Thread by @EnergyVanguard

@prlambert @energysmartwv @curious_founder One per room is way too many. They’re not small enough and the turndown isn’t low enough for that.

@prlambert @energysmartwv @curious_founder We usually do ducted #heatpumps for the whole house in our #HVAC design projects. When people come to us wanting all #ductless units, we try to talk them into ducted for the bedrooms.

@prlambert @energysmartwv @curious_founder Here's an example from a small high-performance house we did in Virginia.

Loads (in BTU/hr)

Bedroom 1
htg = 1,944
clg = 320

Bedroom 2
htg = 1,454
clg = 511

Study
htg = 1,665
clg = 1,063

@prlambert @energysmartwv @curious_founder The wall-mounted ductless unit we usually specify is the Mitsubishi MSZ-FS. The smallest one they make is a 6k unit.

Rated capacity (BTU/hr)
htg = 8,700 (at 47 °F)
clg = 6,000

Minimum capacity
htg = 1,600
clg = 1,700

See the problem?

@prlambert @energysmartwv @curious_founder One of those 6k ductless units in each of those 3 rooms will be running at the bottom of the range pretty much all the time.

The design heating lead is right around the minimum capacity, so you might get some benefit from the variable capacity on the really cold days.

@prlambert @energysmartwv @curious_founder The design cooling loads are way, way lower than the minimum capacity for the two bedrooms. That means they will never get any benefit from variable capacity.

The study has a cooling load closer to the min. capacity than the bedrooms, but it's still only 63%.

@prlambert @energysmartwv @curious_founder That analysis is for what happens at the design loads. 99% of the time, the house operates at part load conditions.

Finally, running at a capacity significantly higher than the load can lead to comfort problems.