Heat anticipator component of a room thermostatThermostat Heat Anticipator Operation
How the heat anticipator works

  • HEAT ANTICIPATOR OPERATION - CONTENTS: explanation of how the thermostat heat anticipator actually works. Difference Between a Heat Anticipator and a Heating Control Device Differential? A heat anticipator will not work accurately unless these conditions are met
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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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What is a Thermostat Heat Anticipator & How Does it Work?

Current path (green) through the heat anticipator of a Honeywell T87 Thermostat (C) DanieL FriedmanWhat's a Heat Anticipator?

A heat anticipator, found in some room thermostats, is a tiny heater that warms the thermostat to cause it to turn the heating system off a bit before the room has actually reached the set-temperature on the wall thermostat.

Heating systems such as hot water heat using cast iron radiators have enough thermal mass that the radiator will continue to emit heat after the boiler has shut down. By turning off the boiler a bit early we prevent the room from overheating.

Because many people find this concept a bit confusing, the rest of this article explains how the heat anticipator works and gives a bit of its history.

For readers who need to know just how to set the heat anticipator on their thermostat, you can skip the details below and go directly to HEAT ANTICIPATOR ADJUSTMENT -.

A heat anticipator is used to avoid room temperature overshoot

Heating technicians call this room over-heating problem room temperature overshoot.

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.

[Click to enlarge any image]

In my photo above the green arrow and dots show the path of electricity through the heat anticipator.

How a Heat Anticipator Produces Its Own Heat

Current flows through a tan wire to the heat anticipator's mounting screw at the left side of the photo, from there through the flat-coiled nichrome wire, up throug the black contact button on the movable adjuster, through the adjuster's copper arm, and back into the thermostat.

The green dots show the electrical current's path through the small left end of the heater that is mostly hidden by copper pointer.

The red arrow, which I regret as it clutters the photos, is a pointer to the wider portion of the body of the nichrome wire heating element. Notice that the little nichrome wire heater is located conveniently just above the coiled bi-metallic spring that turns the thermostat's control wire on and off to turn the heating system on and off.

A Nano-Sized History of the Thermostat Heat Anticipator

The first heat anticipator was produced in 1924 (Walker 2008 cited at REFERENCES) in a design that, like the heat anticipator described in Honeywell's T87, used the heat output of an electric current passing through a tiny heating device (a coil of nichrome wire) to warm the coiled bimetallic spring that in turn was used to turn off the thermostat ahead of the time when the room would reach the set temperature called for on the thermostat control.

This was an improvement to a much-older device, the electric room thermostat patented in 1883 the U.S. by Warren Johnson (Johnson Controls).

Just two years later in 1885 Albert Butz patented a furnace heat output regulator that used a moving flap to adjust air entry into a heating furnace.

Honeywell T832 Day-Night Room Thermostat (C)

Butz's company, The Electric Heat Regulator Company ultimately became the Honeywell Corporation whose 1960 first round wall thermostat is shown above. This classic 1960 Honeywell T8332A round day-night thermostat is illustrated and discussed further at HEAT ANTICIPATOR FAQs.

More details follow below.

A heat anticipator is actually a tiny electric heater inside of the room thermostat

Thermostat heat anticipator explanation (C) Daniel FriedmanThe heat anticipator, not found on all thermostats, is a tiny little electric heater that will, depending on its setting, warm up the inside of the themostat, thus warming the thermostat's room temperature sensor, thus fooling the thermostat into thinking the room is a little warmer than it actually is.

That little heat anticipating joke played on the thermostat's room 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.

The heat anticipator is anticipating the additional heat that is going to arrive and regulating the thermostat accordingly.

Thus we avoid overshooting: we avoid making the room warmer than the thermostat's set temperature.

You won't find a heat anticipator in all thermostats. In fact most newer room thermostats use a thermistor to sense room temperature and they do not usually include a heat anticipator device.

In our photgraph above you can see the critical components of a thermostat heat anticipator such as in the Honeywell T87.

Details of how a heat anticipator works inside of a wall thermostat

The components of the heat anticipator are shown in the photo and are explained in more detail below. They include:

The Nichrome Wire in the Heat Anticipator is a Tiny Electric Heater

The heat anticipator in an electromechanical thermostat includes a tiny heating coil of nichrome wire which gets warm as elecricity (current) flows through the wire.

Moving the pointer along the Amps scale moves the position of the contact that in turn changes the length of nichrome wire that will be used, making the in-use wire shorter or longer.

Meaning of the Numbers on the Heat Anticipator Scale

The heat anticipator numbers along its scale are measurements of current or Amps that will flow through the heat anticipator's little heater at different settings.

At ELECTRICAL RESISTANCE vs HEAT GENERATED we explain that when the electrical resistance of a circuit is higher less current flows and less heat is generated.

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 current numbers on the left) or decreasing (to the lower current numbers on the right) the amps or current flow through this tiny heating 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.

Honeywell T87 Thermostat set at a higher Amps number (C) Daniel Friedman

Above: the heat anticipator pointer is set to about 0.45 Amps.

Longest heat anticipator nichrome wire length = most resistance (Ohms) = least current flow ( 0.1 Amps) = least heat from the heat anticipator = heating system stays on longer

If we move the arm and contact fully to the left to the 1.2A position that's using the maximum length of the nichrome wire or heater length - that's the circuit with the most resistance, the least current flow, so the least in-thermostat-heating, so we turn the actual heating system off later.

Shortest heat anticipator nichrome wire length = least resistance (Ohms) = most current flow ( 1.2 Amps) = most heat from the heat anticipator = heating turns off sooner

If we move the arm and contact fully to the right to the 0.1A position, that's using the shortest length of the nichrome wire heater length - that's the circuit with the least resistance, and thus the highest current flow through the wire, so the wire gets hotter, so it produces the most in-thermostat-heating, so the thermostat inside warms up faster than the room air, so the thermostat will turn off the actual heating system off sooner.

How Adjusting the Heat Anticipator Changes Its Heat Output - Variable Resistor

Thermistor resitance curve 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 thermostat, fooling it into thinking the room is just a little warmer than it is.

The heating thermostat manufacturer's instructions & heat anticipator operation explanation can be a bit confusing, but significantly, as we detail at HEAT ANTICIPATOR ADJUSTMENT

The shortest burner-on time will be when the heat anticipator puts out the most heat. This warms up the thermostat's room temperature sensor and therefore tells the thermostat the room is up to set temperature earliest.

The longest burner-on time will be when the heat anticipator puts out the least heat, thus does not turn off heat early, thus lets the burner keep running longer.

The graph at above left, adapted from one provided by Carrier, illustrates how a different device, 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. That's how newer thermostats work.

Changing the Heat Anticipator's Pointer Changes the Wire Length, Current Amps, Heat Output

Heat anticipator adjustment scale details (C) InspectAPedia HoneywellMore heat output from our teensy electrical resistance heater inside the wall thermostat means more room heat anticipation (more pre-heating of the room thermostat's sensor) and thus a shorter heat-on cycle.

Here we explain heat anticipators a second time to offer another way to understand what's happening in a thermostat that uses a heat anticipator circuit.

Moving the heat anticipator pointer (the open triangle at the top of the copper arm) changes the effective wire length and thus the electrical resistance and thus the heat output of the heat anticipator inside the thermostat.

When we move the adjuster we are essentially using more (longer) or less (shorter) lengths of wire, thus causing more or less resistance through the device.

Key Concepts: Relationship of Wire Length, Resistance (Ohms), Current flow (Amps), Heat Generated, Heat Anticipator Warming, Heating System On-Cycle

If we assume, as is the case for a typical heating system thermostat, that the thermostat voltage level remains fixed (typically 24VAC) then moving the heat anticipator position changes the length of the nichrome wire in the active circuit.

Longer Wire = More electrical resistance = less current flow = less heat generated by the heat anticipator = less pre-warming of the thermostat = longer heat on cycle.

Shorter Wire = Less electrical resistance = more current flow = more heat generated by the heat anticipator = more pre-warming of the thermostat's sensor = shorter burner on time

Below: the heat anticipator pointer has been moved down towards the right end of the scale, and is set to about 0.15 Amps.

Honeywell heat anticipator scale (C) Daniel Friedman

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.

Many of us have found the heat anticipator in thermostats confusing because we think intuitively but incorrectly that more resistance means more heat is generated. No.

That's wrong.

More resistance (higher Amps numbers) = less current flow = less heat is generated = the heat anticipator heats up less = the thermostat turns off the heat later (or it runs "longer").

Why Lower Electrical Resistance Means More Heat is Generated and Why Higher Electrical Resistance Means Less Heat is Generated in a Heat Anticipator Circuit

Details about the relationship between electrical resistance and heat are at ELECTRICAL RESISTANCE vs HEAT GENERATED.

What is the Difference Between the Heat Anticipator and an Aquastat Heating Control Differential?

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.

A heat anticipator will not work accurately unless the following conditions are met:

Useful References for Thermostat Heat Anticipators


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