g/hvac • Posted by u/minisplit_mike • 1 days ago

Heat pump COP dropping in cold weather — expected behavior or refrigerant issue?

Customer has a Mitsubishi MXZ-3C30NAHZ hyper-heat system, two heads. Works great above 25°F. Below 25°F, they're complaining about it "not heating" and the aux electric strip is running constantly.

Checked superheat and subcooling at 15°F outdoor temp — superheat is 18°F, subcooling is 12°F. These are within range. Charge looks fine.

Is COP degradation at low ambient just physics at this point, or should I be seeing better performance? The spec sheet says it's rated to -13°F but doesn't give power factor at those temps.

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u/freon_frank • 1 days ago

Also rule out a refrigerant restriction. A partially clogged filter drier can look fine at moderate temps but show up as poor performance when the system is working hardest. Check your superheat carefully across the full outdoor temp range if you can catch a cold enough day.

u/hvac_hero • 1 days ago

That's mostly physics at -13°F ambient. Hyper-heat units like the MXZ are impressive but COP does fall significantly below 20°F. At 0°F outdoor temp, most heat pumps deliver COP of 1.5-2.0 vs 3.0+ at 47°F. The aux strip running in single digits is normal and expected. Check if the system is in "defrost" mode frequently — that can feel like "no heat" to a customer even though it's working correctly.

u/boiler_bill • 1 days ago

Verify the balance point is set correctly. Most heat pump systems need the balance point (temperature below which aux heat supplements) set during commissioning. If it's set too high, aux runs more than necessary. Too low and the heat pump is undersupported in extreme cold.