Setting a Thermostat Anticipator

On mechanical thermostats, an anticipator is actually a differential control device that uses the heat of the amperes passing through the anticipator wire to provide the 'right amount' of heat to adjust the accuracy of the thermostat, so it doesn't wander more than 1 degree above nor 1 degree below the temperature setting. Without an anticipator a mechanical thermostat might start the heater at one temperature and not shut off until the room was 6 degrees warmer. By using the heat of the anticipator, the themostat element is warmed much sooner to shut off within 2 degrees warmer room temperature. Electronic thermostats use fixed timing to arrive at a similar action.

Older thermostat anticipators were usually set around 0.4 amperes. However, as some zone valves draw 0.9 amps, setting them to 0.4 could mean that just wiring them up without checking the ampere draw with a thermostat anticipator on the wires beforehand could mean that the anticipator fries instantly from twice the expected setting amperes flowing through. Putting the anticipator at 0.7 amps with an actual draw of 0.9 would mean that the room temperature would shut the heat off more quickly; but probably without damage. Leaving the anticipator set at 0.7 with actual ampere loads through the anticipator at a lower value than 0.7 would mean wider temperature swings, but without damage. It would force adjusting more carefully; hopefully with a thermostat amperemeter.

Setting anticipators to the gas valve is very old news, over two decades old when there were standing pilot gas valves. Since then, anticipators have to deal with electronic control boards for electronic ignitions, and/or stack dampers. Setting to a gas valve would mean nothing when the furnace is started by a zone damper or a boiler is started through a zone valve. It would surely mean NOTHING to an oil heater.
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Use a thermostat anticipator ampere meter to be sure. I'm not sure when old, outdated information transitions into an 'old wives tale' told by the guesser to be repeated ad infinitum on the web as newbies come across the story.

Thermostat amperemeters sensitive to less than 1.5 amperes are available at HVAC supply houses for the trade.

The anticipator works off the amount of amperes provided by the transformer that powers the boiler control through the thermostat. When you know that is how the anticipator works and that the numbers on the anticipator dial relate to how much amperes are expected, you realize that the numbers are there to let you measure the amperage with a thermostat ampere meter, then set the anticipator to that number.

If you don't have an amperemeter, you are trying to come to perfection without the instruments to find the answer. So you need to guess what you need by moving the anticipator lever in one direction or other til you get what you want.

Many transformers provide between 0.2 and 0.4 amperes to go through the thermostat, so the starting point is .4 for your guesses.

If you knew that you had 0.4 amperes in the wires that go through the thermostat, you'd set the anticipator dial at 0.4

Moving the lever from 0.4 toward 0.2 would mean the anticipator would heat up more and faster, as the thermostat 'would expect' 0.2 amperes, which would heat the anticipator quicker making the thermostat open the contacts sooner. Thus the burner in the boiler would cycle faster on and off. Just enough cycling and the heat is steady. Too much heat in the anticipator from too low a number selected on the dial would prevent the house from heating enough as the burner cycles would be too short. Setting the anticipator lower than 0.2 with some devices that draw over 0.5 amps can burn the anticipator so it won't work any longer.

Pushing the lever the other way from .4 toward .9 would mean that the thermostat would be expecting more amps than 0.4  (say 0.7 if that is where it is set) That results in the anticipator not heating the thermostat operating bimetal enough, so it would take longer to open the contacts. The rooms would then get warm, but take longer to cool down, so there would be periods of cold on every cycle.

The right guess as to location is when there is enough heat all the time, but no cold periods between. As steam radiators get very hot, and retain heat a long time, the cycles are very long, like 2 or 3 times an hour instead of 6 as in warm air systems.

Most of the repairmen in the trade don't have a clue about this, so don't carry a thermostat amperemeter. The confusion is caused by aggressive marketing where the bimetal coil heater is called an 'anticipator' (to give the clever idea that the thermostat is human and can think) instead of a 'thermostat contact set point range heater'; it is, however easier to say, even if it describes nothing.