Is My Ductless Heat Pump Performing to Par?
July 3, 2014 7:09 AM   Subscribe

We got a Mitsubishi ductless heat pump installed late last Summer. While we have been satisfied with its Winter performance, its cooling capabilities have left us unsatisfied.

A typical condition would be: 90 degrees F outside. 84 degrees in the room. The remote control of the unit set at 68 degrees. The air out of the unit never gets below 78 degrees, regardless of variations in the fan speed and other settings. (I have a pretty fancy indoor air monitor I am using and am confident in the accuracy of my measurements.)

I realize that I might never be able to get my living room down to 68 on a hot day, nor would I want to use that much energy. But is it a reasonable expectation that, when set at 68, the unit would blow out 68 degree air?

I have not been able to find any performance standards for these things. Does anyone want to add to my knowledge base before I contact the company that installed this?
posted by Danf to Home & Garden (4 answers total)
In general for heat pumps, the outlet temperature should be 18-20 degrees lower than the inlet temperature. I don't have a lot of experience with ductless heat pumps, but if it is drawing air from inside the room, it should be blowing out progressively cooler air as the room cools. The temperature on your thermostat will just tell it when to stop blowing air, but in general it doesn't directly control the temperature of the air coming out of the unit.

Unless I'm vastly misunderstanding your unit, it sounds to me like there's either a refrigerant problem, or a problem with the outside unit on the system (which is where the heat is supposed to be shed).
posted by muddgirl at 7:24 AM on July 3, 2014

You say it's 90°F outside, but what's the temperature of the outside unit's cooling fins? If it's sitting in the sun, or if it's installed in such a way as to encourage recirculation of its own exhaust air, it might be running much hotter than you think it is.
posted by flabdablet at 8:35 AM on July 3, 2014

I've seen ductless heat pumps do much better in 90 degree weather than what you're seeing. It's possible it's installed in a bad spot that prevents the unit from cooling properly. It's also possible you need a technician to recharge the refrigerant, check for leaks and otherwise service the unit.

FWIW, ductless heat pumps are often installed on roofs here in Albuquerque (where it's often over 90F with a blazing sun) and they're able to cool rooms well.

I wouldn't expect the unit to put out 68 degree air (as muddgirl notes, that's not how a thermostat works). I would expect you to be able to cool a room (assuming the unit is sized properly and you don't have substantial air/heat transfer between the inside of your home and outside) below 84F.
posted by jeffch at 10:41 AM on July 3, 2014 [1 favorite]

Response by poster: Refrigerant leak, probably on installation. Fixed at no cost. It is very adequately cooling our space now.
posted by Danf at 2:52 PM on August 2, 2014

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