Use existing duct system to allow a ductless minisplit to heat more rooms?
My parents live in Vermont, climate zone 6. The house was built in 2011 and is pretty high performing (ACH50 under 2, etc). They are interested in reducing or eliminating the use of gas in their home; right now it is only used for space heating. They have a forced air gas furnace. There are no great heat pump products today that would be a drop-in replacement for the furnace. Manual J heat load at the design temp of right around zero F is approximately 24 kBTU/hr, although based on their consumption and HDD, with aggressive setbacks they would see gas heat demand closer to 18 kBTU/hr at that temperature.
Options include: 1) wait for better ducted HP options to become available and 2) switch to ductless. I am here to ask about option 2.
The main living space (about 40% of the one-floor home) is quite open and would accommodate a ductless head pretty well, but the three bedrooms and 2 bathrooms would not get great distribution of the heat.
If if they put a ductless head (say a 1.5 ton unit) in the main room, could they run the air handler for the furnace to mix the air in the house and get heat to the rest of the rooms? Using small fans (eg bath fans or HRVs) for this purpose has been debunked on this site, but a full on air handler that is intended to serve a furnace/AC seems like it could work to link the temperatures in the bedrooms with the main space, and thus to allow a ductless head to heat the home. What do you think? Thanks in advance.
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>"There are no great heat pump products today that would be a drop-in replacement for the furnace."
Don't be so sure about that. What are the CFM and BTU ratings of the furnace?
If the gas furnace and ducts are already oversized for the existing load they may be able to accomodate a mini-duct cassette's limited static pressure capability. The Fujitsu 18RLFCD is rated for up to 0.4" water column and can work just fine within a range of pre-existing duct sizing. It's good for a bit more than 18,000 BTU/hr @ -5F, which might be good enough, but maybe not.
On the other hand there are beefier ducted minisplits with flow rated at up to 0.8" water column static pressures and can handle an even broader range of preexisting ducts. It's likely that a 2-ton Fujitsu ARU24RGLX could handle this:
https://ashp.neep.org/#!/product/25350
If not a 2 ton, the 3 tonner for sure:
https://ashp.neep.org/#!/product/25352
The 2- ton Midea DLFSDAH24XAK would handle it comfortably too (and has a pan heater on the outdoor unit for automatic defrost ice management):
https://ashp.neep.org/#!/product/26524
>"If if they put a ductless head (say a 1.5 ton unit) in the main room, could they run the air handler for the furnace to mix the air in the house and get heat to the rest of the rooms?"
An air handler moving room-temperature air around adds nothing but wind chill, not much heat, and may end up creating more infiltration losses than would be "worth it". Blowing 75F air out of the register is the opposite of "comfort" during the heating season, whereas blowing 110-120F air is more like "summer breeze" territory. The only time moving heat with an air handler is reasonably efficient is when the temperature of the air it's moving is substantially higher than the room it's delivering the heat to. Moving 75F air to a 65F room can sometimes take more air handler energy per net BTU delivered than heating the space with an electric space heater.
Thanks, Dana. I really appreciate the quick answer. I think the 2 ton Fujitsu with higher static pressure might work for them - that’s a hard product to find on the Fujitsu website, and the higher static pressure isn’t obvious in the NEEP data.