Can my outdoor minisplit unit benefit from a solar cover in harsh NY winters?
I am brainstorming a greenhouse type cover over my outdoor unit to keep it warmer in the winter to maximize its ability to heat. There are clear corrugated roof tiles at my local home store I am thinking of fashioning into a roof over the unit outside, facing South with possible partial sides. I am aware it needs air flow. My hope is to keep the outdoor unit warmer to avoid it getting too bogged down in the cold. It is a Fujitsu 15RSL3H made for extreme cold. The inside unit is placed centrally in my 2000sqft home, open floor plan with pretty good insulation, always improving it in different ways. The outdoor unit is south-facing 18″ from foundation on a base about 1′ off the ground, in an area pretty protected from winter winds. I would like to design a roof over it to keep the snow off and maximize its ability to heat my home in the winter. Do you have suggestions for building a cover that can improve its winter capabilities?
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I wouldn't worry. My RLS3H units have had no trouble keeping up with Maine winters. As you mentioned, they need lots of air flow. A roof several feet above the unit may help keep the snow off a bit, but it isn't going to be any warmer.
You often see the outdoor unit mounted on the exterior wall so it cannot be obstructed by snowfall (with a roof, as Stephen mentioned).
"protected from winter winds"
I would think that you want those winds to circulate freely around the unit as much as possible. It extracts heat from them.
Pam,
Q. "Can my outdoor minisplit unit benefit from a solar cover in harsh NY winters?"
A. No. This question comes up regularly on GBA. Here are links to earlier discussions of the issue:
Heat pump in an earth-banked greenhouse?
Call me crazy ... again with the ideas!
Does a Heat Pump Condenser Need to Go Outdoors?
You DEFINITELY want to put a roof over it, and mount it so that the bottom is above the local historically deep snowpack.
Being only a foot off the ground means you'll probably be digging it up at least a couple of times per winter unless somehow protected from both falling & drifting snow, while still observing the minimum distances to obstructions. In central MA it takes ~4' of air between the bottom pan and grade on wall-mounted systems to be reasonably assured of keeping it snow-free after a big nor'easter. I would imagine in lake-effect snow zones of NY you'd need even more. A shed roof or awning is often called for to keep it from being clobbered by roof avalanches or icicle fall. Most locations in NY will see at least the occasional 1.5-2' snowfall in a single storm.
I think this gets dismissed too quickly without even a back of the envelop calc. Lets say the 14500 btuh heat pump is running at about 50%, or 2kw heat output. If it has a COP of 3 it's getting 1.33 kw of energy from the air and 0.67 from electricity. Peak irradiance is about 1kw/m^w, So if you had 1 sq m of collecter & could use 1/3 of that, you'd have 1/4 (0.33/1.33) of the energy that the heat pump needs. It wouldn't be a direct input, but if the airflow was properly handled it would surely increase the temperature of the air and the COP (when the sun is shining)
Brad,
You misunderstand. The calculation has nothing to do with the ability of a passive solar greenhouse to gather heat. The calculation has to do with the volume of air that needs to be pulled through the outdoor unit. The volume of air will exceed the volume of a greenhouse enclosure in a matter of minutes.
To put some numbers on what Martin said, mini-split that delivers 14,500 BTU/hr is usually a one-ton or even a 1.25 tonner.
A typical 1-ton mini-split's compressor unit moves ~1000-1500 cfm.
If you have a 2000 square foot 15 foot tall greenhouse it would do a complete air exchange in 20-30 minutes, big enough to maybe do some good, but it's as big as your house(!).
If it were 500 square feet, 10' tall it would deliver a complete air exchange in 3-5 minutes, and the effect on average efficiency would be "in the noise", probably not easily measurable.
If it were a 100 square feet an 8' tall, like a small garden greenhouse you're talking a matter of seconds, not minutes, and completely worthless even on a theoretical basis.
If you built it as a large solar collector with solar spectrum absorber plates it would need to have sufficient air turbulence over the plates for reasonable heat exchange from plates-to-air, yet not so restrictive of air flow as to load the blower motor by very much adding to the total power use. Getting an optimal (or even net-positive) benefit out of something like that is a real engineering project, not a back of envelope math or backyard hack problem to solve. It's do-able for the truly dedicated though.
Thank you, everyone. I knew you had the knowledge to address my idea. These boards helped me learn a lot about mini splits before deciding to get them. I took a chance installing them in western NY where it gets below freezing. Many nay-sayers warned me it was a bad idea. But, I have one upstairs that did an awesome job keeping the entire house cool this Summer. Now, the test is will it keep my home warm enough by itself this Winter. Just in case, we kept our woodstove. But I am seriously trying to avoid using it and become officially fossil fuel free. Putting a clear lean-to cover over the outside unit was my idea of keeping it warmer in the very cold weather, then removing it in the Summer. I never intended to fully box it in a greehouse. I have looked a many roof designs created but never seen a clear plastic one that would benefit from greehouse effect, maybe. Just an idea and I greatly appreciate all of your input.
OK, taking this 1 step further, Using Dana's 1000 CFM, my 1 sq m, 3kw~1000 BTUH, results in ~1 degF temp rise for the incoming air. Not enough for me to try to capture, but if you need a roof anyway it might be worth trying. Probably not enough sun in NY to bother tho.
The consensus seems to be that having some sort of roof over the unit to keep it clear of falling snow would be beneficial.
Going one step farther, if the roof over the unit was designed so that it overhangs the unit enough to to protect it from snow in the winter, enough to shade it from the sun in summer, but not enough to shade it from sun in the winter, you could potentially increase its COP slightly in the summer cooling months as well.
A free window overhang calculator could show you how to do this for your specific location:
http://susdesign.com/tools.php
Lance,
The effect of sun or shade on the outdoor unit has no affect on the outdoor air temperature, and therefore no effect on the unit's efficiency.
Understood, however keeping the sun's radiant energy away from the coils of the unit will keep them cooler. How much cooler and how much more efficient? Who knows, but most modern top-fan air conditioners have full covers for the coils that keep them shaded from being in direct sunlight.
The 2005 SEER 11 (I think?) air conditioner on our house leaves all of the coils exposed to the sun for most of the afternoon hours (it's in a terrible location on the SW corner of the house). The outdoor units of mini-splits generally have their coils on the back and would generally be shaded in a south facing application, but shading it would still make some sort of difference even if it was just keeping the reflected radiation from the house from hitting the back of the unit where the coils are.
There's a good chance the differences would be small enough to be not significant, but if you're putting a permanent structure over the unit anyway it couldn't hurt to shade it in the summer.
Lance,
This has been discussed often here at GBA. The determining factor is the air temperature. After all, the outdoor air is the source of the heat.
In an article published in 2013, there was a discussion of this issue. In Comment #28 on the first comment page for that article, I wrote the following:
"I'll quote from the July 1995 article from Energy Design Update, "The Nonbenefit of Shading Air Conditioners":
"The results of two-year field study by the Florida Solar Energy Center (FSEC) show that energy savings [resulting from shading air conditioners] are minuscule at best. ... The problem is that a typical 3-ton unit moves about 2,800 cubic feet of air per minute or 170,000 cubic feet per hour. In order for a shading device to be effective, it would have to shade the entire area in which that air is contained. ... Shading just the air conditioner does almost nothing, says [researcher Danny] Parker. Even at peak sunlight (1,000 watts per square meter), shading two-thirds of the sunlight form a 3-ton air conditioner would theoretically reduce the cooling air temperature by only 0.3 degrees F and thereby increase the air conditioner efficiency by less than half a percent. ...
"The results [of the study] show that the house with a trellis saved only 3% in energy and the house with plantings actually showed an increase of 18% in energy use after the shading was installed. Parker hypothesizes that the reason for the increased energy consumption was that the plantings caused some of the exhaust air to recirculate back into the air conditioner."
Allison Bailes later published a blog on the topic: How Much Will Shading Your Air Conditioner Improve Its Efficiency?