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Heat anticipator component of a room thermostatHow & Why to Adjust the Heat Anticipator on a Room Thermostat

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Thermostat heat anticipator function & adjustment guide: this article explains what a heat anticipator is on a wall thermostat, where to find it, what it looks like, why we adjust the heat anticipator, how we do so.

We answer: How Does the Thermostat Heat Anticipator Actually Work. What Should Be the Heat Anticipator Settings on a Room Thermostat? Do I need a thermostat with a heat anticipator? Where to Buy a Room Thermostat with a Heat Anticipator.

We list the recommended heat anticipator settings. We explain how to test the heat anticipator on a thermostat, and we describe the conditions necessary for a heat anticipator on a room thermostat to work properly in the first place. We also explain the difference between the job of the thermostat heat anticipator and the differential settings on a heating system aquastat or similar control.

We also discuss when you do or do not need a thermostat that includes a heat anticipator device, modern alternative thermostat controls, and where to buy a thermostat with a heat anticipator feature. 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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How & Why to Adjust the Heat Anticipator on a Room Thermostat

Heat anticipator component of a room thermostatThermostats may also have a "heat anticipator" adjustment which is tuned to the particular heating system to avoid heater on-off oscillating too close to the thermostat's set point.

What is a thermostat heat anticipator? The purpose of a thermostat heat anticipator is to "de-sensitize" the thermostat so that when actual room temperature is hovering close to the set temperature on the thermostat, the thermostat switch won't keep switching the air conditioner or heating system on and off too often - which can damage the equipment.

In our photo you can see our pointer hovering over fine wires wound around a triangular piece of plastic (forming a variable resistor) and you can see at center of the photo a flat copper arm which can be moved to slide a contact to different positions along the wound variable resistor.

The triangular pointer at the top of the copper arm has an opening which helps read the exact position to which the heat anticipator has been set. Behind the pointer you can see a silver scale with different amp readings which are detailed in the table below. On this heat anticipator setting scale settings range from 0.10 to about 2 Amps.

The heat anticipator scale: If you click to enlarge this photo you can see the amperage level to which this heat anticipator had been set when we took this picture. It is below the lowest setting recommended by Honeywell.

Which Way Do I Move the Heat Anticipator Adjustment?

Heat anticipator adjustment scale details (C) InspectAPedia HoneywellTake another look at the finely-wound and flattened wire coil underneath the movable copper arm in our photo just above and at our adapted Honeywell T87 heat anticipator sketch at left.

[Click to enlarge any image]

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.

Since at higher current more heat-anticipator-heat will be produced, the shortest heating cycle or heat-on cyle will be at the higher current (Amps) end of the scale, closer to 1.2A.

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 longest heating cycle or heat-on will at the lower current (Amps) end of the scale,closer to 0.1A since at lower current less heat-anticipator-heat will be produced. The thermostat's room temperature sensor won't be getting much extra warmth from the heat anticipator.

Watch out: be sure to look at both the direction of the arrow and at the amperage numbers on the heat anticipator scale. In our sketch (above) the higher Amps numbers and thus settings for shorter heat-on cycles are at the left end of the scale, but don't trust "left or right" on your thermostat - read the scale.

Depending on your model thermostat and where the heat anticipator is found, left and right or up and down might be reversed from the images here, and some of the thermostat heat anticipators we examined have only a numeric scale, no arrows, so it's important to follow the manufacturer's instructions.

Still it is useful and interesting to understand the principles involved. Below we compare instructions for the Honeywell T87 and the Honeywell CT87 room thermostats, each of which gives similar instructions for which way to move the heat anticipator indicator - to a higher number for a longer burner on-cycle.

Below we provide specific heat anticipator setting recommendations as well as a description of which way to move the heat anticipator lever to change the thermostat's behavior to address room temperature overshoot. You will see that Honeywell's own instructions contradict one another depending on the thermostat model.

What Should Be the Heat Anticipator Settings on a Room Thermostat?

Flair APVOV2 thermostat with heat anticipator (C) Daniel FriedmanIf your heating system seems to be working OK, without excessive burner on-off cycling and without troubling room temperature overshoot, leave the heat anticipator alone.

If however you are an installer and want to precisely set up the heat anticipator on a thermostat (if your thermostat has this feature), there are two approaches:

Our sketch, above left, adapted from a Flair APVO2 two-wire thermostat whose the heat anticipator level and scale (red arrow) on the left side of the device. Interestingly the company referred to this as a "fixed anticipation" thermostat, but its installation instructions indicated a screw-secured heat anticipator pointer and an amperage scale on the device.

This thermostat was designed for use with zone valves or damper actuators.

Table of General Heat Anticipator Settings

Heating System Type Heat Anticipator Setting in Amps
Electric heat 0.3 Amps
Hot Water Heat - radiators, baseboards, hydronic
0.8 Amps
Steam heat
1.2 Amps
Warm air, Standard 0.4 Amps
Warm Air, High Efficiency .8 Amps

Notes:

1. Heat on too long: If the furnace stays on beyond the set temperature on the thermostat, move the heat anticipator DOWN by 0.1 A

2. Heat cycle too short: if the furnace shuts off before teh set temperature on the thermostat has been reached, move the heat anticipator pointer UP by 0.1A

3. Additional heat anticipator settings and details are usually provided on the thermostat installation pamphlet provided by the manufacturer.

Watch out: Honeywell, in instructions for its CT87A,B,J Round Thermostat warns: Never adjust the heat anticipator below 0.3A

...

Table of Thermostat Heat Anticipator Settings for Honeywell Primary Controls - (60 cycle current)

Honeywell Control Part Number Heat Anticipator Setting in Amps
Honeywell L8124,
Honeywell R8182-D-F,H,J
Honeywell R8184G,K,M
Honeywell R8185E

Honeywell V800A-C
Honeywell V801A-C
Honeywell V810A-C
.2 Amps
Honeywell RA116, RA117
Honeywell RA816
Honeywell RA817A
Honeywell R8184L

Honeywell V8254
Honeywell V8280
Honeywell V8292
.4 Amps
RA817C .55 Amps

Notes:

Additional heat anticipator settings and details are usually provided on the thermostat installation pamphlet provided by the manufacturer.

The settings shown here are not likely to be correct for other brands or models of thermostats.

Watch out: as Honeywell warns thermostat installers:

Honeywell heaet anticipator thermostat setting warningThis thermostat has an adjustable heat anticipator and will operate properly only if this [heat anticipator resistor] heater is adjusted to match the current of the valve or relay.

See HEAT ANTICIPATOR MINI AMMETER TEST for details.

Set the heat anticipator to the heating boiler or furnace manufacturer's recommendation, or if you don't know that number, you can either leave the heat anticipator at its factory setting (recommended) or you can use a mini-ammeter to actually measure the heating thermostat circuit and to fine tune the heat anticipator setting, as we discuss further at HEAT ANTICIPATOR MINI AMMETER TEST

For the Honeywell T87F the manufacturer's heat anticipator setting instructions are as follows:

If the T87F is used for 3-wire, SPDT, heating only (Series 20 control), set the heat anticipator to 1.2 (far left end of the scale).

A 25V open circuit shunt type heat anticipator is provided in the 137421A Wallplate for this application.

For other control applications proceed as follows:

Adjust the anticipator to match the current rating of [the] primary control.

Rating is usually stamped on the control plate. Move the indicator to the marking that matches this rating.

Indicator may be moved with fingers or pencil point, pin, or wire through hole shown [in our photos this is the triangular opening in the pointer -Ed.].

If the current rating is not given, proceed as follows before mounting the thermostat:

A slightly higher setting to obtain longer burner-on times (fewer cycles per hour) may be desirable for some systems.

Heat anticipator scale (C) InspectAPedia

  1. Connect an AC ammeter of appropriate range, sensitivity, and accuracy (0 to 2 A VAC between the R and W (standard terminal descriptions) terminals on the wallplate or subbase [NOT in the thermostat as some how to or just-ask fellows recommend elsewhere - Ed.]

    Watch out: a suitable ammeter may not be your typical electrician's clamp on ammeter as some fellows recommend - you need a meter that is capable of precision in the 0 - 1.2A VAC range.

    See HEAT ANTICIPATOR MINI AMMETER TEST for a detailed AMPS measurement example using the proper type of ammeter.
  2. Let the heating system operate for at least 1 minute before reading the ammeter, 5 minutes is better to assure that the system is at normal operating temperatures and consistent behavior.
  3. Move the heat anticipator indicator pointer to match the ammeter reading. For example in our yellow sketch above, our ammeter measured 0.7A so we moved the pointer (shown as a white arrow) to .7 on the heat anticipator scale.

Honeywell thermostat heat anticipator detail (C) Daniel Friedman

Should you vary the heat anticipator above or below the measured thermostat circuit amps?

Possibly yes. Depending on how your heating system is performing (burner cycle time too short or room temperature has too much overshoot) you might want to adjust the heat anticipator a bit further.

Watch out: except where specific primary controls tell you othewise, the heat anticipator amps should not be set below 0.3A.

Here is an EXAMPLE of heat anticipator setting advice from instructions packed with an old Honeywell thermostat:

A slightly higher setting to obtain longer burner-on times (fewer cycles per hour) may be desirable on some systems.

EXAMPLE: if burner-on time is too short with a heater setting of 0.4 adjust to 0.45 setting and check system operation; adjust until the desired burner on time is obtained.

On the original T87 thermostat metal scale (my photo at left) you'll see the word "Longer" stamped into the 0.10 end of the scale. [Click to enlarge any image for more detail]

For a detailed explanation of how the thermostat heat anticipator actually works see HEAT ANTICIPATOR Operation.

Heat Anticipator Adjustment 101: Simplified anticipator operation, setting, and tweaking

Before getting into more complete details below where we describe Honeywell's instructions for adjusting a heat anticipator, this nice summary was provided by reader in a comment originally posted as a comment to this article.

12 April 2015 qprew7u said:

A furnace has two principal heat controls:

1. The thermostat controls the gas valve and burner.

2. The blower temperature switch controls the blower.

Similarly, the oil or gas burner on a hot water heating boiler is turned on or off by the temperature of the boiler water (via the boiler's Aquastast control) while in U.S. installations the thermostat is turning the circulator on or off to send hot water to the spaces to be heated. In Canada typically the circulator runs continuously and the thermostat is turning the boiler on and off directly. - Ed.

The furnace blower continues to run after the thermostat opens, for efficient heat exchange

On a furnace the blower is scavenging the remaining heat in the heat exchanger and it's avoiding heat exchanger damage that could occur if that component overheats and cracks. QP's remarks describe forced warm air furnace heat but keep in mind that heat anticipators are used on hot water heating systems (baseboards and radiators) as well.

Any heating system: forced warm air, gravity warm air, hot water heat through radiators or baseboards, or steam heat, can "over-shoot" and continue to deliver more heat to the occupied space after the thermostat is satisfied. Only if the amount and duration that over-shoot is really a problem should you think about adjusting the heat anticipator. - Ed.

The heat anticipator “anticipates” that extra heat delivery by warming the thermostat to shut it off early.

The anticipator also creates hysteresis, so the furnace does not cycle too often.

QP means by hysterisis that we introduce a lag or delay between the change in room temperature and the response of the thermostat. However the heat anticipator is only operating to turn the thermostat "off" a little "early". It does not introduce a delay in turning the thermostat back on. - Ed.

Setting the Heat Anticipator:

The main reason for the anticipator adjustment is to match the thermostat with different furnaces.

From the factory the anticipator resistor is adjusted so that its warming effect (power dissipation) is nearly the same for different furnaces. That way the thermostat will behave the same way, no matter what furnace it is used with. But sometimes fine-tuning is needed as we explained above.

The heat anticipator is connected in series with the coil that energizes the gas valve [or turns on the hot water circulator if heat is by hot water]. The anticipator resistance is much lower than the resistance of the gas valve coil [or the circulator relay coil]. That means that no matter where the anticipator is adjusted, it does not appreciably affect the current in the circuit.

In other words, the current through the anticipator is constant for your furnace [or hot water boiler or steam boiler], no matter where the anticipator is adjusted.

The warming effect of the heat anticipator is

P = R x I**2

where

“P” is the power dissipated by the anticipator

“I” is the gas valve current rating in amperes, and

“R” is the anticipator resistor.

“I” is relatively constant for your furnace. “R” is adjusted to so that “P” is the nearly the same for different furnaces.

That is why the anticipator is calibrated in amperes, for easy matching to your furnace. It is also why some manufacturers refer to the adjustment as “fixed”.

Tweaking:

OPINION of QP: You can tweak the anticipator if you want to, but that is not the main reason for the adjustment.

Note: as we discuss at HEAT ANTICIPATOR OPERATION there is some confustion between power and heat and current and watts. That article gives a clear explanation of the relationship between electrical resistance and heat - the theory that explains why and how a thermostat heat anticipator works.

Details are at ELECTRICAL RESISTANCE vs HEAT GENERATED.

Honeywell's Instructions for Setting the Heat Anticipator

Heat Anticipator Setting Direction for the Honeywell Traditional T87 Round® Thermostat

Using the Honeywell T87 Thermostat instructions (see [3a] in REFERENCES] as an example, and quoting:

If the T87F is used for 3-wire, spdt, heating-only (Series 20) control (Fig. 6), set the heat anticipator for 1.2 (far left end of scale). A fixed resistor-type heater is provided in the 137421A or 198170A Wallplate for this application.

For other control applications, proceed as follows.

Adjust anticipator to match current rating of primary control. Rating is usually stamped on the control name- plate. Move the indicator to the marking that matches this rating. Indicator may be moved with fingers or pencil point through the hole shown in Fig. 8.

If the current rating is not given, proceed as follows before mounting the thermostat:

1. Connect an ac ammeter of appropriate range (0 to 2.0A, for example) between the R and W terminals on the wallplate or subbase

2. Let the system operate for one minute before read ing the ammeter.

3. Move the anticipator indicator to match the ammeter reading.

A slightly higher setting to obtain longer burner-on times (fewer cycles per hour) may be desirable for some systems. [3a]

According to Honeywell, set the heat anticipator to a lower number to let the heat run shorter or make the room a bit cooler in relation to the set point on the thermostat by avoiding room temperature "overshoot" as additional heat arrives in the room by radiation from heating baseboards or furnace run-on time after the burner on the boiler or furnace has been stopped by the thermostat.

According to Honeywell, set the heat anticipator to a higher number to cause the burner to run longer - with a longer on-cycle.

When we adjust the heat anticipator on a thermostat that has one, we are fine-tuning the amount of heat anticipation that the device is allowing.

Watch out: Some modern DMMs that include a clamp-on ammeter might measure accurately down into the very few milliamp range that is needed, but most do not. In its instructions for using a mini milli-amp meter (described in detail
at HEAT ANTICIPATOR MINI AMMETER TEST) the company makes clear that a precise adjustment of a heat anticipator to a specific individual heating system installation needs to make a precise measurement of the current in the entire thermostat circuit.

Heat Anticipator Setting Direction for the Honeywell CT87A,B,J Round® Thermostat

Honeywell CT87A,B,J Round(R) Thermostat Heat Anticipator SketchUsing the Honeywell CT87A,B,J Round® Thermostat instructions (see [3b] in REFERENCES] as an example, and quoting:

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

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

Never adjust the anticipator below .3 ampere. [3b]

You'll notice that the word "Longest" and the arrow shown in the earlier T87 Thermostat heat anticipator sketch are not shown in this Honeywell illustration, but the explanation of which way to move the heat anticipator indicator for a longer burner on-time remain the same as for the T87: for longer burner on time move the heat anticipator indicator to a higher number on the scale.

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

Heating System Type

Recommended Heat Anticipator Setting
for the CT87 Thermostat

Steam Heat

1.2A

Hot Water Heat

0.8A

High-Efficiency Warm Air Heat
(High Efficiency Furnaces)

0.8A

Standard Warm Air Heat
(Furnaces)

0.4A

Electric Heat

0.3A

Notes:

Adapted from Honeywell CT87A,B,J Round® Thermostat Low Voltage (15 to 30 VAC) Thermostat and Mounting Hardware Installation Instructions Honeywell Corporation, [3b]

Reader Question: which way to adjust the heat anticipator settings?

Lower heat anticipator settings: according to Honeywell, setting the heat anticipator to a lower number (lower Amps number) setting will result in shorter burner on-cycles, ie less room-temperature overshoot. A typical heat anticipator scale runs beetween a low of 0.10 Amps and a high of about 1.2 Amps.

Higher heat anticipator settings: according to the manufacturer, a higher heat anticipator setting will result in less self heating of the thermostat, longer burner on-cycles, and possible overshoot of the room temperature. - Chubby541 10/31/2012

For an explanation of the relationship between electrical resistance and heat that thus explains how a heat anticipator adjustment works, see ELECTRICAL RESISTANCE vs HEAT GENERATED.

Reply: heat anticipator setting numbers versus heat-on cycles

Thermostat heat exchanger scale marking LONGER (C) InspectApedia

Thanks Chubby for prompting us to make sure that our explanation of the thermostat heat exchanger is precisely accurate and more clear.

As you can see in our closeup photo of the heat anticipator adjustment scale, the word "Longer" and an arrow are stamped into the end of the scale with the lowest number. The arrow points away from the low end towards the high end of the scale.

Honeywell is saying for a longer heat-on cycle (less heat anticipation) move the heat anticipator adjustment in the direction of the arrow - towards the higher amps end of the scale.

[Click to enlarge any image]

The heat anticipator is an electrical resistance heater that provides a tiny source of extra heat right inside the wall thermostat of some older room thermostats like the Honeywell T87 bimetallic spring mercury bulb model.

Heat anticipators are found in the T87® and the CT97A,T,J Round® room thermostats by Honeywell as well as in some other thermostat brands discussed here.

Making a little extra heat inside the thermostat means that the thermostat thinks the room is a little warmer than it actually is - which leads the thermostat to stop calling for heat a little early - hence the name "heat anticipator". The thermostat is anticipating that some additional heat will arrive into the room after it turns off the boiler.

More heat inside the thermostat means sooner burner off time (shorter burner on-cycles) and less room heat overshoot.

By Ohms Law, I = E / R or

We can also write

The relationship of electrical resistance to temperature is also derived from Ohms law and is written as follows, where (a) is the temperature coefficient of resistance: [23]

Where for example watts can be giving us energy in the form of light, heat, or most likely a mix of the two. (This is true in general or simplified terms. A true "power" equation that relates power dissipation and current through electrical resistance P = IE ; P = I2R; is a little more complicated thanks to James P. Joule but we'll skip that more technically correct approach for this discussion.)

Details are at Definition of Amps, Electrical Current, and Definition of Ohms, Electrical Resistance

Reader Follow-Up: moving the heat anticipator to lower numbers gave shorter burner on-time

The Honeywell documentation is correct. Moving the anticipator to the RIGHT, i.e. LOWER current numbers, results in SHORTER cycle time. Why am I so sure?

1. Just tried it with my forced hot air system; moved from 0.8 to 0.4. It turns on/off twice as often now.

2. The current numbers on the anticipator aren't how much current it's generating (really, how would it do that?), it's how much current it's EXPECTING.

At lower settings, it is EXPECTING less current; since the amount of current the BOILER produces is constant, this results in more Joule heating of the anticipator and a faster cycle.

3. That "LONGER" text? Yeah, look more closely. It's got an arrow POINTING TO THE LEFT. It's too fuzzy to see in your picture; look on a real device. - C.K. 11/30/2013

The arrow does not point toward higher resistance, it points toward higher EXPECTED CURRENT (those are measurements in Amperes, as you correctly note!) and hence LOWER RESISTANCE: if the current coming from the boiler is HIGHER, the anticipator needs LESS resistance to produce the same amount of heat.  (This is simply keeping I²R, i.e. the total heat output, constant.)

Example: boiler sends 0.4 A through the anticipator.  If the anticipator is set to 0.8 A, it will heat up LESS (because it is expecting MORE current) and hence the cycle time will be LONGER than if the anticipator were set to 0.4 A.

I think the confusion in earlier versions of this article ("I can't explain this [yet] because at lowest resistance you'd think that we are not using the heat anticipator's resistor") stems from the assumption that moving the anticipator toward lower values uses less of the heating element, when in fact it is the other way around (to keep I²R constant).  It's not obvious from looking at a T87 which end of the resistance coil is connected (I would assume the small end, but you seem to assume the large end.)  Have you tested this with a multimeter?

Reply: exactly right: shorter wire, less resistance, more current flow, more heat anticipator heat output, earlier thermostat shutoff, shorter heat-on cycle

Thanks C.K. I'm not sure about your interpretation of "expecting" and anticipation, but Honeywell agrees with you about which way to move the heat anticipator setting for longer or shorter burner on-cycles.

Please click on any InspectApedia image to see a larger, more detailed image, including showing the arrow on the heat anticipator.

And in the REFERENCES section of this article you can find a link to the installation instructions for the Honeywell T87F from which we quote above. I agree with you about the direction of the arrow you cite.

Lowering the heat anticipator to a lower number gives shorter heat-on times

Honeywell Corporation agrees that lowering the heat anticipator from 0.8 to 0.4 will give shorter heat on-times which, in turn, would typically result in more frequent burner on-off cycling - which is on an oil fired system and on some gas fired systems a less efficient way to run a heater.

\That's because lowering the Amps number from 0.8 to 0.4 cuts the current flow (Amps) in half, cutting the heat anticipator output accordingly. By cooling the heat anticipator we stop pre-heating the thermostat's room temperature sensor as much. By stopping the temperature sensor pre-heat we stop turning off the heat early. We turn it off sooner. That gives shorter on-imes and thus more frequent heat operation.

Raising the heat anticipator to a higher number gives longer heat-on times

In their T87 Honeywell states that a higher number on their heat anticipator scale gives a longer burner on-time. That's because the heat anticipator pre-heats the thermostat less at higher Amps numbers, so the thermostat doesn't turn off "early" - it turns off "later" so the heating system will remain on longer.

A slightly higher setting to obtain longer burner-on times (fewer cycles per hour) may be desirable for some systems. 

And again in their CT87A,B,J Round Thermostat instructions Honeywell also states that moving the heat anticipator up will give the burner a longer on time.

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

In preparing the original article and again in 2017 for confirmation of our understanding of heat and electrical resistance and thus of how the heat anticipator actually works, I checked with Dr. Jess Aronstein, an electrical engineer specializing in forensic work. Here is his comment:

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). So if you set to half the resistance, the current in the active portion of the heating element is double the original, and the heat generated is the current squared times the resistance which calculates to double the heat relative to the original setting. - J. Aronstein to D Friedman, private email 2017/11/01

This is not a case of increasing the heat output by increasing electrical resistance in the circuit. It's the opposite.

On a thermostat that uses a heat anticipator, moving the anticipator's heating device sliding adustment to a higher Amps number (more current) will be seen to actually be using a shorter segment of the nichrome wire that comprises the heating element. That's actually lowering the resistance, permitting more current to flow, and generating more heat.

The numbers and resistance and heat effects in the original manufacturer's instructions can be a bit confusing.

In answer to your question of how the heat anticipator would know how much current it is generating, the anticipator is not generating current.

The heat anticipator is a variable resistor that varies by changing wire length. It is resisting current flow and generating heat. When we shorten the anticipator's wire length by moving the adjuster we are sending the same voltage through a lower-resistant shorter wire - the current flow increases and more heat is genearted.

The heat from the variable resistor warms the thermostat sensing bimetallic spring, thus fooling the thermostat into thinking the room is warmer than it actually is.

The heat anticipator is anticipating that additional heat is going to arrive in the room from water (or less-so by air flow).

We can measure resistance or current flow in Amps at different points along a wound resistance coil. The scale on the heat anticipator corresponds roughly to the position along the coil at which the moving contact touches the coil.

Moving the point of contact to a place where electricity has run through more wire means more resistance, while less wire means less resistance.

Bottom line: by anticipating additional heat arrival (hence the device name "heat anticipator") and by shutting off the burner a bit sooner than otherwise, the heat anticipator is reducing room temperature overshoot - warming the room higher than the thermostat requested.

Reader question: the physics of resistance vs amps vs heat anticipators

(Feb 26, 2014) Bill Martino said:

When you raise the current value on your heat anticipator setting, according to Ohms law the resistance goes down as the amperage is inversely proportional to the voltage R=E/I which gives off less heat in the anticipator coil which allows the furnace or boiler to run longer

Reply:

Thank you for the clarification, Bill. This topic has generated many comments, which underscores how confusing it can be for some readers.

Longer burner on time has to be due to less heat coming off of the heat anticipator (less add-on warming of the thermostat's temperature sensor).

(Nov 17, 2014) RH in CT said:

What I find amusing about a thermostat with a heat anticipator is that a thermostat that is NOT controlling any sort of furnace or heat source, but which is adjusted to near the current temperature of the room, can cycle on and off despite the room temperature not changing.

Repy:

RH the thermostat should not be making you laugh - nor cycling on and off as you describe. I'd check further. Check for shorted thermostat wires.

Details are at ELECTRICAL RESISTANCE vs HEAT GENERATED

Should I Change the Heat Anticipator Settings?

It's probably smartest to leave the heat anticipator alone on your thermostat unless you're having a specific problem controlling room temperature. And then, before attacking the heat anticipator setting, check first for more significant problems such as a bad thermostat location (on a cold exterior wall, or in the direct path of warm air from a forced warm air supply register).

What if my Thermostat Does Not Have a Heat Anticipator? Do I need one?

Well you may not need a heat anticipator. As we discuss in detail immediately below, for many types of heating system distribution, heat source, and heat controls, you don't need a heat anticipator, while for certain types of heating systems (such as those using heavy cast iron radiators) using a heat anticipator can provide important improvements in heating system operation.

But first let's define three thermostat heat anticipator categories:

Reasons to Buy A Wall Thermostat that Includes a Heat Anticipator feature

OJ UDG-4999 radiant floor heat thermostat (C) InspectApedia

Heating distribution systems that have high thermal mass (cast iron radiators or cast-iron baseboards) tend to continue emitting heat after the boiler shuts down - thus possibly heating the room well past the thermostat's set temperature.

Popularly, this phenomenon is called temperature overshoot. On the other hand, an improperly adjusted or installed thermostat can also cause too-frequent burner on-off cycles, wasting heating fuel.

In these cases we can use a heat anticipator-equipped thermostat to advantage.

Other heating systems with thermal mass such as an in-floor radiant heat system, particularly if the floor is a concrete slab or a blockbed solar heat system might also benefit from a room thermostat that has a heat anticipator function.

But we notice that at least some thermostats sold for radiant floor heating, such as the OJ programmable floor thermostats UTN-4991 and the OJ UDG-4999 (shown at left), designed to monitor both floor temperature and air temperature, do not include a heat anticipator.

Question: Helpful Pointers Heat-Anticipators

... [other text relocated to pertinent article]

3. Heat-anticipators are typically only found in mechanical (non-digital) thermostats. Whenever installing or servicing a thermostat with a heat-anticipator, the amperage setting on the heat-anticipator should always be verified as matching the amperage draw on the gas valve or relay that it controls. Otherwise the relay or thermostat may not function properly.

- Helpful Pointers Regarding 24V T 10/27/2012

Reply:

Helpful, thanks so much for the detailed tips on wiring 24-volt room thermostats. I've inserted your remarks into the main article body over at THERMOSTAT WIRE CONNECTIONS -

and in that article beginning in a new section at

9 THERMOSTAT WIRING TIPS & COLOR CODE CONVENTIONS.

Actually, though, a recent search (2013) found that there are indeed contemporary digital and programmable thermostats that do include a heat anticipator, though I agree that many models do not include that feature. In a later FAQ just below we point out the types of heating systems that do and don't benefit from a heat anticipator function in a room thermostat.

Working together and exchanging information makes us better informed than any individual can be working alone. - Ed

Types of Heat for Which You Do Not Need a Thermostat with a Heat Anticipator

Honeywell T49 electric heat thermostat (C) InspectApediaLow thermal mass heating systems: other heating distribution systems that have low thermal mass (copper tubing baseboard) or no thermal mass (warm air heat and ductwork) stop emitting heat quickly (thin wall copper baseboard) or immediately (forced warm air heat) when the boiler or furnace is shut off by the thermostat.

Those systems don't need and don't benefit from a heat anticipator.

Fan coil type heating systems are, like forced warm air heaters, an "on-off" type heat with no significant thermal mass. You'll find that thermostats sold for use with fan coil heaters, like PECO's TA180-001 digital programmable thermostat do not include a heat anticipator.

Similarly, electric baseboard heat thermostats or electric floor heating mat thermostats typically won't include heat anticipators, though you can purchase thermostats with varying degrees of room temperature variation such as +/1 1 °F or +/3 3 °F.

Shown at above left is the Honeywell T498B1512 120V wall thermostat used for electric baseboard heat. ($50.) This thermostat is designed to maintain heat within a temperature differential of +/- 3 degrees F.

If you need to control electric heat and want a more narrow temperature differential range, check out the Honeywell TL7235A100 non-programmable line voltage digital thermostat or the White Rodgers line voltage snap action room thermostat model 1A65-641 that controls temperatures to +/- 1 degree F. ($40.)

For a programmable electric heat thermostat see the Dayton 1UHG4 line voltage thermostat. Electric heat thermostats are discussed
at LINE VOLTAGE THERMOSTATS

Non-bounce thermostats: a second type of improvement in heating thermostat controls may also have obviated the need for heat anticipators.

A second reason (besides room temperature overshoot) for the original inclusion of heat anticipators was to allow the installer to widen or narrow the precision of the "on-off" response of the wall thermostat. This prevented a thermostat from bouncing rapidly between on-and off which could happen if the thermostat was both very sensitive to room temperature and the room temperature changed too rapidly. Modern thermostats are quite reliable at holding to a +/1 one degree temperature range.

Homes with stable indoor temperatures: after super-insulating our lab with blown-in foam insulation, we found that its temperature remains very stable - the building is slow to cool down and slow to warm up as outdoor temperatures change. The single largest factor is probably the elimination of uncontrolled air leakage.

Many modern homes are better insulated so cool off more slowly so temperature changes more slowly, and in some homes installers have got smart enough in locating the wall thermostat that it's not placed on a cold outside wall nor in direct path of blowing hot or cold air.

What happened to the bimetallic spring and mercury bulb in thermostats? What about the heat anticipator?

Modern digital room thermostats don't use a bimetallic spring to sense room temperature. And for environmental reasons (mercury is highly toxic) manufacturers no longer use a mercury bulb to control the switching of a wall thermostat.

Rather they use a solid state thermistor. A thermistor is a device whose electrical resistance changes in response to temperature. A microprocessor chip in the digital room thermostat converts that change in resistance to a temperature reading.

How & Where to Buy a Room Thermostat with a Heat Anticipator

White Rodgers 1C20-101 thermostat with heat anticipator (C) InspectAPediaThere are definitely modern wall thermostats that still use a heat anticipator, though its physical form has changed from the tiny coil over which a pointer slides to a thin wire mounted on a disc, still encompassing a movable arm that contacts the wire at different points along its length.

The operating principle is the same: shorter wire, less resistance, more current flow, more heating. Here are some room thermostats that include a heat anticipator:

Question: Where can I buy a room thermostat that has a heat anticipator? Big box stores don't carry thermostats that include a heat anticipator

Stopped at a local big box store and looked at the models on display, called Honeywell while there and was told that typical retail outlets like these DON'T carry thermostats that have anticipator functions or a heat control differential and they are set to stay within +/- 1 degree at all times. The Honeywell tech could not give me part numbers but said that those units are normally only sold to HVAC installers through their distributors. - Mike at Longmeadow MA

Reply: Where to Buy a Room Thermostat with a Heat Anticipator Feature

Thanks for this field report on the availability of room thermostats that include a heat anticipator.

You're sure right. I stopped by a local Home Depot store, checked out a local Radio Shack, and also shopped at the one remaining lumber yard/building supplier in our area who hasn't been driven out by big box store competition. None of them stocked a room thermostat that included a traditional heat anticipator, though some newer room thermostats provide a very similar function in the form of micro-toggle-switches inside the unit.

But it is in fact still easy to find a wall thermostat that includes a heat exchanger by a little careful shopping, as I detail below. I think part of what you heard from the Honeywell tech was less than clear or less than forthcoming. Or maybe s/he was not fully informed about the company's products. Or maybe the tech didn't even know what a heat anticipator is or why they are sometimes used in thermostats.

Following some explanation, just below at I include a list of room thermostats that include a heat anticipator - including Where to Buy a Room Thermostat with a Heat Anticipator g from Honeywell, Lux, Robertshaw and White-Rodgers. Perhaps you'll want to forward a copy of this article to the Honeywell "tech" with whom you spoke by telephone.

Recap of Thermostat Heat Anticipator Functions

A heat anticipator is basically a tiny heater placed inside of a room thermostat to slightly shorten the heat-on-cycle called for by the thermostat.

The heat anticipator in the traditional Honeywell T87 thermostat and older models is a simple electrical resistance heater of very small size, placed close to the bimetallic spring that operated the thermostat.

In case it's not clear, a bimetallic spring - two different metals laminated together and then shaped into a coil - expands or contracts in response to temperature changes, causing the coiled spring to open or close.

In the old mercury bulb thermostat shown in the article above (and in some other thermostat designs) moving the spring tipped the bulb that moved the mercury to close (on temperature fall) or open (on temperature rise) the switch that turned the heating system on or off.

When the heat anticipator's teeny little nichrome wire heater warms up the inside of the room thermostat it fools the bimetallic spring into thinking the room is a bit warmer than it really is, so the heat is turned off "early".

The question of whether or not you need or should want a thermostat that includes a heat anticipator feature is discussed in the article above, at Do I need a thermostat with a heat anticipator?.

More Information about Heating System Controls

This article series on room thermostats and heat anticipators continues discussion of the basics of heating or cooling system thermostats, their use, setting, and adjustment. Here we provide A Guide to Finding, Using, and Adjusting Thermostats for Heating & Air Conditioning Furnaces & Boilers, Heat Pumps or Electric Furnaces or Boilers.

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards.

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Continue reading at HEAT ANTICIPATOR Operation or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see TEMPERATURE RESPONSE of ROOM THERMOSTATS

Also see DEFINITION OF AMPS, ELECTRICAL CURRENT,

Also see DEFINITION OF OHMS, ELECTRICAL RESISTANCE

Or see ELECTRICAL RESISTANCE vs HEAT GENERATED

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