Thermostat heat anticipator function & adjustment FAQs:

Questions & answers about how a room thermostat heat anticipator works.

This article series explains how adjusting the heat anticipator pointer changes the heat output of the anticipator that in turn changes the behaviour 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 heat anticipator assembly.

Thermostat Heat Anticipator Questions & Answers

These room thermostat heat anticipator questions and answers were posted originally

at HEAT ANTICIPATOR OPERATION - be sure to review that article's explanation of how a heat anticipator works and why it's used in some thermostats.

[Click to enlarge any photo]

On 2018-11-13 by (mod) - years of confusion about which way the heat anticipator slides to give longer or shorter heat-on cycles

Tom:

We agree that the circuit as well as the specific resistance device in the heat anticipator set the current flow and thus the heat generated inside the thermostat - warming or not warming it up and thus and thus shortening or not-shortening the length of the boiler or furnace "heat-on" cycle.

However in asserting that we made up the illustrations in this article series, not so. You are simply mistaken.

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-resistance 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.

"LONGER" is taken to mean the heat will be on longer - the anticipator will be doing less pre-heating of the thermostat.

Pushing the pointer in the direction of higher resistance decreases the current flow. Since it's the current that's squared in (heat = I² x R) that should tell us something, right?

We took both the sketches from Honeywell's literature (installation instructions packaged with the T87) - which itself changed from time to time as more people than you might guess were confused by both the original drawings and the actual stampings in the steel of the heat anticipator plate.

Click to enlarge the photo of the T87 heat anticipator scale shown here and you'll see Honeywell's own arrow stamped into the steel just below the 0.12A - near the low-current end of the scale is pointing away from the low-numbers and towards the high numbers.

Look closely if you can and you'll also see the arrow stamped into the steel and pointing in the same direction - to the left.

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.

On 2018-11-13 by Tom

The error in the picture here is in the red color labeling of "shorter" and "longer", and (1) these do not appear to be part of the Honeywell graphic, and (2) they are also opposed to what the T87 manual says. 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). While some of your statements below are correct, you have again repeated a mistake in assuming that it is the (very small) resistance of the anticipator resistor that is governing current flow. It is not. 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 (mod) - 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.

I do not think the image is mistaken, though 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, towards higher resistance.

I've commented on that point in several articles. Clarification is always welcome and helpful. I think I've got this right that at lower resistance more current flows and more heat is produced in the wire.

More heat in the wire means the bimetallic spring is heated sooner and the heat shuts off sooner in the heating cycle.

Therefore pushing the indicator in the direction of the "longer" ARROW, that is, towards higher numbers (more resistance).

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 = I² x R). - Aronstein to Friedman cited in the original article at HEAT ANTICIPATOR ADJUSTMENT .


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.

On 2018-11-13 04:55:07.706434 by Tom

This is the offending image -- completely backwards from reality

On 2018-11-13 04:48:01.142398 by Tom

yes, speaking as a PhD electrical engineer myself, you DO need to look a little deeper. I haven't missed anything & I do understand how the anticipator works. I'm honestly not trying to be rude, but please read my response carefully enough to understand it, and maybe you'll stop providing misguided information to the public.

On 2018-11-12 19:21:09.576553 by (mod) -

thank you for your comments Tom, I guess the other electrical engineers who reviewed this for me were mistaken. I'll take another look at the text.

I think you may have missed how heat anticipator is actually functioning - It's a 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're generally 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 .

On 2018-11-12 16:24:52.419009 by Tom

Your article about anticipator settings has seriously incorrect information. 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.).

You have incorrectly deduced that the I^2R heat load delivered will be higher at the larger numbers because you think that the larger numbers will somehow control the thermostat to allow a larger current flow. 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 result is that your picture above is seriously misleading, as 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. It's rather alarming that a site trying to be authoritative delivers erroneous logic and erroneous information.

On 2018-04-09 23:43:59.241157 by (mod) -

Thomas,

Thank you for posting your question and opinion.

My 0.3A quote is excerpted from Honeywell's "general" guidance for the heat anticipator setting.

But I agree that it makes perfect sense to follow the instructions on the specific heating equipment you've installed.

Another approach that's beyond the scope of most service techs is to actually *measure* the current draw on the heating thermostat control circuit. That requires using a special low-range ammeter or a digital VOM/DMM with a suitable range sensitivity. Most techs wing-it.

The apparent contradictions in heat anticipator setting advice aren't as ugly as they may seem.

In the article above on this page you'll find a summary table titled

Table of Heat Anticipator Setting Recommendations for the Honeywell CT87A,B,J Round® Thermostat

there you'll see a range of recommended heat anticipator settings that depend on the type of heat being controlled. Those numbers range from 0.3A (electric heat) to 1.2A (steam heat). And yeah, you'll see Honeywell's recommendation for forced hot water heat as 0.8A at the heat anticipator.

Keep in mind that Honeywell recommended these settings as a general number - the company (and I) can not possibly know the specific performance or behaviour of every installed heating system and I think Honeywell's engineers would agree that further adjustment could be needed.

Certainly you can try lower settings down to 0.1 if your particular heating system is continuing to overheat or over-shoot.

Compare the actual room temperature - measured independently but AT THE THERMOSTAT with the thermostat's set-temperature.

For example if the thermostat set temp is 68F and you see that after your boiler turns OFF your heat continues to rise to say 70F or more, then you might try setting the heat anticipator to a lower number.

Finally, don't forget to look for other reasons for heat overshoot, such as a zone valve that's not closing or a check valve at the boiler that's stuck open when it should be closing.

On 2018-04-09 21:34:07.279120 by Thomas Spero

I am reading your section on " How &why to adjust the heat anticipator on a room thermostat" and my oil burner service man installed a new control on my baseboard oil/hot water burner lately. It has a L7224/L7248 Electronic Aquastat next to it. When I take the cover off ther is a piece of cardboard that tells you to set the Thermostat anticipator to 0.1 amps.

The oil burner serviceman had it set to 0.2 amps and the burner seemed to be overshooting the thermostat reading. I just turned it down to 0.1 amps. I question your article that says you should never turn down below 0.3 amps. Can you help me there? I have copper tubing/finned baseboard heat in a small apartment so NO mass (cast iron radiators).

Your article also says the recommended setting to be 0.8? Can you help me out with these contradictions? Thank you, Thomas Spero

On 2018-02-23 02:13:43.108631 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.

On 2018-02-22 20:20:51.929298 by Ivan

Where should the heat anticipator be set for a Majestic gas fireplace so it doesn't short cycle.

On 2018-01-03 23:24:19.464411 by Anonymous

Corinne,

Can you use the picture icon next to the Comment button to attach sharp photos of yoyr thermostat and the control in question?

On 2018-01-03 22:50:13.245860 by Corinne

Thermostat for old oil Furnance has 3 numbers on bottom, 10, 5 ,0. Assuming that's the heat adjuster what should it be on. Furnance doesn't kick on unless I move it

On 2017-07-09 01:42:28.510409 by (mod) -

Jorge, you can consult the setting tables on the article above, or if we know nothing else,

Adjust anticipator to match current rating of primary control. Rating is usually stamped on the control name- plate.

On 2017-07-08 20:20:54.700031 by Jorge Acosta

Where should i set the heat anticipater on a white rodger thermostat?

On 2017-03-11 16:38:38.960824 by (mod) -

Bob

Thanks for the interesting and important comment about heat anticipators on thermostats controlling steam heating systems. The total amount of anticipation time effected by those thermostats that sport a heat anticipator is not that great that in my opinion there is any need to try to remove the feature. One could simply set it to its minimum or maximum.

Remember that the anticipator is functioning by basically turning the call for heat off slightly early - that's all.

On 2017-03-11 14:02:27.040127 by bob

Good article - but in steam systems, you should tell people to cut out the anticipator entirely. Because the steam systems take so many minutes between the time the thermostat detects a need for heat and the time the boiler boils water, delivers it to the radiators, then heat up and finally warm the room, the anticipator function isn't really needed. I didn't see any instructions for how to cut it out entirely, using the other lug or a drop of solder on the coil. - Bob

On 2017-01-17 00:08:37.813283 by (mod) -

Seems very unlikely to me Steve.

On 2017-01-16 21:27:01.625603 by Steve_Schmidt

My unit in heat mode seems to be short cycling, on 5 min off 5 min could the anticipatory be causing this

On 2016-12-15 04:31:36.430537 by greg riutzel

An alternative to using a Mini-Ammeter is using a loop of wire with 10 wraps of wire; such as 10 wraps of 16 or 18 gauge wire around 4 fingers. Hook this up in series with the thermostat just as you would the mini-ammeter.

Put the jaw clamps of any journeyman's type ammeter through the loop and divide the reading by 10 for the systems' current draw. This way you won't need an extra, sensitive low reading ammeter. You can also use any number of wraps for sensitivity or ease of division.

Most good DMM's also now have a current scale for measurements up to 10 amps. The point is, as you make it, is to MATCH the thermostat to the system. Then, any fine tweaking is easier from there. Thanks for a great article. Greg

On 2016-02-12 02:41:04.013897 by (mod) -

Sorry you found the article confusing, Joe. Since not all heat anticipators work the same, and since many thermostats don't have one, your best bet is to take a quick look at the start of this article where we have a section titled

Which Way Do I Move the Heat Anticipator Adjustment?

On 2016-02-12 00:04:37.779891 by joe

All this technical mumbo jumbo...Bottom line. WHICH way to I set the heat anticpator to get MORE longer heat?

On 2015-04-13 16:59:26.098040 by (mod) -

Thanks QP

What I mean is that as shipped from the factory the typical wall thermostat - among those that had a heat anticipator adjustment such as the famous Honeywell T87, will work well with most heating systems. Unless the installer reads the data tag giving different heat anticipator adjustment recommendations (we show this tag in the article) and matches it to the heater then she or he should leave it alone.

Your point about matching the gas valve is helpful and certainly applies to gas fired heating equipment. Keep in mind that this thermostat is very widely used on furnaces, but also on hot water and steam heating systems and with both oil and gas fired equipment. So there is not a single "right" number.

On 2015-04-13 15:25:54.022040 by Qprew7U

Thank you for incorporating my comments.

There is one mistake: "From the factory the anticipator resistor is adjusted".

That's not what I am saying.

The anticipator needs to be adjusted at installation to match the furnace gas valve. Doing that will make the thermostat behave the same way for any furnace.

For example, my furnace gas valve is rated at 0.6 amperes. So the anticipator should be set at 0.6.

Another furnace gas valve might be rated at 1.0. So the anticipator should be set at 1.0.

The 0.6 setting for my gas valve will give the same thermostat behavior as the 1.0 setting for the other furnace.

On 2015-04-13 02:41:28.039930 by (mod) -

Qprew7U & other readers please Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies.

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Recommended Articles

• HEAT ANTICIPATOR ADJUSTMENT
• HEAT ANTICIPATOR ADJUSTMENT - T87
• HEAT ANTICIPATOR EFFECTS on ROOM TEMPERATURE
• HEAT ANTICIPATOR SET & TEST by AMMETER
• HEAT ANTICIPATOR THERMOSTAT SOURCES
• HEAT ANTICIPATOR OPERATION

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