Sizing a Minisplit System
so I hired an out-of-town engineer to do a Manual J, but forgot that the blower door number I had was before we sprayfoamed attic/top of balloon framing plates, replaced basement back door that was not meant to be an exterior door, and went from no insulation in attic to r-34ish (and polyiso strips on rafters).
Still, their manual J isn’t far off the estimate of ‘Out with the Old, in With the New.”
Their estimate for my heating upstairs, half my finished square footage, is 30,468 btu/hr (before ducts are considered; currently I have radiator heat)
My estimate for whole house is 45,883 using the 1.4 multiple, using actual natural gas use data (including hot water/stove draw; without that, it’s about 42k). It’s a duplex, with no west windows, most windows on east.
One of the HVAC quotes I have for a ducted minisplit to serve the upstairs is for a Daikin unit whose submittal data says it can provide between 5,500 and 24,000 in cooling and 6,700 and 26,000 btu per hour in heating at 47 degrees https://daikinac.com/content/assets/DOC/SubmittalDataSheets/FDMQ24RVJU-RX24RMVJU.pdf.
Here’s my question. I have radiator heat, and am keeping it for now, though one day I might go to an all-electric house. We are not putting any minisplits on first floor right now (though we’ll see what it feels like having to close up the first floor because the second floor minisplits are running.)
The manual J estimates I need 25,462 for cooling, combining sensible and latent (latent is about a third of the load in this 4a Washington, D.C. climate).
So is that Daikin an appropriate size, do you think?
Also, is it important to have a register in the hall near the bathroom for future-proofing, if I do want to use it for heat one day? I don’t care about it for cooling now, though the sales guy said it could help cool the downstairs, since it would be at the top of the stairs. (Bathroom has a radiator).
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