Heat Pump Cold-Weather Operation Reference

Why this matters

Heat pumps work great in moderate weather + struggle in cold. Customers in cold-climate regions often complain "my heat pump isn't keeping up" - sometimes the heat pump is fine + their expectation is wrong, sometimes there's a real problem. The tech who understands cold-weather heat pump operation can diagnose + educate effectively. This is the field card.

Heat pump basics in heating mode

A heat pump moves heat from outdoor air INTO the home. Even cold outdoor air contains heat energy (anything above absolute zero has heat). The compressor + refrigerant cycle pulls that heat in.

Two modes:

• Cooling: standard AC operation; refrigerant absorbs heat from indoor air, dumps outdoors
• Heating: reverse cycle; refrigerant absorbs heat from outdoor air, dumps indoors

Reversing valve switches the cycle.

Why heat pumps struggle in cold

As outdoor temp drops:

• Less heat available to absorb
• Refrigerant pressures drop
• Compressor works harder for less output
• Capacity (BTU/hr delivered) decreases
• Efficiency (COP) decreases

At very cold temps:

• Standard heat pumps lose significant capacity
• May not keep up with house heat loss
• Auxiliary heat kicks in (electric resistance, OR gas furnace in dual-fuel)

Balance point

The outdoor temperature where heat pump capacity = house heat loss = "balance point."

Example:

• House heat loss: 30,000 BTU/hr at 0°F outdoor
• Heat pump capacity: 30,000 BTU/hr at 30°F outdoor (drops as temp drops)
• Balance point: 30°F

Below balance point: heat pump can't keep up alone; auxiliary heat needed.

Balance points typically:

• Standard heat pump: 25-35°F balance point
• Cold-climate heat pump: 0-15°F balance point (much lower)

Auxiliary heat types

Electric resistance (strip heat):

• Most common
• Coils in air handler
• 5-20 kW typical
• Expensive to run vs heat pump (3-4× the cost)
• "Emergency heat" mode = strip only

Gas furnace (dual-fuel):

• Heat pump for moderate weather
• Gas furnace below balance point
• Best of both worlds: efficient heat pump, gas backup when needed
• More expensive to install (two systems)

Hydronic backup:

• Less common
• Boiler / hot water as backup heat
• Found in some installations

Defrost cycle

When heat pump runs in cold + humid weather, outdoor coil collects frost:

• Refrigerant is below freezing
• Moisture in outdoor air freezes on coil
• Frost reduces heat transfer
• System enters defrost mode

Defrost mode:

• Reversing valve switches to cooling mode
• Outdoor coil becomes the hot side
• Frost melts off
• Indoor coil becomes cold; auxiliary heat may run to keep house warm
• Lasts 5-15 minutes typically
• Then switches back to heat

Defrost initiation:

• Time + temperature method (most common): defrost every 90 min if coil cold
• Demand defrost (newer): pressure sensors detect frost buildup
• Newer = more efficient (less unnecessary defrost)

Normal cold-weather operation symptoms

Customer may notice + flag as "broken":

Steam from outdoor unit:

• Defrost cycle melting frost; condensation evaporating
• NORMAL
• Not exhaust, not refrigerant

Outdoor unit "off" for periods:

• Defrost cycle
• Short pause between heat / defrost cycles
• NORMAL up to 15 min

Auxiliary heat running:

• Indicator light on thermostat
• Higher utility bills
• May be temporarily normal during cold weather

Slower heating:

• Lower capacity in cold
• Longer runtime
• NORMAL

Cool air from registers:

• Heat pump delivers heated air; typically 90-105°F vent temp
• Lower than gas furnace (130°F+ vent temp)
• Feels "cool" compared to gas; customer often complains
• Customer EDUCATION matters here

Problem symptoms (not normal)

These suggest real issue:

Outdoor unit not running at all:

• Compressor fail
• Reversing valve stuck (in cooling position)
• Control board failure
• Refrigerant charge low

Outdoor unit running but no heat indoors:

• Reversing valve stuck (in cooling position)
• Refrigerant charge low
• Indoor coil iced up

Constantly in defrost:

• Sensor failure
• Outdoor coil restriction
• Low charge

Aux heat running constantly even in moderate weather:

• Thermostat misconfigured (set to emergency / aux only)
• Heat pump not running (use aux as primary fallback)
• Refrigerant problem

Outdoor unit icing badly + not defrosting:

• Defrost control failure
• Refrigerant charge low
• Indoor / outdoor coil restriction

Running but house not warming:

• Capacity not matching load
• Iced coil
• Refrigerant issue
• Auxiliary heat NOT enabled (some installs)

Customer education topics

"My heat pump puts out cool air":

• Vent temp ~90-105°F
• vs. body temp 98.6°F = feels cool/lukewarm
• vs. gas furnace ~130°F+ = feels hot
• Not broken; just the technology
• Some prefer it (steadier, less hot blasts)
• Some hate it (want hot register air)

"My utility bills are higher":

• During cold snaps, aux heat runs
• Aux heat 3-4x cost of compressor heat
• Some increase expected; severity depends on weather + system

"It's running constantly":

• Heat pumps often run longer cycles than gas furnaces
• Lower-temperature delivery + slower house warming
• Often actually MORE efficient than short hot cycles
• "Steady is good"

"The outdoor unit is steaming":

• Defrost; melting frost = water vapor in cold air
• Looks alarming, not a problem

"I never see snow on top of my outdoor unit":

• Heat pump is melting it
• Normal in winter

Cold-climate heat pumps (CCHP)

Modern heat pumps designed for cold operation:

• Variable-speed compressor
• Maintains capacity to much lower outdoor temps
• Higher COP (efficiency) at low temps
• Some operate down to -15°F or lower with usable capacity
• Premium vs standard heat pumps

If customer in cold climate + new equipment: cold-climate heat pump is the better choice.

Mini-split cold-weather operation

Ductless mini-splits in cold:

• Many models cold-climate rated
• Heads pump directly into living space (no ductwork loss)
• Each head with own thermostat
• Defrost cycles same as ducted

Cold-climate mini-splits (e.g., Mitsubishi Hyper-Heat) work to -13°F or lower.

Service procedures (cold-weather specific)

Charge verification:

• Refrigerant pressures vary dramatically with outdoor temp
• Use manufacturer charge chart at the outdoor temp during service
• Don't add refrigerant based on cooling-mode pressures in winter

Coil inspection:

• Check outdoor coil for ice / frost
• Clear any obstruction
• Verify defrost cycle completes

Reversing valve operation:

• Verify mode change (heat to defrost to heat)
• Listen for solenoid click
• Common failure point

Auxiliary heat operation:

• Verify strips operating correctly (read current draw)
• Verify limit switches operational
• Sequencer control sequence

References

• ACCA Manual J + S (load + equipment selection)
• ASHRAE Standard 90.1 (heat pump efficiency)
• AHRI Directory (cold-climate heat pump ratings)
• NEEP (Northeast Energy Efficiency Partnerships) cold-climate heat pump list
• Manuall internal: Heating Systems Comparison, Refrigerant Types Reference