Room-to-room temperature differentials, Manual J, and Manual D
I am in climate zone 4a. The upstairs of my house has 3 bedrooms — two of which are in use, the third is a guest room. So I will focus on the two bedrooms that are used. (The house was built in 2010 and is pretty standard builders grade–R13 fiberglass insulation, an ACH50 around 3, double-pane windows. HVAC and ducts were sized per Manual J and Manual D.).
One of them (called it “Bedroom 1”) has east and south facing exterior walls and a significant percentage of windows. The other (“Bedroom 2”) has north and west facing walls and a smaller percentage of windows. Because of east and south facing walls/windows, Bedroom 1 starts heating up in the morning almost as soon as the sun comes up. Bedroom 1 can easily be 2-3 degrees warmer than Bedroom 2 before the A/C ever starts running, and Bedroom 1 has significantly more heat gain throughout the entire morning and early afternoon than Bedroom 2.
To try to keep Bedroom 1 comfortable, we have adjusted the dampers in our ductwork (at the takeoff collars at the supply plenum) to provide somewhat more air to Bedroom 1 and somewhat less air to Bedroom 2. Static pressures remain decent (but not great — around 0.3″ on the supply side) even after damper adjustments. The result is that, when the A/C kicks on, the two bedrooms end up being a relatively similar temperature until around bedtime.
Problem is, by the time the sun goes down (or even by the time its late afternoon), Bedroom 2 now has a heat gain load that is similar to or greater than Bedroom 1. So over the course of a night, the temperature differential between the two rooms changes, and by 6 am, Bedroom 2 is ~3 degrees warmer than Bedroom 1. We could solve this nighttime temperature differential problem by tweaking the duct campers at the plenum, but that would simply reverse the damper adjustments we made to address the daytime differentials.
My question is this: Is zoning (or individual ductless heads in each bedroom) basically the only way to really solve a problem like this? It seems to me that, when two different bedrooms have significantly different cooling loads throughout the day, the problem I describe above will inevitably occur unless you can adjust the A/C supply capacity to each room throughout the day as the load changes.
Am I correct about that, or is there a way to solve this problem apart from zoning?
I suppose the better insulated the house is and the slower the heat gain/loss, the problem could be minimized. But ripping out the walls and spray foaming the bedrooms isn’t really something I want to do. I would consider zoning… or else I will just live with 2-3 degree temperature differentials like a normal person who is not obsessed with temperature and air quality! 😉
Thanks for any feedback.
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