Minisplit blowers mounted low or high?
okapi222
| Posted in Mechanicals on
When your primary need for a minisplit system is heating rather than cooling, would it make sense to mount the blower low (near the floor) rather than high (at the ceiling)? I spoke to someone at Mitsubishi who explained that they were designed to go high, even for heating. But I don’t totally buy it. Why design them that way if heat rises?
Daniel
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Replies
Daniel,
You aren't the first person who has thought that it might be a good idea to install the indoor unit of a ductless minisplit unit near the floor. Even though the method is not recommended by minisplit manufacturers (as far as I know), it is advocated by Peter Talmage, an energy and design consultant and an instructor in the Renewable Energy & Energy Efficiency program at Greenfield Community College in Massachusetts.
Here is a link to a blog post by Peter Talmage in which he makes the case for mounting the units low on a wall: 7 Tips to Get More from Mini-Split Heat Pumps in Colder Climates. The blog was published on the BuildingGreen website.
Talmage wrote,
"For heating, the interior unit should be mounted about 18 inches off the floor and should have a good, clear shot into the living space. Mounting the unit low has many benefits for heating:
First, it operates more efficiently because it is pulling in cooler air to warm up.
Second, the warmed air is blown out across the floor and mixes with the cold air at floor level.
Third, the air isn’t blowing directly on occupants, which can cause discomfort in the winter unless the moving air is very warm.
Fourth, it is very easy to access the filters for cleaning."
The photo below was published as part of Talmage's blog.
.
The theory is great and all, but without careful and calibrated measurement to prove otherwise I don't see how it is going to be dramatically different than mounting it higher up. When there is any significant heating load the cfm output of the head is considerable, and mixing of room air quite good. It would take a fairly low-R house to have much stratification of the room air at the air movement volumes delivered by a mini-split, no matter where you mount the head.
If the house has a vented crawlspace, and minimal to no floor insulation you'd probably reap the benefit of a low mount approach though.
In fact, the exit air of most better mini-splits will be well above human body temp, and there is more of a "warm summer breeze" effect than a wind chill effect (which is probably why the dog in the pic is happiest right in front of the head.)
BTW: It looks like the side framing they built for the shed roof over the Fujistu on that blog page violates best practices by restricting too much of the air flow to the intake side of the unit:
https://www2.buildinggreen.com/sites/buildinggreen.com/files/Blog_Images/Paula_Blog_Images/mini_split_sun_porch.png
Even a 3" clearance between the side-boards an the unit would make a difference, 6" is probably better. It looks like they have only about 3" of clearance to the wall, leaving it to suck air through the slot between the unit and the (as yet uninstalled) shed roof from the top and 3" or so slot at the back on the bottom, none from the now-blocked in sides. I doubt it'll hit it's numbers when running full out if you restrict it that much.