How cold can a heat pump go before it stops?
It depends on the type and the model. Some installation manuals list a low limit where the unit shuts off; others give none. To check yours, look up the manual for the model number on the outdoor unit's nameplate.
- A standard heat pump falls below its rated heating capacity under the mid 40s and gives only a fraction of it by the teens, according to the U.S. Department of Energy's Building America Solution Center.
- A cold climate heat pump certified by ENERGY STAR keeps working below 5 F, though pairing it with backup heat is the most efficient way to heat when it gets colder still.
- Some models list a floor. Lennox's EL18KSLV variable capacity heat pump heats down to -22 F. Below that it stops, restarts on its own when it warms up, and its manual recommends a secondary heat source.
What matters more is the balance point, where it stops keeping up with your house.
Expect cooler air at the registers than from a furnace: Lennox's homeowner notes explain that a heat pump delivers large amounts of warm air, a gas furnace less air that is considerably hotter.
How cold does a Northeast Ohio winter get, and for how long?
Long and cold, with short spells of deep cold. At Cleveland Hopkins Airport the normal January low is 22.3 F, per the National Weather Service Cleveland office.
NOAA's 1991 to 2020 climate normals for the airport station count the cold days:
| Daily low at or below | Average days per year |
|---|---|
| 32 F | 110.8 |
| 20 F | 38.4 |
| 10 F | 12.6 |
| 0 F | 2.7 |
The all time record low is -20 F, set on January 19, 1994, according to the National Weather Service. Those are airport readings, and inland can run colder: that same day, Akron-Canton reached -25 F, the National Weather Service noted.
For a heat pump, that means a long season of nights in the 20s and 30s, when a standard model still heats but below its rated output, and a handful of subzero nights, when output is lowest and the house loses heat fastest.
Cuyahoga, Summit, Lorain, Medina, Lake, Geauga and Portage counties all sit in climate zone 5A on the 2021 International Energy Conservation Code map, according to the Department of Energy's climate zone guide. The Northeast Energy Efficiency Partnerships (NEEP) wrote its cold climate specification for zone 4 and colder, which takes in all seven counties.
What makes a heat pump cold climate rated?
An ENERGY STAR cold climate rating means the heat pump was tested at 5 F and met minimums for both output and efficiency there. ENERGY STAR's criteria require at least 70 percent of its 47 F heating capacity at 5 F, and a COP of at least 1.75 at 5 F. COP, the coefficient of performance, is heat delivered per unit of electricity used.
NEEP keeps its own specification and a public product list. Its Version 4.0 specification requires a variable capacity compressor and a COP of at least 1.75 at 5 F at maximum capacity, but sets no minimum output at 5 F and accepts engineering data as well as lab tests.
| Standard heat pump | Cold climate heat pump | |
|---|---|---|
| Compressor | Single stage, two stage or variable capacity | Variable capacity (NEEP requires three or more speeds, or continuously variable) |
| Test at 5 F | Optional | Required for the ENERGY STAR cold climate designation |
| ENERGY STAR minimum output at 5 F | None | 70 percent of its 47 F capacity |
| ENERGY STAR minimum COP at 5 F | None | 1.75 |
NEEP adds a caution: meeting the specification does not make a unit right for every house, so check the home's heating load and the equipment's capacity at design temperatures. That is why we size every heat pump with a Manual J load calculation, which works out how much heating and cooling your house needs as it stands. Our heat pump page explains the approach.
What is the balance point?
The balance point is the outdoor temperature where the heat pump's output exactly matches the heat your house is losing. Below it, the Building America Solution Center says, the heat pump cannot fully meet the home's needs without backup heat.
It depends on the house and on how the heat pump was sized. In the Solution Center's worked example for a Minneapolis house, the balance point ranged from 40 F for a heat pump sized for air conditioning to -15 F for a cold climate unit sized to carry the whole heating load.
The Solution Center's installation and commissioning guide lists making sure backup heat, gas or electric, runs only below the balance point as a setup check. On a dual fuel system, which we recommend for most Northeast Ohio homes, the controls hand the heating to the furnace at a set switchover temperature.
What is auxiliary heat?
Auxiliary heat, shown as AUX on many thermostats, is the backup heat the system calls on when the heat pump alone is not keeping up. In an all electric system it is usually an electric resistance element in the indoor air handler. In a dual fuel system it is the furnace.
The Solution Center calls electric resistance backup inefficient and typically expensive, and notes that an undersized heat pump leans on it more. AUX coming on is normal below the balance point, and during defrost: Lennox's manual notes the thermostat may bring on auxiliary heat during a defrost cycle. If AUX shows most of the day in mild weather, have the system checked.
What is emergency heat, and when should you use it?
Emergency heat, often EM HEAT on the thermostat, locks the heat pump out so all heating comes from the backup heat, Lennox's EL16KP1 manual explains. Use it when the outdoor unit has a problem, not because it is cold outside: Lennox says it is usually used during an outdoor unit shutdown.
| Mode | What runs | What turns it on |
|---|---|---|
| Heat pump heat | The heat pump | The thermostat, in HEAT mode |
| Auxiliary heat | Backup heat, with or instead of the heat pump | The system, automatically |
| Defrost | The outdoor unit melting its own frost | The outdoor unit's control, automatically |
| Emergency heat | Backup heat only, heat pump locked out | You, by hand |
Reasonable times to switch to emergency heat and call:
- The outdoor unit stays encased in ice through several defrost cycles.
- It makes a loud grinding, scraping or banging noise.
Lennox's manual adds a case: if the power was off over an hour and it is below 50 F outside, run emergency heat for at least six hours so the compressor's crankcase heater can protect it, then return to HEAT. Check your own manual.
On most dual fuel systems, emergency heat runs the furnace alone; your thermostat's manual will say. On an all electric system it runs only resistance heat, which is costly, so switch back once the problem is fixed.
Why is my heat pump covered in frost, and what is defrost mode?
A thin layer of frost on the outdoor unit in winter is normal. Defrost mode is how the unit clears it.
Lennox's manual explains that frost forms on the outdoor coil when the heat pump is heating below 45 F, and an electronic control runs a defrost cycle of 5 to 15 minutes at set intervals to clear it. You may see steam rising from the unit during defrost, which is also normal.
What you can safely do, from Lennox's homeowner notes (check your own manual too):
- First, switch the unit's power off at its disconnect switch before clearing anything off or around it, and keep hands and tools away from the fan.
- Clear leaves and debris that block airflow around the unit, without taking any panels off.
- Keep snow below the unit's louvered panels.
- If snow or ice keeps falling onto the top of the unit, have it looked at rather than moving it yourself; the fix can be more clearance, which is installer work.
- Do not chip, scrape or heat the ice off, or speed up defrost in any way the manufacturer does not recommend.
When to call Raber HVAC
Before you call, run the checks Lennox lists for a system that will not run:
- The thermostat is set to HEAT, above room temperature.
- Every disconnect switch for the system is on, and no fuse has blown or breaker tripped. Reset a tripped breaker once; if it trips again, leave it off.
- The access panels are on the unit.
- The air filter is clean.
Call us at (216) 888-4822 if your heat pump cannot keep the house warm, AUX runs most of the day in mild weather, you have had to switch to emergency heat, or the outdoor unit stays iced over. We are open every day from 8:00 AM to 10:00 PM, and our 24/7 emergency line covers no heat, active leaks and sewage backups.
If you are weighing a cold climate heat pump or a dual fuel system, we also check the ductwork's static pressure (how hard the ducts resist airflow), since a heat pump moves more air than a furnace. At start up we verify the refrigerant charge by superheat and subcooling, pressure and temperature checks on the refrigerant lines, rather than assuming it. To book, pick a day and an arrival window online, and the office calls to confirm.
Sources
- ENERGY STAR, Heat Pump Equipment Key Product Criteria
- ENERGY STAR, Air-Source Heat Pumps
- NEEP, Cold Climate Air Source Heat Pump Product List and Specifications
- NEEP, Cold Climate Air Source Heat Pump Specification, Version 4.0
- Building America Solution Center (U.S. DOE), Cold Climate Heat Pump Sizing and Selection
- Building America Solution Center (U.S. DOE), Heat Pump Quality Installation and Commissioning
- PNNL for the U.S. Department of Energy, Guide to Determining Climate Zones by County
- NOAA NCEI, U.S. Climate Normals 1991 to 2020, Cleveland station USW00014820
- National Weather Service Cleveland, Cleveland Normals
- National Weather Service Cleveland, Cleveland Records
- National Weather Service Cleveland, Coldest Temperature in 15 Years
- Lennox, EL18KSLV Heat Pump Installation Instructions
- Lennox, EL16KP1 Heat Pump Installation Instructions