Hi I have all the pieces in place for a passive solar house with wood stove assist. I am leaning toward a low-tech approach to maximize safety and preserve options in case it doesn't work. I'd like to know what you think.
 
The house is 1600 sf: 1000 sf downstairs in a single room and 600 sf upstairs in two bedrooms and a bath. The heat downstairs will be radiant in a concrete slab. The heat upstairs will be forced air. The design heat loss is 25,000 btu/ hr at 10 degrees. The house has lots of mass -- 50,000 pounds of concrete in the floor alone -- so the average temp of 30 degrees in Dec and Jan. will probably mean more than design.

The south wall of the great room has 40 linear feet of windows six feet high. In theory, there will be enough heat coming through the windows to generate thermal lag (heat the slab to the point that it will carry the house overnight.) I want forced air upstairs to move excess heat between the floors on super bright days (and to distribute air from an HRV.) But in Dec particularly, I'll have the opposite problem. There are long stretches in so central Pa when the sun doesn't shine.

The theory anyway is to use a wood stove to induce thermal lag on cloudy days. The stove is a Stratford Eco-Boiler http://www.eco-boiler.com/ It puts out 70k btu/hr max, with 41k btu going to water. I figure that putting 25k btu/hr into the slab in hot, efficient burns of eight hours to 10 hours will get the job done.

Of course all of this is conjecture, particularly the solar part, so the mechanical design needs to be flexible and incremental. What if I started out by using a 150-gallon hi-temp polypropylene tank, vented and insulated to R15, as an indirect tank? Connected to the stove by thermosiphon, the tank would be for heat storage only. If my math is right, it would take the stove 3.5 hours to heat it from 100 degrees to 180, at which point it would be storing about 90k btus. The slab removes any possibility of overheating, except in malfunctions. It could soak up 200k btu before anyone would say 'hey, this floor is hot.' The stove also has an automatic shutdown feature, reading the temperature of the return water.
 
The question is how to get the heat out of the tank efficiently -- and inexpensively in case I have to scrap much of this and start over. It would be simple enough to coil up 40 or 50 feet of copper tubing and drop it in. In this approach, I would use the tank to preheat potable water for DHW and hydronic, straight through an electric boiler in the case of DHW, and through the boiler into a heat exchanger in the case of radiant. (I'd use antiscald valves and mixing valves to keep temperatures in range.) Do you think a homemade heat exchanger could keep up with the stove? Will an electric boiler stand up to feeds >150 degrees? If passive solar is a complete bust, how you would add a high-efficiency oil boiler? Scrap the electric? Separate DHW and radiant?
 
What's missing here is quick response, even from the wood stove. I could add hydronic to the forced air for a fast response. Sort of. (In spring or fall, it could take the stove several hours to get the indirect tank hot enough to make a difference; I am reluctant to tamper at all with the thermosiphon loop.) Can you do a threeway out of the electric boiler: DHW, HX to hydronic slab, HX to hydronic forced air? With the air handler pulling air from opposite ends of a single room downstairs, and with an open, central stairwell as a return, is that enough circulation to heat both floors in a tight house?
 
The house is in the woods, so I can't say this point whether I can mount a solar water heater above the leaves, or what the house will be like in late Oct. before the leaves fall. A heat pump could be handy early and late. Even in the winter it would be fairly efficient. Sorry to be so vague. This is one where I'm going to have to live in it first. I am just trying not to do anything terminally stupid.