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The heating anticipator
In order to get the thermostat to keep from overshoot and lag, we need something that makes the thermostat close early and open early. If we place a small heater near the bimetal that is energized when the thermostat is closed, the heat will cause the bimetal to bend and open the thermostat earlier than the change in temperature of the structure would allow. As we discussed above, a thermostat set at 70F would open at about 71F. With the anticipator, the thermostat would open at about 69F and the furnace fan would clear the heat from the heat exchanger, bringing the temp to 70F.
When the thermostat opens, the heater is also shut down. As the heat added by the anticipator is removed, the bimetal senses the ambient temp that is dropping as the structure cools. This allows the thermostat to close a bit early, thus "anticipating" the drop in temperature. When properly set, the anticipator will effectively keep the structure temperature within one or two degreesF. The video below explains lag and overshoot and provides an illustration.
When the thermostat opens, the heater is also shut down. As the heat added by the anticipator is removed, the bimetal senses the ambient temp that is dropping as the structure cools. This allows the thermostat to close a bit early, thus "anticipating" the drop in temperature. When properly set, the anticipator will effectively keep the structure temperature within one or two degreesF. The video below explains lag and overshoot and provides an illustration.
Anticipators on mechanical thermostats vary with the manufacturer. What they have in common are some sort of dial or slide that exposes ore or less of a heater wire. The more wire exposed, the more heat given off and the shorter the cycling. In the video below, you can see some of the types of anticipators used and how to adjust them.
Above we have seen how to adjust and anticipator. But how do we know how to actually do it. We are trying to match the amperage draw of the control circuit so that the anticipator puts out a specific amount of heat necessary to provide the proper anticipation. If the amperage draw of the control circuit is high, less of the heater wire is exposed.
When setting the anticipator, you must know which wire to check amperage draw. A clamp ammeter is used for this. When checking amp draw, the heating control wire must be clamped. This wire is usually colored white. The video below goes thru the operations necessary on a gas furnace to do this operation.
When setting the anticipator, you must know which wire to check amperage draw. A clamp ammeter is used for this. When checking amp draw, the heating control wire must be clamped. This wire is usually colored white. The video below goes thru the operations necessary on a gas furnace to do this operation.
Some homeowners will like to set the anticipator to their own tastes. This quite simple to do. Set the anticipator to the amp draw of the circuit and show the customer where the adjustment is. If the customer wants longer cycles, all he has to do is move the adjustment toward longer cycles about 1/4" at a time. The video below explains how.
Troubleshoot the heating anticipator
When the anticipator is maladjusted, it can cause 2 problems. If it adjusted to too short of cycles, the furnace will come and off in very short cycles. It may not keep up with heat loss of the structure. If it is set too long, the temperature in the room will raise beyond the temp setting, then drop below the setting and the complaint will be too hot then too cold. Sometimes, the anticipator will burn in two. If it is burned in two, heat will not come on at all. This usually results from a short in the control system. Anticipators seldom fail by themselves. If the anticipator does fail, the thermostat must be replaced. In order not to ruin the new thermostat, the heating wires to the thermostat (usually red and white) should be shorted together with a clamp ammeter around the white wire. If the amp draw is excessive (over 2 amps), you have a short in the controls. If there is a fuse in the control circuit, it may blow. At this point, you must find the short before continuing. Check here for instructions for finding the low voltage short.