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Old 12-09-2017, 03:09 PM   #15
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Join Date: Nov 2011
Posts: 1,161
Default Re: Looked at new thermostats, nah I'll DIY this

Originally Posted by redwire View Post
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 ?
An HVAC plant oscillates, by definition (unless you have infinite insulation). How do you consider the heat coming on, then off, then on, again? Using whatever control strategy you choose!

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.
Not all HVAC plants have fans. Ever live with baseboard radiators? Steam heat? Radiant tile?

The anticipator plays the role of the D term in a PID controller. It aims to compensate for lag in the controlled system -- one of the toughest issues to address in any control system!

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.
Modern furnaces "run on" for a fixed time after the thermostat has finished calling for heat -- usually a minute or two (DIP switch on the controller inside the furnace). The manufacturer's intent is that this open-loop run-on can be tweeked (via that DIP switch) to flush the duct work (duct work represents transport delay -- lag!) of the remaining warm air along with move residual heat from the heat exchanger into the duct. Without pushing "cooler" air through in the process (once the warm air has been exhausted).

Our plant used to "coast" a full degree beyond the upper setpoint due to the amount of stored energy in the control system when the thermostat stopped calling for heat. This caused the temperature variation that we encountered in the house to be wider than we'd have liked.

[In general, more hysteresis -- a larger "comfort zone" -- is more efficient but less comfortable]

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.
Ideally, a control should map onto the intuitive expectations of its user. The control system should do what it needs to do in order to facilitate that mapping, within the other constraints placed on it.

The only reason for a "1 degree" change is because a digital thermostat displays the temperature in 1 degree increments. If the thermostat turned off at "68 degrees" (when it was really 68.932 degrees) instead of when the display indicated "69 degrees", the user -- lacking those extra digits beyond the decimal point) would consider it as misbehaving -- especially as it would start calling for heat at 68 degrees, as well (68.0000) instead of 67 degrees.

Note that legacy "analog" thermostats made it difficult for a user to determine if the thermostat was cycling on/off at 68, 68.2, 67.9, 68.7, 69.5, etc. degrees so the user didn't fret over the action of the controls. As long as the indoor temperature was "about 68" he was happy.

Specifying a too narrow comfort zone for a given controlled system could lead to lots of short-cycling which can be hazardous to the equipment and wasteful ($$). Furthermore, conditions may change in the controlled environment (e.g., windy or damp days may "feel" colder and/or alter the response of the controller in a less-than-desireable way).

Anything that is software-based should be smart enough to recognize these sorts of conditions and tweek its behavior accordingly -- just like a user knows to bump the setpoint up a degree or two when it "feels colder" (even though the thermostat ensures that it ISN'T colder, indoors!)

Otherwise, you'll be hard pressed to beat the cost, ease of use (i.e., no batteries to change) and reliability of a coiled bimetal spring and mercury switch!
Curious.George is offline   Reply With Quote