Heating a very challenging old house.

1700s timber frame structure with 1800s addition.
2.5 stories over laid-up stone foundation throughout. 3,200 sq/ft conditioned.
The house is in its final phase of an interior gut rehab.
As improved heat load of roughly 65K BTU/hr @ 1F (climate zone 6). 1st story heat load of roughly 30K BTU, with remaining 35K BTU load attributed to the 2nd story and conditioned attic.
60K BTU modcon combi boiler serves roughly 24K BTU of plated underfloor radiant tubing for the entire 1st story. Boiler fires down to 15K BTU.
The existing furnace (located in the attic) is grossly oversized for the improved envelope, even in low stage. The existing ductwork, which serves the entire 2nd story, is capable of supporting 1,200 CFM at acceptable static pressure.
The longer term plan is to utilize an air to water HP for both heating and cooling the 2nd story. In the immediate term I need to pursue a more basic strategy for the heating side of things.
Initial question: Do I opt for the smallest modulating furnace my supplier offers (60K BTU, with low fire at 24K BTU), or do I go with a hydronic air handler to replace the furnace completely?
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Replies
Unless you don't need cooling, replace the furnace with a heat pump. This can now be sized for the new load and the existing ducting is definitely big enough to support it. Simple install and no need for any venting. Make sure to go for one of the newer cold climate units that will deliver near nameplate heating capacity down to 0F.
I have an ATWHP and still use a heat pump for cooling. Cooling with water is not worth it if you already have the ducting.
Thank you Akos.
My HP guy only installs Mitsubishi with Hyper Heat. It’s a fine system, just mighty pricey. I’ll revisit the conversation with him.
Around me the BOM cost of a furnace+AC is about the same as budget cold climate heat pump and the install is simpler. Your HVAC guy might try to upcharge for it, but there really should not be much difference there.
Mitsubishi is not cheap but not that expensive either, you can see internet costs on-line to compare.