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Best Practices

Base Pan Heaters for Heat Pumps

To protect equipment from frost, this heat pump accessory is a must in cold climates

Base pan heaters prevent damage from ice and frost build up on heat pumps in cold climates.

One of the things I try to do in my GBA column is write about my mistakes. Mistakes are almost always learning opportunities, and I hope that by writing about mine, I can help others avoid repeating them. I also find that mistakes can bring to light principles and phenomena that get overlooked when everything goes as planned. The mistake I’m writing about here was a bit of a doozy, and I caught it just in time.

In June, I helped film a series of training videos on heat pump installation. We shot the videos at my cabin in central New York. I installed a single-zone minisplit system with the indoor head in the cabin’s main room. The matched outdoor unit, the Mitsubishi MUZ-FS12, is designed for cold climate use, delivering its nominal 1-ton heating capacity down to -5°F. It comes in two otherwise identical models: The MUZ-FS12NAH has a base pan heater built in, while the slightly less expensive MUZ-FS12NA does not.

While I caught my mistake just in time to not let any damage happen to the heat pumps outdoor unit, it taught me the necessity of a base pan heater for heat pumps in cold climates.

What is a base pan heater?

A base pan heater is a small electric resistance element that sits at the bottom of the outdoor unit. Its purpose is to prevent meltwater that runs off the outdoor coil during the defrost cycle from refreezing before it can drain. In milder climates, base pan heaters are generally not needed; if daily high temperatures rise above freezing, ice doesn’t build up to problem levels. (In milder areas, it’s still necessary to elevate the unit on a bracket, stand, or blocks to allow unimpeded drainage.)

In colder climates,…

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4 Comments

  1. Josh_Dillingham | | #1

    Thank you for writing this article, it was very informative for me. I have the exact same model and somehow never realized that they sold models with and without the pan heater. It seems odd that Mitsubishi would sell a cold climate heat pump made to operate down it -15f and not include a pan heater If it is needed. After reading this I went outside to my condenser and didn't find any ice build up. This is my second winter running the unit with no pan heater and I haven't had any issues with ice build up. I'm in Vermont so most of the winter is below freezing. So I'll definitely be monitoring this closely going forward.

  2. D_Hallowell | | #2

    Thanks for this article.

    I have two Mitsubishi heat pumps (MXZ4C36NA2U1 & MXZ3C30NA2U1) without pan heaters in Chicago and have worried about ice build up since I learned about the issue a year after the units we're installed. (No mention of pan heaters from the Mitsubishi recommended installer.)
    I ordered pan heaters but haven't had a chance to install them. I've been checking the units for ice whenever it's below freezing and there's precipitation or high humidity. I've noted some ice, but never enough to plug the drain hole or build up over the fins.
    I'll have to check again, but I think I read specifications for my units that said the pan heater would energize when the temperature drops below 40 degrees, which is most of our winter.
    After 3 years with no issues, I'm now hesitant to install the pan heaters because of the high electric penalty. I'm considering trying to hack a separate controller or timer so the heaters wouldn't run continuously below 4o.

    Any thoughts?

  3. Sundogger | | #3

    Running two Daiken Aurora series ,12k & 15k here in zone 8 Alaska. In the second winter now and very impressed with cold wx performance down to 20 below. No drain pan heaters. Fairly large accumulations of ice occur below the units (wall mount- wouldn’t do that again) Maybe I should be more worried about accumulations below the exchangers, but they seem to hold steady. Caveats: we don’t have long durations of humid cold wx, mostly below 60% RH, and these days, we do get chinooks every few weeks. Last winter had several weeks of deep cold on a row and no issues. Anything above 20 below and our PGH is toasty. Lots of variable to be sure, but the drain pan design on those units seems to work well.

  4. scsiguy | | #4

    The Nordic air source heat pumps have no drip tray on their outdoor units, completely avoiding this issue. This, combined with the compressor being indoors, is one of the reasons I selected their product over other options. (Boulder, CO - Climate Zone 5)

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