Will it still be warm when the thermometer reads ten below?

A heat pump on a mild day is easy. What decides whether you are happy with it is how it behaves when the weather is at its worst. In Syracuse that means a handful of nights a year, but they are the nights you remember.

On the 1991 to 2020 normals for the Syracuse airport, the temperature falls to zero or below about seven nights a year. It falls to 20 or below on more than fifty. A system chosen for an average winter day may not be chosen for those nights.

The City of Syracuse's property code bases heating capability on an outside temperature of minus ten degrees, which is where this guide gets its name.

The question is not whether heat pumps work here. It is what yours will do at the bottom of the thermometer, and what covers the gap.

The US Department of Energy's cold climate sizing guide defines the balance point as the outdoor temperature at which the heat pump's output equals what the house needs. Below it, the unit cannot meet the whole load alone, and a backup source has to carry the rest.

Every heat pump has one for every house. A contractor can calculate it from the unit's published output at low temperatures and your house's load. Ask for the number, in writing, and ask what turns the backup on.

The same guide sets out four approaches. You can size for cooling and treat heating as secondary. You can size for cooling but pick a unit with strong heating output. You can size to cover most of the heating load with backup heat for the coldest days. Or you can size to cover all or nearly all of the load.

The first approach gives the warmest balance point, which means more hours on backup. The last gives the coldest, so the heat pump carries more of the winter. None is the wrong answer in itself. The right one depends on your house, your backup and how you want the system to run.

Ask the contractor which approach the quote follows. If the answer is that they size for cooling and that is that, ask what warms the house below the balance point.

Backup heat comes in several forms. Some houses use electric resistance strips, built into the air handler. Others keep the existing furnace or boiler to take over in the coldest weather. Others rely on a second heat source in a particular room.

The federal guide says electric resistance backup is an inefficient and usually expensive form of heat, and that it should be kept to a minimum in every heat pump project. Ask the contractor how many hours a year the backup is expected to run, and what it does to your electric bill.

If you have a gas furnace now, ask whether it can stay as the backup. That keeps your existing venting question alive, so ask what happens to the chimney and the water heater in that case.

The Northeast Energy Efficiency Partnerships publish a specification for cold climate air source heat pumps. Its version 4.0 asks for a variable capacity compressor and a minimum efficiency at 5 degrees Fahrenheit.

NEEP warns that meeting its specification does not make a product right for every home. Both the contractor and you should review the building load and the unit's capacity at the design temperature before choosing.

In practice, ask for the unit's output at the coldest temperature in its table, and compare it with your load at your design temperature. If the contractor cannot tell you either number, ask for both.

NYSERDA says oversizing a heat pump can cause effects on comfort and energy use starting at 120 percent of the building load, with severe effects above 150 percent. It adds that an oversized heat pump is a bigger problem than an oversized furnace or boiler.

Inverter units run best in a middle band of their output. A unit that is too large spends more hours below that band and cycles, and NYSERDA says that can cost as much as 25 percent of efficiency in low load periods.

It follows that a bigger unit is not a safer unit. Ask the contractor for the load worksheet behind the size, as you would for a furnace.

A heat pump either uses ducts or avoids them with indoor heads on the wall. If it uses your ducts, ask whether they can carry the airflow. If it uses heads, ask how many rooms each one serves and what happens in the room farthest from a head.

A house that never heated evenly may have a duct problem that a new unit will inherit. Read our guide on sizing and our page on duct work before you decide.

Syracuse airport records an average of about 124 inches of snow a year. An outdoor unit that sits at ground level can be buried, and drifts form along walls. Ask the contractor how high the unit will stand and where the snow will pile.

Ask also how the unit drains in cold weather and where melt water goes. A good plan shows the pad or bracket, the height and the clearance from the wall on the quote or in a note.

The same question applies to a side wall vent on a furnace. Both are exposed to the same snow, and both belong on your walk around the house with the contractor.

Look for these lines on any heat pump quote: the outdoor and indoor models, the output at a low outdoor temperature, the balance point, the backup heat and the point where it starts, the load worksheet, and what changes in the ducts. A quote that has all of them is easy to compare.

Should a line be absent, request it before you decide. The missing line is often the one that matters on the coldest week.

Some contractors reassure you that a heat pump will keep up. Ask how they know. A good answer mentions your load worksheet, the unit's published output at low temperatures and the backup plan.

A weak answer mentions only the brand or the size on the nameplate. Ask what the unit puts out at the coldest outdoor temperature, and ask what the backup will do on that night. A contractor who can give you both has done the arithmetic.

Before any visit, write down three questions. What is the balance point? What is the backup, and when does it start? What load worksheet produced the size?

Then compare quotes. The ones that answer all three can be compared with each other. The ones that do not need a phone call.

We do not size or install heat pumps. We give you the questions and introduce a local contractor. You decide whether anyone visits and whether you book.