Fan operation switch setting in thermostat (C) Daniel FriedmanThermostat Internal or Hidden Switch Settings
How to Set the Thermostat Cycle Rate Switches, Fan Operation Switches, Emergency Backup Heat, Reset, & other switches inside of room thermostats
     


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This article describes the purpose and setting of tiny switches found inside of some room thermostats. These little slide or toggle switches may control the heating system on-off cycle rate or burner-on time in a manner similar to the heat anticipator feature found in other room thermostats.

Here we describe various types of internal or "hidden" switches found in some wall thermostats and in a table form we summarize their proper setting.

In most cases you can safely install a new room thermostat without worrying about these switches - just leave the switch or switches in the factory-set position. But in some circumstances setting the switches correctly may involve a change .

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How to Set the Cycle Rate or Fan Operation Switches on a Room Thermostat

Here we describe various types of internal or "hidden" switches found in some wall thermostats and in a table form we summarize their proper setting. In most cases you can safely install a new room thermostat without worrying about these switches - just leave the switch or switches in the factory-set position. But in some circumstances setting the switches correctly may involve a change .

How to Set the Room Thermostat Cycle Rate Switches (set switch to ON or OFF)

Termostat cycle switch settings (C) Daniel Friedman Heating System Type
Switch 1
Switch
2
Steam or gravity-hot water
ON
ON
High efficiency forced warm air (90%+ efficiency), hot water, or heat pump system
OFF
ON
Gas or oil forced warm air (this is the factory switch setting)
OFF
OFF
Electric forced warm air heat
ON
OFF
 

How to Set the Room Thermostat Fan Operation Switch (slide switch to left or right)
Example: Honeywell RTH2300 / RTH221 series wall thermostats

Fan operation switch setting in thermostat (C) Daniel Friedman Heating System Type
Switch Position


Gas or oil (this is the factory switch setting) for systems that control the fan in a call for heat)

 

LEFT
Electric heat or heat pump (for systems that allow the thermostat to control the fan on a call for heat - a fan wire is connected to the "G" terminal)
RIGHT
 

How to Set the Room Thermostat Fan / Electric Switch (slide switch to F or E)
Example: Honeywell CT2700 Electronic Round Programmable Thermostat

Thermostat F or E switch (C) Daniel Friedman Heating System Type Switch Position

 

Factory = (Gas or oil fuel) This is the factory default position and will be correct for most heating systems

 

F (left)
Electric = (Electric heat 1) Use this position if this (low voltage) thermostat is controlling electric heat such as an electric furnace or electric boiler. The E setting allows the fan (if one is used) to turn on immediately with the heating or cooling equipment in a system in which the G thermostat wiring terminal is used.
E (right)
 

How to Set the Room Thermostat HVAC Type & Heat Source Switches
Example: 3M Filtrete™ 7-Day Programmable Thermostat Model 3M-22

Watch out: the Filtrete™ 3M-22 is a battery-operated thermostat. If the batteries fail the thermostat may fail to provide heating or cooling and the building may suffer accordingly.

Filtrete 3M Thermostat switch settings (C) InspectApedia HVAC Switch Selections HVAC Type
Left Switch
Heat Source
Right Switch
NORM = conventional gas, oil, electric heat
This is the factory setting (blue arrow)
NORM (left)  
HP = heat pump HP (right)  
GAS = conventional gas or oil heat or heat pump with auxiliary gas or oil heat
This is the factory setting (red arrow)
  GAS (left)
ELEC = normal electric heat or if you have a heat pump with auxiliary electric heat   ELEC (right)
Emergency heat switch, 3M Filtrete thermostat (C) Daniel Friedman Emergency (Backup) Heat Switch for Heat Pumps
3M-22
Normal Emer-gency

An emergency backup heat control switch is provided for the 3M Filtrete™ 7-Day Programmable Thermostat Model 3M-22. This switch is located at the top right side of the thermostat beneath the removable top cover.

If you have set this thermostat for HP (heat pump - see the HVAC switch selections explained above), then an EMER or emergency function is available. This is a manual override switch.

Set this switch to OFF (right) for normal operation.

If you set this switch to ON or EMER (left) this will disable the heat pump and will mean that you are relying on your auxiliary heat or backup heat system only. This makes sense if your heat pump is in trouble or inadequate and you want to force backup-heat to be used instead.

Watch out: as purchased, in our thermostat we found this switch had been set to ON (to the left) as you can see in our photo.

OFF



(right)

EMER
=
ON

(left)
Thermostat reset switch, 3M Filtrete thermostat (C) Daniel Friedman Thermostat Reset Switch
3M-22
   

The 3M Filtrete™ thermostat also includes a hard reset button under the top cover and located to the right of the two HVAC switches discussed above.

If your thermostat is not working properly, and provided you've already checked the obvious faults such as no power, bad batteries, loose wiring, improper settings, or manual switches in the wrong position, then you might try pressing this button.

This will re-boot the 3M-22 thermostat without losing its pre-programmed settings.

An alternative and more involved hard reset of the thermostat could also be performed by removing its batteries and disconnecting its thermostat wiring.

Button

Press once to reset the thermostat

 

How to set the Swing Rate or HVAC Cycling Rate on a Programmable Thermostat
Example: 3M Filtrete™ 7-Day Programmable Thermostat Model 3M-22

3M Filtrete 3M-22 Thermostat Swing Rate Cycle Rate setting (C) Daniel Friedman Swing or Cycle Rate Setting
3M-22
Factory Range

The 3M Filtrete 3M-22 thermostat provides an optional adjustment to the HVAC cycling rate that the company calls Swing Rate: the desired variance in temperature between the 3M-22 setting and the room temperature before the heating or cooling system will turn on. The swing rate can be varied from 0.5o to 2.0o F.

Press SWING found under the DAY icon (second button from left, top row). Press the + / - buttons to select the desired swing from 0.5o to 2.0o F. Then press the Home button (leftmost button, bottom row) to store the setting.

Watch out: unless you have bright light and good eyes the names, labels, and functions of the forest of setting buttons on this thermostat are quite difficult to read. Check to be sure you are pressing the correct button.

Watch out: this is a battery-operated thermostat. If the batteries fail the thermostat may fail to provide heating or cooling and the building may suffer accordingly.

+/-
1o F
+/-
0.5o -2.0o F
 

How to Change the Temperature Readout Calibration on a Programmable Thermostat
Example: 3M Filtrete™ 7-Day Programmable Thermostat Model 3M-22

How to calibrate the 3M-22 room thermostat (C) Daniel Friedman Thermostat temperature display calibration
Thermostat engineers showing off or gone wild
Factory Range

The 3M Filtrete 3M-22 thermostat also provides an optional adjustment to its temperature readout described as "calibration". The thermostat is factory calibrated to an accuracy of +/- 1o F but users can deliberately change the temperature display readout to be "off-calibration" by +/- 6o F.

The 3M-22 operating guide suggests you might want to do this to make the thermostat "agree" with another in the building or "agree" with the old thermostat that it is replacing - reasons that we consider ... well, frivolous.

Imagine the confusion down the road when the next user goes crazy fiddling with the thermostat because it's "off" in temperature reading.

There might be a more subtle reason that you'd deliberately change the temperature readout to an inaccurate number, following thinking similar to changing the swing cycle discussed just above.

If the temperatures for the area being controlled by this thermostat always overshoot substantially past the thermostat's set point - something that might happen if the heat is provided by heavy cast iron radiators or baseboard - you could fudge the system by telling the thermostat that the room is warmer than it is.

Similarly if a bad thermostat mounting location means it turns off the heat while the space is still too cold you might be tempted to fudge the calibration rather than moving the thermostat.

Frankly we think changing the calibration on a thermostat is usually asking for trouble and confusion. Leave the calibration where it was set by the factory.

As we explain at THERMISTORS this thermostat is capable of very good accuracy.If your building temperatures overshoot, try adjusting the swing cycle discussed just above rather than messing with the thermostat's calibration.

+/-
1o F
+/-
6o F
       

Notes:

These examples will not necessarily fit other brands or models of room thermostats. Consult the installation and operating instructions for your individual thermostat. Contact us by email if you are unable to find instructions for your room thermostat.

At HEAT ANTICIPATOR Operation we explained that on some room thermostats internal switches are used to control the heating system on-off cycling rate, sensitivity, or fan functions in a manner that provides benefits similar to the heat anticipator found on other room thermostat devices.

 

Continue reading at THERMOSTAT SWITCHES, EXTERNAL or select a topic from the More Reading links shown below.

Or see HEAT ANTICIPATOR Adjustment

Or seeTHERMOSTAT DIAGNOSTIC GUIDE

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THERMOSTAT SWITCHES, INTERNAL at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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