Heat Pump Bad Sizing?
I have a ~2500 sq foot colonial with no AC and hydronic nat gas based heating. We’ve decided to put in one or two heat pumps with ducted air handlers. Both air handlers would be in unconditioned spaces (basement and attic) and cover the respective adjacent floors. We got several quotes from different contractors and all of them sized our house with a 5 ton multi-zone Mitsubishi MXZ-SM60 unit (non hyper heat). After reading the great resources on this website and getting feedback from a local non-profit I suspected the heat load was far less. None of the contractors will do a full room by room manual J so I took it into my own hands. I did a manual J using an online tool (load-calc) and used my natural gas usage data with the boiler sizing article (https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler) from here. All come out to between 38 and 40kBTU/hr at the design temperature in my area.
To be clear, my goal isn’t to 100% replace my hydronic system but rather reduce it’s use to colder weather and hot water (indirect tank). I suspect the natural gas numbers align with the load calculation because they don’t consider that the new ducts and air handlers will be located in unconditioned space. I’ve read that the losses from the air handlers through the ductwork can be 15 to 40%.
Question 1) Is the 5 ton unit correct because of these losses? The calculation I did, did not ask about location of ducts.
Question 2) The turn down ratio (modulation) of the MXZ-SM60NAM (60kBTUh to ~16kBTU) vs MXZ-SM60NAM2 (60kBTUh to ~30kBTUh) seems to have dramatically changed for the cooling according to NEEP . Why is this?
Question 3) Because of the above info and the concern about AC not being able to modulate in the fringe months, I’ve considered instead going with a 4ton hyper heat multi unit. Thoughts?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part