Minisplit Design for Whole House
Hello, first of all thank you GBA for all of your articles, I have read many and found them to very helpful and interesting.
I am renovating my house (in Tampa, FL) and installing a new AC system, I will be installing minisplits most likely DIY, but will also talk with installers. I want to see what GBA users recommendations are for how I should design my system.
I did a detailed load analysis using http://www.loadcalc.net and it came to 23k btu cooling and 98k btu heating. Which seems very low to me, but I actually think it would be even lower in real life, since I have a cement tile roof on most of the house which reflects a ton of heat and radiation, and a TPO foam insulated roof on the rest, and my house is shaded all morning by a giant oak tree, so it doesn’t get any sun until noon. This has me quite confused as to what my cooling load really is, especially since all sellers say to use around 40k btu based on my square footage.
I could go with a 4 head system with 1 condenser, but due to the layout of my house figured it might be better to to use 2 dual head systems, 2 heads on 2 condensers. I like that it would give me fault tolerance if one system fails I still have half a cool house. I was thinking of going with 9k for the 3 bedrooms, and using a ducted 12k or maybe 18k unit for the main room and other bedroom. This adds up to 39k btu which is double the 23k load calc (using full sun remind you, which my house doesn’t get).
So then I thought since my actually cooling load is probably 15-20k (accounting for a tile roof and my giant oak) maybe I should just get one 24k unit and put it in the middle of my house, and then put vents that draw air in each bedroom and dump it back to the main room (so they have air movement), this is would save me thousands and be a much simpler design, but haven’t read of anyone doing something like this. And if it didn’t work no big deal, just buy more mini splits and install them, so it seems like it might be worth a try.
I would love to hear your thoughts and advice if you have experience, thank you!
Floorplan attached, red are 4 heads on 2 condensers, center unit is ducted (purple lines).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Generally, vents to move around air that is only a few degrees different isn't very effective. On the other hand, open doors and hallways move so much air that they are pretty effective (in a well insulated house).
First of all, there is no way that a 1,614 square foot house in Tampa, Florida can have a design heat load of 98,000 BTU/h. Either that is a typo, or you need to go back to square one and check your calculations.
A second point: your calculation showed that your 1,614 square foot house has a design cooling load of 23,000 BTU/h, and you wrote that this number "seems very low to me."
Reality check: according to a recent article by Allison Bailes, the average area in square feet for houses he's worked on recently is 1,431 square feet per ton of cooling load. If your house was "average" (for the type of house for which Allison does cooling load calculations), then your 1,614 square foot house would have a design cooling load of 1.128 tons, or 13,524 BTU/h. So unlike you, I don't think that your calculation of 23,000 BTU/h "seems very low."
Thank you for the answers. I redid the calculation and found I was entering flooring sqft incorrectly causing the heating btu to be way off, and I adjusted the sqft and ceilings. The cooling figure changed to 33k btu, and heating is now 28k btu. My house square footage is 2200sqft, but if you count it from interior dimensions of rooms its 1900, the 1614 figure is not counting tiled areas. So given 2200 sqft which is the figure I assume Allison would use, it works out to 19k btu cooling. At least all btu figures are in the ballpark now.
If I go with my original plan of 3 9k units, and one 12k unit in the main room, for a total 39k of capacity, my bedrooms are obviously way over capacity is the problem, but there are no smaller units. My only concern is with short cycling and not removing enough humidity, since in FL that is a main issue with comfort, but from what I understand mini-splits can operate at a fraction of their capacity, and as a dehumidifier so should it be good?
What about the layout of the system, are there any issues with it?
You might want to install two ducted units. One ducted unit could go in a dropped ceiling in your master bedroom closet; this unit could have three ducts serving three rooms: your master bedroom, your kitchen (with a duct terminating in a wall register above your wall cabinets), and the room labeled "18x12." (If you are willing to put up with a soffit to hide a long duct, this unit might also serve Bedroom 4.)
Another ducted minisplit could go in a dropped ceiling above the room labeled "7x5," and could have two ducts: one serving Bedroom 2, and one serving Bedroom 3.
Even 28KBTU/hr is a gia-normous heat load for a ~1600' house in Tampa, at Tampa's +40F outside design temperature. Is that with or without glass in the windows?
For reference, the measured heat load at my ~2400' 2x4 framed antique in MA comes in at about 17,000 BTU/hr @ +40F.
Looking at your summary load calculation image, I don't quite understand how a 70F house loses 8000 BTU/hr through a slab floor to ground in a location with deep subsoil temperatures in 72-73F range. Or is this pier foundation with R13s under the floor?
The software asks for linear feet for floor so it must be calculating floor loss at the perimeter rather than vertically. Do you really have no insulation in your walls?
The house is 2200 sqft. That is with double paned windows and sliders. I don't understand it either. I believe I entered everything correctly. It is a normal slab in the ground, nothing special.
All exterior walls are cinder block with stucco on the outside and a moisture barrier with furring strips and a thin layer of insulation, maybe R1-3. Is there another load calc site I can use? All others I've found are not detailed.
I would put some effort into more wall and edge insulation before planning the AC system. Also air sealing (probably worse than you think).