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HVAC Mini-Split Sizing

mgensler | Posted in Energy Efficiency and Durability on

We are starting the process of a major remodel of our kitchen. As part of the process, we decided to improve the building envelope and install mini-split systems. It’s a ranch house built in 1960 with a major addition (with sloped ceilings) put on in 2004 by the previous owners. The total envelope square footage is around 4200. They created several energy loss issues that we are working to correct. The major items are: Replacing 40 non-ic rated can lights with air-tight ic-rated led’s, removing 4 HVAC ducts that were routed outside the envelope, removing all of the flex ductwork which was poorly installed, upgrading the original attic insulation to r-56 with spray foam around the perimeters, air sealing around all other wall and ceiling penetrations.

Our existing systems are 3 dual fuel HP with 90% gas furnaces.

I did a fuel-based consumption calculation and came up with a 99% design heat load of 57kBtu/h. Upsizing this per ASHRAE by 1.4x comes to 80kBtu/h. I also did a Manual J which came out around 100kBtu/h.

Our plan is to do the improvements above and replace the current HVAC with 4 Fujitsu mini-splits. One will be ductless and the other three ducted. I am trying to guesstimate the energy savings from the envelope improvements and came up with 24% improvement which comes out to around 60KBtu/h. Based on these calculations I spec’d mini-splits that produce 68,800 KBtu/h at 5 degrees outside and 65 degrees inside. Our 99% design temperature is 10.3 degrees and we normally heat the house to 67 degrees. Our current HP’s are rated at 50KBtu/h at 7 degrees but really only hold the house temperature when it’s about 15 degrees outside.

Any thoughts on the sizing and savings estimates would be appreciated.

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