9.5 Heat Pump Types, Cold-Climate Performance, and Sizing Approaches

9.5 Heat Pump Types, Cold-Climate Performance, and Sizing Approaches

The Heat Pump sheet of the HVAC Excellence Competency and Task List carries more asterisked competencies than any other area. Those asterisks mark items contributed by Pacific Northwest National Laboratory, a U.S. Department of Energy laboratory, to align the credential with national electrification goals. They are also the newest material on the exam, and HVAC Excellence's Heat Pump Installer and Heat Pump Service programs are recognized by DOE as Energy Skilled.

Sections 9.1 through 9.4 cover the refrigeration cycle, defrost, balance point, and troubleshooting. This section covers equipment selection, performance across temperature, and sizing.

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1. Heat Pump Types

By heat source and sink

Why ground-source performance is flat: the earth a few feet down stays near the annual mean air temperature. The heat pump therefore always works across a modest temperature lift, so compression ratio, capacity, and COP barely change between October and February. An air-source unit in the same building faces a lift that grows every degree the outdoor temperature falls.

Ground loop configurations: closed loop (horizontal trenches, vertical bores typically 150–400 feet deep, or pond/lake loops) circulating water with antifreeze; open loop pumping well water through the heat exchanger and discharging to a second well or surface. Open loops are cheaper but vulnerable to water chemistry, fouling, and local discharge regulations.

By distribution and configuration

• Ducted split — outdoor unit plus indoor air handler or furnace coil; the dominant residential form.
• Ductless mini-split — one or more wall, ceiling, or floor cassettes fed by line sets; no ductwork losses (see Chapter 14).
• Packaged — all components in one outdoor cabinet, ducted through the wall or roof.
• Packaged terminal heat pump (PTHP) — a through-the-wall unit in hotels, apartments, and offices; Type I EPA territory because most hold under 5 lb of refrigerant.
• Variable Refrigerant Flow (VRF) — many indoor units on a common refrigerant circuit with inverter compressors and electronic expansion valves; heat-recovery versions heat and cool simultaneously.

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2. Performance Across Temperature

Two things fall together as outdoor temperature drops: the heat pump's capacity and the building's available heat source. Meanwhile the building's load rises. The point where the two lines cross is the balance point (Section 9.3).

AHRI 210/240 rating points for air-source heat pumps are 47°F and 17°F outdoor dry bulb. A unit rated 36,000 BTU/hr at 47°F may deliver only about 22,000 BTU/hr at 17°F — a 40% loss for a conventional single-stage machine. Manufacturers' extended performance tables, not the rating plate, are the only reliable source for capacity at any other temperature, and reading those tables is an explicit task-list competency.

Cold-climate heat pumps

Modern cold-climate models hold capacity far better:

• Inverter-driven compressors run above nominal speed in cold weather, trading efficiency for capacity when the building needs it most.
• Vapor injection (enhanced vapor injection / economizer injection) taps partially expanded refrigerant into a mid-compression port, increasing mass flow through the condenser without increasing suction-side displacement, and simultaneously cooling the compressor.
• Result: rated capacity maintained to roughly 5°F, with useful operation down to about −13°F on many models.
• Defrost still governs the real-world result. Frost forms fastest between about 28°F and 40°F with high humidity, and each defrost cycle costs capacity and comfort, which is why demand defrost (Section 9.2) matters more in mild-humid climates than in dry cold ones.

Comfort differences versus fossil-fuel heating

A DOE-contributed competency asks the technician to "explain differences in operation and comfort between heat pumps and fossil fuel heating systems." The teachable points:

• Supply air temperature. A gas furnace delivers 120–140°F air in short bursts. A heat pump delivers 85–105°F air for long periods. That air is warmer than skin temperature but feels cool to a customer used to a furnace, and it is the single most common source of "the new system doesn't heat" complaints. Explain it at the sale, not at the callback.
• Run time. Heat pumps are designed to run long and steady. Continuous operation is normal, is more efficient than cycling, and improves temperature uniformity and filtration.
• Setback. Deep thermostat setbacks are counterproductive on a heat pump, because recovery calls in expensive auxiliary electric heat. Use a thermostat with intelligent recovery or a shallow setback.
• Auxiliary heat lockout. An outdoor thermostat that prevents strip heat above a chosen outdoor temperature is often the single highest-value adjustment on a heat pump installation.

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3. The Three Sizing Approaches

A DOE competency asks the technician to "explain different approaches for sizing heat pumps: sizing for cooling, partial heating, full heating." These are genuinely different design decisions with different consequences.

Climate decides. In a cooling-dominated southern climate, sizing for cooling is correct and the winter shortfall is small. In a heating-dominated northern climate, sizing for full heating is attractive, but it only works well with inverter-driven variable-capacity equipment, which can turn down far enough in summer to avoid the short-cycling penalty. This is exactly why cold-climate heat pumps are almost always variable capacity.

Other sizing inputs the task list names

• Ductwork insulation: when replacing a ducted fossil-fuel system with a ducted heat pump, verify the ducts are insulated well enough to prevent condensation. Ducts sized and insulated for 130°F furnace air carrying 55°F cooling air through a humid crawlspace will sweat.
• Airflow. Heat pumps require higher airflow than furnaces — commonly 350–450 CFM per ton, and toward the high end in heating. A duct system sized for a furnace's lower CFM will produce high static pressure, low airflow, low supply temperature, and nuisance high-pressure trips.
• Customer goals and limitations. A DOE competency states it directly: the customer's budget, comfort priorities, noise tolerance, and willingness to change habits all legitimately affect equipment selection.
• Outdoor unit installation requirements: set the unit above expected snow depth on a bracket or riser pad, provide clearance for airflow and defrost drainage, keep it out of roof-drip lines, and route condensate away from the pad so ice does not build under the coil during defrost.

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4. Assessing Electrical Panel Capacity

Two separate task-list entries — one on the Heat Pump sheet and one on the Electrical sheet — require "assessing an electrical panel to determine if the building's electrical capacity can account for a heat pump's load." This is now routine work.

What to check

1. Service size (100 A, 150 A, 200 A) at the main breaker, and whether the panel bus rating matches.
2. Existing calculated load using the NEC Article 220 standard or optional method: general lighting and receptacles, small-appliance and laundry circuits, fixed appliances, range, dryer, water heater, and existing HVAC.
3. New heat pump load: the outdoor unit's MCA and MOP from the nameplate, plus the air handler blower, plus auxiliary electric heat, which is the big number. A 10 kW strip heater at 240 V draws about 41.7 A; 15 kW draws about 62.5 A.
4. Physical space for the new breakers, and whether the panel accepts tandem breakers.
5. Article 220.83 (existing dwelling with additional load) allows an optional calculation that credits the largest of the heating or cooling load rather than adding both — heating and cooling do not run simultaneously.

When the panel is short, the options in ascending cost are: reduce auxiliary heat capacity (often possible with a cold-climate unit and a proper balance-point analysis), use a circuit-sharing or load-management device listed for the purpose, replace the panel, or upgrade the service. Modern practice frequently avoids a service upgrade simply by right-sizing the strip heat, which is why the balance-point calculation in Section 9.3 has direct dollar consequences.