Dual-zone heat pump quandary – GreenBuildingAdvisor

We recently gut-renovated our 100yo brick home in Cleveland, OH (Zone 6A). We modernized it with two heat pump systems covering 4 distinct zones. The upstairs zone – the original 2nd floor + original bonus room above garage – has a Mitsubishi system with total capacity of 3 tons, and is the subject at hand. Bear with me…

That system consists of a 24,000 BTU SVZ ducted blower for the main area and a 12,000 BTU ceiling-mounted mini split head for the 19 x 11 x 7 bonus room, hooked up via a branch box to a single 36,000 BTU M-series hyper-heating pump. Because we kept the original house largely uninsulated, primary heat comes from the original steam radiators. We took out the steam radiators in the bonus room, so we added a 4,000 BTU electric baseboard. This means the heat pump is relied on for summer cooling plus primary heat in the shoulder seasons, and supplemental heat only for the bonus room in the winter. We do not use the 2-ton blower for winter heat.

The main upstairs area is fine. Unlike that area, the bonus room was moderately insulated in the renovation (R-15 rockwool + 1” exterior XPS). It’s south-facing and has stucco vs. brick exterior (lower thermal mass), and it’s my home office which means it’s used during daytime. So I end up relying on the mini split mostly for cooling and very moderately for heating. The 1-ton capacity is clearly massively oversized, resulting in lots of short cycling (both for heating and cooling) and humidity control issues. I often use it when the 2-ton system is dormant, which means the pump’s minimum capacity is all going to the mini-split. I’ve resorted to using a de-humidifier, which easily fills its 8L tank daily in the summer. Oh also, the heat pump is mounted to the wall of this room and vibrates like crazy especially on heating cycles, but that’s neither here nor there…

The obvious thing I need to do is swap that 1-ton head for one half the size, the smallest one Mitsubishi makes. Even 6,000 BTU is likely oversized given the size of the room (and I have about R50 worth of blown cellulose in the attic). This would require me to downsize the pump as well, to a 30,000 BTU unit. (While I’m at it, I’d put it some shock mounts to mitigate the vibration.) The problem is even that unit has a min load of around 12,000 BTU, so in frequent scenarios where I’m running the mini-split alone it’s not a very efficient setup. My understanding is given the choice of running a system a lot at min load vs. hitting max load more often, the latter is generally preferable (assuming it’s still sufficient for comfort). 

If I move to a 6,000 BTU head in the office and keep the 24,000 BTU SVZ blower unchanged, is this system going to run any better with a 30,000 BTU pump? My current 3-ton pump apparently has a min cooling capacity of 15,500 BTU while the 2.5-ton unit is 12,600 BTU, which is better but still very oversized for a single head running. I looked at undersizing with a 2-ton pump, but strangely the min capacity is the same. Yes, I realize what I really need is to run two single-zone systems… it is a bit of a pain to retrofit, but after typing all this out I’m growing resigned to the fact that this may be the only real solution.

(Also if you’re wondering why I’m not discussing all this with my installer, who are very nice people… well, they’re the ones who put this system in, and you know how that goes…)