Design upgrade: two hybrid LP gas furnace + heat pump

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Hi everyone,

Long time, first time :)

This is about upgrading an <80% LP gas furnace and AC to a 92+ and heat pump (HP). One of my current LP furnaces needs replacing soon, so work is inevitable.

Designing an upgrade. Two systems, dual fuel/hybrid-- Propane furnace coupled to a heat pump (HP), so burn LP on cold days, HP on mild winter days, and HP for AC in the summer.

Stats: I'm in southeastern Massachusetts (zone 4), and burning $4K+/year in propane. House was built in 2001, size 3000 sqft, 4-inch walls, lots of large Andersen windows, and I feel it could better insulated, so considering spray-foam on the attic (on roof underside) to let the attic act as added insulation and make it usable for storage. (It's sizzling in the summer and freezing in the winter)

Current HVAC: two separate systems -- both Heil/ICS/ComfortMaker RPJ-II LP gas furnaces + AC, forced air. #1 is 100K-btu, #2 is 75K-btu, not sure of the AC tonnage. Solar panels too, but our usage is well above production and we run out of credits in Oct-Nov.

1st story is ~2000sqft with full/unheated basement underneath; ~1000sqft 2nd story is semi-separated from 1st story: it's a family room over the garage and a small office room partly over the master suite's bath on the 1st floor. This 2nd story space is next to a large attic which is over the 1st story, and is where the 2nd story's furnace+air-handle is located. Both HVAC systems share the same single-flue chimney, and LP hot water heater. Basement is unfinished, but we're slowly building out areas, so any necessary duct changes need to be done soon, or opportunity is forever lost, we wont factor in basement for heat load (workshop, exercise room, cold storage).

I fantasize about combining the two systems into one. but I understand that two systems appropriately sized can be more efficient than one larger system, especially when heat is generated close to the room vents.

For main house, a 95% furnace and HP. Done.

For upstairs, could use a 95% furnace and HP, venting out the eve or roof.

Also considering a ducted mini-split for the smaller 2nd-story space, meaning one head outside feeding a distribution inside. A ducted mini-split works just like a single interior ductless wall unit, but it's a single air handler (usually flatter than a big box bolted to a furnace) installed in the ceiling (attic in my case) and uses dampers to direct air to each heating zone duct(s). These multiple zones allow it to provide the function of multiple wall units (perhaps in separate rooms), but with single unit (coil, fan, etc). Typically the ducts are smaller and more in number, since it needs to move more air which is not as warm as that of an oil/gas based furnace. But since this attic space is next to the conditioned space, duct and register changes are easy, and I could greatly reduce the overall duct lengths since it lets me move the air handler away from the chimney and closer to the heated space. THE QUESTION is whether a HP-only can keep up with the heating load for this space. Maybe electric backup (vs propane), even tho Mass is approaching 25¢/KWh electricity... this space gets minimal use and so electric backup may work... that and a small glass-front LP fireplace for ambiance (w air jacket +fan, ducted straight to an outside stainless stack)

I could insulate the attic roof underside and closing in the eves (or add knee walls) -- would eliminating sizzling hot and freezing cold conditions around the ducts (not to mention damage to stored items from intense heat.)

Not sure if a one-to-many heat-pump/air-handler arrangement is possible or practical (??) Meaning that one large outside unit feeds both a new hybrid LP furnace/AC air handler for the main house (HVAC system located in basement), as well as the air handler in the attic. Y'know, like mini-splits can use a single outside head for multiple interior wall units. I know those work in both heating and cooling modes. maybe this can't be done for operation in dual heating modes-- HP for low heating load, and LP gas for high heating load(??). If this arrangement IS possible then what brands do you suggest?

Minor stuff:
Remote thermostat apps--I currently use Radio Thermostats so I can talk directly to them via the phone app, without using their Internet Mothership (security issue). It appears there are lots of high-tech touch-panel thermostats, and remote apps exist, and interfaces to security systems (e.g. we used to have the thermostat connected to our alarm system). Any favorites for this application?

Again, I fully appreciate keeping it simple. In my previous house the orig cast-steel boiler quit. We replaced with another small boiler (hydronic baseboard), because we had gutted the house and exterior siding, replacing everything and re-insulated walls and attic. It was so well insulated that oil use was very low; the cost of a complex system would have led to a 10+ year payoff, or never. And we wound up only living there for 4 years.

Any other ideas would help greatly--

Thank you!
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