 17 and 5°F (and lower if published), plus the COP at each.
Cooling: total, sensible and latent capacity at your summer design temperature and expected airflow.
Minimum capacity for variable-speed units, which decides how well they run in mild weather.

4. Decide what sets the size

Compare the two loads:

Cooling-dominated (hot summers, mild winters): size to the cooling load, so humidity control stays good. Accept that backup heat covers the coldest hours.
Heating-dominated (cold winters): a variable-speed or cold-climate heat pump can often be sized closer to the heating load, which cuts backup heat, as long as cooling isn't badly oversized and the unit's minimum output is low enough to run long cycles in summer.

Manual S sets limits on how far capacity may exceed the loads, tighter for cooling than for heating, and the equipment chosen has to sit within them.

5. Find the balance point

Plot the house's heating load as a straight line, from zero at about 60–65°F to the design load at the design temperature, and plot the heat pump's capacity curve. Where they cross is the thermal balance point. Above it the heat pump carries the house alone; below it backup heat helps.

For example, with a 42,000 BTU/h heating load at 6°F, a generic 3-ton cold-climate curve in the balance point calculator crosses the load near 19°F. A typical year at Denver International Airport spends only a few hundred hours colder than that, so the heat pump would deliver the large majority of the season's heat.

6. Size the backup heat

At the design temperature, the gap between the load and the heat pump's output is what backup has to cover:

Backup kW = (design load − heat pump capacity at design) ÷ 3,412

In the example above, the heat pump makes about 28,600 BTU/h at 6°F against a 42,000 BTU/h load: a gap of about 13,400 BTU/h, or 3.9 kW. Many installers round up to the next heat-kit size, and some add margin for defrost or for emergency heat if the heat pump fails. Local codes and utility programs may set their own rules.

With a furnace as backup (dual fuel), the furnace usually runs instead of the heat pump below the switchover, so it has to carry the whole design load.

7. Check cooling and airflow

Whatever sets the size, check the cooling side: the equipment's sensible and latent capacity at design should cover the cooling load without excessive oversizing, especially in humid climates. Then check that the airflow the equipment needs matches what the ducts can deliver. That's Manual D territory, and it's where many retrofits fall short.

A quick checklist

Room-by-room loads at local design temperatures
Extended performance data at 47, 17 and 5°F, plus minimum capacity
Sizing priority chosen for the climate
Balance point found, backup sized at design
Cooling sensible and latent capacity checked
Airflow matched to the ducts

Where calc.blue fits

calc.blue does this on an iPad: scan the house, get room-by-room loads with a likely range, and compare heat pumps and furnaces against the house at its own design temperatures, with the balance point and backup heat on one chart. It's built on ACCA Manual J 8th edition procedures. Join the waitlist.

ACCA, Manual J, Manual S and Manual D are trademarks of the Air Conditioning Contractors of America. calc.blue is not affiliated with or approved by ACCA.

Questions

How do I know what size heat pump I need?: Start from a room-by-room load calculation at your local design temperatures. Then compare candidates' real heating and cooling capacity at those temperatures, from the manufacturer's extended performance data, with the loads. Square footage and the old unit's size are not reliable guides.
Should a heat pump be sized for heating or cooling?: It depends on the climate and the equipment. In cooling-dominated climates the cooling load usually sets the size, and backup heat covers the coldest hours. In cold climates a variable-speed heat pump can often be sized closer to the heating load, as long as its lowest output still lets it run properly in cooling.
What happens if a heat pump is oversized?: In cooling it short-cycles, removes less moisture and lets temperatures swing. A variable-speed unit tolerates some oversizing better, because it can slow down, but only down to its minimum capacity.
What happens if a heat pump is undersized for heating?: Below its balance point it can't keep up on its own, so backup heat runs more often. That is expected on the coldest days; if it happens for much of the winter, running costs rise and comfort suffers.