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Diagnose & fix heat that won't turn off when it should: here we discuss various causes of too much heat or of a heating system that does not shut off whenit should.
We explain how to diagnose and fix thermostat switch settings, switch settings right at the air conditioner or furnace air handler, and we review thermostat wiring problems that cause a heating system or cooling system to stay ON when it should not.
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 also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Thermostat Troubleshooting: heating or cooling system won't turn OFF
Here we discuss how to diagnose & fix a thermostat that keeps calling for heating or cooling when it should not.
A separate article, HEAT WON'T TURN OFF, gives diagnostic procedures for other reasons why building heat wont' turn off besides problems at the thermostat itself.
If the heating system does not stop 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.
If the cooling system wont' turn OFF, that is if cool air keeps being delivered even when you don't want it, we do the opposite: set the thermostat to its highest temperature, or if your thermostat has a HEAT OFF COOL or FAN AUTO OFF control, set the control to OFF or HEAT.
If the cooling system stops then the problem is most likely cause of the problem was the thermostat setting,
the THERMOSTAT CALIBRATION, (including placement on the wall)
If the cooling system does not stop 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.
Step 2: confirm that the problem is in the thermostat or its wiring
Forced Warm Air Heat Won't Stop
If your heating system is by forced warm air (a furnace system) then check two switches that can cause the blower fan to run continuously even if the furnace is not heating the air:
FAN WONT STOP - THERMOSTAT SWITCH - separate switches right on the room thermostat (photo at far left) can turn the blower fan on continuously. If you are having trouble finding switches on your thermostat
FAN WONT STOP - LIMIT SWITCH - a manual push-pull switch right at the limit control in the air handler (photo at close left) can set the blower fan to run continuously. If you are having trouble finding the blower assembly limit switch
While you may find good reason to keep the blower fan running continuously (discussed
at BLOWER FAN CONTINUOUS OPERATION) for now, to diagnose the problem make sure that both of these switches are set to FAN OFF.
Watch out: not all forced warm air furnace heating systems have a Manual-On switch on the limit switch, and not all thermostats include a FAN-ON switch on the thermostat.
All Heating Systems: forced air, warm air, hot water, steam, heat pump, electric: stuck "on" step 2 - continued
Disconnect the thermostat from the heating or cooling system - at the heating or cooling equipment.
It's easy to isolate the room thermostat and its wiring from the heating or cooling system: we simply remove the two thermostat wires right at the primary control for the heating or cooling system involved. Typically the two thermostat wires are red and white and are connected in the controller to terminals T1 and T2.
Our photo (left) shows the red and white thermostat wires connected to a heating boiler aquastat control - look at the top left corner of the photo. [Click to enlarge this or any photo or chart seen at InspectApedia.com]
If heating or cooling system keeps running (more than a couple of minutes if you have warm air heat), something else is wrong. Since we have eliminated the thermostat we need to check for damaged or shorted thermostat wires. Continue to STEP 4 below.
OK so it's not always as trivial as I just said. Multi-zoned heating systems may mean you have to follow the individual thermostat's wires to find just which control they operate.
If you are diagnosing a heating system that is controlled by a single room thermostat, then there will be one wall thermostat, one pair of wires that run between the thermostat and a primary heater or air conditioner controller, and one primary controller at the heater or air conditioner.
An air conditioner or heat pump will be controlled by thermostat wires connected to a control board or relay in the air handler and/or compressor/condenser unit.
But in multi-zoned heating systems there may be multiple hot water zone valves or multiple air duct zone dampers. In those systems you will need to follow the individual problem thermostat wires to the specific controller such as an individual hot water heating zone valve to make the disconnection we discuss here.
Keep in mind that a zone valve can become stuck in the OPEN position or it may even be manually latched "open" to continue to allow heat to flow.
Our photo above shows a rats nest of wires at an older hot water heating system zone valve. Two of these, red and white, will be coming from the thermostat and will tell the zone valve to open on a call for heat. Additional wires from the zone valve tell the circulator pump to run.
Many room thermostats for heating and cooling have more than two wires connected to the thermostat. But in just about every case, just two wires are actually being used to turn the heater, air conditioner, or heat pump ON or OFF.
Disconnect the thermostat wires at the thermostat: when the heat is on and running and the thermostat is set to say 68F, when the room reaches 68F (measured at the thermostat on its scale), just disconnect the thermostat wires right there at the thermostat. (See our warning at Step 2.)
Our photo (left) 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.
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.
Some room thermostats include a "safe" mode that will call for heating at a pre-set temperature should the system lose power. If the thermostat requires batteries to operate, be sure that it has new fresh batteries installed.
For cooling problems check that you were setting the thermostat all the way UP and that room temperature is BELOW the SET temperature.
If the heating or cooling system does not stop, after you disconnected the thermostat wires at the thermostat, then disconnect the thermostat wires at the heating or cooling equipment again as we described at
If the heating or cooling system stops then most likely the thermostat wiring is damaged, probably shorted together. Shorting the thermostat wires together is the same as a thermostat calling for the heating or cooling system to run.
If you cannot find and fix the shorted thermostat wire fault (check first around the wall thermostat and around the primary controller at the other end of the same wires) you'll need to re-wire the thermostat.
If the heating or cooling system does not stop then the problem is not the thermostat and not the thermostat wiring. Go to Step 4
Step 4: what to do when the problem is not in the thermostat or thermostat wiring
Turn Off Electrical Power to the Heating or Cooling Equipment. Call your heating or air conditioning service company for further diagnosis and repair help.
Watch out: Don't cycle heating or cooling equipment ON and OFF rapidly using the electrical power switch or any other control.
For heat troubleshooting, wait at least five minutes between on-off cycles;
For cooling system troubleshooting wait at least fifteen minutes to minimize the risk of equipment damage.
On rare occasions malfunctioning controls on heating equipment may continue to feed fuel to an oil or gas burner, requiring that fuel also be shut off. Turn off the fuel supply following instructions at OIL or GAS SHUTOFFS.
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 sticking and keeps heat on even after the thermostat has said "enough".
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
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.
Examples of Diagnosing Too Much Heat or Heat Won't Turn Off - starting at the room thermostat
Reader Question: how do I fix a thermostat that I have to set lower than the level of heat I want?
My traditional honeywell simple dial-type thermostat on the wall has two pointers and two temp. scales. However I have to set it on 60 degrees to get heat temp to 70 degrees.
What can I do? It was working fine until my condo changed the air/heating system this summer. - Jeannette 1/23/2013
Reply: check these reasons for a room thermostat that keeps calling for heat
On most two-scale room thermostats, one scale reads the current or actual room temperature while the other scale is the set temperature - and is adjustable. When you move or adjust the thermostat to call for a different room temperature you will see the scale pointer move to your new setting on the "set" scale.
You are describing having to set the thermostat lower (60F) than the desired temperature (70F) to get the proper level of heat in your home. In other words, you say that the room gets too hot - hotter than you asked-for. Here are some things that would cause that problem:
The thermostat or its wires have been damaged and are not reliably signaling the heating system primary controller.
I wouldn't normally place this guess first, but as you think the problem began after someone worked on the system it's worth a check.
Look for a loose connection or intermittent short in wiring. For example if two thermostat wires short together that's the same as constantly calling for heat.
The thermostat is being blocked from properly sensing room heat. That could be due to movement of the warm air supply registers to a new, more distant location, or more often, due to furniture, drapes, or even dust and crud that block the air inlet openings around the thermostat that allow it to sense room temperature.
Dust can also interfere with moving parts in older spring-type thermostats.
The thermostat is tilted out of level. If the thermostat is an older mercury-bulb type then it needs to be level on the wall.
The thermostat is located on a cold outdoor wall or where cold air blows on it; if your thermostat was not moved and is on an interior wall and in a draft of cold air, this isn't likely to be the cause of the problem you describe.
The primary controller on the heating system is not working properly; on occasion we find that the primary control on a heating system, for example an internal relay, is sticking and keeps heat on even after the thermostat has said "enough".
This is easy to check. When the heat is on and running and the thermostat is set to say 68F, when the room reaches 68F (measured at the thermostat on its scale), just disconnect the thermostat wires right there at the thermostat.
If heat keeps running (more than a couple of minutes if you have warm air heat), something else is wrong.
There is some other problem we haven't thought-of.
Reader Question: possible shorted thermostat wires
Our furnace thermostat failed after an adjacent water heater installation. I noticed that wires were pinched between the water heater and the gas pipe.
After I loosened the safety straps and freed the wire, the thermostat works. (I reset the furnace).
Were squished wires the cause of a temporary short and will the wires keep working if they are undisturbed? - Larry 8/2/11
Reply: check for shorted wires and for thermostat transformer damage as well as for blown fuse
Yes it's surely possible that your thermostat wires were shorted.
Now a short in those wires sometimes lets you off the hook with no trouble, since the wires and thermostat are basically an "on-off" switch that calls for heat (or no heat).
But a thermostat wire short at the transformer could burn out the low voltage transformer that powers the thermostat and heating controls or maybe even damage the thermostat itself.
Find and check the low voltage transformer (often it's mounted near your furnace, on an electrical junction box, or it may be integral in your heater control unit).
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Question: we replaced the thermostat and now our living area is too hot - and the heater won't turn off
Our living area has been really hot and the old thermostat did not have an ON/OFF switch. We changed the thermostat today and switched it off but the heater is still on. We cannot find a reason why and we have verified the wiring and all is correct. Is it possible that something else may not be working? Maybe the transformer or some kind of regulator?
First disconnect the thermostat wires right at the heater itself, at it's control. That's the same as turning down the thermostat. If the heater stops then you know the trouble is in the thermostat wiring thermostat settings, is the thermostat itself.
As your problem showed up after installing a new thermostat, I would be sure to check that you made the right connections, that the thermostat switches are properly set, AND that the thermostat wires were not accidentally shorted together.
Question: heat keeps running even if I turn off the thermostat
(June 7, 2014) Billy-Joe said:
My old style t86a thermostat wont shut off normally. the only way to shut it off is to crank the heat to the max. I switched the wires around and still the same problem its a T 86a Honeywell thermostat.
(Nov 22, 2014) Chuck said:
heat continues to run even after turning down thermostat.Temperature reached 80 degrees and then I turned off breaker that shut unit down. Should I replace the thermostat?
I'd look for thermostat wires that have become shorted together. Disconnect and remove the thermostat from the wall.
Then if heat still runs disconnect the thermostat wires at the heater-end.
If heat still runs then the problem is in a control board or relay at the heater.
Before replacing the thermostat check for thermostat wires that are shorted together.
Question: blank display on thermostat
(June 16, 2014) ijay said:
my thermostat shows no display
When a room thermostat display goes blank the usual reason is that electrical power is off to the heating or cooling system to which the thermostat is connected. Also check for a dead internal battery in the thermostat.
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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
Toll Free (U.S.): 866-IP-LIVING (866.475.4846)
Fax: +1.978.692.3378 - Email: email@example.com or Customer support: firstname.lastname@example.org Website: 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: email@example.com. 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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