Yesterday I did a consultation with a homeowner in Michigan. He’s got a 60 year old house with a boiler and radiant heating system that has become a burden. The pipes for hydronic distribution system just developed their seventh leak since he’s lived in the house. To repair the leak would require tearing up the kitchen, so he’s looking for other solutions.
One possibility is going with all ductless mini-split heat pumps, and one HVAC contractor proposed 7 indoor units for his 2,100 square foot, 4 bedroom, house. That’s one ductless unit for each bedroom plus three for the downstairs. The mini-splits might be sized appropriately, but it would only be by accident. The contractor didn’t do or even propose doing a load calculation.
Can you oversize a mini-split?
And that gets me to the real point of this article. A lot of people seem to think that you don’t have to worry about oversizing a mini-split heat pump, whether ductless or ducted, because these systems have variable capacity. The heating or cooling output drops when the load is lower, so it’s OK to put in a unit that’s too big, they argue.
But is it true? No! Here’s why. Mini-splits do ramp down in capacity as the load changes, but they don’t go all the way to zero. There’s a bottom they won’t go below.
For example, the Mitsubishi FS06 wall-mounted ductless unit has a rated cooling capacity of 6,000 BTU per hour and goes down to a minimum capacity of 1,700 BTU per hour. That’s a turndown percentage of 28% (1,700 ÷ 6,000). If you put that unit in a room with a load of 1,500 BTU per hour, it will bottom out nearly 100% of the time it’s running in cooling mode.
Sometimes you’ll see turndown ratio, which is the same concept as turndown percentage but upside down. It’s the ratio of maximum to minimum capacity. For the example here, it would be 6,000 ÷ 1,700 = 3.5. So the turndown ratio is 3.5 to 1, sometimes written 3.5:1.
What happens at part-load?
One of the great advantages of using mini-splits is their variable capacity. By oversizing them, you can lose that advantage completely. That 6,000 BTU per hour mini-split in a room with 1,500 BTU per hour of cooling load doesn’t act at all like it has variable capacity because it’s always running at the lowest capacity except on the really extreme temperature days.
When the bottom end of the capacity range is at or above the heating or cooling load, there’s no benefit in part-load conditions. As the heating or cooling load drops from the design load, you want a system whose capacity also drops.
That 6,000 BTU/hr system with a bottom end of 1,700 BTU/hr serving a load of 1,500 BTU/hr doesn’t vary at all in part-load conditions.
When the load is 1,500 BTU/hr (the design load), the system will give you 1,700 BTU/hr. When the load drops to 1,000 BTU/hr, the system will give you 1,700 BTU/hr. When the load is only 500 BTU/hr, the system will give you 1,700 BTU/hr.
Yes, oversizing is a problem
So, yes, you absolutely can oversize a mini-split. And when you do, you end up with some of the same problems you get from oversizing conventional systems: poor humidity control, short cycling, and wasted money.
When a contractor proposes 7 indoor ductless units for a 2,100 square foot house without doing a load calculation, there’s close to a 100% chance that the systems will be oversized. But this problem happens with new homes that get load calculations, too. You’ve got to look at the specifications for any mini-split heat pump and put in one that will take full advantage of the variable capacity.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to the Energy Vanguard newsletter and follow him on LinkedIn.
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