There is a phenomenon out here that locals call the “death fog,” and it has proven to be the ultimate death fog heat pump stress test. While it brings temperatures in the low 20s and thick rime ice, it’s important to note that it does get even colder here, and yet the system continues to work great.
The most surreal part is how localized it is. Often, if you drive just five miles in any direction, the fog lifts, the sun comes out, and the temperature jumps ten degrees. But when you’re stuck in the thick of it, the humidity is high, the air is biting, and you quickly learn exactly what your equipment is made of. This isn’t just about thermostat settings; it’s about how your home handles constant defrost cycles and sub-freezing moisture accumulation over several days.
The 7-Degree Reality Check
My home is relatively new (built three years ago) and roughly 1,200 square feet. When I was designing the system, I originally planned for the extended capacity “hyper heat” version of the Mitsubishi. However, due to availability issues when it came time to pick it up, I ended up with the standard 1.5-ton Mitsubishi heat pump.
Based on my calculations, I estimated my balance point—the outdoor temperature where the heat pump can no longer keep up with the home’s heat loss—to be 7oF.
Over the years, those calculations have been tested:
The Backup: I do have electric resistance backup, but I’ve only had to engage it twice when the indoor temperature dipped below 65oF. My backup is only 5 kW. Most contractors want to install enough to heat the house assuming the heat pump will fail on the coldest day. I have my opinions on this subject. I’ll write about it another time.Dry Cold: The unit successfully heated the house down to that 7oF mark without help.
The Snow Factor: When it hit 12oF and started snowing, the performance shifted. The unit began defrosting much more frequently to shed the ice, which naturally limits its heating uptime.
The “Death Fog” and the Ice Pile
One of the most visual parts of running a heat pump in these conditions is the defrost cycle. I’ll share some photos below of the pile of ice that accumulates directly under the unit as it sheds the frozen moisture from the “death fog.” You’ll also see the rime ice on the trees—it’s stunning, though I’m usually ready for it to melt away after a few days!
Why is the heat pump so high off the ground? I have dogs that have previously enjoyed eating lineset insulation. Four feet from the ground stopped that issue!
Know Your House, Not the 1980s Playbook
The most important lesson here is that if you know your house and your ducts, you can size your system appropriately.
Contrast my experience with a neighbor nearby. They have a 1,000-square-foot house—smaller than mine—but their contractor installed a 3-ton unit with 10 kW of strip heat. When the temperature dropped below 35oF, the contractor’s advice was to “just turn off the heat pump and run the electric strips.”
That is advice straight out of the 1980s. With modern equipment and a well-insulated thermally efficient shell, you don’t need to over-size your equipment or rely on inefficient heat strips the moment it gets chilly.
Precision matters. Well… not precision. Get close. Guess as close as you can get. But include calculations with those guesses. When you size your system based on data rather than “rules of thumb,” you get a home that stays comfortable even when the death fog arrives.
While the rime ice is beautiful, it eventually has to go away. Knowing your house and your ducts allows you to size your equipment for these exact conditions or a bit worse. That doesn’t mean putting in a heat pump that will heat a house 3x the size. If you want to see the specific math I used to avoid the “1980s playbook” of oversizing, check out my deep dive on Manual J and Manual S calculations.