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Thermostat heat anticipator function & adjustment: how a thermostat heat anticipator works. We explain how adjusting the heat anticipator pointer changes the heat output of the anticipator that in turn changes the behavior of the room thermostat to turn the burner off sooner or later with respect to the actual room temperature. Our page top photo illustrates key parts of a traditional room thermostat including the temperature sensing device, thermostat switch, and the heat anticipator assembly.
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The heat anticipator in an electromechanical thermostat includes a tiny heating coil or wire which warms the thermostat's room temperature sensing bimetallic spring or in newer thermostats the temperature-sensing thermistor (see graph just below).
Warming up the thermostat's temperature sensor causes it to "open" its contacts (stop calling for heat) a little before the room temperature actually reaches the thermostat's "set" temperature.
Stopping the call for heat a little early allows for the delivery of residual heat that is already in the boiler or furnace but that has not yet reached the living space.
By turning off the call for heat a little early we avoid "overshooting" or making the room warmer than the thermostat's set temperature.
The heat anticipator is anticipating the additional heat that is going to arrive and regulating the thermostat accordingly.
How Adjusting the Heat Anticipator Changes Its Behavior - Variable Resistor
That tiny resistance wire or on older thermostats a wire wound into a flat coil is a tiny electrical resistance heater that puts some heat into the interior of the wall thermostat, fooling it into thinking the room is just a little warmer than it is.
In my photo at above left the blue arrow points to a round dark button - that's the contact point between the copper arm and the surface of the coiled resistance heating element (red arrow).
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When we move the heat anticipator adjusting arm we are moving an electrical contact (blue arrow) along the flat-wound resistor wire, effectively increasing (to the higher numbers on the left) or decreasing (to the lower numbers on the right) the amps or current draw of the device by lengthening or shortening the total length of wire included in the heater circuit.
Increasing or decreasing the wire length included in the circuit changes how hot the wire gets as current flows through it. We review Ohms law to explain this in still more detail at Questions & Answers at the end of this article.
If we move the arm and contact fully to the left to the 1.2A position that's using the maximum amount of the heater length - that's the most resistance. The heating thermostat manufacturer's instructions & heat anticipator operation explanation can be a bit confusing, but significantly, as we detail at HEAT ANTICIPATOR Adjustment:
Our graph at above left, adapted from one provided by Carrier, illustrates how a thermistor translates temperature changes into a change in electrical resistance that in turn can be used by a room thermostat to control building heating equipment.
How Changing the Heat Anticipator's Temperature Changes the Thermostat's Behavior
More heat output from our teensy electrical resistance heater inside the wall thermostat means more heat anticipation (more pre-heating of the room thermostat's sensor) and thus a shorter heat-on cycle.
Moving the heat anticipator pointer (the open triangle at the top of the coppe arm) changes the resistance and thus the heat output of the heat anticipator inside the thermostat.
Why Honeywell printed the "Longer" arrow on the low end of the resistance scale but included an arrow pointing towards the higher end of the scale is something I've asked the company with no result It may simply be a matter of where there was convenient, visible space on the stamped heat anticipator scale - Ed.
Watch out: before changing the heat anticipator setting you might want to read both the manufacturer's actual recommended settings and our explanation about matching the heat anticipator to the actual installed-thermostat circuit's current draw in the rest of this article.
The differential is the temperature (or pressure) change or "differential" between the LOW and HIGH settings of a heating system control. See AQUASTAT CONTROL Functions for an example of control differential settings on heating equipment.
What is the relationship between setting the heat anticipator higher or lower and the resulting room temperature?
Typically if you set the heat anticipator lower you are narrowing the "gap" or amount by which the heat anticipator anticipates the amount by which heat delivery will "coast" when the thermostat stops calling for heat.
In our example photo (left) moving the pinter to the left is setting the heat anticipator to a lower resistance (turn heat off later) and moving the pointer to the right is setting the heat anticipator to a higher resistance (turn heat off sooner).
If you click to enlarge our heat anticipator photo you'll see that the settings range from 0.10 to about 2 Amps - a scale quite similar to the mini Ammeter used to fine-tune thermostat heat anticipators based on the circuit resistance (Ohms) and current flow (Amps) as we discuss above.
Notice that at the left end of the heat anticipator wiper dial at the lowest setting Honeywell has imprinted the word "LONGER". This means the heat will run longer or cut off later and the room will be warmer when the thermostat turns off the heating boiler or furnace.
Details about Heat Anticipator Function and Settings
The heat anticipator, by warming the thermostat bimetallic spring heat sensor, causes the thermostat to stop calling for heat sooner. The result is that the room temperature will be a bit lower when the thermostat turned off the heating system.
Put another way, because the heat anticipator is basically a resistor that itself heats up the wall thermostat in order to cause it to turn off the heat ahead of the room temperature set point on the thermostat, moving the heat anticipator to a higher setting (typically to the right) is moving it to a position of greater resistance in the circuit.
Lower resistance in Ohms, or lower Amps draw, means that less heating of the thermostat bimetallic spring will be caused by the heat anticipator circuit all mean that the heating system will remain on longer before the thermostat reaches the set temperature and turns off the heater.
See HEAT ANTICIPATOR Adjustment for details on how to set the thermostat heat anticipator.
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