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How to use an ammeter to adjust or repair room thermostats by fine tuning the heat anticipator circuit
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• 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?

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

Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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

Since a thermostat is 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

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?

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

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.

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THERMOSTATS
  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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