Heat Pump Balance Point and Aux/Emergency Heat

What it is

A heat pump's heating capacity isn't constant — it drops as the outdoor temperature drops, because there's less heat available in colder air to pull indoors. Meanwhile the building's heat load goes up as it gets colder outside. Plot both against outdoor temperature and the two lines cross at one point. That crossover is the balance point: the outdoor temperature at which the heat pump's output exactly equals the building's heat loss.

• Above the balance point: the heat pump alone can keep up.
• Below the balance point: the heat pump can't make enough heat by itself, and something has to fill the gap — that's auxiliary heat.

How it works

Think of two lines on a graph (outdoor temp on the bottom):

• Building load rises as it gets colder (a straight-ish line climbing to the left).
• Heat pump capacity falls as it gets colder (a line dropping to the left).

Where they intersect is the balance point — often somewhere around the mid-20s to mid-30s °F for a typical residential system, but it depends entirely on the equipment size and how leaky/well-insulated the house is. A tight house with an oversized heat pump has a low balance point (rarely needs backup); a leaky house with a modest heat pump has a high balance point (leans on backup often).

Auxiliary (aux) heat is the backup that supplements the heat pump when it can't keep up — electric resistance strips, or in a dual-fuel system, the gas furnace. It runs alongside or instead of the heat pump as needed.

Emergency (em) heat is the SAME backup heat source, but the homeowner (or the control) has commanded the heat pump compressor OFF entirely and is running on backup alone — used when the heat pump itself has failed (bad compressor, bad reversing valve, iced outdoor unit). Em heat is a manual "the heat pump is broken, run on backup until I fix it" mode. Same heat strips, different reason for running them.

The practical difference: Aux = heat pump PLUS backup, automatically, when it's cold. Emergency = backup ONLY, on purpose, because the heat pump is down. Running em heat all winter because it "feels warmer" is just running expensive resistance heat and skipping the efficient heat pump — a common and costly customer mistake.

In the field

When should aux come on? Aux is brought in when the heat pump can't satisfy the call. Controls trigger it different ways:

• Staging on the thermostat: the heat pump runs as first stage (Y/W1 on many heat-pump-stat setups); aux is second stage (W2), brought on when the room temperature keeps falling or droops a set number of degrees below setpoint despite the compressor running.
• Time/droop logic: a smart stat brings aux after the indoor temp drifts too far below setpoint for too long.
• During defrost: aux comes on to temper the cold supply air (see the defrost article).
• Outdoor thermostat (lockouts): see below.

Outdoor thermostat lockouts are a key efficiency control:

• A low-ambient lockout on the compressor can shut the heat pump off below some very low outdoor temp where it's no longer worth running.
• More commonly, an outdoor thermostat locks OUT the aux heat strips above a certain outdoor temperature — so the expensive strips can't run when it's 45°F out and the heat pump can easily handle it alone. Set it near (a bit above) the balance point so strips are only available when actually needed.

Normal values & targets

• Typical residential balance point: often ~25–35°F, but it's design-specific — depends on equipment capacity vs building load. Don't assume; it varies widely.
• Outdoor thermostat for aux lockout: commonly set somewhere around the balance point (e.g., locking out strips above ~35–40°F so they only assist in genuinely cold weather). Field/design dependent.
• Aux/em heat source: electric resistance strips (commonly sized in stages, e.g., 5 kW / 10 kW / etc.) or a gas furnace in dual-fuel.
• Resistance heat is ~100% efficient but expensive — every watt becomes heat, but you're paying full electric resistance rates instead of the heat pump's multiplier (COP > 1).

Common faults & what they mean

• Heat strips run constantly / huge electric bills — could be: stuck in emergency mode, aux lockout outdoor thermostat missing or misadjusted, heat pump not actually producing heat (so aux carries the whole load), thermostat staging set wrong, or a wiring issue energizing W2 with the call.
• House can't keep up in cold weather, aux never helps — aux not staging in (bad W2 path, sequencer/relay, blown strip, outdoor lockout set too low so strips never get permission). Verify aux actually energizes on a hard call.
• Cold air during defrost — aux not engaging during defrost; check the defrost-board aux output.
• Heat pump short of capacity well above the expected balance point — likely undercharged, dirty coil, or airflow problem making the real balance point higher than design. Fix the heat pump before adding backup logic.
• Em heat works, normal heat doesn't — points at the compressor/outdoor unit or its control, since em heat bypasses it. Good diagnostic split.

Tech tips & gotchas

• Aux and em heat are the same heating elements — the difference is whether the compressor is allowed to run. Teach customers: leave it on normal "Heat," not "Emergency," unless the heat pump is broken.
• Set the strip lockout near the balance point. No reason to let resistance heat fire when it's 40°F out and the heat pump is loafing. That single adjustment saves real money.
• If a customer complains the heat pump "blows cool air," remember a heat pump's supply air is cooler than a furnace's (often ~90–105°F supply vs a furnace's 120°F+). That's normal heat-pump behavior, not a failure — it's still well above body temperature and adding net heat. Don't let that complaint push you to run em heat as a "fix."
• A higher-than-expected balance point is a symptom. If the system needs backup way sooner than it should, the heat pump is underperforming — charge, coil, airflow — not "just how heat pumps are."
• Verify which terminal stages aux (usually W2/AUX) and that the thermostat is configured as a heat pump (not conventional) so it stages correctly.

Safety / code notes

• Electric heat strips draw large currents — confirm conductor sizing, breaker/fuse protection, and the sequencers/relays are rated and functioning before energizing (see the sequencer and transformer articles).
• Strip heat staging via sequencers is intentional — it brings elements on in steps to limit inrush. Don't defeat it.
• In dual-fuel, the changeover logic must lock out the heat pump compressor when the furnace fires (and vice-versa) — see the dual-fuel article. Running both heat sources together can be hard on the equipment and isn't the intent.