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How to use an ammeter to adjust or repair room thermostats by fine tuning the heat anticipator circuit InspectAPedia®  -     How to Use a Mini Ammeter to Check Heat Anticipators and Thermostats for Proper Adjustment How Do I Install & Wire Up a Room Thermostat like the Honeywell Round Wall Thermostat? Questions & Answers about how thermostat heat anticipators work and how to set the heat anticipator

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 explain how heat anticipators work and how to adjust the heat anticipator on a room thermostat.

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This article is part of our Guide to Finding, Using, and Adjusting THERMOSTATS for Heating & Air Conditioning Furnaces & Boilers, Heat Pumps or Electric Furnaces or Boilers. This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs.

We describe how to inspect residential heating 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.

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER CONTROLS & SWITCHES, and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators. If your building uses warm air heat, see  FURNACE CONTROLS & SWITCHES. If your building uses steam heat see STEAM HEATING SYSTEMS. Also see Heat Won't Turn Off - Stop Unwanted Heat.

Readers needing to find and fix un-wanted air leaks, heat losses, or other energy wasters should see HEAT LOSS DETECTION TOOLS. Readers should see ENERGY SAVINGS RETROFIT CASE STUDY and also see HEAT LOSS DETECTION TOOLS and INSULATION INSPECTION & IMPROVEMENT for energy saving retrofit detailed guides. Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks.

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

Why 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.

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, 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 at left.

Since we no longer service heating systems, we'll give our tool free to a trained heating service technician who'll pay the shipping postage.

How to Connect the Ammeter to Check the Thermostat Circuit and the Heat Anticipator

With 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.

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.

The mini ammeter for adjusting thermostats to work precisely (shown at the top of this page) was produced by and may still be available from T.D. Instruments Corporation, 180 Charlotte St., Rochester, NY 14067 - 716-232-4208. We used model T.D. 2, which operates between 0 and 1.2 Amps A.C.

Questions & Answers about how thermostat heat anticipators work and how to set the heat anticipator

Question: what is the relationship between setting the heat anticipator higher or lower and the resulting room temperature?

I want to know that if the heat anticipator sets was too low, whether the room temperature lower or higher? - Thanks Hyuk Lim.

Reply:

Typically if you set the heat anticipator lower you are narrowing the "gap" or amount by which the heat anticipator anticipates the amount by which heat delivery will "coast" when the thermostat stops calling for heat.

In our example photo (left) moving the pinter to the left is setting the heat anticipator to a lower resistance (turn heat off later) and moving the pointer to the right is setting the heat anticipator to a higher resistance (turn heat off sooner).

If you click to enlarge our heat anticipator photo you'll see that the settings range from 0.10 to about 2 Amps - a scale quite similar to the mini Ammeter used to fine-tune thermostat heat anticipators based on the circuit resistance (Ohms) and current flow (Amps) as we dosciss above.

Notice that at the left end of the heat anticipator wiper dial at the lowest setting Honeywell has imprinted the word "LONGER". This means the heat will run longer or cut off later and the room will be warmer when the thermostat turns off the heating boiler or furnace.

This "coasting" effect is particularly significant with hot water or steam heating baseboards or radiators because those devices have plenty of thermal mass that keeps on giving heat even after the heating boiler has stopped running. So even though the heating boiler has stopped firing, heat will continue to flow into the rooms from the hot radiators.

In order to keep the room from becoming much hotter than the thermostat set point, a heat anticipator turns off the heating boiler ahead of time or before the room temperature has actually reached the thermostat set point.

A smaller or lower heat anticipator gap or setting should result in a HIGHER room temperature and conversely setting the heat anticipator higher will result in a LOWER room temperature.

Details about Heat Anticipator Function and Settings

The heat anticipator, by warming the thermostat bimetallic spring heat sensor, causes the thermostat to stop calling for heat sooner. The result is that the room temperature will be a bit lower when the thermostat turned off the heating system.

Put another way, because the heat anticipator is basically a resistor that itself heats up the wall thermostat in order to cause it to turn off the heat ahead of the room temperature set point on the thermostat, moving the heat anticipator to a higher setting (typically to the right) is moving it to a position of greater resistance in the circuit.

Higher resistance in Ohms, or higher current (Amps) draw, means that the greater resistance in the heat anticipator circuit causes heat anticipator to get hotter, which causes the heat anticipator to cause the thermostat itself to warm up more, which causes the thermostat to shut off sooner which means that the room will be slightly cooler.

Lower resistance in Ohms, or lower Amps draw, means that less heating of the thermostat bimetallic spring will be caused by the heat anticipator circuit all mean that the heating system will remain on longer before the thermostat reaches the set temperature and turns off the heater.

See HEAT ANTICIPATOR Adjustment for details on how to set the thermostat heat anticipator.

Also see Definition of Amps, Electrical Current, and Definition of Ohms, Electrical Resistance

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THERMOSTATS, HEATING / COOLING
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT, Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat

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• Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
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• Installation Guide for Residential Hydronic Heating Systems
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