How does a dual-fuel system decide when to switch from the heat pump to the furnace?
The thermostat or control switches at an outdoor temperature you set: above it the heat pump heats the house, below it the furnace takes over. Set that point at the warmer of two temperatures: where the heat pump stops keeping up with your house, and where furnace heat becomes cheaper at your rates.
Key points
- Dual fuel pairs a heat pump (in place of your AC) with a gas or propane furnace on the same ducts.
- Set the switchover at the warmer of two points: where the heat pump can't keep up, and where the furnace becomes cheaper.
- You can work out the cost point from your own bills and the heat pump's COP; no rate forecast needed.
- CenterPoint's ducted heat pump rebate requires pairing with a gas furnace; co-ops such as Connexus and Dakota Electric run dual-fuel programs.
- A healthy existing furnace can usually stay; the heat pump replaces the air conditioner.
A dual-fuel system switches at an outdoor temperature you choose. Above it, the heat pump heats the house; below it, the gas or propane furnace takes over, and the thermostat or control reads the outdoor temperature to decide. Set that switchover at the warmer of two points: where the heat pump stops keeping up with your house, and where furnace heat becomes cheaper than heat pump heat at your rates. Both can be worked out from your house and your own bills.
The setup itself is simple: a heat pump and a furnace sharing one set of ducts. The heat pump takes the place of your central air conditioner, heats the house in mild and moderate weather, and cools it in summer. The furnace takes the cold end of winter. After the install, the one decision that really matters is where the handoff happens, and it is worth setting on purpose instead of leaving it at a default.
What are the parts of a dual-fuel heating system?
- Outdoor unit: a heat pump in place of the AC condenser.
- Indoor coil: sits on the furnace where the AC coil would. The heat pump moves heat through it in winter.
- Furnace: its blower moves the air for both systems, and its burners are the backup.
- Thermostat or control: one that supports a heat pump with fossil-fuel backup and reads the outdoor temperature.
For normal heating, the controls run the heat pump or the furnace, not both. The heat pump's coil sits in the furnace's hot discharge air, and furnace heat blowing across it while it is heating would drive its pressures too high. Mitsubishi's ducted version, intelli-HEAT, adds a second trigger: Mitsubishi says it also switches to the furnace when the heat pump can't hold the indoor temperature for a set time in extreme cold.
Two switchover points, and you use the warmer one
Two temperatures matter.
The comfort point (thermal balance point) is where the heat pump's output falls to your house's heat loss. Below it, the heat pump can't keep up alone. A leaky 1950s rambler reaches it at a warmer temperature than a tight 2010s two-story with the same heat pump.
The cost point (economic balance point) is where a unit of heat from the heat pump costs the same as a unit of heat from the furnace. Heat pump efficiency falls as it gets colder, so below some temperature the furnace is the cheaper heat.
Set the switchover at whichever of the two is warmer. If the heat pump can keep up down to 5°F but the furnace becomes cheaper below 20°F, switch at 20°F. If the heat pump stays cheaper well below zero but can only keep up to 10°F, switch at 10°F.
How do you work out the dual-fuel cost point from your bills?
You don't need anyone's rate forecast. You need three numbers: what you pay per therm of gas, what you pay per kilowatt-hour, and the heat pump's COP (heat delivered per unit of electricity) at the temperature in question.
- A therm is 100,000 BTU. Through a 96% furnace it delivers 96,000 BTU of heat.
- A kilowatt-hour is 3,412 BTU. At a COP of 2.0, a heat pump turns it into about 6,800 BTU of heat.
- So at COP 2.0, about 14 kWh of heat pump electricity matches one therm burned in a 96% furnace.
| Heat pump COP at that temperature | kWh that match one therm (96% furnace) |
|---|---|
| 1.75 | about 16 |
| 2.0 | about 14 |
| 2.5 | about 11 |
| 3.0 | about 9.4 |
Divide your price per therm by your price per kWh. If the result is bigger than the number in the table, the heat pump is the cheaper heat at that COP. Use all-in prices from your bills, including delivery and fuel-adjustment charges, rather than the base rate alone. The NEEP cold-climate list shows COP at 5°F and at milder test temperatures for listed systems. COP drops as the temperature drops, so your cost point is the temperature where your ratio stops beating the table. For propane, the gallon-based version is in our propane-to-heat-pump guide.
Putting both points together, in order:
- Get the heat pump's heating capacity and COP at 47°F, 17°F and 5°F from its submittal sheet or NEEP listing.
- Get your house's heat loss from the Manual J load calculation done for the install.
- Find the outdoor temperature where the heat pump's output drops to that heat loss. That's the comfort point.
- Work out your price ratio from your bills and find the COP where the heat pump stops winning. Match that COP to a temperature on the submittal. That's the cost point.
- Set the switchover at the warmer of the two, watch the first cold month, and adjust a few degrees if the house or the bills say so.
How can utility programs change the dual-fuel switchover?
Your utilities can move the math, so ask before settling on a switchover:
- CenterPoint Energy offers a ducted air-source heat pump rebate only when the heat pump is paired with a gas furnace as a hybrid system.
- Xcel Energy says electric customers who heat mainly with a heat pump can get a reduced space-heating rate. Ask whether your dual-fuel setup qualifies.
- Connexus Energy runs a reduced-rate heat pump demand-response (dual-fuel) program, and Dakota Electric lists a rebate for first-time dual-fuel program enrollment.
Which bills you're comparing depends on where you live. In Saint Paul, Xcel Energy supplies both gas and electricity, so one utility's rates set both sides of the ratio; in Minneapolis, gas comes from CenterPoint Energy and electricity from Xcel. Our pages on heat pumps in Saint Paul and heat pumps in Minneapolis cover how we set dual fuel up in each city's older houses.
Dual-fuel and demand-response programs generally let the utility switch off the electric heat during peak periods, and the furnace covers those hours. That is one more reason the furnace has to be healthy. Terms and rates change, so get the current rules from the utility itself.
Can you add a heat pump to the furnace you already have?
If the furnace is in decent shape, you don't have to replace it. We check:
- Furnace age and condition. Pairing a new heat pump with a furnace near the end of its life means a second project soon. We usually replace a 20-year-old furnace at the same time; a 7-year-old one usually stays.
- Blower and airflow. A heat pump needs steady airflow across its coil. A variable-speed blower handles that well; an older single-speed blower needs its settings checked.
- Coil and line set. New heat pumps use lower-GWP refrigerants such as R-454B or R-32, so the old line set is checked for size and condition, and sometimes replaced.
- Thermostat. It has to support dual fuel. Our thermostats and zoning page covers the options.
A heat pump with us runs $7,500–$14,500 installed, and an add-on to a healthy furnace usually sits in the lower part of that range. If it is time for both, a new furnace runs $4,500–$9,500. Our heat pump installation cost guide walks through what moves a job up or down.
After the install: settings and troubleshooting
- Switchover temperature, set from your house and your rates.
- Defrost behavior. Many dual-fuel controls fire the furnace briefly during defrost so you don't feel cool air. Our defrost post explains what to expect.
- Emergency heat, which runs the furnace alone. Use it only if the heat pump fails, and switch back to normal heat afterward.
| What you notice | Likely cause | Fix |
|---|---|---|
| The furnace runs on mild fall days | Switchover set too warm | Reset it from your bills and the heat pump's COP |
| The house falls behind on the coldest nights | Switchover set colder than the comfort point | Raise the switchover a few degrees |
| Cool air from the registers for a few minutes | Defrost with no furnace assist | Have the control set to fire the furnace during defrost |
| The heat pump never seems to run | Emergency heat left on, or the switchover set very warm | Return to normal heat mode, then check the setting |
| The outdoor unit is buried in snow or ice | Drifts, roof runoff, or a unit set too low | Clear snow gently, never chip ice, and call if it keeps icing |
To have the switchover worked out for your house, call 612-429-3465. For how heat pumps hold up in the deepest cold, read do heat pumps work in Minnesota winters, and see our heat pump page for what an install includes.
Questions
What temperature should a dual-fuel system switch to the furnace?
Set it at the warmer of two points: the temperature where the heat pump stops keeping up with your house, and the temperature where furnace heat becomes cheaper than heat pump heat at your rates. There is no single right number. It varies a lot between a drafty older house and a tight newer one, and between natural gas and propane.
Do the heat pump and furnace ever run together?
Not for normal heating. The heat pump's coil sits in the furnace's hot discharge air, so dual-fuel controls run one or the other. The exception is defrost: while the heat pump briefly reverses to melt frost off the outdoor coil, many controls fire the furnace so the registers keep putting out warm air.
Can I add a heat pump to a propane furnace?
Yes. Dual fuel works the same way with propane as with natural gas. The switchover math uses your propane price per gallon instead of a price per therm, and with propane the heat pump often stays the cheaper heat down to a colder temperature, so it carries more of the winter.
Does CenterPoint give a rebate for dual fuel?
In a way, yes. CenterPoint Energy lists a ducted air-source heat pump rebate that applies only when the heat pump is paired with a gas furnace as a hybrid system. Amounts and rules change each year, so check CenterPoint's current page before you buy. We help with the paperwork where the program allows it.
Sources
- NEEP cold-climate list ashp.neep.org
- ducted air-source heat pump rebate centerpointenergy.com