Two contractors look at the same house. One says the heat pump should hand over to the furnace fairly early in winter. The other says it can keep going much longer. Before deciding who is overselling what, ask a smaller question: is that cutoff about enough heat, or cheaper heat?

Those are different calculations. A useful dual-fuel proposal explains both, then tells you which one drives the controls. “We always set them there” is not much of an answer when you are the person paying the winter bills.

Checked September 17, 2026. This is a way to compare proposals, not instructions for changing thermostat settings or removing backup heat.

The first question: can it keep up with this house?

The thermal balance point is where the heat pump's available output meets the house's heating demand at the same outdoor temperature. Below it, that heat pump cannot cover the full load on its own. PNNL's sizing guide compares the home's load with the exact equipment's performance across outdoor temperatures.

Here is a deliberately fictional example. At one cold outdoor temperature, a house needs 30,000 Btu/h. The proposed heat pump can deliver 24,000 Btu/h there. The shortfall is 6,000 Btu/h, or 20% of the home's demand. A description saying the unit “operates down to” that temperature does not fill the gap.

These figures are not a recommendation for backup size. They show why a minimum operating temperature and sufficient heating output are different promises. Ask the contractor for the home's calculated load and the proposed indoor/outdoor equipment combination's output at the same temperature.

If air sealing or insulation is still planned, start with the sizing-after-weatherization guide. Changing the house changes the calculation.

The second question: which heat costs less at that point?

The economic balance point asks when the other heating system becomes cheaper to run. That depends on delivered electricity and fuel prices, plus the efficiency of the two systems. A heat pump might still have enough capacity when a cost-based control chooses the furnace.

CEE's design guidance treats this as a separate calculation. It also cautions that changing energy prices require seasonal review, that savings can be modest, and that this price comparison leaves carbon costs out. “Cheaper today” is not the same as “best for every goal.”

Ask which rates were used and when they were checked. An old national average is not your current bill. Also ask whether the proposal prioritizes running cost, heat-pump use, comfort, or a combination. The answer should be stated before anyone argues over one outdoor-temperature number.

A cold-climate label doesn't settle either question

ENERGY STAR's current cold-climate criteria include a COP of at least 1.75 at 5°F and heating capacity at that test point of at least 70% of the specified 47°F capacity. These are equipment qualification criteria, not a promise that the system supplies 70% of your house's needs.

That small wording difference matters. A qualifying machine can still be too small for a particular house, while a properly designed system may intentionally retain another heat source. The label helps identify equipment; the proposal still has to explain the match.

Put these two columns beside the quote

| Ask about enough heat | Ask about running cost | |---|---| | What is our calculated heating load at the local design temperature? | Which delivered electricity and fuel prices did you use? | | What can this exact equipment combination deliver there? | Which equipment-efficiency assumptions support the comparison? | | Where does demand exceed heat-pump output? | Where does the price comparison favor the other heat source? | | What covers any shortfall? | When should these assumptions be reviewed? |

Then ask one connecting question: “Which of these results determines the proposed changeover, and how will you verify the controls at commissioning?”

Keep the reply with the proposal. It gives the next contractor something concrete to respond to, and gives you a useful reference when rates or the house change. You do not need to win an argument about heat pumps in winter. You need to know which winter problem your quote actually solves.

For the surrounding project order, return to the air-sealing and energy-upgrade checklist.