I've got my nighttime setback device up and running and am happy with it. After I sketched out the basic design (shown in an earlier post above), I realized that I could add a bit of versatility to it by including a switch (SPDT Center Off) that would allow me to:
Operate it in the timed setback mode, or
Take it out of the circuit completely so that the stove operates normally, or
Run it fulltime at the setback temperature (like when I'm away all day).
Here are the resulting diagrams of the device and of the entire setup.
And here are a couple of pictures of the finished product. If you're really sharp-eyed you can see that the actual wiring deviated just a tiny bit from that in the diagram, though they are electrically equivalent. I drew the diagram for ease of comprehension, and wired it for ease of construction.
Quickly reviewing the design, in normal usage the stove is controlled by a thermistor, which is a resistor whose value changes with temperature, and in the opposite direction from the temperature change. What this device does is place a resistor (I used a trimpot, which is simply a small, variable resistor) in parallel with the thermistor, under control of either the relay or the switch. This lowers the resistance seen by the stove and fools it into believing that the room has suddenly gotten warmer. So it cranks its heat back through its normal processes. The resistance of the thermistor increases as the room temperature drops, until the combined parallel resistance reaches the value the stove is set to expect as normal. Then the stove does its regular thing to keep the temperature at about that level. Taking the resistor out of the circuit returns the thermistor to sole command, and the stove heats the room back up. The stove never actually shuts itself off or exhibits any error indication; it just goes through its regular adjustment processes, though the adjustments it makes are greater than those it would usually encounter in normal operation.
Before connecting this to the stove I used my multimeter to set the trimpot to 30 kOhms as recommended by 603BOB. I haven't run this long enough to get a reliable estimate of the actual setback that this accomplishes, but it seems to be a bit under ten degrees. Theory says it should be in that ballpark when room temperature is around the normal comfort range. I might tweak it a little bit, but I saw no reason to make the adjustment accessible from outside the box. It will probably stay untouched forever after I've finished tweaking. I think you could easily substitute a fixed resistor of about 30 kOhms and get quite satisfactory results. And save a dollar or two. A resistor of less than 30 kOhms will produce a greater setback; a greater resistance will produce a smaller one. Wattage of the trimpot (or fixed resistor) is not critical. This is a very low current situation, so most any wattage will work fine.
If you do build one of these and want to use a trimpot but don't have a meter to set it with, be sure to adjust the trimpot to somewhere around the middle of its range before plugging it in. (Or buy a meter at Harbor Freight for $3.99. Everybody should have one!) You wouldn't want to take a chance on the setting being so low that it damages the control circuitry in the stove. And adjust it in small steps, never letting it get down too close to zero. My trimpot requires many turns to go through its full range, but I've seen others that have a very coarse adjustment and could easily get set too low.
The trimpot and the relay are secured in place with mounting foam. Nothing moves around inside.
There are lots of ways this could be connected into the sensor cable. I chose to use 3.5mm audio jacks and plugs since I already had them in stock and they presented a neat appearance. I made a short adapter cable to connect the box to the existing crimp terminals on the sensor cable, since I wanted to leave it unmodified in the event of warranty repairs. And I made up my own cable to connect the box back into the stove: crimp connectors on the stove end and audio plug on the other. Polarity doesn't matter; either way is fine. I would have done something like that anyway, because I wanted to mount the sensor farther away from the stove than its own cable would permit. If ease of disconnecting isn't important to you, you could simply run one pair of wires out of the box and connect them to the existing cable. It's not really necessary to run the sensor circuit through the box as I did.
Do be careful to ground yourself to discharge any static electricity (especially common during these winter months) before touching any part of the device that connects back into the stove. You don't want to take a chance on zapping anything in there. And keep the AC and the sensor circuitry well separated from each other. Note the shrink tubing on the relay terminals, where the two are closest to each other.
I'm thinking about putting in a pilot light that will tell me when the box is getting AC from the timer, if I can find one small enough to fit and that's capable of still providing the necessary safe separation of the AC from the sensor circuitry.
I'll confess to one embarrassment. The timer I used is one I had already set up for a day/night program, so I just left it as is. That is, until I realized that the settings were backwards, and the stove was giving out regular heat when it should have dropped down to low. That was an easy fix.
As always, measure, test, be careful, keep the AC well insulated and well separated from the control circuitry, know what you're doing, and proceed at your own risk. Neither 603BOB nor I can guarantee that your stove is configured the same way that ours are or that something along the way won't go horribly wrong.