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Thermostat Internal or Hidden Switch Settings
Set Thermostat Cycle Rate Switches, Fan Switches, Emergency Backup Heat, Reset, & other
switches hidden inside of room thermostats
- POST a QUESTION or COMMENT about how & when to set the internal dip or slide switches inside the room thermostat
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 require a change.
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- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?
How to Set the Internal Switches on a Room Thermostat
Here, using several popular room thermostat brands and models, we describe various types of internal or "hidden" switches found in some wall thermostats.
In 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 and an improvement to how your heating or air conditioning system works.
Watch out: your thermostat may have none or only some of the internal switches we describe below. So check the thermostat manual for your device and then check the settings we list below to find more-detailed explanation about what each setting does.
Article Contents
- THERMOSTAT CYCLE RATE SWITCHES - CT87: choose heating system type: Steam, Hot Water, Heat Pump vs GasOil/Electric Forced-air (Factory setting)
- THERMOSTAT FAN SWITCH - HEATING TYPE Example 1 - RTH2300 / RTH221: Gas/Oil (Factory) vs Electric or Heat Pump
- THERMOSTAT SWITCH - HEATING TYPE Example 2 - CT2700 F (Factory) = gas/oil vs E (Electric)
- THERMOSTAT SWITCH - HEATING TYPE Example 3 - T87K F or E: Steam, Hot Water, Warm Air, Heat Pump, Gas/Oil/Air, Electric Air
- THERMOSTAT SWITCH for HEATING TYPE FAN OPERATION - T87K F or E: Gas/Oil (Factory) vs Electric or Heat Pump
- THERMOSTAT SWITCHES - 3M Filtrete: Norm, HP, Gas, Electric
- THERMOSTAT SWITCH for BACKUP HEAT - 3M-22
- THERMOSTAT RESET SWITCH - 3M-22
- THERMOSTAT SWING SETTING / CYCLE RATE - 3M-22 Filtrete
- THERMOSTAT CALIBRATION - 3M-22 Filtrete Calibration Settings to adjust thermostat accuracy
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Thermostat Cycle Rate
How to Set the Room Thermostat Cycle Rate Switches (set switch to ON or OFF)Example: Honeywell CT87 Thermostat |
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Heating System Type | 1 |
2 |
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| Steam or gravity-hot water | ||||
| High efficiency forced warm air (90%+ efficiency), hot water, or heat pump system | ||||
| Gas or oil forced warm air (this is the factory switch setting) | ||||
| Electric forced warm air heat | ||||
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Thermostat Fan Operation: Heating Type - Gas/Oil vs Electric or Heat Pump
How to Set the Room Thermostat Fan Operation Switch (slide switch to left or right)
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Heating System Type | |||
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| 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) | ||||
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Thermostat Switch for Heating Type: Gas/Oil vs. Electric
How to Set the Room Thermostat Fan / Electric Switch (slide switch to F or E)
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Heating System Type | Switch Position | ||
Factory = (Gas or oil fuel) This is the factory default position and will be correct for most heating systems
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| 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. | ||||
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Thermostat Internal Switch: Heating Type, detailed choices
Honeywell T87K Internal Switch Set for Heat Type: Steam, Hot Water, Warm Air, Heat Pump, Gas/Oil/Air, Electric Air |
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Heating System Type - Internal Cycle Rate Switches | |||
Internal Cycle Rate Switches |
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| Switch 1 Position | Switch 2 Position | |||
Steam Heat:
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ON |
ON |
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| High Efficiency (90% or more) warm air (furnace) | OFF |
ON |
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| Gas or Oil fired warm air (furnace) [Conventional furnaces - this is the factory setting] | OFF |
OFF |
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| Electric warm air (furnace) | ON |
OFF |
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T87K Thermostat Internal Switch: Fan Control for Gas/Oil vs Electric
Honeywell T87K Internal Switch Settings for Fan Control: Gas/Oil vs Electric/Heat Pump |
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Heating System Type - Internal Fan Control Switch | Internal Fan Control Switch | |
| Gas or oil fired heating systems [this is the factory setting] | F |
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| Electric or heat pump system Note 1 | E |
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Notes to the table above
1. This setting is for systems that allow the thermostat to control the fan in a call for heat, if a fan wire is connected to the G terminal.
2. Thanks to reader Fred Gruber for asking about switch settings in the CT87K, 6 March 2016. Fred asked:
Please just tell me which settings give the longest cycle on the CT87K.
Reply:
Fred, There is no traditional heat-anticipator adjustment as was found on some older thermostats we describe in this article series. We should simply follow Honeywell's recommendations for the CT 87K thermostat
In the CT87 owner's manual given below you'll see that the CT87N is for 24 Vac heating and cooling systems while the CT87K is for 24 Vac heating systems - that is the N model controls both heating and air conditioning.
[Thanks to InspectApedia.com Paul for correcting our mix-up on which thermostat is which, 29 Nov. 2019]
See THERMOSTAT INSTALLATION & WIRING MANUALS
3. Illustration and table above adapted from Honeywell's CT87 Owner's manual cited here: Source: Honeywell Owner's Manual, CT87N / CT87K Round Thermostat, [PDF] Honeywell International, Inc., 1985 Douglas Drive North, Golden Valley MN, 55422 or in Canada: 35 Dynamic Drive, Toronto, Ontario M1V 4Z9, Canada, Website: http://yourhome.honeywell.com retrieved 2016/03/06.
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3M Filterete Thermostat Internal Switch Settings
How to Set the Room Thermostat HVAC Type & Heat Source Switches
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HVAC Switch Selections | HVAC Type Left Switch |
Heat Source Right Switch |
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| 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) | |||
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3M-22 Thermostat Heat Pump Backup Heat Switch
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Emergency (Backup) Heat Switch for Heat Pumps Example: 3M-22 |
Normal | Emergency | |
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
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EMEr = On (left) |
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Thermostat Reset Switch
Example: 3M22 Room Thermostat reset.
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Thermostat Reset Switch 3M-22 |
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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 |
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Programmable Thermostat Swing Rate / Cycle Rate Setting
Example: 3M-22 Filtrete Thermostat swing rate setting in degrees F.
How to set the Swing Rate or HVAC Cycling Rate on a Programmable ThermostatExample: 3M Filtrete™ 7-Day Programmable Thermostat Model 3M-22 |
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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 |
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Definintion of Thermostat Swing Temperature
A thermostat's Swing is the sensitivity of its response to changes in temperature. On a typical room heating or cooling thermostat the thermostat swing is one half of a degree or +/- 0.5°F.
That means that when the temperature drops 0.5°F below the thermostat's set temperature the thermostat will turn heat ON and when temperature riese to about 0.5°F above the thermostat's set temperature the thermostat will turn heat OFF.
We set the swing at a thermostat to avoid un-wanted rapid "on-off" cycling of the heater - a condition that can lead to heat or A/C equipment damage or failure and thus loss of heating or air conditioning in a building.
Some thermostats include controls that allow you to explicitly set the swing temperature.
Other thermostat include an indirect control that affects its swing adjustment by specifying the fuel (electricity, gas, oil, solar, etc) and the heat top (hot water or forced warm air).
Still other thermostat do not have any adjustment to the swing temperature range.
An example of a thermostat on which the swing can be set is this FCP Non-Programmable and Programmable Fan Coil or PTAC Thermostat User Guide [PDF] from Johnson Controls. - retrieved 2025/04/21 (local copy saved as FCP-Thermostat-Manual-Johnson-Controls.pdf)
Excerpts:
The FCP-NA-701 and FCP-PA-701 are non-connected Fan Coil Unit (FCU) and Package Terminal Air Conditioning (PTAC) Thermostats. ...
The FCP thermostats are compatible with 2-pipe and 4-pipe FCU, conventional PTAC, and heat pump PTAC with or without auxiliary heat. ...
The FCP Programmable Fan Coil or PTAC Thermostat without Fan (FCP-PA-701-NF) is a new FCP model designed for 2-pipe or 4-pipe FCU with independent fan control, or radiant type under floor heat-only applications that do not require a fan control output from the thermostat.
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Calibrate Thermostat Accuracy
How to Change the Temperature Readout Calibration on a Programmable Thermostat
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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 in ROOM THERMOSTATS 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. Also be sure to read HEAT ANTICIPATOR OPERATION |
+/- 1o F |
+/- 6o F |
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Notes to the table above
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.
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Continue reading at THERMOSTAT SWITCHES, EXTERNAL or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
Or see THERMOSTAT SWITCHES, INTERNAL FAQs - questions & answers posted originally at the end of this page.
Or see these
Recommended Articles
- HEAT ANTICIPATOR ADJUSTMENT
- HEAT ANTICIPATOR OPERATION - 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.
- MANUALS for HEATING & A/C SYSTEM CONTROLS
- THERMOSTAT DIAGNOSTIC GUIDE
- THERMOSTAT INSTALLATION & WIRING MANUALS
Suggested citation for this web page
THERMOSTAT SWITCHES, INTERNAL at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.
Or see this
INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS
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Citations & References
In addition to any citations in the article above, a full list is available on request.
- [3] Thank to Mr. Scott Meenen , G&S Mechanical Services , for providing some common thermostat wiring codes also found at Mr. Meenen's web page Malware Deleted 12/9/2014 . Mr. Meenan provides heating, heat pump, and air conditioning repair services in Maryland, Washington D.C., and northern Virginia.
He can be contacted at 301-591-1646 or by Email to Malware Deleted 12/9/2014 - 10/2010. Quoting:
We service American Standard, Amana, Arco, Arco-Air, Bryant, Carrier, Coleman Evcon, Comfortmaker, Day/Night/Payne, Dunham-Bush, Fedders, Fredrich, Goodman, General Electric, Heil, Intertherm, ICP, Janitrol, Lennox (Armstrong, Johnson Air-Ease), Miller, Modine, Nordyne, Rheem/Ruud/Weatherking, Sears, Stewart Warner, Trane, Weather King, Williams, White-Westinghouse, Whirlpool, Weil Mclain, York, (Frasier Johnson/Borg Warner) and others. - [4] Azel Technologies Inc., P.O. Box 53138 10 Royal Orchard Blvd. Thornhill, Ontario, Canada L3T 7R9 Ph: 905-223-5567 Fax: 905-223-3778 Email: info@azeltec.com, Website: www.azeltec.com.
- [5] Honeywell Controls, the company wants you to use their contact form at this web page: http://www51.honeywell.com/honeywell/contact-support/contact-us.html
Honeywell Consumer Products, 39 Old Ridgebury Road Danbury, CT 06810-5110 - (203) 830-7800
World Headquarters, Honeywell International Inc., 101 Columbia Road, Morristown, NJ 07962, Phone: (973) 455-2000, Fax: (973) 455-4807 1-800-328-5111- Honeywell product model numbers & instruction Manuals: see http://yourhome.honeywell.com/home/Applications/FindYourModelNumber.aspx
- [6] White Rodgers Thermostats and HVAC controls,
Homeowner information: http://www.emersonclimate.com/en-US/brands/white_rodgers/Pages/wr-homeowner-info.aspx
Contractor information: http://www.emersonclimate.com/en-US/brands/white_rodgers/wr_contractor_info/Pages/white-rodgers-contractor-info.aspx
White Rodgers Product Catalog (don't misspell the company's name as White Rogers Thermostats) -
http://www.emersonclimate.com/Documents/thermostats.pdf - Thermostat Catalog - [7] White Rodgers 1F90 Low Voltage Digital Comfort-Set thermostat Installation Instructions, PN 37-3654, White-Rodgers Division, Emerson Electric Co., 9797 Reavis Rd., St. Louis MO 63123
- [8] "Automatic Oil Burner Controls - Thermostats", Domestic and Commercial Oil Burners, 3rd Ed., Charles H. Burkhardt, McGraw Hill, 1969 (and later editions), ASIN B0000EG4Y8
- [9] Thermostat wiring color codes & conventions, Thanks to reader " Helpful Pointers" Regarding 24V T, 10/7/2012
- [10] Domestic Central Heating Wiring Systems and Controls, 2d Ed., Raymond Ward, Newnes, ISBN-10: 0750664363, ISBN-13: 978-0750664363,
- [11] Proliphix Corporate Headquarters [Website: proliphix.com] , 3 LAN Drive Suite #100, Westford, MA 01886 Phone: +1.978.692.3375 Toll Free (U.S.): 866-IP-LIVING (866.475.4846) Fax: +1.978.692.3378 - Sales: sales@proliphix.com Marketing: marketing@proliphix.com Customer support: support@proliphix.com http://www.proliphix.com/ - quoting from the company's website:
All Proliphix Network Thermostats come with our free Uniphy Remote Management Service. This unique offering lets you monitor and control your HVAC systems by simply pointing your Browser to our secure Proliphix Web Site. Enjoy the convenience of programming a thermostat from any location, using a simple graphical interface.
No computer equipment or software is required. And since Proliphix takes care of the network configuration for you, you’ll be up and running in no time.
We’ll even proactively monitor your thermostats and send you an immediate email or SMS message when an HVAC problem is detected. - [12] "Heating Control Handbook for the Installer and Service Man,Oil Burner, Gas Burner and Stoker Controls", Honeywell Corporation, March 1949 [copy on file as HoneywellControlsHandbookSA1399-2-1949.pdf] . Some of the controls discussed in detail here include the
- Honeywell T1 and T11A = Series 10
- Honeywell T21A (T2) = Series 20
- Honeywell T847A = Series 80
- Honeywell RA117A (RA1) = Series 10
- Honeywell LA101A = Series 10,
- Honeywell LA419A (LA4) = Series 40
- V155A = Series 10, V435A = Series 40, V575A = Series 50, V835A = Series 80
- [13] Trane TCONT800 Series Touch Screen Programmable Comfort Control Ownes Guide, American Standard, Inc., Troup Highway, Tyler TX 75711, January 2005, Telephone: Customer Service: 1-877-3381, website: www.trane.com
- In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
CONTINUE READING or RECOMMENDED ARTICLES.
- Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The HOME REFERENCE BOOK - the Encyclopedia of Homes and to use illustrations from The ILLUSTRATED HOME .
Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.
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