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Q&A Spotlight

Tweaking Plans for a Minisplit System

With heating and cooling load calculations in hand, a homeowner tries to fine-tune specs for a minisplit system

An HVAC contractor is recommending a combination of ductless minisplits, like this one, and ducted minisplits for heating and cooling. A homeowner is looking for advice on optimizing the system.
Image Credit: Brian Post

A GBA reader by the name of Green Heron has recommendations in hand from an HVAC contractor for heating and cooling a Climate Zone 2 house currently undergoing renovations. But he’s not sure whether the recommendations make sense.

The contractor has proposed a four-zone system using a mix of ductless and ducted minisplits, Green Heron explains in a post in GBA’s Q&A Forum. A single 3-ton compressor would run the four indoor heads — two ducted units installed in the attic, and ductless units in both the kitchen and the living room.

“I don’t like the idea of putting units in the attic,” Green Heron writes. “I’m also concerned about sizing. Seems like there’s less room for error with minisplits.”

Green Heron has some questions about the specifics of the proposal, such as how difficult it would be to mount a ducted minisplit on a ceiling and run ducts through interior walls, and whether he could use a single ductless unit for both the living room and kitchen.

Overall, the heating and cooling loads as calculated by a third party are giving Green Heron pause for thought. In some rooms, heating loads are twice as high as cooling loads. Taking the house as a whole, the heating load has been calculated at 27,230 Btu/h with a cooling load of 22,197 Btu/h.

“As you see from the load calcs, some rooms have a big difference between cooling and heating loads,” Green Heron says. “If I size for the cooling load (due to my climate zone), some rooms won’t keep up with heating demand in the winter. If I size to the heating load, some rooms will be significantly oversized in the cooling season.”

Are Green Heron’s concerns well placed? That’s the topic for this Q&A Spotlight.

Heating more than one room with a single unit

One ductless unit could be used to heat and cool the master bedroom and bath, GBA senior editor Martin Holladay says, but there are some caveats.

“The answer depends on your expectations,” he says. “If the ductless unit is in the bedroom, then the temperature of the bathroom is likely to differ by a few degrees, especially if the bathroom door is kept closed for long periods of time.”

Green Heron also wonders whether a single ductless unit could heat and cool the kitchen and living room if there is a 40-square-foot opening between the two rooms. According to Holladay, that would probably would work.

As to the differences between calculated heating and cooling loads, he adds, it’s not unusual. “The equipment should be sized to meet the highest load,” Holladay says.

A smaller compressor might work

The load calculations as presented by Green Heron suggest that a 3-ton compressor would be wandering into the “overkill zone,” says Dana Dorsett. Each ton of capacity equals 12,000 Btu/h, so a 3-ton compressor’s maximum 36,000 Btu/h output is well above the 27,230 Btu/h heat load Green Heron has been given, and far beyond the calculated cooling load of 22,197 Btu/h.

“There are 2-ton and 2.5-ton three-zone multi-splits out there that might be more appropriate for your loads than the 3-tonner,” he writes. “Most load calculations have a bit of built-in slop, overestimating the actual loads, and 3 tons of compressor for a calculated whole house cooling load of about 27,000 Btu/h is edging into the overkill zone (even if that were the actual load rather than a slightly padded number.)”

One of Green Heron’s questions is whether a single 15,000 Btu/h unit could take the place of two 9,000 Btu/h units for the kitchen and living room, and here Dorsett suggests that the position of the head plays an important role.

“The location of the ductless head and the direction it is blowing relative to larger archway or opening makes a difference in balancing temperatures,” he says. “Blowing directly toward the opening is far superior to having a head around the corner and directed elsewhere, counting on convection to drive the air exchange.

“If the combined cooling load of the kitchen and living room is only 11,995 Btu/h, there’s no need to go for a 15,000 Btu/h head, since most 12,000 Btu/h wall coils are capable of delivering about 13,000 Btu/h of cooling into a 78°-80°F room,” Dorsett continues. “The minimum modulated output of the 15,000 Btu/h head needs to be compared to that of the 12,000 Btu/h head if you’re tempted to oversize. A 12,000 Btu/h ceiling cassette mounted in the kitchen ceiling near the opening between the rooms can work pretty well too.”

Dorsett explains that most minisplit heads and cassettes can deliver more than their “rated” or “nominal” cooling capacity. For example, the Mitsubishi FH09 is rated when delivering 9,000 Btu/h but is “fully capable” of delivering up to 12,000 Btu/h of cooling. The FH06, rated at 6,000 Btu/h, can actually deliver 9,000 Btu/h, he says.

“When looking at which and how many heads/cassettes to hang on a 2-ton or 2.5-ton compressor, see if you can’t get away with undersizing at the nominal rated output, by seeing if the maximum capacity still covers your 1% and 99% loads,” Dorsett says.

Vented crawl space can be a problem

Green Heron lives in a muggy Climate Zone 2, on the Gulf Coast, and the 1,500-square-foot house has a mix of slab and vented crawl space foundations. Currently, the HVAC system’s return vent runs through the crawl space and into an unsealed cavity beneath the air handler.

For a house of that size, Dorsett says, true cooling and heat loads should come in at less than 2 tons and under 20,000 Btu/h after air-sealing and insulation upgrades. A vented crawl space, however, can add a lot of latent load in a hot, humid climate.

“In Zone 2A, if there is any air leakage on the return runs located in a vented crawl space, it’s dragging in a significant latent load into the cooling system,” Dorsett says. “Simply having the crawl space vented is also a significant latent load for the house.”

At the very least, he adds, Green Heron should put down a vapor retarder over the ground in the crawl space and air-seal the foundation walls to limit the infiltration of outdoor air. “That brings the crawl space into the conditioned space,” he says. “The subfloor and joists will run drier, for low/no mold risk, and it should reduce the latent load measurably.”

“I doubt that 2.5 tons would be undersizing, even if the crawl space is left vented,” Dorsett adds. “With the air handler and ducts all inside the pressure and thermal boundary of the house, your true total load would likely come in around 1.5 tons, give or take.”

Our expert’s opinion

GBA technical director Peter Yost adds this:

As usual, the expertise of our GBA Q&A regulars is thorough and impressive. I have just two points to add:

First, getting all ducts and equipment into conditioned space is key. Crawl spaces are often “confused” spaces, neither completely in nor completely outside the building’s air and thermal boundaries. Getting mechanical systems sized right and delivering comfort often depends on eliminating this confusion.

Second, achieving conditioned comfort means including latent load management, especially in a climate like Zone 2, and especially during part-load conditions. It’s hard to get either central forced air or minisplit heat pump systems to manage relative humidity adequately, even if they are properly sized for the total cooling load. At a minimum, make sure the sensible heat ratio (SHR) of all your units is 0.70 or less. But best practice means incorporating whole-house dehumidification into your mechanical system.

Latent load management becomes even more important as we improve building enclosure performance; better enclosures mean longer shoulder seasons, when comfort is more about reducing indoor relative humidity than lowering dry bulb temperature.

One Comment

  1. Ronald Booker | | #1

    Mini split
    We added a small 180 sqft solar room two years ago. Since we live in upstate New York had high SGHC windows installed. Our south facing glass box is seperated from the rest of the the rest of the first floor living area (living room, kitchen, dining area) by two sets of patio doors when open the total open floor space is 900 sqft. We expected to keep the doors shut during the peak of the heating and cooling seasons, We installed a Mitsubish MSZ-FH12na in the solar room primarily to heat/cool the solar room. Given the amount of glass I wanted a bigger unit. However, our HVAC contractor was convinced the the 12,000 could easily handle the job. Turns single our MSZ-FH12na manages to maintain the entirety first floor at a temperature of 75-78 floor even when the outside temperature is up to 100 F. How is it possible for a 12,000 Btu unit to cool a space of twice the cooling load? Our house is well insulated, tight and we use a variety of window treatments to help control heat loss and gain through our windows. This includes using outside curtains to block solar gain in the solar room.

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