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Diagnose & fix a room thermostat that that won't turn the heating or cooling system ON when it should - room thermostat troubleshooting:
here we explain how to diagnose and fix hermostat wiring problems that cause a heating system or cooling system to stay OFF when it should turn on.
We provide a sequence of diagnostic steps that determine if the problem is at the thermostat, the thermostat wiring, or at other heating or cooling system controls. We include some simple VOM tests or DMM tests to check for a broken or "open" thermostat wire and tests to check for thermostat wires that are shorted together.
Thermostat Troubleshooting: heating or cooling system won't turn ON - Step 1
Here we discuss how to diagnose & fix a thermostat that seems to refuse to call for heating or cooling when it should. We explain how to rule out the thermostat in a "NO HEAT" diagnosis procedure.
If you're confident that the thermostat is not the problem, the see this separate article, HEAT WON'T TURN ON where we give diagnostic procedures for other reasons why building heating or cooling system will not turn on when it should.
Change the thermostat setting up or down
For heating: if the heating system won't turn ON, that is NO heat (or cooling) is being delivered when you want it, set the thermostat to its highest temperature setting and wait three to five minutes. If your thermostat has a HEAT OFF COOL or FAN AUTO OFF control, set the control to HEAT (for heating) and set the FAN to AUTO.
If the heating system does not start continue to STEP 2 below.
Watch out: for warm air heat blower units behave differently than hot water or steam heat systems: to avoid damage to the heat exchanger the air handler or blower unit will normally keep running for a minute or even a few minutes after the thermostat has stopped calling for heat.
So for warm air heating systems, give the system a few minutes to shut down.
For hot water and steam heating systems, because the heating baseboards, convectors, or radiators will be hot and because they have some thermal mass, heat continues to be delivered for a longer interval even after the heating system itself has stopped.
Therefore to check whether or not setting the thermostat temperature down to its lowest setting (or to OFF) has turned OFF the heating system you need to listen to the heating unit itself to observe that it has stopped.
For air conditioning: if the cooling system wont' turn ON, that is if no cool air is being delivered even when you want it, we do the opposite: set the thermostat to its lowest temperature, or if your thermostat has a HEAT OFF COOL or FAN AUTO OFF control, set the control to COOL and FAN to AUTO.
If the cooling system does not start continue to STEP 2 below.
Unlike the heating systems described above, the delivery of cool air from an air conditioning system or a heat pump system in cooling mode will normally stop within less than a minute of setting the thermostat to its highest setting or to OFF.
Thermostat Troubleshooting Step 2: confirm that the problem is in the thermostat or its wiring
Remove the thermostat face or body to expose the thermostat wires themselves. If you simply connect the two thermostat wires, usually the red and white (for heat), the heating system should start or the zone valve open or boiler or furnace start operating.
When you connect the thermostat wires you're doing the same thing that the thermostat does itself when it "calls for heat".
Our photo shows the simplest case: a red and white wire from the wall thermostat turn a heating unit on (if they are connected by the thermostat or manually by touching them together) or off if they are disconnected.
Pulling the thermostat off of its mounting plate in this instance accomplishes "disconnect the thermostat wires at the thermostat" instruction - you can see that the red and white wires simply end at individual screw terminals.
(Those terminals are contacted and connected or not by the thermostat when it is restored in its normal position).
Since a room thermostat is basically an ON-OFF switch that calls for heating or cooling in response to room temperature, when we disconnect the thermostat itself, that's the same as NOT calling for heating or cooling.
If when you connect the red and white wires the heating system operates then the problem was the thermostat itself.
If when you connect the red and white wires the heating system does not operate then we suspect a problem with the thermostat wires (a wire is cut or open), or with the wire connections at their other end - at the heating system.
Thermostat Troubleshooting Step 3: check for damaged thermostat wiring: shorts or open wires
Reader question: the thermostat won't trigger the boiler nor the zone valve
I have a hot water heating system with oil furnace for heating water. I replaced the old low voltage thermostat (circular mercury one) with low-voltage Honeywell RTH 5+2 day programmable thermostat. There are only two wire white and red which I correctly connected to the new thermostat.
Now I do not have heat upstairs as thermostat does not trigger either boiler or the zone valve. I called Honeywell tech support and they asked me to check the voltage across the wire and I did. The voltage is between 8-10 VAC.
The rep at Honeywell told me that for RTH to work the voltage should be in the range of 20-30VAC. Later, I checked the transformer and it showed me a voltage of 24VAC.
I tried manually opening the zone valve to see if it will trigger the system but no avail. I have old thermostat in the basement when I turn that to higher temperature the system seems to run fine for the basement (different zone valve for the basement).
I am wondering if old thermostats ran on very low voltage or there is substantial loss in the voltage for the upstairs or the zone valve is gone bad.
Also, I am wondering for upstairs if I remove the thermostat and connect two wires together then the circuit must be complete and the system must trun on continuously?- Akram Khan, Fredericton, New Brunswick, Canada. 12/4/2013
Reply: thermostat wire integrity tests: check for open thermostat wires & for shorted thermostat wires
I suspect you may have mis-wired the thermostat connections at the thermostat, the zone valve, or the boiler, but assuming you've first double-checked those connections, we'll check out the thermostat and its wires for a problem.
Since you found 24V at the transformer (LOW VOLTAGE TRANSFORMER TEST) but not at the wires at the thermostat one suspects that there is a bad connection or perhaps even a partial short circuit between the wires somewhere in their passage.
Here are a few thermostat wire integrity tests you might try:
Disconnect the thermostat wires at the heater end and disconnect them from the thermostat upstairs at their other end.
Tie the thermostat wires together at one of those two locations.
Measure resistance (OHMS) using your VOM or DMM at the other end - between the two wires. It should be close to zero - showing continuity.
Now disconnect the thermostat wires where they were tied together - so both ends of the pair of wires are unconnected to anything.
Measure resistance again between the two wires - you should get infinite resistance - no current flows. If you find anything less there is probably a short between the wires.
Finally, yes, if the thermostat wires are intact and un-damaged throughout their length, then presuming the wires are connected properly at the heating system controller, if at the thermostat end you simply connect the two wires together, that should turn on the heat - you are doing exactly what the thermostat itself would do when room temperature falls below the set point and the thermostat calls for heat.
Watch out: Don't cycle heating or cooling equipment ON and OFF rapidly using the electrical power switch or any other control. For heat, wait at least five minutes between on-off cycles; for cooling wait at least fifteen minutes to minimize the risk of equipment damage.
The primary controller on the heating system may not be working properly; on occasion we find that the primary control on a heating system, for example an internal relay, is refusing to pull "in" to turn heat on even though the thermostat has said "gimme heat!".
Depending on the type of heat your building uses your service person will check one or more primary or additional heater controllers for a stuck relay or other defects.
Details about the heating or cooling device controller operated by the thermostat depend on which type of equipment the thermostat is controlling, and are found at
If the warm air heating or air conditioning or heat pump air handler, blower unit, or fan won't start, see these two diagnostic procedures:
FAN WONT STOP - LIMIT SWITCH - a manual push-pull switch right at the limit control in the air handler can set the blower fan to run continuously to at least confirm that the blower motor and fan can operate
Watch out: as we warned earlier, warm air heat blower units behave differently than hot water or steam heat systems: to avoid damage to the heat exchanger the air handler or blower unit will normally keep running for a minute or even a few minutes after the thermostat has stopped calling for heat.
For warm air heating systems, give the system a few minutes to shut down.
Continue reading at AIR CONDITIONER WON'T START if you're having a cooling problem, or see HEAT WON'T TURN ON if you're having a heating problem, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Questions & answers or comments about heating, air conditioning, and heat pump thermostat installation and wiring
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 Thanks to reader S.R. for discussing loss of heat due to a thermostat wiring mistake, October 2010
 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: email@example.com Marketing: firstname.lastname@example.org Customer support: email@example.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.
 "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
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. 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 & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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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