Slow1 said:
I would want the thermostat to regulate the air so as to maintain a steady heat output. By making regular adjustments to the air input (constant fiddling if you may) it could in theory even out the hills and valleys even more than the thermal mass of the soapstone does. Net effect? Well - I would expect that as the fuel is consumed and goes through the various stages of burn (from rapid outgassing through to coaling stage) the air would be adjusted to keep the effective heat output more steady, less air during the outgassing to temper the burn down more, then more air during coaling stage to keep the heat up and 'burn down' the coal bed and get the maximum heat out of the stove at that time.
Wow, that's a tall order, don't you think? Here's a few things that I can think of that will make that quite a challenge:
1. The blessing of 640 pounds of soapstone will prove to be a curse if they make any attempt to regulate the burn by using feedback from the stove temp to make adjustments to the air intake. The stove simply won't be able to react in time. Here is where I feel soapstone's stellar qualities will work against it. Because of the high specific heat of soapstone compared to steel and iron, it will take a lot more heat to raise the temp of the material up in temperature by the same amount. By the time the stove gets up to a higher temp, the conditions that caused it to rise in the first place would now be different. As an engineer, I'm sure you are familiar with this as
hysteresis. The lag time between changes in air intake and changes in stove temp would be too long to be useful unless a "history" of how it got there was part of the control mechanism.
2. Any attempt to regulate the burn using a thermocouple in the flue might be even more problematic. This stove seems to burn at an amazingly uniform flue gas temp over a wide range of conditions and heat outputs. The stove would somehow have to "know" the conditions that are responsible for that flue temp at that precise time. It could be 275º on the way up from startup, or 275º at the end of the burn. How will the thermostat know what way the stove wants to adjust the air? Giving more or less air will not always have the same results in different circumstances.
3. Any attempt to control the stove by using flue gas analysis with sensors would be prohibitively expensive and ridiculously complicated. Granted, this setup would be the tits if they could accomplish it, but I don't think they are going in that direction. As a matter of fact, when I proposed such an idea to Tom last year he just kinda stood there blinking at me. I took that as a "You gotta be kiddin' me?" look.
I have a stove with thermostatic air control, and for me, I have found I have a love/hate relationship with it. My VC stove has a block cast into the back of the stove with a bi-metallic coil that opens and closes the primary air as the stove body temp changes. It's a crude little mechanism, but an elegant solution to the problem... when it works correctly. Here's a few examples of where and when it fails.
- In the morning when I go down to a cold stove, the air flapper is wide open since the stove continued to cool as the burn died down, and that causes the flapper to open to allow more air in. I may start it there to get the fire going closed well, but five minutes later I have to close it down before I walk away from it or I risk overfiring the stove. At five minutes into the burn, the bigger wood is just beginning to pyrolyse but the stove temp hasn't budged all that much. If I close the flapper too far, when the fire gets really going and outgassing begins in earnest, and the stove need more air to burn off those gasses, the increasing heat will work against this and actually close the draft more. This isn't just a hypothetical, I experience this phenomenon every morning of the burn season.
- At the end of the active burn and just before the coaling stage, the stove will be at its hottest. The flapper will be as closed as it is going to get at that point as well. Everything seems to be working fine until the coaling stage is reached. At that point, the feedback from the still hot stove is causing bi-metallic coil to keep the flapper closed, even though the inside of the box is crying for more air to keep those coals burning. By the time the stove has cooled sufficiently for the flapper to allow more air in, it's too late and the coal bed has cooled and collapsed to a point when only a fresh load will burn up those coals. Plus, that wide-open flapper is allowing the heat inside to get drawn up the flue without check, even though the air velocity past the coals isn't great enough to make them burn hotter. Less heat transfer going on and no way to reverse it because the flapper won't be closing down again until the stove either gets hot again (and, of course, it won't) or until I manually move the lever to the right. Which I don't do, because I'm either asleep or am not home.
There are several other faults with the VC system that will take too long to explain, but it is not the "set it and forget it" convenience it is touted to be, at least not in my experience. No, it is not a sophisticated control system, but I hope I have explained why I believe a better one is not an easy thing to engineer. Kudos to them if they can pull it off in a convincing manner. As for me, I'd be just as satisfied to keep to the K.I.S.S. principal in play and go with the stove as it is currently being offered. One lever, set it where you want it, and learn what it will do at that setting and in those conditions. Maybe I'm just a control freak (ask Lady BK if she disagrees), but I feel more comfortable being the one in control of my heater rather than some mechanical contraption making all the decisions, no matter how effective it might be at making them.