What you describe is a control problem.

Explosion is not the issue, though any water pressure vessel can explode, it is the fire hazard. Wood is filthy, inevitably degrading IAQ and requiring considerable (hopefully) combustion air which is 100% parasitic. I use a small wood stove but don't have any allusions of it's efficiency (about 75% not counting loss of conditioned air up the archaic lined chimney), but also aspire to more sophisticated and automatic systems.

I would be surprised if a wood stove reached 75%, 80 to 90% you are talking gassifiers.

http://www.woodboilers.com/product-detail.aspx?id=50 Just an example, wood boilers have moved on a lot.

There are wood stoves with make up air options the same as there are for most pellet stoves.

So the real negative is if you do not want to bring wood inside the building, pellets may be a cleaner option if that is a big issue for someone.

Como, mine is a passive solar house rather a passive haus. According to my modeling, no added heat would mean 55 degrees at the house's coldest, in December. That is a nonstarter with the missus. I figure hydronic gives me better control of supplemental heat. I have temperature sensors in the slab so I'll know by late afternoon by how much the sun has fallen short of generating thermal lag. (Cloud cover is the problem in Pa.) Radiant happens slow enough that I should be able to hit the mark consistently. All this is conjecture of course. If the house needs little extra heat, I may be able to run supplemental on a thermostat. I'll have 200 gallons of water heat storage also, primarily to heat DHW without running the stove every day.

As Badgerboiler points out, another advantage is that a radiant slab still works with 110-degree water, extending storage capacity and improving efficiency across the board. Solar hot water works better with a high delta T between tank and collector, for example. You can also run a smallish heat pump at optimal COPs: afternoons only, say, or on a warm day preceding a winter front.

Finally, there will be times after trips that we return to 150 tons of cold concrete. On such occasions, the wood stove/radiant combination is a way to get the floor at least back to comfortable temperatures in a reasonable time frame. I figure a degree an hour.

Badgerboiler's concerns are easily addressed by buying a stove with outside air supply, or, as in my case, isolating the stove from the house. When I close the glass doors on my "fireplace," the stove will be separated from the house and burning outside air. There will be almost no radiant output downstairs, or hydronic either if don't switch on the pump. I should be able to heat DHW through much of the shoulder seasons.

For my money, an outside wood boiler idling on a warm winter afternoon is genuinely filthy business. I hope to get by on a tenth of the wood required and a fraction of the particulates generated. My stove is inside because it is an option rather a necessity. (Knock on firewood.)

Bruce, I sent my insurance agent an email yesterday explaining the difference between UL and CE and asking if her underwriters will have a problem. She is a marvel; she got me a 12-month builders risk policy when everyone else said no way. We'll see. There is no question of legality here. The EPA standards for boilers are voluntary. And according to most U.S. codes, a vented (non-pressurized) boiler doesn't require UL certification.

Sounds like we are in very different climatic regions. I know a bit about the weather in Delaware, presumably similar to yours?

Where I am we have massive temperature swings, 30F is normal, can be more. So even on a really cold day, if as is quite usual it is sunny then we can get a lot of sudden solar gain. So what is required is something that is much more quick acting, both to provide heat and stop providing heat.

www.thermaskirt.com or a couple of radiators for example. Passivhaus was of course designed for Germany, some of the issues make less sense for Colorado.

PS Modern wood boilers can not idle, hence the need for storage.

Delaware has the coastal thing going: more heat and humidity. I have the mountain thing, including more cloud cover. Tuffest thing about passive solar. Every spot is different.

I have temperature sensors in the slab so I'll know by late afternoon by how much the sun has fallen short of generating thermal lag

Todd, where in the slab did you set the sensors and how did you make that decision? And, I'm sorry if I missed it in earlier posts, but did you lay 4" or 6" for those slabs?

I have planned on installing a large wood burner in my passive solar as insurance against loss of grid, but as action time draws near, I am increasingly thinking about getting some of that heat output into the hydronic heating system. The sealed fireplace has the ability to blow heat to 2 "remote zones" and I am thinking about putting an air to water heat exchanger in there to intercept some of that, so your descriptions have been interesting.

I have one sensor in the den for a loop that will be on a separate thermostat, and the other two in what I anticipate will be the hottest and coldest spots in the great room: At the greatest concentration of pex where they enter the slab, and at the corner with 24 feet of windows running one way and 16 feet the other. It is a 4" slab. Given the amount of concrete in the house, I figured that response time was more important than storage capacity. Radiant types say you shouldn't heat slabs above 85 degrees because of comfort and expansion considerations.

I considered your approach before it dawned on me that I could use eBay to shop in Britain. These folks have some very nice fireplaces that qualify for the (reduced) $500 tax credit: http://www.icc-rsf.com/en/rsf/rsf-fireplaces-woodburning-fireplaces. Note that they require a proprietary and pricey chimney pipe. There are folks in the OWB industry who can tell you what size air to water heat exchanger you need. I found a guy in MN who was quite helpful but I can't come up with his company's name any more. I was asking for an exchanger that could capture most unto all of the heat. His answer was that it could be done with some qualifications. I vaguely remember air speed as the sticky variable. Ask for HX names at Hearth.com. RSF's technical guy came to the phone too.