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Heat anticipator adjustment tool from Amps Check Heat Anticipator Precise Adjustment Methods
Use an ammeter: adjust / repair room thermostat, fine tuning

Thermostat heat anticipator testing & fine tuning procedure.

This article explains how and why we fine-tune a room thermostat or wall thermostat by checking its heat anticipator using a mini ammeter.

We describe the use of the T.D. Amps-Check mini ammeter specifically designed for thermostat heat anticipator adjustment.

This mini ammeter gives precise amps readings in the 0 - 1.2A AC range. By measuring the current (amperes) flowing through the thermostat contacts on a call for heat we can adjust the heat anticipator precisely to its optimal setting.

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How to Use a Mini Ammeter to Set or to Check Heat Anticipators and Thermostats for Proper Adjustment

Heat anticipator adjustment tool from Amps CheckWhy do some thermostats include a heat anticipator?

Since a room heating or cooling thermostat is really just a simple "on-off" switch, some fuzziness needs to be built into the thermostat's control of the air conditioner or heating system, lest it cycle on and off too frequently, oscillating very closely around the set temperature.

Article contents

The job of the heat anticipator circuit in a wall thermostat is to prevent heating or cooling "overshoot" too far past the set temperature, but to permit some overshoot to prevent system on-off oscillation.

When we serviced heating systems and our clients sometimes complained that the thermostat did not seem to be responding as desired to room temperature.

The thermostat might permit the room to get much warmer, or much cooler than the temperature to which the thermostat was set.

This was a reason to whip out our little ammeter to see what was really going on with the thermostat circuit.

A second reason we'd use this ammeter to check the current draw of the thermostat circuit was to allow proper setting of a heating or air conditioning system control set which was made by a different manufacturer from the one who made the wall thermo sat.

For example, if the air conditioning or heating system control being switched on and off by the thermostat was not one of the ones in Honeywell's list of heating controls

(given in HEAT ANTICIPATOR ADJUSTMENT) ,

one of the checks we'd make is to actually measure the heating control circuit ampacity using a special mini ammeter sold just for that purpose, and shown in our photo above.

How to Connect & Use a Low-Range or Mini Ammeter to Check the Thermostat Circuit and the Heat Anticipator

Thermostat heat anticipator ammeter measurement connections (C) InspectApediaWith the heat set "down", power to the equipment on, and the thermostat set to heating mode, the alligator clips are connected to the thermostat wire terminals in the thermostat.

Usually these are "R" (red) and "W" (white wire) or "R" and "Y" terminals on thermostats.

Just connect the ammeter's leads to the thermostat terminals where you see the red and white wires coming from the heating system are already wired.

(You'll have to remove the thermostat cover to perform these steps.)

The current flow (Amps) is read on the meter, and the heat anticipator is set to match the actual current that was just read.

How to Make the Correct Current (Amps) measurement at the thermostat heat anticipator

  1. Turn power on to the heating system.
  2. Set the thermostat to call for heat
  3. Adjust the set temperature on the thermostat to well above current room temperature so that the heating system will operate
  4. Run the heat for at least five minutes to warm up the system to normal operating temperatures.
  5. Remove the thermostat cover (if necessary) to expose the R and W terminals. Connect the T.D. Amps-Chek or equivalent ammeter test leads onto the two thermostat terminals that make (connect or close) on a call for heat. These are the R and W terminals on most thermostats.
  6. Read the current flow indicated on the meter.
  7. Remember to re-set the room thermostat to the desired room temperature

Watch out:  T.D. points out that a few controls, such as motorized gas valves, draw more current while operating than when they are in the fully-open position.

So if your ammeter shows an unusually high reading (say more than 1.2A) you should hold the ammeter leads on the terminals for an entire minute. By that time the motorized valve should be fully open and you should see a lower AMPS reading.

Examples of equipment where you will face this problem include

Watch out:  the amps reading made at the room thermostat can also indicate a system or wiring problem: if the meter continues to read current over 1.2A there is probably a system problem, risking damage to the thermostat itself.

How to Use an Ammeter to Set the Heat Anticipator

Honeywell heaet anticipator thermostat setting warning

Watch out: as Honeywell warns thermostat installers:

This thermostat has an adjustable heat anticipator and will operate properly only if this [heat anticipator resistor type] tiny in-thermostat heater is adjusted to match the current of the valve or relay.

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 SET & TEST by AMMETER

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 SET & TEST by AMMETER 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.

Watch out: on some heat anticipators the movement must be in the opposite direction. See the instructions for your model.

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.

Really? Well those are the company's instructions. What's confusing is that the heat anticipator recommended settings given the table above (from Honeywell) that show that heating systems where we want more heat from the heat anticipator and an earlier turn-off of the boiler the table uses larger numbers such as 1.2A for steam heat and lower numbers such as x for electric heat.

You generally want shorter heat on cycles with heating systems that have a lot of thermal mass (steam radiators) and longer heat-on cycles with heating systems that have little or no thermal mass (forced warm air furnaces).

So what's going on?

Imagine a little electric heater made of wire whose length you can vary by sliding a pointer: if we send the current through more of the wire is it generating more heat? That would be consistent with Honeywell's instructions.

When the heat anticipator puts out more heat the bimetallic room temperature sensor will be fooled into turning off the heat sooner - a shorter "heat-on" cycle.

[Click to enlarge any image for more detail]

For a detailed explanation of how the thermostat heat anticipator actually works

see HEAT ANTICIPATOR OPERATION.

What is the Exactly Correct Heat Anticipator Setting when Using an Ammeter?

Heat anticipator scale (C) InspectAPedia

Set the heat anticipator pointer to the same AMPS reading on its heat anticipator scale as the actual AMPS or current read on the mini ammeter.

Example: thermostat 24VAC circuit reads 0.7A. Set the pointer to 0.7A on the scale as shown in our sketch at left.

What do we gain from fine-tuning the thermostat's heat anticipator adjustment?

This places the thermostat heat anticipator in exactly the right setting for the equipment to which it is attached. Then we simply removed our test leads and re-set the thermostat to the desired room temperature.

When the heat anticipator is working correctly, it prevents too much temperature "overshoot" when the thermostat is turning heating or air conditioning equipment on or off. Ultimately this means that the thermostat will maintain room temperature more accurately and more closely to the "SET" temperature set by the occupants.

Watch out for static cling that can foul up an ammeter reading when measuring a thermostat circuit

Precaution when using this equipment: if the meter is wrapped in plastic there may be a static charge when you unwrap it. Because it is very sensitive, any static charge on this meter (or many other ammeters or VOMs or multimeters) can cause the dial movement to show an erroneous reading. Just wait 5 minutes before using the meter, allowing the static charge to dissipate.

Where to Buy the Mini-Ammeter Amps-Chek® from T.D. Instruments or equivalent low-range high precision amps measuring instruments


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