You could also couple a forced air hydronic air handler with one designed for both heating and cooling, install an air-to-water heat pump that would provide hot or chilled water for heating and cooling and use the Rinnai as a backup.

It’s not commonly done in residential systems yet but it will become more common in residential high end, high efficiency high flexibility installations.

BTW: That’s a very large capacity boiler for a home that needs only 60,000Btu. I understand it's for instantaneous domestic hot water. You might benefit from an indirect water heater that would also serve as a buffer tank to not only provide DHW but to prevent short cycling supplying a 60,000Btu hydronic forced air heating coil.

TuboMax Thermo 2000

IMPO

SR

Yes, it can be done. I personally wouldn't (but I wouldn't have a tankless water heater, either). Check with the local Rinnai dealer for more information.

The first comment above not surprisingly shows a knowledge gap - most gas tankless water heaters modulate, meaning - if the circulating water comes to the input port needing just a 20F degree temperature rise, the unit will fire only at the reduced heating level needed to reach the set temperature, and rarely at full power once it has been on for awhile and maybe never. Also, I would hope that the unit would have a minimum temperature setting to allow it to pulse on and off as the water temp changes through the set range.

The gap puts the rest of the comment into context. In the US, an area he's unfamiliar with, even with the wide range of climate zones, most homes need both heating and air conditioning, and there hasn't yet been any movement toward water systems in homes. If and when gas appliances reach a sunset of permissibility, expect heat pumps using refrigerant to become more common, with forced air distribution remaining most common.

Installing a water to air heating system is not a trivial exercise and isn't well suited for DIY handling. If you're not skilled in the HVAC trade, consider hiring an expert. Even among HVAC contractors, many don't do such systems.

Don't really need air conditioning being so close to the ocean. Our 1896 Victorian home interior never gets above 78 degrees, but does drop down to 55 degrees at night..

Manual J says I need 40,000 BTU for the first floor and 20,000 BTU for the second floor. It also recommends a two stage 60,000 BTU furnace until second floor heating has been completed.

The 199,000 BTU natural gas tankless water heater has not been installed, so was going to exchange for a Rinnai condensing boiler. The reason for the high BTU tankless water heater was to supply enough hot water for the first floor and the second floor with two bathrooms.

Current forced air horizontal heating setup installed in the crawl space is for the first floor only . Heating source for the second floor has not been determined as yet. Second floor is 1600 square feet with 9 foot ceilings.

Consensus seems to be replace with either a heat pump and air handler, or conventional two stage gas furnace.

"Consensus seems to be replace with either a heat pump and air handler, or conventional two stage gas furnace."

I'd expect so.