# Heat Pump Auxiliary Heat: Backup Settings That Keep Bills Down > Heat pump auxiliary heat explained: heat strips, emergency heat, Seattle's 40°F and 35°F lockouts and dual fuel switchover settings. Check your thermostat. URL: https://seattleheatpumpco.com/guide/heat-pump-backup-heat-settings/ Updated: 2026-10-21 *Backup heat is where a well-chosen heat pump either saves money or quietly wastes it. Here is how auxiliary heat, emergency heat and dual fuel changeover should be set in Seattle.* Heat pump auxiliary heat is the backup that helps when the heat pump alone cannot keep the house at the setpoint, usually electric resistance heat strips in the air handler or, in a dual fuel system, a gas furnace. It should run rarely. Seattle's energy code keeps supplemental heat off above 40°F and requires the lockout set to 35°F or lower at final inspection. [The Seattle Heat Pump Company](https://seattleheatpumpco.com/) plans ducted heat pump and dual fuel systems across Seattle and King County, including how the backup is staged and controlled, with installation by licensed and insured contractors. This guide explains the difference between auxiliary and emergency heat, how heat strips and furnaces behave as backup, how to think about a dual fuel switchover temperature, and the thermostat settings that keep backup heat from quietly running up the bill. **Key takeaways** - Auxiliary heat runs automatically; emergency heat is a manual mode that shuts the heat pump off. - Seattle's code bars supplemental heat above 40°F and requires the lockout at 35°F or lower at final inspection. - Resistance heat strips have a COP of 1; a heat pump in typical Seattle winter weather delivers several times as much heat per kWh. - Deep thermostat setbacks are the most common reason backup heat runs when it shouldn't. ## Auxiliary heat versus emergency heat The two terms appear on most heat pump thermostats, and they mean different things. **Auxiliary heat** is automatic. The thermostat brings it on when the heat pump is running but the house is still falling behind, when the setpoint jumps several degrees at once, or briefly during a defrost cycle. The heat pump keeps running alongside it. When the gap closes, the backup shuts off. **Emergency heat** is manual. Selecting it turns the heat pump off completely and heats the house with the backup alone. It exists for one situation: the outdoor unit has failed, is iced over beyond normal defrost, or is damaged. Left on by mistake, it runs the most expensive heat in the house all day. ## How heat strips and furnaces work as backup ### Electric resistance heat strips An electric air handler can hold one or more resistance heating elements, called heat strips, sized in kilowatts. They sit in the airstream after the indoor coil and work like a toaster: current flows through the element and it gets hot. Every kWh becomes one kWh of heat. That is a coefficient of performance, or COP, of 1. Strips can be staged, with one element coming on first and a second only if the first is not enough. Staging matters for the bill and for the electrical service, because a large strip kit can draw several times the current of the heat pump. The [electrical panel guide](https://seattleheatpumpco.com/guide/heat-pump-electrical-panel-requirements/) shows how strip size can decide whether a panel upgrade is needed. ### A gas furnace in a dual fuel system In a [dual fuel heat pump](https://seattleheatpumpco.com/dual-fuel-heat-pump/), the heat pump's coil sits on top of a gas furnace. The furnace is the backup. Instead of adding heat alongside the heat pump, the system usually switches between them: the heat pump runs above a changeover temperature and the furnace runs below it, because the coil usually sits downstream of the furnace and the furnace's hot air would drive the heat pump's refrigerant pressures too high. Furnace backup avoids resistance strips and their electrical load entirely. Whether to keep the furnace or retire it is covered under [furnace to heat pump conversion](https://seattleheatpumpco.com/furnace-to-heat-pump-conversion/). ## Heat pump heating versus resistance heating The reason backup heat settings matter is the gap between these two ways of making heat. A heat pump moves heat from outdoor air into the house. In mild, damp Seattle weather, a modern unit delivers several units of heat for every unit of electricity. As the outdoor temperature drops, that ratio falls, but it stays above 1 well below freezing. Resistance heat delivers exactly 1 at any temperature. On City Light's current Seattle rate of 13.38 cents per kWh, one million BTU of heat from resistance strips costs about $39 in energy charges. The same heat from a heat pump averaging a COP of 3 costs about $13. Every hour the strips carry the house instead of the heat pump costs roughly three times as much for that hour. The [operating cost guide](https://seattleheatpumpco.com/guide/heat-pump-operating-cost-seattle/) works through the full comparison, including PSE's higher rate. That is also why "will a heat pump raise my bill" questions often come back to backup heat. A system that leans on strips because it is undersized, poorly configured or short of airflow can cost far more to run than one that rarely needs them. ## What Seattle's code requires Seattle's energy code addresses backup heat directly. Heat pump controls must minimize the use of supplemental heat, supplemental heat may not run when the outdoor temperature is above 40°F, and the auxiliary heat lockout must be set to 35°F or lower at final inspection. In practice, that means the thermostat or control board needs an outdoor temperature reading, either from an outdoor sensor or from the outdoor unit, and the lockout has to be programmed rather than left at a factory default. The inspector checks the setting. The code's intent matches good practice: with the heat pump sized to a [Manual J load calculation](https://seattleheatpumpco.com/guide/heat-pump-sizing-manual-j/) for Seattle's 24°F design temperature, backup is needed only on the coldest hours. Seattle's weather makes this a light constraint. Sea-Tac averaged about 25 nights a year at or below freezing over the 1991 to 2020 normals and about one night a year at 20°F or below. The [Seattle winter guide](https://seattleheatpumpco.com/guide/do-heat-pumps-work-in-seattle-winter/) covers what happens on those nights. ## Choosing a dual fuel switchover temperature The switchover temperature, also called the changeover point or compressor lockout, is where a dual fuel system hands off from the heat pump to the furnace. Two different balance points inform it. **The thermal balance point** is the outdoor temperature below which the heat pump can no longer meet the house's heat loss on its own. It comes from the load calculation and the model's published low-temperature capacity. **The economic balance point** is the outdoor temperature below which gas becomes cheaper per unit of heat than the heat pump. It depends on your rates. Using current energy charges and PSE gas at about $1.67 per therm, a heat pump beats a 95% efficient gas furnace whenever its COP is above about 2.2 on City Light's Seattle rate, about 2.4 on City Light's suburban rate, and about 3.4 on PSE's electric rate of about 20.6 cents per kWh. Those figures lead to different settings in different places. Inside Seattle, a modern heat pump's COP stays above 2.2 through most winter weather, so the economic case for a high switchover is weak, and the thermal balance point usually governs. On the Eastside, where PSE sells both fuels, the heat pump's advantage narrows as it gets colder, so a higher switchover can make economic sense. PSE customers with both PSE gas and electric service can also qualify for PSE's hybrid heat pump rebate, covered in the [PSE rebate guide](https://seattleheatpumpco.com/guide/pse-heat-pump-rebates/). PSE has also requested further increases of 17.75% for electricity and 12.93% for gas, which are not approved; if approved, they would push the PSE break-even higher. The exact temperature depends on the specific model's COP curve, which is why it should be set from the manufacturer's data rather than a default. One Seattle dual fuel owner asked whether raising the switchover to 45°F would hurt anything because the heat pump seemed to run constantly. It would not harm the equipment, but on City Light rates it would usually cost more. A heat pump that runs constantly in mild weather is often doing exactly what a [variable-speed unit](https://seattleheatpumpco.com/guide/variable-speed-vs-single-stage-heat-pump/) should. If the house is actually falling behind, sizing and airflow are the first suspects, and [checking the existing ductwork](https://seattleheatpumpco.com/heat-pump-existing-ductwork/) is where airflow problems get found. ## Setbacks and morning recovery A deep nighttime setback is the most common way to waste backup heat. With a furnace, dropping to 62°F overnight and jumping back to 70°F in the morning saves fuel. With a heat pump, the thermostat sees an eight-degree gap at 6am, decides the heat pump cannot close it fast enough, and brings on the strips. Small setbacks of a couple of degrees, or none at all, usually cost less with a heat pump. Many heat pump thermostats offer adaptive or smart recovery, which starts warming earlier using the heat pump alone. Some also let you set a temperature differential, sometimes called droop, that must open up before auxiliary heat is allowed. ## Thermostat settings to check A thermostat left on furnace settings can undo good equipment. At commissioning, and any time the thermostat is replaced, these should be configured: - system type set to heat pump, with the correct reversing valve setting for the brand - number of compressor stages and auxiliary heat stages - outdoor temperature source connected and reading correctly - auxiliary heat lockout at 35°F or lower, and no supplemental heat above 40°F - for dual fuel, the compressor lockout or switchover temperature - recovery behavior and any auxiliary heat differential ### Thermostat compatibility Some variable-speed systems use a communicating control that only works with the manufacturer's own thermostat; a third-party thermostat may run the system in a reduced mode. Others accept any heat pump thermostat that supports the right number of stages. Check compatibility before buying a smart thermostat. Installed, a smart thermostat runs about $200 to $500 in national figures (HomeGuide). Commissioning should end with someone walking you through these settings. The [installation process guide](https://seattleheatpumpco.com/guide/heat-pump-installation-process/) covers the full commissioning checklist, and the [how a heat pump works guide](https://seattleheatpumpco.com/guide/how-a-ducted-heat-pump-works/) explains defrost and why backup sometimes runs during it. For a system planned with the backup and controls designed together, start with [dual fuel heat pumps](https://seattleheatpumpco.com/dual-fuel-heat-pump/) or a [ducted heat pump installation](https://seattleheatpumpco.com/ducted-heat-pump-installation/). ## Questions ### Why does my thermostat say aux heat when it isn't very cold out? Usually because the thermostat is asking for a large temperature jump, such as morning recovery from a deep setback, and it calls backup heat to close the gap faster. Defrost cycles can also bring aux on briefly. If it happens often above 40°F, the lockout or staging settings are worth checking, along with airflow and the system's charge. ### What is the difference between auxiliary heat and emergency heat? Auxiliary heat runs automatically alongside the heat pump when the heat pump alone cannot keep up or during defrost. Emergency heat is a manual mode that shuts the heat pump off and runs only the backup. Use emergency heat only if the outdoor unit has failed or is damaged, because running on backup alone costs far more. ### Should I raise my dual fuel switchover temperature so the furnace runs more? It depends on your rates and your equipment. Inside Seattle, on City Light electricity, a heat pump beats a 95% gas furnace whenever its COP is above about 2.2, so raising the switchover usually costs money. On PSE electricity the break-even is about 3.4, so a higher switchover can make sense. If the heat pump struggles in mild weather, check sizing and airflow first.