Kinarfi said:
Unless you actually use the fan switch, I would just forget about it and leave it on automatic, which is no operation from the t stat
This is NOT TRUE. In AC mode, the thermostat controls the fan.
In HEAT, the FURNACE controls the fan except for the ability for the thermostat to cause continuous operation.
FAN
ON - Fan is on continuously regardless of heat or cool
OFF - System is OFF. It does NOT mean that the fan will turn off immediately if in HEAT mode.
AUTO - The thermostat in AC mode or the the furnace in heat mode controls the fan,
Since you are designing a thermostat, it is important to add what's called an anti short-cycle timer whose operation is as follows. If the Y terminal turns off, it then can only turn on after it has been off for x minutes. 2 is usually a good number. In a thermostat that draws power from the furnace, that timer starts immediately on power up. It acts as a Y_enable and prevents the AC system from restarting immediately after a power failure. The Fan CAN start right away.
Let's explain the "Thermostat controls the fan a bit better"
Furnace gets a "Call for heat".
furnace says, the blower will blast perceivably cold air if I turn the blower on immediately, so the furnace can do two things.
1) Just delay the fan turn on or b) use plenum temp with a timeout.
2) Use Plenum temp (If Tp > x) or (Fan is in ON mode) then turn on blower.
When the "call for heat" is satisfied, then the flame turns off.
The fan turns off either :
1) after a set delay or
2) after (Tp<x).
The job of the "anticipator" in the old stats does create a sort of hysteresis and it heated the bi-metalic spring when the there was a call for heat. This had the effect of turning off the heat sooner,
My current stat does know the BTU's of the furnace and the cooling system, the outside temp and the time. I'll bet it uses this info to do a better job of not overshooting the setpoint when in setback mode. The stat is also smart and is able to report furnace errors and save history. It can report blower RPM and CFM values.