CLEANING UP WOOD HEAT FOR 1982-83

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To provide a means of adjustment, we added a second diaphragm, which is controlled by a knob. By moving the rubber membrane in or out, we can alter the volume — and therefore the pressure — in the system. During normal operation, the sensor will be under a slight vacuum, since residual tension in the control diaphragm closes the valve at atmospheric pressure.

The thermostat is first tuned in by operating the stove at the highest desired temperature (with the door open slightly to admit additional air) . . .turning the adjustment diaphragm knob so that there's maximum volume in the chamber . . .and then plugging in the tubing. Once the door is closed, the stove will begin to cool ... because the valve will be lightly closed at atmospheric pressure. Then, as the stack gases cool, the pressure will drop in the system, and the valve will be drawn open. The net result is that the thermostat will maintain a smoke temperature between 10 and 20°F cooler than the initial setting.

Now, to reduce the stove's operating temperature, the adjustment knob can be turned counterclockwise to decrease chamber volume and increase pressure. The valve will then close and remain shut until the sensor cools enough to reduce pressure to the valve's opening point. Once an output level has been selected, the valve will respond to changes in pressure as small as a quarter-inch of water column . . . which corresponds to a rise or fall in temperature of 2 °F. Furthermore, it will begin to move nearly as quickly as a temperature change could be registered on a thermocouple thermometer. Consequently, the valve will have to move only a small distance to control output at normal operating temperatures, and the fire condition can be controlled in very small increments.

We've run our thermostat at a sampling of burn rates varying from 1-1/4 to 4-1/2 pounds of air-dry maple per hour. At the lower settings, the device is able to hold stack temperature to within 5°F above or below the desired level. However, at the higher burn rates, control does degenerate to plus-or-minus 10°F smoke temperature. Any such fluctuation is generally quite short-lived, though, and the readings will almost always settle back to the desired number within five minutes.

CONCERNS

To achieve the accuracy that our unit is capable of, it was very important that the friction in the system be kept low. Therefore, though the assembly process for the thermostat isn't difficult, it's absolutely imperative that the holes in which the control shaft rides be correctly aligned. The system must be leakproof, too, and we suggest that you test the sensor and the diaphragm assembly (lightly pressurize them with an air hose — at no more than 5 PSI — and submerge them in water) before installation.

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