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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 .
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)
Heating System Type
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
How to Set the Room Thermostat Fan Operation Switch (slide switch to left or right)
Example: Honeywell RTH2300 / RTH221 series wall thermostats
Heating System Type
Gas or oil (this is the factory switch setting) for systems that control the fan in a call for heat)
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)
How to Set the Room Thermostat Fan / Electric Switch (slide switch to F or E)
Example: Honeywell CT2700 Electronic Round Programmable Thermostat
Heating System Type
Factory = (Gas or oil fuel) This is the factory default position and will be correct for most heating systems
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.
How to Set the Internal Switches & Fan Control in a Honeywell CT87K Thermostat
Heating System Type - Internal Cycle Rate Switches
Internal Cycle Rate Switches
Switch 1 Position
High Efficiency (90% or more) warm air (furnace)
Gas or Oil fired warm air (furnace) [Conventional furnaces - this is the factory setting]
Electric warm air (furnace)
Heating System Type - Internal Fan Control Switch
Internal Fan Control Switch
Gas or oil fired heating systems [this is the factory setting]
Electric or heat pump system Note 1
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.
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 24V Heating systems while the CT87K is for 24VAC heating and cooling systems - that is the K model controls both heating and air conditioning.
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.
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.
HVAC Switch Selections
NORM = conventional gas, oil, electric heat
This is the factory setting (blue arrow)
HP = heat pump
GAS = conventional gas or oil heat or heat pump with auxiliary gas or oil heat
This is the factory setting (red arrow)
ELEC = normal electric heat or if you have a heat pump with auxiliary electric heat
Emergency (Backup) Heat Switch for Heat Pumps
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.
Thermostat Reset Switch
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.
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
Swing or Cycle Rate Setting 3M-22
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.
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
Thermostat temperature display calibration
Thermostat engineers showing off or gone wild
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.
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.
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 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.
 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
 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
 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
 Thermostat wiring color codes & conventions,
Thanks to reader "
Helpful Pointers" Regarding 24V T, 10/7/2012
 Domestic Central Heating Wiring Systems and Controls, 2d Ed., Raymond Ward, Newnes, ISBN-10: 0750664363, ISBN-13: 978-0750664363, Quoting from Amazon.com:
This unique A-Z guide to central heating wiring systems provides a comprehensive reference manual for hundreds of items of heating and control equipment, making it an indispensable handbook for electricians and installers across the country. The book provides comprehensive coverage of wiring and technical specifications, and now includes increased coverage of combination boilers, recently developed control features and SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) boilers ratings, where known.
In addition to providing concise details of nearly 500 different boilers fuelled by electric, gas, oil and solid fuel, and over 400 programmers and time switches, this invaluable resource also features numerous easy-to-understand wiring diagrams with notes on all definitive systems. Brief component descriptions are provided, along with updated contact and website details for most major manufacturers.
 Proliphix Corporate Headquarters, 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: firstname.lastname@example.org Marketing: email@example.com Customer support: firstname.lastname@example.org 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.
 "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
 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
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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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