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Old 12-09-2017, 12:33 PM   #14
Badcaps Veteran
Join Date: Dec 2010
City & State: Alberta
My Country: Canada
Posts: 1,171
Default Re: Looked at new thermostats, nah I'll DIY this

The "heat anticipator" rheostat built into mercury switch thermostats, I've never seen it work. It adds positive feedback and you get an oscillator. You want PWM in your home heating ?

The intent was to model overshoot- when the furnace burner shuts off the heat exchanger and ducts still have heat so the home heats up beyond the thermostat's setpoint, until the fan shuts off.
A thermostat on old furnaces switched the gas valve (typ. 24VAC 0.28A).
Modern furnaces control board have a dummy ~1k ohm resistor load. So the anticipator rheostat doesn't work anymore. Hurrah.

OP's thermostat, I would think
it should have hysteresis, usually a +/- 1 degree.
A long average (say 1 minute) temperature to decide when to start heat/cool.
A short average (say 15 sec) to decide when to stop heating/cooling.
redwire is offline   Reply With Quote