Sizing a Heat Pump for a Cold Climate

Why this matters

The furnace coming out says 80,000 Btu/h on the plate, the AC beside it says 2.5 tons, and the quick answer is a 3-ton heat pump because that is a little bigger than the AC. In a cold climate that number is a guess dressed up as a size. It may be too small to heat the house on the coldest nights, which the customer discovers in January when the backup strips run for a week and the bill arrives. Or it may be oversized for cooling, which the customer discovers in July as a damp house that cycles every few minutes. The only defensible size comes from two documents read together: the load calculation and the manufacturer's extended performance data.

The existing Heat Pump Sizing for Cold Climate Reference lays out the overall steps. This card goes deep on the part people skip: reading capacity at temperature, finding the balance point, and deciding which way to err. How the equipment behaves in the cold, defrost included, belongs to the Heat Pump Cold-Weather Operation Reference.

Start from the load, never from the old equipment

The furnace's nameplate tells you what someone bought, not what the house needs. Furnaces in older houses are routinely sized at one and a half to two times the actual heating load, because the contractor rounded up, then rounded up again, and the envelope has since been improved with windows and attic insulation. Sizing a heat pump to match an oversized furnace oversizes the heat pump, and that costs more on a heat pump than on a furnace, for reasons covered below.

So the first document is a room-by-room heating and cooling load calculation, ACCA Manual J or an equivalent your jurisdiction accepts, run at the design conditions for your location. The heating design temperature in Manual J is the 99 percent dry-bulb from published climate data: the outdoor temperature that is equaled or exceeded 99 percent of the hours in a typical year. It is not the record low. About 88 hours a year, on average, are colder than it. See related: Manual J Load Calculation Reference.

What the load calc gives you, for this card's purposes, is two numbers: the heating load at the design temperature and the cooling load at the cooling design temperature. Everything else hangs from those.

Read capacity at temperature from the extended data

A heat pump's nominal size, the "3-ton" in the model name, is its cooling capacity at standard rating conditions. Its heating rating on the spec sheet is taken at 47 F outdoors. Neither number tells you what it delivers at 5 F.

Heating capacity falls as the outdoor temperature falls, because the refrigerant has less heat to pick up and the compressor moves less mass per stroke. An inverter-driven unit fights this by speeding the compressor up, which is why cold-climate units hold capacity far better than single-speed ones. But they all fall.

The manufacturer publishes extended performance data, sometimes called expanded tables, that list capacity and power input across a range of outdoor temperatures, usually at minimum, rated and maximum compressor speeds. That table is the second document. Read the maximum capacity column at your design temperature, because that is what the unit can do when the house needs everything. Read the minimum capacity column at mild temperatures, because that tells you how gently it can run when the house needs little.

Capacity in the extended data is usually listed for a specific matched indoor unit. A different coil or air handler pairing can change it. Confirm the combination on the certificate you are reading matches what you are installing.

The cold-climate specifications

Efficiency organizations publish specifications for what counts as a cold-climate heat pump. ENERGY STAR's cold-climate criteria and the Northeast Energy Efficiency Partnerships (NEEP) specification are the two most referenced. In the versions current at writing, the ENERGY STAR cold-climate criteria require a coefficient of performance (COP, delivered heat divided by electrical input) of at least 1.75 at 5 F, and heating capacity at 5 F of at least 70 percent of the capacity at 47 F. Both organizations revise their criteria between versions, so pull the current one before you quote it to a customer or a rebate program.

Use these specifications as a filter, not as a size. A unit that meets them holds its capacity well, which makes a high balance point less likely. It does not tell you whether that unit is big enough for this house. NEEP's product list is useful precisely because it publishes the low-temperature capacity data in one place across many brands, which saves digging through individual spec sheets.

The balance point, and why you compute it

The heating load of a house is close to a straight line against outdoor temperature: zero at some outdoor temperature where internal gains and sun cover the losses (commonly taken around 60 to 65 F), and the full design load at the design temperature. The heat pump's maximum capacity is another line, falling as it gets colder. Where the two lines cross is the thermal balance point: above it the heat pump alone keeps up; below it something else has to make up the difference.

Computing it tells you two things at once: how many hours a year the backup will run, and how big the backup needs to be at design.

Worked example: three candidates for one house

The house: Manual J heating load 38,000 Btu/h at a design temperature of 5 F, and a cooling load of 22,000 Btu/h. Take the house's zero-load point at 65 F, so the load line falls by 38,000 over a 60 F span, about 633 Btu/h per degree. The capacity figures below are illustrative of the shape real extended tables show; read the real table for the real unit.

LOAD LINE: 633 Btu/h per F below 65 F
outdoor  house load   2-ton max   3-ton max   4-ton max
 47 F     11,400       26,000      36,000      46,000
 30 F     22,200       22,000      32,600      42,600
 17 F     30,400       19,000      30,000      40,000
  5 F     38,000       16,000      27,000      38,000

min at 47 F           6,000        9,000      12,000

The 2-ton. At 30 F it delivers 22,000 against a load of 22,200, so its balance point sits at about 30 F. At design it covers 16,000 of 38,000, which is 42 percent of the load. The gap at 5 F is 38,000 minus 16,000, which is 22,000 Btu/h, about 6.4 kW of backup. In most cold climates a large share of heating hours sit below 30 F, so backup heat runs much of the winter. Cooling: 24,000 nominal against 22,000, a good match. It is sized for cooling and starved for heating.

The 3-ton. Between 17 F and 47 F its capacity falls by 6,000 over 30 degrees, 200 Btu/h per degree. Set capacity equal to load: 30,000 plus 200 times (T minus 17) equals 633 times (65 minus T). That solves to about 17.5 F. At design it covers 27,000 of 38,000, which is 71 percent. The gap at 5 F is 11,000 Btu/h, about 3.2 kW, so a 5 kW strip covers it with room. Hours below 17.5 F are a minority of heating hours in most cold climates, so the backup runs on the coldest days, not all winter. Cooling: 36,000 nominal against 22,000 is 1.64 times the load.

The 4-ton. It covers the full 38,000 at design, so on paper no backup is needed. But its minimum capacity at 47 F is 12,000 against a load of 11,400 at that temperature, so through the whole shoulder season it cannot turn down far enough and will cycle on and off. Cooling: 48,000 against 22,000 is about 2.2 times the load. It will short-cycle in summer, run too briefly to pull moisture out of the air, and leave the house cool and clammy.

The 3-ton is the call for this house. It leaves a modest, well-defined gap that a small backup fills, runs continuously and efficiently across most of the winter, and is oversized for cooling by less than the 4-ton. The backup decision itself, strips against a furnace against none, belongs to Backup Heat: Dual Fuel or Resistance.

Why oversizing hurts a heat pump more than a furnace

An oversized furnace wastes some efficiency and cycles more. An oversized heat pump does the same, and then pays three more costs.

• Cooling. A heat pump is also the air conditioner. Size it for a cold climate's heating load and it is often well past the cooling load. ACCA Manual S sets how far equipment may exceed the cooling load, with a looser allowance where heating dominates and the equipment is variable speed; read the edition you work to before you push past the conventional limit.
• The modulation floor. An inverter unit is efficient because it runs long and slow. If its minimum output is above the house's load across the mild months, it cannot run slow and the efficiency advantage disappears in exactly the weather where most heating hours sit.
• The service. A bigger unit has a bigger minimum circuit ampacity and a bigger breaker. In a house where the service was already the constraint, the 4-ton may be what forces the upgrade. See related: Measuring Existing Load Before You Sell a Service Upgrade.

Undersizing has its own failure: the 2-ton above heats the house on its strips for weeks at a time, at half or less of the efficiency the heat pump would have delivered at those temperatures.

What changes the answer

A heating-dominated house with almost no cooling. If the cooling load is tiny, the cooling-oversize objection weakens, and sizing closer to full design heating may make sense, provided the minimum capacity still turns down to the shoulder-season load.

Planned envelope work. If the customer is air-sealing or adding attic insulation next year, size to the improved load, not today's. Running the load calc for both and showing the customer the two sizes is often what sells the envelope work.

Two units instead of one. A house split by an addition or a second floor that the ducts never served well may be better as two smaller systems, each matched to its zone, than as one large one. See related: Ductless Heads in a House With Ducts.

A dual-fuel house. If the gas furnace stays as backup, the heat pump's job is only to cover the hours above the switchover temperature, which can justify sizing closer to the cooling load.

How to verify you got this right

Before the quote goes out, check four things. The load calc exists, is room-by-room, and uses the published design temperature for the location rather than a rule of thumb. The capacity you are relying on came from the extended data at the design temperature, for the matched indoor and outdoor pair you are installing. The balance point is computed and written down, with the backup size it implies. And the minimum capacity at 47 F is at or below the house's load at 47 F, so the unit can turn down in the shoulder season.

After the first winter, the thermostat or the unit's own logs can show backup run hours. If backup ran far more than the balance-point estimate predicted, check whether the strips are locked out properly before you blame the size. See related: How to Set Expectations for the First Heat Pump Winter.

References

• ACCA Manual J (residential load calculation) and Manual S (equipment selection), in the editions your jurisdiction or program requires.
• ENERGY STAR cold-climate heat pump criteria, current version; NEEP Cold Climate Air Source Heat Pump Specification and product list.
• Manufacturer extended performance data for the matched system being installed.
• See related: Heat Pump Sizing for Cold Climate Reference; Heat Pump Cold-Weather Operation Reference; Manual J Load Calculation Reference.
• See related: Backup Heat: Dual Fuel or Resistance; Measuring Existing Load Before You Sell a Service Upgrade.