Designing heat pump system transitioning from natural gas furnace – GreenBuildingAdvisor

1550 square foot duplex home built in 1900 in Western Massachusetts

3 floors: first floor is one unit, second and third floor is a second unit

Currently heated by a 90k btu natural gas furnace (way oversized, I calculated roughly 45k btu needed using Dana Dorsett calculations based on gas bill) with 6inch ducts to first and second floors. passive registers let heat up to third floor

-Never had an issue with third floor being too cold in winter (this is without an active heating on that floor)

-There are 2 ac window units on the third floor.

-The third floor is has a hallway and two bedrooms, one of the bedrooms is split into 2 rooms. 2 foot high knee walls so much of the space has sloped ceilings.

This is a rental unit. Currently I am paying for the heat. I would like to split the heating system in two and put each unit on its own meter for tenants to pay heat.

Option 1

-Use a ducted heat pump to heat/cool the first floor and close off ducts to 2nd floor.

-1 multi-split ductless unit for 2nd floor with a head for the bedroom and a head for the living room/kitchen space

-1 one-to-one unit for third floor with a floor-unit in the hall (this would be primarily for cooling, 624 square feet with 2 foot knee walls)

Option 2

-Replace gas furnace with another gas furnace for the first floor

-Upstairs is same as Option 1

Option 3

Remove ducted system all together and install multi-splits for each unit

-Seems like one multi-split condenser for second and third floors would be super inefficient because third floor will only be running mostly during summer(cooling) and second floor during winter (heating), causing an oversized unit for most of the year.