Heat pump sizing calculator

What size heat pump do I need for my house?

Heat pump sizing follows a different rule than central AC sizing. An AC gets sized for the hottest 1 percent of summer hours. A heat pump has to be sized for the coldest 1 percent of winter hours, because heat pump capacity drops as outdoor temperature falls. Use the AC tonnage rule of 1 ton per 500 to 600 sq ft and you will undersize the heat pump by 20 to 40 percent in zones 4 and colder. The calculator above runs the proper heating load math.

Quick reference for a typical mixed climate (Zone 4) with average insulation at a 70°F indoor setpoint:

• 1,000 sq ft: 2-ton heat pump (24,000 BTU nameplate)
• 1,500 sq ft: 2.5 to 3-ton heat pump
• 2,000 sq ft: 3 to 3.5-ton heat pump
• 2,500 sq ft: 4-ton heat pump
• 3,000 sq ft: 4 to 5-ton heat pump

In Zone 6 or 7 (Minneapolis, northern Maine), the same homes step up one half-ton because the heating load goes up faster than the cooling load shrinks. A cold-climate heat pump from Mitsubishi, Daikin, Bosch, or Carrier with proper sizing can carry an entire home through a Minnesota winter without resistance backup heat.

How to use winter design temperature for heat pump sizing

The single most important number for sizing a heat pump is the 99 percent winter design temperature for your location. This is the outdoor temperature your area beats only 1 percent of winter hours, sourced from ASHRAE weather data. Quick reference by DOE climate zone:

• Zone 1 (Miami): 45°F design temp
• Zone 2 (Houston, Orlando): 30°F design temp
• Zone 3 (Atlanta, LA, Phoenix): 22°F design temp
• Zone 4 (Nashville, DC, Denver): 10°F design temp
• Zone 5 (Boston, Chicago, Seattle): 0°F design temp
• Zone 6 (Minneapolis, Burlington): -10°F design temp
• Zone 7 (Duluth, northern Maine): -20°F design temp

A Manual J load calculation uses your exact ZIP code design temperature, which can differ by 10 degrees within a single state. The calculator above uses zone-level averages, which is close enough for shopping purposes but should not replace a full Manual J for a permitted install.

What is heat pump balance point and why it matters

Balance point is the outdoor temperature where your heat pump's capacity exactly matches your home's heat loss. Above balance point the heat pump easily covers the load. Below balance point you need auxiliary heat to make up the gap. The lower your balance point, the more hours per year your heat pump handles solo, and the less you pay for backup resistance heat at three times the cost per BTU.

A well-sized cold-climate heat pump from Mitsubishi Hyper-Heat or Daikin Aurora should hit a balance point at or below your zone's design temperature. That means the heat pump handles 99 percent of winter hours without help. A standard heat pump in the same install might have a balance point of 25 to 35°F, which means it relies on backup heat for the bottom 20 to 40 percent of winter hours. In Massachusetts at $0.33 per kWh electricity, that backup cost runs $300 to $600 a winter extra.

Standard vs cold-climate vs premium heat pumps

Three tiers of residential heat pump performance, defined by how much rated capacity the unit holds at 17°F and 5°F outdoor temperature:

• Standard heat pump: 60 percent capacity at 17°F, 35 percent at 5°F. Goodman, Rheem base, basic Carrier and Trane lines. $10,000 to $14,000 installed.
• Cold-climate heat pump: 85 percent at 17°F, 65 percent at 5°F. Carrier Infinity 24 VNA, Trane XV20i, Lennox SL25XPV. $14,000 to $18,000 installed.
• Premium cold-climate heat pump: 97 percent at 17°F, 85 percent at 5°F. Mitsubishi Hyper-Heat, Daikin Aurora, Bosch IDS Premium Connected. $16,000 to $22,000 installed.

For zones 1 through 3, a standard heat pump is fine because the design temperature stays above 20°F and capacity loss does not matter much. For zones 4 and 5, a cold-climate unit pays for itself in a few winters of lower aux heat use. For zones 6 and 7, the premium cold-climate units are the only ones that make financial sense because they avoid the resistance backup heat that otherwise wrecks the operating cost.

Why sizing a heat pump for cooling load alone fails

An old contractor habit is to size the heat pump like an AC, picking based on the cooling load. That works in zones 1 through 3 where the cooling load is bigger than the heating load. In zones 4 through 7, heating load is usually 20 to 50 percent bigger than cooling load, so an AC-sized heat pump comes up short on the coldest days. Symptoms: the heat pump runs continuously, the house never quite reaches setpoint, and the resistance backup heat strips run for hours every cold morning.

The opposite problem also happens: a contractor oversizes the heat pump for heating load in a southern climate, then the unit short-cycles in summer because the cooling load is too small for the equipment. ACCA Manual S addresses this by sizing the heat pump for heating load and then verifying the cooling capacity matches the cooling load within 15 percent. The calculator above gives you the heating-side number. Run our AC tonnage calculator to verify the cooling side.

Dual fuel: when to keep a gas furnace as backup

Dual fuel pairs a heat pump with a gas furnace and lets the thermostat switch between them at a programmable cutoff temperature, usually 25 to 35°F. Above the cutoff the heat pump runs because it is cheaper at $0.18/kWh than gas at $1.40/therm. Below the cutoff the furnace runs because the heat pump's COP has dropped below 2 and resistance backup would cost three times as much per BTU.

Dual fuel makes financial sense in zones 4, 5, and 6 where you already have a working gas furnace and natural gas is available. The combined system installed runs $12,000 to $18,000, only $2,000 to $5,000 more than either system alone. In zones 1 through 3 a straight heat pump always beats dual fuel because heating hours are too few to justify the furnace. In zone 7 the cold-climate heat pump alone usually beats dual fuel because the premium units hold capacity down to -5°F.

How to read AHRI capacity ratings on a heat pump spec sheet

Every heat pump certified through AHRI lists rated heating capacity at two outdoor temperatures: 47°F and 17°F. The newer cold-climate certification adds a 5°F rating. The 47°F rating is the "happy day" number used for SEER2 and HSPF2 calculations. The 17°F and 5°F numbers are what you actually need to compare units. A heat pump with 36,000 BTU/hr at 47°F and 30,000 BTU/hr at 17°F holds 83 percent capacity at 17°F, which is in the cold-climate range. Same nameplate with 22,000 BTU/hr at 17°F holds only 61 percent and is a standard-tier unit.

Ask any HVAC contractor for the AHRI certificate number on the heat pump quote. Look it up at ahridirectory.org to see the full capacity curve. If a contractor quotes you a "cold-climate" heat pump but the AHRI ratings only show 60 percent capacity at 17°F, the marketing is ahead of the equipment.