Output air temperature of mini splits on heat?
whitenack
| Posted in Green Products and Materials on
I have a little experience with heat pumps and gas furnaces, and have always noticed the difference in temperature of the air coming through the registers. I read somewhere that heat pump air temps are usually 95* where gas furnaces get the air temp up to 115*.
I have looked around but can’t find the output temps of mini splits. Is it the same as conventional heat pumps? I know that mini splits are supposed to be able to operate in lower outdoor temps compared to regular mini splits, and wondered if that translated to a higher output air temp.
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Replies
Clay,
The output air temperature from a ductless minisplit is usually higher than the output air temperature from a conventional air-source heat pump connected to forced air ductwork.
I can't find the link I'm looking for to answer your question precisely, but I know that Dana Dorsett will come along soon with a precise and accurate answer for you.
Clay, I have had a Fujitsu ductless minisplit for about 3 years and I had a conventional, ducted heat pump before that. I am still amazed at the air temperature that the minisplit can produce. So, a few days ago, when it was 12F outside, I used my IR gun to check some of the unit's surfaces, immediately next to the air outlet. The temperature range was 90-93F
How warm your house is is more important than output temperature, I think. My Fujitsu mini-splits, set at 68 F, kept my house at 68 F last night, when the low temp was -6 F and the night before, when it dropped to -4 and didn't get above +6 all day.
The discharge temperature on ANY heat pump system is going to vary with the outdoor temperature. Ignoring supplemental electric resistance heat for a moment...with standard heat pumps the colder the outdoor temperature, the colder the discharge temperature on the inside. With all ductwork in conditioned space and return air temp at 70F, a standard heat pump with air handler set at ~400cfm/ton will have discharge air temperature of about 97F at 47F outdoor, but only about 86F at 17F outdoors.
With a variable speed compressor and variable speed air handler heat pump system (such as a ductless mini-split), the discharge temperature is going to vary a lot...based on outdoor temperature, compressor speed and air handler speed. For a given heating load, the control logic on the ductless units tends to err on the side of better occupant comfort (higher temp. discharge air by lowering fan speed) rather than better efficiency (higher fan speed and lower temperature discharge air). I think this is the same for the ducted systems. Discharge air temps for ductless mini-splits can easily exceed 120F in some cases. Furnace discharge air temperature can be well over 120F as well.
See Tables 5 & 6 in Appendix A:
http://www.nrel.gov/docs/fy11osti/52175.pdf
Clearly the output temp depends on the blower rates and outdoor air temps, but for the most part except at the extremes the bench tested exit air temps at the heads were north of 100F, and usually north of 110F, for both the Fujitsu and Mitsubishi 1- tons.
At 2.8F outdoors. 68F indoors the Mitsubishi running at max blower speed was delivering 95F air, but at min-speed it was delivering 119F air. At -9,7F running flat-out it was delivering 88F air. At warmer outdoor temps it was mostly in the 120F+ range, comparable to condensing gas furnaces or hydro-air systems.
These were bench tests, forcing the compressor and interior heads to different ranges, but it's indicative of the range to expect in normal modulating operation. Most of the time the output will be warmer than 110F.
Thanks to John S. and Dana D. for your detailed and helpful responses.
You guys rock! Thanks so much.