HVAC sizing for cooling: Manual J vs. HVAC contractor?
We are still in the planning stages of a 2 story single family whole house remodel in the San Francisco Bay area. The HVAC system is going to be a Mitsubishi multi-split. My question lies in the 2nd floor which will use a fully ducted system (ducts buried in attic insulation) with one high static pressure air handler for the entire 2nd floor (PVFy-P12NAMU-E1). A manual J was performed that I have attached, which calls for a cooling load of 12892 BTU. The capacity of the specified air handler is 15998 BTU (11999 Btu sensible, 4000 Btu latent). Despite the HVAC design specifying 1 ton air handler, the HVAC subcontractor my general contractor works has quoted us a 2.5 ton air handler, probably based on rule of thumb. When asked about the discrepancy, the HVAC subcontractor said that he thinks putting a 1 ton air handler is a big mistake since the Manual J, D, S calculations do not allow for extremely warm days or if there are several people at the house.
Looking at the Manual J, the design temp is listed at 85F. Our climate occasionally gets into the mid 90s. Despite reading through all the HVAC posts I can find on this fantastic website and trying educating myself, the HVAC subcontractor has made me question whether a 1 ton air handler will be enough to keep the house comfortable at temps outside of the design range. Obviously I would prefer a right sized system, and not fall into the trap of a grossly oversized system that I have read about and the associated short cycilng. Should I insist on a 1 ton air handler (and risk annoying the HVAC sub), or compromise by going with a 1.5 ton air handler? Or is there something else that I am not thinking of?
Thanks in advance and for all the great advice on this website.
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