How to Determine if a Minisplit System is Right-Sized
We’re trying to troubleshoot a heating issue in California (Zone 3C), and wonder about the sizing of the two ducted mini split zones we have. Are they sized wrong?
Our house is from 1961. The second floor has baseboard heaters. The first floor was upgraded with a two-zone ducted mini split system from Carrier, with the head units in the crawl space under the house.
On the first floor, zone one covers the entry, living, dining, kitchen, laundry, and utility rooms, for a total of 1,450 sq ft. Zone two covers the master bedroom and bath, for a total of 350 sq ft. Both zones have 24,000 btu head units. That already seems odd, because the square footage is so different… Read on for, maybe, why it’s like that.
We set a fixed temperature of 67 all winter, day and night. We live in California where it gets down into the low 30s just a couple days a year. Our master bedroom zone has no trouble maintaining the set temperature. Our living area zone mini split maintains the temperature within half a degree as long as it stays above 45 or so at night. By the time it drops below 40 outside, it can’t keep up and it gets up to 5 or 6 degrees colder inside. It takes a few hours after sunrise to catch up, and even then it needs help from some sunshine.
First: is it normal for a right-sized mini split to fail to maintain temperature when it’s cold out?
We had a professional do a full Manual J load calculation and conclude that we need 42,000 btu for the first floor. (I tried to repeat the calculation with CoolCalc, and despite entering every single room one by one, my number was way off from that at 21,000 btu heating and 2.5 ton cooling. I don’t trust my numbers, because I’m not an HVAC professional.) So it seems like 48,000 for the two head units should be enough, overall, though I’m worried about how much difference there is in the square footage that’s covered.
One possible reason for the 24,000 btu unit for the master bedroom is that it has a really long duct run, and maybe having some extra air pressure helps with that?
About the ducts: The head units connect by flex duct to the vents. The duct runs are very long, traveling more than 50′ from the head units to the farthest vents. I guess the reason is that it was difficult to move the head units through the crawl space to get them any closer. It probably would be possible to put them closer if that would help.
So I guess the questions are:
* Is it possible that everything is actually fine, and we’re just new to mini splits and find it odd that the temperature drops an extra 5 degrees at night when it’s cold?
* Or does it sound like an undersizing problem, like maybe we should have a smaller head unit for the master bedroom, and a bigger one for the main living area? Carrier makes a 36,000 btu.
* Or might it be the length of duct runs that’s an issue, and if we could shorten them by 50′ that might fix it? Or something else?
Any thoughts appreciated!
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