Why Your HVAC System Struggles During Extreme Cold or Heat
When outdoor temperatures swing from triple-digit heatwaves to sudden winter cold snaps in the Pacific Northwest, your home’s heating and cooling equipment takes the hit.
Most residential HVAC systems simply aren’t designed for extreme weather patterns. This guide from Carrier Northwest explains why HVAC systems struggle during these conditions and how to protect your equipment when severe weather hits.
Understanding Your HVAC System’s Operating Limits
Standard air conditioners, furnaces, and heat pumps are sized around regional weather averages—not weather extremes.
When contractors install a system, they use a design temperature metric that covers roughly 99% of typical local weather conditions. That rare 1% of extreme heat or severe cold is intentionally left out.
Why?
Sizing a system for those rare days would mean installing an oversized, inefficient unit that wastes energy and struggles to control humidity during normal, everyday use.
Air Conditioners & Heat Pumps (Cooling Mode): When outdoor temperatures push past 90°F, it gets much harder for your outdoor unit to release heat absorbed from inside your home. The compressor has to run significantly longer just to move that same amount of heat.
Air-Source Heat Pumps (Heating Mode): When temperatures drop toward freezing, there’s simply less heat outside for your system to pull in. To make matters worse, frost builds up on the outdoor unit, forcing your heat pump to shut down so it can thaw out.
Gas Furnaces: While furnaces generate heat easily, extreme cold causes your home to lose heat through walls and windows much faster than usual. A properly sized furnace may end up running continuously just to offset this rapid heat loss.
The 20-Degree Rule for Air Conditioning
Most air conditioners are built to cool your home’s air by roughly 20 degrees below the temperature outside.
If it’s 105°F outside, a system running at peak efficiency can realistically only pull your indoor air down to around 85°F.
Dropping your thermostat to 68°F during a 100-degree heatwave won’t make the air blowing out any colder. It just forces your system to run nonstop, spike your energy bill, and risk freezing up the cooling coil.
Is There a 20-Degree Rule for Furnaces?
No. Because a furnace recirculates and heats the air already inside your home, it can easily keep your house at 70°F even when it’s 0°F outside. If your furnace struggles during a deep freeze, it isn’t because the unit hit a temperature limit—it’s simply because heat is escaping your home faster than the system can replace it.
HVAC Operating Tips for Severe Weather in the Pacific Northwest
Taking a few practical steps before extreme weather hits helps your equipment run efficiently and prevents unexpected breakdowns.
How to Protect Your Cooling System in High Heat
Raise the Temperature: Set your thermostat to 75°F–78°F during peak afternoon hours. Expecting 68°F inside on a triple-digit day puts severe strain on your system.
Pre-Cool Your Home: Drop your thermostat by a few degrees early in the morning. Cooling your home’s walls and furniture before the outdoor heat arrives helps your system maintain a comfortable temperature later.
Replace Air Filters: A dirty filter chokes off airflow, which can freeze the cooling coil or trip the compressor.
Clear the Outdoor Unit: Keep at least two feet of open space around your outdoor condenser so the unit can dump heat efficiently.
Block Direct Sunlight: Close blinds and shades on south- and west-facing windows to block radiant heat before it bakes your living space.
How to Keep Your Furnace and Heat Pump Running during a Deep Freeze
Keep to One Setting: Maintain a steady thermostat temperature day and night during deep freezes. Dropping the temperature overnight requires your heating system to work that much harder during the cold morning hours.
Clear Snow and Ice from Heat Pumps: Clear snow drifts and ice buildup around the sides and top to allow air to circulate through the coil.
Check the Flue and Intake Vents: For high-efficiency condensing furnaces, make sure the outdoor PVC intake and exhaust pipes are above the snow line.
Keep Interior Vents Open and Unobstructed: Never close supply vents in unused rooms during extreme cold. Blocking vents alters system static pressure, reducing airflow efficiency and potentially overheating the heat exchanger.
Pacific Northwest Extreme Weather FAQ
Why do heat pumps blow cool air during deep freezes in Western Washington and Oregon?
Heat pumps blow air at around 95°F, while gas furnaces deliver air at 120°F or higher. Because heat pump air is cooler than your body temperature (98.6°F), it can feel lukewarm coming out of the vents, even though it’s actively warming your home.
During a thaw cycle, it might even blow cool air for a minute before backup heat kicks in.
What should I do when freezing rain or ice storms hit the Northwest?
Freezing rain poses a severe risk to heat pumps by encasing outdoor coils and fan blades in solid ice, which restricts airflow and can damage the fan motor.
If an ice storm is forecast, clear overhanging gutters or rooflines directly above your outdoor unit to prevent water from dripping onto the fan guard and freezing solid. If the outdoor unit becomes heavily encased in thick ice, switch your system to “Emergency Heat” and contact a nearby Carrier technician rather than trying to knock ice off the fragile aluminum fins with tools.
Prepare Your HVAC for Extreme Weather
Don’t wait for the next heatwave or freeze to find out if your system can handle the strain. Carrier Northwest has a network of Carrier Factory Authorized Dealers in Oregon, Montana, and Washington, ready to assist you.