If you were going to heat water for hydronic heating you would use and appliance designed for that purpose (boiler) Being that your in SF, I doubt your water temp would need to exceed 160 degrees. Your best bet is to use a modulating condensing boiler that would have efficiencies up to 96% with nat gas.

In a heating-only environment I would never consider forced-air heat in new construction or in a studs-out remodel. Hydronic radiators or in-floor is the way to go. It is most quiet and comfortable. Adding AC changes EVERYTHNG. If you are going to have two systems you add expense and that kills it for most people as others have ably pointed out. In addition to that, if you don’t have in-floor radiant, you double the amount of heating/cooling “equipment” in the room, meaning that in addition to the supply and return grills, you have to deal with the radiators. The take-home message for the next paragraph is this, if you have a leaky house, including the ducts, you will have a dry house in the heating season and a damp, clammy house in the humid, cooling season. If you have a tightly-sealed house with good ventilation control, you will have a more comfortable home. It does not matter how the heat is provided via blowing air or flowing water, heat pump or boiler/burner. I will elaborate in the following paragraph. Any time that you hear or see something that indicates that says that forced-air, hydronic, or heat pumps are inherently more or less drying, raise your shields because the horse-hockey is flying. The relative humidity depends on the temperature and on the amount of water in the air. That noted, you have to think about what affects the water in the air. Input is obvious, cooking, bathing and breathing are the big contributors. If you have a big crop of plants growing inside, that is a factor too. In Winter, water goes out of a home via air leaks. In Summer it enters via air leaks and condensed by AC evaporator coils. Forced-air heat is probably perceived to be dry because it is associated with leaky ducts. Duct systems that leak up to 5% are considered good though 5% seems like a lot to me. When these leaks are outside of the home’s envelope, and they usually are, they cause pressure differentials. Your in-attic supply ducts are blowing your nice, moist heated air outside which causes (winter dry) outside air to be sucked in somewhere else to make up for it. Return air duct leaks can depressurize parts of the house as well. That is why, in addition to less noise and having the air blowing around, hydronic is better in a heating environment. Whenever you design a house with a big blower and ducts, you are going to generate pressure differentials Heat pumps may be perceived to be less drying because they don’t suck in dry outside combustion air, like a conventional furnace. A condensing furnace will not do that, but it will still have those pesky leaking ducts. OP, note that your scheme to add humidity to your home in the winter with the conventionally-designed AC-only ducts, grills and blowers probably will not work. In order to add significant water to any indoor air being circulated in those ducts, you have to raise the temperature significantly. Even if you did not have to inject the ducts with heat at the same time you add water, you would not like much the cool air blowing on you.

I have no idea whether the photovoltaic panels could produce enough energy enough of the time to offset the small cost disadvantage of electricity, but your numbers are right. Rather than using an electric resistance water heater (the standard type) have you considered a heat pump water heater? This would be two to three times as efficient as a standard water heater, so you could give yourself a huge cost advantage over gas. Then you could use the power from the PV panels for something else. I am not sure about the heat recovery rates of this type of water heater, so in many places I don't know how well it would work for space heating, but since it's not too cold in San Francisco, it might be worth considering. Here is a link that might be useful: Heat Pump Water Heaters

Did you make provisions for fresh air intake to the room? I don't recall the sizing recommendations on our brand new 70k BTU boiler, but it was stated that if the boiler is in a small room, intake air must be allowed to come in through large vents at the top and bottom of the room. Even our own municipal code requires a six-inch air intake on our garage because that's where our boiler and water heater are. 47 years is a great life span for a boiler. I just replaced a 140k BTU boiler with a 70k BTU unit. The new boiler is the Weil-McLain GV3. Our heat loss calculation showed that our home should lose about 55 BTU / hour at -10°F. But since we haven't been in the house through a winter, we were being cautious. If you choose to replace it, you'll need a heat loss calculation to size the new one correctly. No doubt that you can find a unit that will be much more economical. I found myself unwilling to go for a new 'modulating' boiler due to my concerns of its reliability, so I went with a unit that's a 15 year old Condensing design. When researching, look into Outdoor Temperature Reset units, which adjust the temperature of the boiler supply water to be in line with the heat loss of the house in warmer weather. (For example, my boiler has been running at around 120°F while the temperature outside is hovering around freezing. As it gets to 10°F outside, the boiler is gradually ramped up to 180°F. This saves a bundle on fuel usage.)