Thermostat Heat Anticipator FAQs-2

Q&A on how a heat anticipator works

Electrical Engineer's Explanation of the Heat Anticipator Scale

Clarifying How a Heat Anticipator Works: a PhD EE speaks up. Says lower numbers need more resistance to generate needed heat load.

The numbers on the anticipator do NOT adjust the amount of current delivered across the anticipator resistive heating element.

In contrast, they are intended to be set to match the current that is drawn by the system actuator (relay, gas solenoid, etc.).

The current flow is almost unaffected by this setting because the resistances of the heating coil are so small.

The setting is instead selecting the appropriate resistance to match the current flow of the system, so larger numbers (i.e., larger current flow drawn by system) means a smaller resistance is needed to generate an appropriate anticipator heat through I^2R.

Lower system currents (corresponding to the lower numbers on the dial) nead a larger resistance to generate the appropriate heat load.

The larger numbers lead to LONGER cycle times, not shorter.

This is consistent with (1) the arrow on the dial, indicating that longer cycle times are achieved by going to larger numbers, and (2) a Honeywell manual that will explain this to anyone who is interested.

- On 2018-11-12 by Tom

Reply by (mod) -

Thank you for your interesting and helpful comments Tom, I guess the other electrical engineers (Aronstein, EE PhD & others) who reviewed this for me were mistaken - or I misunderstood them.

But I agree that if we move the heat anticipator pointer UP to a HIGHER NUMBER on the Amps scale the heat will stay on LONGER.

What has confused countless technicians as well as our readers is that while the arrow on the heat exchanger scale points to the left towards higher numbers, the location of the word "LONGER" appeared (where there was room for it) at the right end of the scale.

The heat anticipator is simply tiny heater next to the bimetallic spring in the thermostat, adding heat in order to warm the spring before the building air alone would warm the spring enough to turn the call for heat to the off position.

That's why it's called a heat anticipator.

In any case we have been excerpting Honeywell's T87 installation literature, drawings, and we're using photos of an actual T87 - actually several of them of different ages.

I don't claim that the heat generated is higher at higher numbers on the current (amps) scale.

I claim that

1. When less heat is generated inside the thermostat by the heat anticipator, the thermostat is warmed up less by the anticipator so the heat will run "longer"

2. When more heat is generated by the heat anticipator the thermostat is warmed up more and so turns the heat off a bit "early" or the heat runs "shorter"

3. More resistance = less current flow = less heat generated by the heat anticipator

4. The anticipator heater operates with a constant voltage applied to it.

When you decrease its resistance, at constant applied voltage, the current increases (E=IR).

The heat generated is proportional to the current squared times the resistance (heat = I-squared x R). - Aronstein to Friedman cited in the original article at HEAT ANTICIPATOR ADJUSTMENT .

by Tom

I haven't missed anything & I do understand how the anticipator works.

This is the image -- completely backwards from reality [NOTE: the image below has been edited and corrected - Ed. 2026/01/20]

by (mod) - These Images here are from Honeywell's heat anticipator documentation

Thanks Tom. Notice that I'm using an image provided by the manufacturer of the thermostat - it's from Honeywell's T87 instruction sheet.

LONGER appears at the low end of the scale but its arrow points to the high end of the scale.

I do not think the image is mistaken, though as I said before, I have always been troubled by this sketch as I think their choice of label location and arrow was confusing: the word "LONGER" appears at the lowest resistance numbers on the scale, yet the arrow under the word LONGER points to the left.

Honeywell's documentation confirms that the numbers on the scale measure Amps.

The arrow points towards higher current settings that should be matched to the amps of the primary control for the heater.

At HEAT ANTICIPATOR ADJUSTMENT we quote Honeywell's instructions:

If the furnace stays on beyond the thermostat set temperature, move the anticipator down by .1 ampere.

If the furnace shuts off before the set temperature is reached, move the anticipator up by .1 ampere.

Notice this table of recommended heat anticipator settings from Honeywell's documentation for heat anticipator adjustment:

So we see that for heating systems that tend to overshoot the thermostat temperature settings Honeywell says set the heat anticipator to a LOWER number by 0.1A. That should be turning the system off slightly "early" (system is on for a "shorter" time) to avoid making the room hotter than desired.

And conversely the company says to move the heat anticipator to a HIGHER number by 0.1A if the heater shuts off before the room is warm enough - that is, to get more heat we set the number up.

It's too bad that there's that confusing arrow under the word "LONGER" on the device.

I've commented on that point in several articles. Clarification is always welcome and helpful.

I think that while it's not a topic on which the company published anything I can find, on later versions of the same traditional T87 thermostat that labeling changed, so perhaps someone else at the company agreed that it was confusing.

In my lab as a TBD project I've got one of these old T87s for which I intended to measure the actual resistance at different positions to compare with the scale.

We used to use a special heat anticipator ohmmeter that was specifically calibrated for use by heating technicians to measure the actual resistance of the thermostat circuit for a particular installation in order to match the setting properly.

COURTEOUS discourse, without insulting language is AWAYS welcome, and invited at InspectApedia.com. We are completely dedicated to the most-accurate, unbiased information we can provide. Working together makes us smarter.

by Tom: Longest heat on cycle using the Heat Anticipator

Longest cycle is at the left end of the scale with the largest numbers (1.2), while the shortest cycle is at the right end of the scale with the lowest numbers (.10).

The current flow is governed by the resistance of the actuator in the furnace; it is that resistance that dominates and controls the current flow.

As a result, the numbers on the anticipator do not set current flow, rather they select the appropriate small resistance to result in an appropriate amount of resistive heat for the current flow of your system given by the reading on that dial.

The appropriate amount of heat is indeed to raise the temp of the bimetallic coil and trip the mercury switch early so residual system heat does not overshoot the thermostat setting. On 2018-11-13 by Tom

by (mod) - years of confusion about which way the heat anticipator slides to give longer or shorter heat-on cycles

Tom:

You're right that I might ADD red or yellow or other colored text to point out a feature in a photo or drawing.

The word "LONGER" and the arrow pointing as we indicate are original, appear in the original product literature and appear stamped into the steel of the T87 itself.

I think it was the combination of Honeywell's stamping "LONGER" at the low end of the scale along with the arrow under the word "longer" pointing towards the high end of the scale that has generated so much discussion and confusion.

The photographs are of actual Honeywell T87 thermostats of several generations in which we observed that the confusing stamping and location of the word "LONGER" with and without an arrow and ultimately removed entirely, waxed and waned in the T87's over decades.

This has been a very popular topic as you'll read across years of comments, FAQs, and quotes from the company's own literature as well from electrical experts whom I consulted to be sure we had this exactly right.

Thanks for the discussion.

Heat Anticipator Setting vs Short Cycling of the Heater FAQ

Could the heat anticipator be making my heater short cycle?

My unit in heat mode seems to be short cycling, on 5 min off 5 min could the anticipatory be causing this - On 2017-01-16 by Steve_Schmidt

Reply by (mod) -

Possilby, Steve.

Calling for a longer heat-on cycle can reduce short cycling, though there are other causes of that problem too.

At HEAT ANTICIPATOR ADJUSTMENT we quote these Honeywell's instructions:

If the furnace stays on beyond the thermostat set temperature, move the anticipator down by .1 ampere.

If the furnace shuts off before the set temperature is reached, move the anticipator up by .1 ampere.

You can interpret that to also mean that if you move the anticipator up by .1 Amps you may cause a slightly longer heater ON cycle.

Other causes of boiler short cycling - if your heat is by hot water - can be poor aquastat temperature sensing, as we explain

at AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND 

Heat Anticipator setting for a Majestic gas fireplace to avoid short cycling

Where should the heat anticipator be set for a Majestic gas fireplace so it doesn't short cycle. On 2018-02-22 20:20:51.929298 by Ivan >

Reply by (mod) -

Ivan,

if your gas fireplace is a model that permits operation with a conventional low voltage wall thermostat then I would set the heat anticipator at
0.4A,
the same as for a conventional forced warm air furnace heating system. My reasoning is that there's no temperature overshoot in a room where the heating system in use does not have a high Thermal Mass radiating device such as a cast iron radiator.