Backup is a design choice, not a universal requirement
Some heat pump systems use electric resistance elements in the air handler. Others retain a gas or oil furnace and switch between fuels. Some designs can meet the specified heating load without a second heat source. PNNL describes backup and cold-weather capacity as part of system selection. The correct arrangement depends on the house, equipment, climate, and desired contingency plan.
Distinguish three reasons for backup: a heat shortfall in very cold weather, temporary support during certain operating modes, and heating during a compressor fault. These can lead to different control decisions. Ask which of those needs the proposal addresses.
Electric strips versus dual fuel
| Issue | Electric resistance backup | Furnace backup |
|---|---|---|
| Energy source | Electricity | Gas, propane, or oil depending on equipment |
| Installation question | Circuit, service capacity, staged output | Existing furnace condition, fuel and venting |
| Variable operating cost | Electricity rate and hours used | Fuel rate, furnace efficiency and hours used |
| Control question | When and how many stages energize | When the system changes heat sources |
| Ongoing obligation | Electrical and HVAC service | Combustion equipment maintenance and fuel service |
ENERGY STAR identifies retaining a furnace with a heat pump as a dual-fuel option. Compatibility must be verified for the exact indoor and outdoor combination and controls.
What one hour of backup can cost
The following numbers are illustrative assumptions, not typical equipment sizes or utility rates. A 10 kW resistance heater operating for one hour uses 10 kWh. At $0.18/kWh, that costs $1.80. At three full-output hours per day for 30 days, the heater alone would use 900 kWh and cost $162. Partial staging and cycling change the actual result.
A heat pump delivering the same 10 kWh of heat at COP 2.5 uses 4 kWh of electricity, costing $0.72 at that rate. A furnace delivering equivalent heat at 95% AFUE uses about 0.359 therm, costing about $0.54 at an assumed $1.50/therm. The furnace comparison excludes blower electricity and fixed charges. EIA’s energy units provide the conversion basis.
These calculations explain why long periods of resistance backup can materially affect a bill. They do not prove that a furnace is always cheaper: different rates and efficiencies can reverse the comparison. Evaluate the hours and conditions in which backup will actually operate.
Capacity balance and economic balance are different
The capacity balance point is where the building’s heating demand meets the heat pump’s available output. The economic switching point is where the delivered-heat cost of one source overtakes the other. A furnace may be cheaper before the heat pump runs out of capacity, or the heat pump may remain cheaper until backup is physically needed.
Do not choose a universal outdoor lockout temperature from an internet comment. Ask the contractor to use manufacturer performance information and your tariff. The dual-fuel break-even guide shows the fuel-cost calculation; it does not replace installer commissioning.
Controls deserve their own conversation
Ask how thermostat setbacks affect recovery, whether the display distinguishes compressor and auxiliary heat, and which conditions trigger each backup stage. A large manual temperature increase can affect the control sequence on some systems. Use the manufacturer-approved settings and instructions rather than changing installer menus to suppress backup indiscriminately.
Also ask what “emergency heat” means on your thermostat. It is not a generic way to make normal heating faster. Follow the equipment instructions and service advice. If the heat pump fails, the permitted temporary operating mode depends on the installed arrangement and the nature of the fault.
Specify the handover information
The completed proposal should state backup type, rated capacity, electrical or fuel work, normal control sequence, and user instructions for a fault. For all-electric equipment, ask the electrician to evaluate the full proposed load; an empty breaker position alone does not establish service capacity. For retained gas equipment, ask who verifies safe operation and venting.
Record the settings and expected behavior at handover. If a later bill rises, that baseline helps distinguish colder weather from an unexpected change in backup use. The heat pump electricity guide provides a worksheet for separating compressor and backup consumption.