Decoding Defrost, & Saying Goodbye to W'26
Our conclusion? Daikin & Mitsubishi seem to have the more aggressive defrost vs. LG.
In late February 2026, Massachusetts got hit with a second major winter storm, after a month of extreme cold. Cambridge got about 16 inches by my account, but someplace in the snow drift went up to 18.
Needless to say, it is an extremely snowy winter. And personally, I think it's the toughest winter since 2015 (which I was here for!).
This particular winter gives us a real natural experiment in how well heat pumps perform in, extreme conditions. Our conclusions, based on the second blizzard, is that is that if you shovel halfway through the blizzard to ensure that snow never covers the bottom of the heat pump above the 18 inch stand, you've got about a 90% chance that the heat pump will make it through an 18 inch blizzard.
And that number goes up to about 99% if we're talking about Mitsubishi and Daikin specifically, with drain pan heaters.
Read on for the analysis. And if you want to just remember the winter, skip to the end.
What does defrost look like?
To give you a clear picture of what happens during the defrost cycle, I mounted off the shelf, remote temperature sensors on a series of heat pumps in the Cambridge area.
Here's what they look like when the defrost cycle is on:
You can see that on a regular basis, the temperature spikes up. I believe this is the few seconds immediately after the defrost cycle switch is off, and the fan comes back on, blowing the warm air through the front grill onto the sensor.
What could cause units to ice up?
If you read my article from the first snowstorm, you'll know that a number of heat pumps iced up. (This was picked up by awesome journalist Alison Takemura!)
At this point, I have three theories:
There was legitimately simply too much snow (e.g. the unit gets covered in snow, pass the 18 inch stand)
The defrost mode was not aggressive/frequent enough, or wasn't kicking on when it was supposed to
The unit is low on refrigerant, which potentially impacts the effectiveness of the defrost
In the best case scenario, your heat pump would be connected to some sort of remote diagnostic sensor that can help you figure out which one of these it is. Between startups like Quilt or OEMs like Daikin (with Skyport Care), we are starting to see more and more of this in the field these days. If there's nothing wrong, with the refrigerant pressure reading, or if you refilled on refrigerant shortly before a snowstorm, I would suspect #2, which may be a control board issue since that determines the logic of the defrost cycle.
Every outdoor unit these days comes with their own defrost algorithm, and usually, you can’t change it. (Perhaps this is due to the fact that your heating efficiency may be lower if you run defrost way too frequently when you don't need to.)
However, I think the future is remote diagnostics. I want to actually manually override the automatic defrost algorithm & make it run more frequently ahead of extreme weather events. This is far more preferable to finding out that 4% of your units iced up after a top 10 blizzard when it's already got a thick layer of ice on it.
This is something that we will be looking more into in 2026, and 2027. We will likely prioritize equipment that has such capabilities in the future.
Could I just.. cover the unit?
Yeah. You can put something like this around your heat pump:
Honestly, kinda worth it if you have a lot of runoff from your roof/know you’re in an alley that gets a ton of snow.
What else matters?
You can make sure that your heat pump has a drain pan heater. This ensures that any melted water from a defrost cycle ends up draining out the bottom.
This helps, but probably isn't as important as the defrost cycle itself; the LG units we had in the field had heaters built-in by default, but ultimately, the Daikin units with drain pan heaters performed the best out of the bunch. And a fair number of Daikin units made it through without drain pan heaters.
How much should I worry about icy conditions?
Not often. Most winters are mild these days. Take a look yourself:
Honestly, every single heat pump we installed would probably be OK in a mild or cold year. Check out the difference between this year & 2023 on awesome weather website, WeatherSpark:
Because I am a weather nerd (thank you elementary school Scholastic book orders), I pulled all the NOAA data into a database and categorized the toughest winters and toughest snowstorms.
Here's my list of major snowstorms in the past 15 years:
Now, there is disagreement as to where this winter ranks for the past 50 years since the infamous 1978 winter snowstorm. But there's no disagreement that this is the toughest winter since 2015. As somebody who was a student back in 2015 who didn't have to shovel, I have to say: you guys are built different out here. I don’t know how you do it.
Much has changed in my life since 2015. So I want to take this moment to just remember the cold winters in Boston, which I can now say I have experience twice!
Not bad for a Californian, eh?
The heat pump canopy looks interesting. However, doesn't the heat pump cover reduces the air flow of the heat pump and decrease efficiency? Thank you.
Need taller stands/platforms for New England.