A properly installed forced hot air and AC system can be comfortable and energy efficient. A radiant floor system is very nice, but it can be very expensive. It comes down to budget and how to spend the money. My vote would be for a the forced hot air system with a variable speed furnace and 2-stage AC. It is a little more money, but it is the quietest and most comfortable set up. It will also give you the best performance for lowering humidity in the summer. For a house of your size I strongly recommend either making the first and second floors separate zones, or have separate systems for the first and second floors. If you do get two systems I would want them both in the basement. HVAC equipment should not be installed in an unconditioned attic. It is done all the time, but I would pay extra not to have it. The primary reasons houses are dry in the winter is due to outside air infiltration. New construction is tight and should not have a low humidity problem. However it is not too costly to add a humidifier to the furnace. You can add the humidifier later if you want to first try without it. Just let the contractor know your intentions.

Radiators do not dry the air any more than any other heat source. All other things being equal, the only things that affect the final relative humidity are the absolute humidity in the air and the air temperature. What makes indoor air dry in winter is excessive air exchange with outdoor air. The moisture that is produced by ordinary living (breathing, cooking, bathing and cleaning) leaks out. Poorly designed and installed ducted, forced air, heat can make matters worse by inducing pressure differentials turning the forced air system into a power vent. If you have excessively dry air, put money into air sealing your structure. It will pay off handsomely in summer and winter. Steam heat radiators that are installed well in a good design and maintained properly can be silent, or nearly so. There is something wrong with your system. I am not saying that you can easily fix it, but there is something wrong. A competent steam contractor will know how to add a steam heat exchanger to your current cooling duct set-up. High temperature steam is ideal for a relatively compact heat exchanger. The main problems might be how to run the pipes and if there is room for the heat exchanger. You may not be able to move enough air to heat even though you can move enough in the current ducts to cool sufficiently. It may not be practical for those considerations. If cost is no object, you might be able to add radiant heat to the floors via a steam to water heat exchanger with an associated circulator pump and expansion tank. You may be able to find a contractor on a forum that has a lot of "steam heads" posting. Try heatinghelp.com and terry love.com.

Agreed Bry, that's a great point. In renovation or replacement comparison situations when a household is consistently using the baseline allowance, using the next tier rate up would be more accurate. It's also tough figuring out how much of the minimum monthly gas fee to include. I would guess the water heater may account for ~50% of usage so more accurate water heater gas costs would be somewhere near $302. Much tougher figuring the added comfort and space conditioning benefits of a HPWH and if the gas combustion appliance is naturally vented, there is probably added moisture and potentially health costs involved. For new construction, not knowing specific situations I think average kwh rates are where to start, and it's only the most energy intensive homes that would recover new infrastructure costs. Those outliers might be wise to improve the design. An energy rater would be helpful for plan specific comparisons. Mr. Fudd's Tier 2 kicks in at 202 kwh, lower than baseline allowances I saw digging around on the PG&E site. It must be an energy intensive area because most Tier 2 levels I saw were higher. This suggests the averages I linked to are close. Most tier talk for PG&E (serving a third of CA households according to this page) is about to be irrelevant as they are transitioning to Time Of Use rates for everyone. This makes sense for the increasingly renewable electric grid. For example, it will encourage electric vehicle owners to charge during off-peak hours and will keep electric water heating costs competitive with gas, even for replacement situations and even in places of high electric/ low gas rates. This requires the effort of putting a timing control device on a tank water heater, something those with basic electrical skills could DIY. Tying this back to topic, new construction skipping air-conditioning could use electric resistance (or heat pump) radiant floor heating during off-peak hours. The floor mass releases heat during on-peak hours of the day and an efficient envelope would keep it in. No combustion inside home necessary and cost effective if done right. A guy is doing it off-grid in Saskatchewan so it might be easy in CA. Here's a link to the controller he built for the PV to electric resistance radiant floor (or battery) interface.

A big yes to all the ground insulation and vapour barriers but ‘cost notwithstanding’ the approach I would favour would be close spaced in-floor hydronic radiant for the lowest water distribution temperature and greatest efficiency, supplemented by hydronic reversible fan coil units that could be used to raise temperatures quickly in heating mode and provide air-conditioning and dehumidification in cooling mode. The response of the slab will be dependant on insulation, thickness of the concrete and depth within the slab at which the piping is installed, among other factors. Piping installed closer to the slab surface will respond faster if thermostat is bumped up; piping sunk deeper into the concrete results in a more stable but slower responding thermal mass. Take your pick. With in-floor radiant you basically heat the first 6-feet from the floor up and who cares about the rest as long as you and your feet are warm. Although ceiling fans would also be useful. With forced air you have to heat the entire volume of the envelope! Radiant is great for working on vehicles in winter as it keeps the floor drier and warmer. The room also recovers faster when garage doors are opened and closed due to the energy stored in the thermal mass of the slab. Also much more comfortable to work on your back underneath a vehicle - that is dry. One integrated energy efficient system with either an air-to-water or geothermal liquid-to-liquid heat pump as an efficient source. The thermal mass of the slab can also be used as a backup during a power failure by just keeping the circulator(s) running continuously with a very small generator to keep the all pipes from freezing for most outages. Your existing electric boiler can be a backup during extreme weather and for compressor lockout or other HP failures. Something else I can tell you from experience, particularly if you plan to own this for some time is that what looks like an astronomical installation cost today will look like an absolute real bargain 15-years from now - if properly designed and installed! IMPO SR