# What is heat pump aux heat, and when should it run in Sacramento?

> Heat pump aux heat explained for Sacramento: when backup strips should run, why tule fog means frequent defrost, lockout settings, and dual fuel as backup.

URL: https://tulesacramentoheatpump.com/guide/heat-pump-aux-heat-defrost/

Learn

How backup heat strips, defrost cycles and thermostat controls work together, why the valley floor rarely needs strips for capacity, what they cost to run on each utility's rates, and when a furnace makes the better backup.

Aux heat is the backup a heat pump calls on when it cannot meet the load alone, usually electric resistance strips in the indoor air handler. On the Sacramento Valley floor it should run rarely: winter design temperatures sit around 30°F and nights at or below 20°F are almost unknown. Most aux heat here should come from defrost cycles on foggy mornings, not from cold.

[Tule Sacramento Heat Pump](https://tulesacramentoheatpump.com/) installs ducted heat pumps across the Sacramento region, with licensed and insured contractors doing the work, and setting up backup heat controls is part of every commissioning. This guide explains what those settings decide.

Key takeaways

- NOAA normals show 0.0 to 0.2 nights a year at or below 20°F at the Sacramento airport stations
- January mornings around 41°F with dew points near 41°F make frost and frequent defrost the main winter issue
- Backup strips heat at a COP of 1, which costs more per unit of heat than a 95% gas furnace even on SMUD's off-peak rate
- Title 24 does not allow electric resistance as the primary heat in a replacement system, apart from listed exceptions
- Lockout settings and thermostat setup decide whether strips run only when needed

## What counts as backup heat

### Electric strips

Resistance coils in the air handler, switched on in stages by the thermostat. They turn electricity into heat one for one. Simple and reliable, but the most expensive heat in the house.

### A gas furnace

In a [dual fuel system](https://tulesacramentoheatpump.com/dual-fuel-heat-pump/), the existing furnace stays as the backup. The controls switch from heat pump to furnace below a set outdoor temperature, and the two never run against each other.

### No backup

Some valley-floor houses with a well-sized system and good envelope could run without strips for capacity. Title 24 also limits electric resistance as primary heat in a replacement system. Many designs still include a small strip stage for defrost comfort and for the occasional colder morning.

## How cold the valley floor gets

NOAA’s 1991-2020 normals count about 15 nights a year at or below freezing at Sacramento Executive Airport and about 23 at Sacramento Metro Airport, with 0.0 and 0.2 nights at or below 20°F. Woodland averages about 9 freezing nights, Davis about 18.

The foothills differ. Placerville averages about 30 freezing nights and 0.4 at or below 20°F, and its 0.2% heating design is 26°F, as are Rocklin’s and Citrus Heights’. Capacity at low temperatures matters more there, as it does for homes in [Rocklin](https://tulesacramentoheatpump.com/service-areas/rocklin/) and [El Dorado Hills](https://tulesacramentoheatpump.com/service-areas/el-dorado-hills/) at the region’s edges.

| Station | Nights at or below 32°F | Nights at or below 20°F |
| --- | --- | --- |
| Sacramento Executive Airport | 15.4 | 0.0 |
| Sacramento Metro Airport | 22.8 | 0.2 |
| Davis 2 WSW | 17.7 | n/a |
| Auburn | 19.6 | 0.2 |
| Placerville | 30.0 | 0.4 |

### Why cold-climate equipment is not required here

Cold-climate heat pumps are built to hold capacity far below freezing. The valley floor rarely asks for that. Equipment sized for Sacramento’s summer afternoon usually has enough heating capacity at a 31°F design morning, which the [sizing guide](https://tulesacramentoheatpump.com/guide/heat-pump-sizing/) explains. Foothill homes are the exception worth a closer look.

## Tule fog and defrost

The real winter test is damp air near freezing. NOAA’s hourly normals at Sacramento Executive Airport put the January 7 a.m. normal at 41.6°F, with one morning in ten at 32.3°F or colder, and the mean January dew point at 41.3°F.

### Why frost forms

To pull heat from outdoor air, the outdoor coil runs colder than the air around it. On a damp morning that coil drops below the dew point and below freezing, so moisture freezes onto the fins. Frost blocks airflow and cuts capacity.

### What a defrost cycle does

The control senses frost, through coil temperature, run time or both depending on the design, and reverses the system into cooling mode for a few minutes. Hot refrigerant runs through the outdoor coil, the frost melts, and the outdoor fan usually stops so the coil warms faster. You may see steam and water running off the unit. Then it switches back.

### What you feel indoors

During defrost the indoor coil is briefly cooling. Without backup, air at the registers turns cool for a few minutes. That is the main job for a small strip stage in this climate, and it is why aux heat often lights up on fog-season mornings.

### Normal frost versus a problem

Light frost that clears every cycle is normal. A coil that stays iced, ice building on the base or top, or a unit encased in ice points to blocked airflow, a defrost control fault or low refrigerant. Those need [heat pump repair](https://tulesacramentoheatpump.com/heat-pump-repair/), not a thermostat change.

> Drainage matters more than snow
>
> NOAA shows no measurable annual snowfall at valley-floor stations, so snow stands are not needed. The pad should let defrost water drain away from the unit and the foundation so it cannot refreeze underneath; the
>
> placement guide
>
> covers pads and clearances.

## What backup heat costs to run

Electric strips have a COP of 1. Working from published tariffs and PG&E’s residential gas baseline, the break-even COP against a 95% gas furnace is about 1.41 on SMUD’s winter off-peak rate and about 3.13 on PG&E’s E-ELEC off-peak rate. Strips fall below both, so every hour on strips costs more per unit of heat than the furnace would have.

The heat pump itself clears the SMUD bar easily. That is why the controls matter: a system that uses its strips freely can lose much of the savings that justified it. The [electric bill guide](https://tulesacramentoheatpump.com/guide/heat-pump-electric-bill-sacramento/) runs seasonal costs by utility.

Set up backup heat that runs only when it should

Commissioning covers defrost, strip staging and the thermostat lockouts, with the settings explained before handover.

## Controls that keep strips in check

### Outdoor temperature lockout

An outdoor sensor tells the system not to run strips above a set temperature, except during defrost. The setting belongs at or below the balance point from the load calculation.

### Setback recovery

A deep night setback followed by a jump in the morning can make the thermostat call for strips to catch up quickly. Smaller setbacks, or a thermostat with heat pump recovery logic, let the compressor do the work over a longer ramp.

### Staging

Multi-stage strips should come on one stage at a time, with the heat pump running first.

### Thermostat compatibility

A heat pump thermostat controls the reversing valve and backup stages separately; a communicating system may need the matching control from the equipment maker. Title 24 requires a digital setback thermostat when refrigerant components are replaced, and SMUD’s rebate requires a Wi-Fi seven-day programmable connected thermostat; the [SMUD rebate guide](https://tulesacramentoheatpump.com/guide/smud-heat-pump-rebates/) lists its other conditions.

## Supply air and comfort expectations

A heat pump’s supply air is milder than a gas furnace’s and runs for longer cycles. Held up to a register, that air can feel cool next to a furnace’s blast, even while the room holds its setting. Strips raise supply temperature but at high cost, so they should not be used to make air feel warmer.

## Power outages

Every option here needs electricity. A heat pump and its strips stop in an outage, and so does a gas furnace, whose blower, igniter and controls are electric. Backup heat strategy is about cold and cost, not outages.

## Choosing the backup

On SMUD power a heat pump with a small strip stage suits most valley homes. Where electricity costs more, as on PG&E rates, or where a furnace is young enough to keep, [dual fuel](https://tulesacramentoheatpump.com/dual-fuel-heat-pump/) can make sense, and strip size affects the [panel question](https://tulesacramentoheatpump.com/guide/heat-pump-electrical-panel/). To plan backup heat around your house and rate, call (916) 516-1454 or [request an estimate](https://tulesacramentoheatpump.com/contact/).

## Questions about this guide

### Is it a problem if the aux heat indicator comes on during defrost?

No. During a defrost cycle the heat pump briefly runs in cooling mode to warm the outdoor coil, and many systems bring on the backup strips for those minutes so the air at the registers does not turn cool. The indicator should go off when defrost ends. Aux heat that stays on for long stretches on a 40-degree morning is the pattern worth having checked.

### What is the difference between aux heat and emergency heat?

Aux heat is automatic: the thermostat adds backup heat alongside the heat pump when it is needed, then drops it. Emergency heat is a manual setting that shuts the heat pump off and runs backup alone, meant for when the outdoor unit has failed. Leaving a system on emergency heat runs resistance strips at a COP of 1, which costs far more than the heat pump.

### Above what outdoor temperature should backup strips be locked out?

It depends on the house. The lockout belongs at or below the balance point, the outdoor temperature where the heat pump's output matches the home's heat loss, which comes from the load calculation and the equipment's capacity table. Set well below that, the strips run only when the heat pump truly cannot keep up, plus during defrost if the system is set up that way.

Want this answered for your house?

Call with your current system, the rooms that bother you and your ZIP code.

## Related service

Dual fuel heat pump

A heat pump added to a gas furnace that still has life in it, with the changeover point set from your utility's rates and the house's heat loss.

## More guides

- Learn [How is a heat pump sized for a Sacramento home?](https://tulesacramentoheatpump.com/guide/heat-pump-sizing/)How a load calculation turns your house, your town's design temperatures and your ducts into a capacity, why the old condenser's tonnage is a poor guide, and what Title 24 and the City permit allow.
- Learn [What do SEER2 and HSPF2 ratings mean for a Sacramento heat pump?](https://tulesacramentoheatpump.com/guide/seer2-hspf2-ratings/)How the cooling and heating efficiency ratings are built, how to turn HSPF2 into a COP you can compare with your utility's break-even figure, and why installation quality decides whether a system reaches its rating.
- Learn [When does a heat pump need an electrical panel upgrade?](https://tulesacramentoheatpump.com/guide/heat-pump-electrical-panel/)What a heat pump adds to a home's electrical load, how a service-load calculation decides whether the panel can carry it, what an upgrade typically costs, and where SMUD's panel and circuit rebates fit.
- Decision Guide [Is a heat pump or a gas furnace better for a Sacramento home?](https://tulesacramentoheatpump.com/guide/heat-pump-vs-gas-furnace-sacramento/)How the two compare on Sacramento's winter mornings, what each costs to run under SMUD, Roseville Electric and PG&E rates, how they differ in comfort, safety and upkeep, and which households should keep gas for now.
- Learn [How much will a heat pump add to your electric bill in Sacramento?](https://tulesacramentoheatpump.com/guide/heat-pump-electric-bill-sacramento/)How heating moves from the gas bill to the electric bill, what a therm of heat costs under each local rate plan, how summer peaks and fixed charges work, and the settings that keep a heat pump's bill from creeping up.