Backup Heat: Dual Fuel or Resistance

Why this matters

Every cold-climate heat pump job carries a second decision hiding behind the first: what heats the house in the hours the heat pump cannot. Get it wrong one way and the customer keeps a gas furnace, its monthly gas charge and its combustion risk for a backup that runs a few days a year. Get it wrong the other way and a 10 kW strip heater carries half the winter at a third to a half of the heat pump's efficiency, the service upgrade nobody priced turns up on the electrician's load calculation, and the bill becomes the customer's whole opinion of electrification.

There are three choices: keep a fuel-burning furnace as backup (dual fuel), install electric resistance strips, or install no backup at all. This card lays out what each costs in comfort, in operating cost and in electrical service, and how to compute the one number that sets a dual-fuel system up correctly. How big the gap is, the balance point itself, belongs to Sizing a Heat Pump for a Cold Climate.

The three options side by side

Three mechanics behind that table are worth understanding.

A dual-fuel system is either-or. In the usual arrangement the heat pump's indoor coil sits on the furnace's discharge. In heating mode that coil is the heat pump's condenser. If the furnace heats the air before it reaches the coil, the refrigerant's head pressure climbs and the heat pump trips on its high-pressure safety. So the control runs one or the other, never both, and below the switchover temperature the furnace carries the whole load. Defrost on a dual-fuel system typically brings the furnace on briefly to temper the air, which the control also manages.

Strips work with the heat pump, not instead of it. Resistance strips sit in the air handler downstream of the coil and stage on in increments while the heat pump keeps running. That is the advantage: below the balance point the heat pump still delivers what it can, and the strips cover only the gap. It is also why strip size and staging control matter so much; a control that brings on all 10 kW at the first call for heat, rather than the 5 kW the gap needs, doubles the backup's cost.

No backup is a real option, with a real exposure. A cold-climate unit that covers the full design load needs no backup for comfort. But design temperature is the 99 percent condition, so about 88 hours a year are colder, and a unit that fails during a cold snap leaves the house with no heat at all. In a house with water pipes in exterior walls, that is a freeze risk, not just a comfort complaint.

The number that sets up dual fuel: the switchover temperature

A dual-fuel control switches from heat pump to furnace at an outdoor temperature someone has to choose. There are two candidate temperatures and they answer different questions.

The thermal balance point is the outdoor temperature below which the heat pump alone cannot keep up. Below it, the furnace is needed for comfort.

The economic balance point is the outdoor temperature below which the furnace delivers heat more cheaply than the heat pump. It comes from comparing two costs per unit of delivered heat, and you can compute it without a single price in currency, using one ratio.

Let r be the price of a therm of gas expressed in kilowatt-hours: "a therm costs what r kWh cost" on this customer's bills. A therm is 100,000 Btu, which is 29.3 kWh of heat. A furnace at efficiency AFUE delivers 29.3 times AFUE kWh of heat per therm, so its heat costs r divided by (29.3 times AFUE) kWh-prices per kWh of heat. A heat pump's heat costs 1 divided by COP kWh-prices per kWh of heat. They are equal when:

break-even COP = 29.3 x AFUE / r

Above that COP, the heat pump is cheaper. Below it, the furnace is. Read the outdoor temperature at which the heat pump's COP falls to that value from the extended performance data, and that is the economic balance point.

For propane, the same logic uses a gallon instead of a therm. A gallon of propane is about 91,500 Btu, which is about 26.8 kWh of heat, so the constant becomes 26.8 instead of 29.3.

Set the switchover at the higher of the two balance points. If the economic point is higher, switching there saves money and the thermal point never comes into play. If the thermal point is higher, the furnace has to take over there regardless of cost, because the heat pump cannot hold the house.

Worked example: a furnace worth keeping

The house has an eight-year-old 95 percent AFUE gas furnace in good shape, and the customer is adding a 3-ton heat pump with the furnace as backup. From the last few bills, a therm of gas costs about what 11 kWh of electricity costs, so r is 11. The heat pump's extended data gives these heating COPs at maximum capacity (illustrative; read the real table):

Break-even COP: 29.3 times 0.95 is 27.8, and 27.8 divided by 11 is 2.53.

The heat pump's COP passes 2.53 between 35 F (3.1) and 17 F (2.3). Over those 18 degrees the COP falls by 0.8, about 0.044 per degree. From 17 F, the COP has to rise by 2.53 minus 2.3, which is 0.23, and 0.23 divided by 0.044 is about 5.2 degrees. So the economic balance point is about 22 F.

The installer's balance-point worksheet put the thermal balance point at 12 F. The economic point, 22 F, is higher, so the switchover goes at 22 F. Between 12 F and 22 F the heat pump could hold the house, but the furnace does it more cheaply on this customer's rates.

Now the same house without the furnace. The gap at the 5 F design temperature, from the sizing worksheet, is 9,500 Btu/h, which is 9,500 divided by 3,412, about 2.8 kW. A 5 kW strip covers it. That strip runs only below 12 F. And every unit of heat it delivers costs, against the gas furnace, 2.53 times as much, because resistance has a COP of 1 and the break-even COP was 2.53. Against the heat pump at 5 F, strip heat costs 1.8 times as much.

Electrically, a 5 kW strip at 240 volts draws 5,000 divided by 240, about 20.8 amperes. Fixed electric space heating is treated as a continuous load in NEC Article 424, so its circuit is sized at 125 percent of that, about 26 amperes, which lands on a 30-ampere circuit, in addition to the heat pump's own circuit. In a house with service headroom that is a routine addition. In a house already near its limit it is the line that triggers an upgrade. See related: Measuring Existing Load Before You Sell a Service Upgrade.

So for this customer, dual fuel is the cheaper system to run and the cheaper one to install. That is not the end of the conversation, for the reasons below.

What changes the answer

The fixed charge. Keeping gas for backup keeps the monthly gas customer charge, which does not shrink with use. A furnace that runs only below 22 F may burn so little gas that the fixed charge outweighs the fuel saved. If the water heater and range are going electric too, removing the gas service entirely eliminates that charge, which can tip the whole-house economics toward strips even though each unit of strip heat costs more. See related: Keeping Gas for One Appliance.

Rates move. The switchover temperature is a function of r, and r changes every time either utility changes its rates. If gas rises so a therm costs what 14 kWh cost, the break-even COP falls to 27.8 over 14, about 2.0, and the economic balance point drops to about 10 F, below the thermal point, so the switchover moves down to the thermal point. Set the switchover from current bills, and tell the customer it can be revisited. A time-of-use electric rate makes r different at different hours, which some controls can follow and most cannot. See related: Time-of-Use Rates and Equipment Settings.

The furnace's age and condition. An eight-year-old furnace is worth keeping as backup. A 25-year-old one with a cracked-heat-exchanger history is not a backup; it is a combustion appliance you are choosing to keep in the house. Replacing a failing furnace just to serve as backup is rarely the cheaper path.

The customer's reason for converting. A customer converting to get combustion out of the house will not want dual fuel at any economic balance point. Take that at face value.

The setup mistakes that undo the choice

Strips with no outdoor lockout. Without an outdoor temperature lockout, many controls bring strips on during morning recovery from a setback even at 45 F, because the heat pump takes longer to recover than the thermostat is willing to wait. A common shop default is to lock strips out above about 35 F except during defrost and emergency heat, then tune it to the house's balance point.

Dual-fuel switchover left at the factory default. The control ships with a switchover setting that knows nothing about this customer's rates. Set it from the worksheet and write the value and the r it came from on the equipment record.

No-backup systems with no plan. If the customer chooses no backup, write down what happens during a January failure: which circuits can carry a portable heater, where the water pipes are vulnerable, and how quickly you can respond.

How to verify the system was set up as decided

At commissioning, record the switchover or strip-lockout temperature, the r and AFUE used to compute it, and the thermal balance point. Watch one changeover if the weather allows, or simulate the outdoor sensor reading at the control's test mode if the manufacturer provides one, without forcing any combustion safety or pressure safety out of the loop. After the first winter, compare backup run hours against what the balance-point worksheet predicted. A large overrun usually means the lockout or staging is wrong, not that the heat pump is too small.

References

• Manufacturer extended performance data (capacity and COP by outdoor temperature) for the matched system.
• NFPA 70, National Electrical Code, Article 424 (fixed electric space heating) and Article 220 (load calculations), as adopted by your jurisdiction.
• Manufacturer installation instructions for the dual-fuel control or air handler strip kit (switchover, lockout, staging and defrost settings).
• See related: Sizing a Heat Pump for a Cold Climate; Heat Pump Cold-Weather Operation Reference; Heat Strip Vs Dual Fuel Cold Climate Decision Matrix.
• See related: Keeping Gas for One Appliance; Time-of-Use Rates and Equipment Settings.