Heating & Cooling Thermostat FAQs set 2

FAQs on room thermostats.

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. Our page top photograph illustrates a traditional round Honeywell T87 room thermostat.

Room Thermostat Diagnostic FAQs set #2

These questions and answers were posted originally at THERMOSTAT DIAGNOSTIC GUIDE - be sure to see that article as it's the starting point for diagnosing and fixing problems with heating and cooling thermostats in buildings.

This photo shows an antique room thermostat still in use at the FDR Estate in Hyde Park, NY.

[Click to enlarge any image]

On 2017-03-19 by (mod) -The boiler may be running to keep a minimum temperature

The boiler may be running to keep a minimum temperature - especially if it's making domestic hot water with a tankless coil, or using an aquastat that is set up as if there WERE a coil even if you don't have one.

On 2017-03-19 by Amy melvin - my heater comes on when it wants to wake up in the middle of the night

How about my heater comes on when it wants to wake up in the middle of the night it's cold and below the temp I had it set for, so I set the thermostat below the temp in the house wait till it clicks then turn it back up waiting for heat to start never does finally give up and like two hours later I hear it come on.

What is the cause of this???

On 2017-03-06 by (mod) -

Timmy

Besides the general thermostat tests described inTHERMOSTAT DIAGNOSTIC GUIDE, further tests of a mini-spit air conditioning unit probably need an onsite service technician: the thermostat itself doesn't provide extended diagnostics of the air handler nor compressor that it is controlling.

In other words let's be sure that the thermostat itself is working properly first.

Then let's see that each of the controls the thermostat offers, such as temperature increase/decrease and fan controls will cause the AC or heat pump to respond properly. If it doesn't, start by being sure that a remote control thermostat (if you're using one) can "see" (has line of sight access to) the wall unit and that its batteries are good and its display is active.

On 2017-03-06 19:02:11.123071 by Timmy - CCHA

how to run a diagnostic through the thermostat on an American Standard mini split

On 2017-02-08 18:22:38.410396 by (mod) - replaced a Carrier Tstat with a Honeywell WIFI model TH8321WF1001.

Please post questions on just one page - thus avoiding doubling our work. Thanks - Mod.

Curious indeed. When a condenser fan won't stop running I suspect either a wiring error (even control wires shorted together) or a control or relay problem. What happens if you completely disconnect the thermostat wires at the air handler and / or compressor/condenser?

On 2017-02-07 15:27:51.440259 by Bob

Trying to replace a Carrier Tstat with a Honeywell WIFI model TH8321WF1001.

While in the programming, I set the reversing valve to O (O/B on Cool). Turn on the heat and it blows cold air. Switch to cool and hot air.

Feel pretty confident I have the Tstat wired correctly so changed the reversing valve setting to B (O/B on Heat). Provides the correct temp air - Cold on cool and Hot on Heat. But now once the room reaches demand temp in heat, the condenser fan will not stop running.

I can turn unit off at tstat and outdoor fans run. Only way to get them off is to turn off safety switch at outdoor unit. System is heat pump with 2 stage compressor. I have gas for AUX heat. and all seems to work correctly except outside fans will not shut themselves off when heat is no longer a demand.

On 2016-12-19 by Anonymous - Honeywell 6000 series programmable thermostat

I have a Honeywell 6000 series programmable thermostat. I've operated in manual mode successfully for several years.

Recently, the heat has stopped coming on when the set temperature is higher than the room temperature.

I've found that I can get it back by cycling the heat button on and off, but it only lasts 10 or 12 hours before reverting to a non-working state. I changed the batteries, and that made no difference. Should I replace the thermostat?

Jon

On 2016-11-23 by Jonthedow

My gas boiler runs every 3-4 hours it won't turn on when the thermostat is calling for heat. I changed my aquastat and it still running the same way. Like it has a mind if it's own. Guys any idea what can it be?

On 2016-11-20 by (mod) -

Kim:

Are you running a heat pump system and using its backup heat? If so I suspect a failed control or blown fuse.

On 2016-11-20 by Kim

Plz tell me easy fix! We just got home after 3 months in the truck. I forgot to turn the air off when we left.

No biggie because it was set at 75 so it wasn't running constantly. So got home and flipped off the air and turned on the heat. All was fine, heat ran for about 30 minutes and then a little later we noticed it was getting chilly.

So I tried to turn it back on and nothing not even emergency heat. Tried breakers and nothing. We have our unit serviced 2x a year so it's not been neglected.plz help and tell me it's an easy fix! Thank yoKim

On 2016-11-10 09:30:13.895207 by Tracy Hite

Could you call me I have a few questions please. Thank you:)

On 2016-08-15 22:43:54.116211 by (mod) -

Diana:

Guessing: when you touch the thermostat to change its setting, the display shows the SET TEMPERATURE that you are selecting.

When you step back to admire your work, in a second or two, the thermostat display will change to show the ACUTAL ROOM TEMPERATURE.

On 2016-08-15 18:03:24.199897 by diana wilson

thermostat set at 70 degrees. not programmed. then it jumps up to 75 mysteriously.
what is wrong?

On 2016-08-12 20:10:36.986429 by (mod) -

Chelly

When an LCD display has part of its letters or numbers missing I suspect that the display unit is failing; try new batteries and be sure that the thermostat has electrical power. Then try cycling through settings to watch the display. If it doesn't self-correct you probably need to replace the thermostat.

On 2016-08-12 03:16:33.983325 by Chelly

My thermostat is turning on its not showing amy signs on low battery but the numbers seem to be cut off so i jave no idea what temp i have the AC aet on. Right now its showing cool on aux on. Any idea what the problem may be. I think it may be down to low im freezing! But i have no way of knowing considering i cant read the numbers

On 2016-07-04 16:57:00.985943 by (mod) -

Good question, Lindsey, I'm not sure. If the location is exactly the same, ask if other conditions have changed such as the position of air supply registers - air blowing on or near the thermostat, thermostat in shade vs sun, cessation of air movement in a wall cavity; A 7 degree difference between 2 thermostats is indeed odd. Be sure you have reviewed the thermostat setting instructions for the new unit.

On 2016-07-04 12:41:21.685762 by Lindsey

We just replaced our rather old thermostat with a programmable Honeywell. Our old one we could easy set at 77 degrees and be fine, we have now had to set the new one at 70 degrees or lower to even get the house cool, what could be going on?

On 2016-06-14 14:40:54.329054 by (mod) -

If the room temperature increases after turning on your A/C system there are several possible causes including

- blowing hot air into the room from hot air ducts in an attic or other high-temperature space at the start of the cooling cycle

- blowing warm air into the room continuously by pulling return air from a warm location combined with an A/C system that is not cooling

- warm air leaks INTO the return duct system

- an external cause: e.g. sunlight on a thermostat

Search InspectApedia for LOST COOLING CAPACITY to read some diagnostic suggestions; I suspect your compressor/condenser is not working

On 2016-06-14 03:41:03.595094 by Karen

Cont....the temperature in my hallway will go from 85 up to 90 degrees...it never satisfies itself and turns off

On 2016-06-14 03:38:55.565862 by Karen

When I turn my air conditioner on, the temperature reading on the thermostat actually goes HIGHER instead of lowering. Example, it might be reading 85 degrees and w when I turn on my a/c, within about a half hour it will read that the temperature in my halfway had give up to

On 2016-06-04 14:46:02.634062 by (mod) -

If your A/C system is unable to keep up with the cooling demand, I would expect the indoor temperature to be higher than the set temperature on the thermostat. That's not a thermostat problem it's an A/C system problem that needs service.

If you mean that your indoor thermostat is showing a temperature that is higher than you think is accurate, it may be that it's exposed to sunlight or some external heat source. You might need to move it.

Thermostats typically operate by a temperture sensing thermistor (search InspectApedia for THERMISTOR to read details) or older units by a bimetallic spring that respond to air temperature right at or in the thermostat itself. Rarely we find dust clogging the air inlet of the thermostat -something else to check. Also search InspectApedia for THERMOSTAT ACCURACY

On 2016-06-03 23:29:58.350371 by MIKY

I have two new Lennox XC14 AC's and two XC25 Lennow air handler. The systems are controlled by a Honeywell Pro 4000 thermostat and a Focus Pro TH6320R. On cool days the thermostat shows indoor temperature same as the temperature setting.

On very hot days the indoor temperature shows as much as 10 degrees warmer than temperature setting. As a result the home is very warm. I was told the indoor reading is the RADIANT temperature and nothing can be done. Anyy suggestions?

On 2016-03-30 21:28:43.480923 by (mod) -

OK thanks.

If by A/C fan turns on you mean the outdoor compressor/condenser, if that's turning on in "heat" mode that might be correct IF your system is a heat pump.

If you mean the indoor air handler blower fan turns on at a call for heat but you get no warm air, just cold air, then there is a wiring or control board or relay error. IF the system was working properly before, I'd suspect a wiring error or perhaps wires accidentally shorted together.

On 2016-03-30 16:52:55.592484 by Jonathan

Hi Dan,

I have a gas boiler and baseboard heat. My thermostat in the 3rd zone controls A/C and the 3rd zone heat. When I set the thermostat to HEAT and Fan as AUTO only the A/C fan turns on. The heat does not come on in the 3rd zone.

Heat is working fine in the other 2 zones. (They do not control the A/C, only heat)

On 2016-03-30 16:25:00.618046 by (mod) -

Jonathan: help me out with some clarification: you refer to "my fan turns on" (that's a forced warm air heating or furnace heating component) but you refer to your "boiler" not going on. Is this a water-to-air heating system or are we just a little off on what terms we're using?

First check the switch settings on your thermostat to be sure it's in HEAT position and FAN is in AUTO.

Next check your thermostat wiring against the installation instructions.

On 2016-03-30 14:42:28.198789 by Jonathan

I have 3 zone heating. I recently replaced my thermostat on the 2nd floor (3rd zone). Now my heating wont turn on for the 3rd zone. When I set the thermostat to heat my fan turns on. Nothing else does, the boiler remains quiet and so does the a/c unit. I know the boiler is working because the other zones work without issue.

If I toggle between AUTO FAN and ON the fan will briefly turn off then turn back on when set to AUTO.

Any thoughts? I'm at a loss.

On 2016-03-03 by (mod) - trouble shooting at the thermostat

Lance:

If you disconnect the TT completely from the circuit you eliminate it from blame.

If you then disconnect the TT wires at the furnace end you eliminate the problem of wires shorted together.

Watch the furnace and its limit switch.

If you have been in a heating cycle, it is normal for the blower to continue to run for a short time after the furnace turns off - cooling down the plenum and heat exhanger.

Sometimes if the air flow is restricted the blower may cycle a time or two - something to be investigated and repaired.

To me this sounds like shorted TT wires or a control board problem at the furnace.

See also COMMON WIRE at THERMOSTATS

On 2016-03-03 by Lance E.

Thanks for responding. I had done some trouble shooting at the thermostat myself. Basically, when in the OFF state, the thermostat takes about a couple of minutes to power on. Steps that I have taken is below:

1. Swapped out the current thermostat for another exact same the thermostat yielded the same result. This rules out the defective thermostat.

2. I soldered two wires to the C-wire and the R-wire to measure the VAC. When the thermostat is in normal operation (ON state), the voltage between C-wire and R-wire is 26.5 VAC.

3. When the thermostat is turned OFF, the voltage between C-wire and R-wire is approximately 0.5 VAC.

4. When the thermostat turns OFF, the furnace is turned OFF immediately. However, the blower is still going. After about 2 minutes of blowing, the thermostat turns ON, and the voltage comes back at 26.5 VAC. Perhaps, after 2 minutes of cooling down, the power is back on.

I am pretty convinced at this point that the issue is heat related. I am planning to climb up to the attic to see what i can do. In the meantime, any suggestion of where to look and what should be done would be most welcome.

Thanks again,

Lance E.

On 2016-02-29 1 by (mod) -

I've not seen this problem Lance and am puzzled too. Isn't the same 24VAC transformer providing power to the thermostat regardless of whether it's in heating or cooling mode? If so I suspect the trouble is elsewhere, perhaps in a control board or relay or less likely, a loose short in the thermostat wiring.

On 2016-02-26 23:18:44.060667 by Lance E.

I have a Honeywell Wi-Fi thermostat (model RTH9580WF) controlling a central heating (natural gas) and AC system. I installed it myself and it has been working fine for 6 months.

I just discovered a strange problem, that is when I use the "heat" mode, the thermostat power cycling (off-on) every 5 mins. But on the "AC" (cooling) mode, it works just fine without any problem.

Could the AC transformer that provides power to the thermostat has problem when heated up? Any insight leading to a resolution is greatly appreciated.

On 2016-02-21 by (mod) -

Anon

Either your heating system or at least the circuit that is powering the 24VAC transformer that powers the thermostat is losing power (or has a loose connection) or the thermostat batteries are dead, or both.

On 2016-02-21 by Anonymous

Thermostat show blank sceen,will come and blow for a minute and shut off again happen twice what could be the problem.

On 2016-02-16 by (mod) -

Jesse,

Possibly, yes. Look for dirt or dust clogging the thermostat bimetallic spring or for a thermostat not mounted level on the wall.

On 2016-02-1 by jesse

I have a honeywell mercury thermostat, i have the heat set at 70° but the heat drops all the way down to 60° before kicking the furnace on again. It's 9 years old. Furnace is 6 years old. Is this a thermostat problem?

On 2016-01-24 by (mod) -

I'd turn the system off and call for repair; Please post questions in just one place, Kim.

On 2016-01-23 by Kim

Hi, I have a honeywell brand heat pump and the outside unit will not come one and has ice on the fan what can I do

On 2016-01-10 by Doug

RE: Hi, I have a Honeywell digital wall thermostat (RTH230B) that has been working fine with baseboard heating until last night...

Thanks for your reply Dan. Turns out it was just a matter of all the heat being taken out of the insulation in the walls, furniture, interceiling air space from the bathroom fan, etc.

Took over 24 hours to reheat the ambient temperature back up to 70. Bumping up the temperature now working fine, since starting from a basically warm unit. Cheers, DOug

On 2016-01-10 by dennis

I have a white Rogers 1f8-344 thermostat and it got hit or something and the wiring is hanging off the wall. And also the thermostat itself doesn't adjust temperature and give off the wrong temperature throughout the house so nothing turns on. Should I replace it?

On 2016-01-09 by (mod) -

Jessica:

compare the thermosat set temperature to an independent measure of room temperature right at the thermostat. If room temp is well below the set point the thermostat should be calling for heat and if it won't it may need repair or replacement.

On 2016-01-09 by (mod) -

I'm not sure, Dan, I'd start by checking for a loose wire.

On 2016-01-08 by dan

my thermostat is starting but only a sec then stop..whats the problem? thanks!

On 2016-01-08 by jessica

My furnace will not turn on when turned to auto. When you just set it to on no problem. The furnace runs fine. We are not sure what the issue is any ideas where to start.

On 2016-01-05 by (mod) -

Doug:

Is it possible that because the indoor temperature was quite low that an indoor heating baseboard pipe froze? If so and if it didn't burst it ought to thaw; feel along the piping to see if it is getting hot on a call for heat; if not hot, either the circulator is not running or the system is airbound, or possibly a section somewhere is frozen.

On 2016-01-0 by Doug

Hi, I have a Honeywell digital wall thermostat (RTH230B) that has been working fine with baseboard heating until last night. Other units in my building are working as usual. I used to keep my temp at 68, and bump it up to 70-71 for an extra 2H of warmth. Sometimes it would not revert back to the programmed 68 degrees. But I was warm, until last night (40 degrees outside) I had to open a window and turn on the bathroom fan for some cooking odors. I left the house for at most an hour and when I cam back, lowered the window and shut off the fan.

From that time on I have not been able to get heat above 66 degrees, even if I bump the temp up to 77 etc. Flame indicator comes on and there IS a little baseboard heat to fingers, but not enough. Can I have damaged something with my "defumigation"? And now what? - thanks you! Doug

On 2015-12-29 by (mod) -

Red

It's common for the air handler to continue to run a bit after the compressor/condenser shuts down, longer if in heating mode.

On 2015-12-29 by Red

Hi. Just a question when your ac clicks off but the indoor unit still runs for a minute is that normal? Is that the fan? The outside unit stops running though when it clicks off its just the indoor unit continually runs for a minute but shuts off. Nothing wrong with the air it's blows cold air and nothing g wrong with thermostat. Just want to know if it's normal. Thank you.

On 2015-12-01 by (mod) -

Yes Carol, I've seen exactly this problem: for example a thermostat located in a dining/kitchen area consistently thought that the room temperature was much warmer than did the occupants. I found that on the other side of the wall on which the thermostat was mounted was the boiler room - a consistently hot place.

A couple of steps can improve this problem, in this order:

1. Inspect and remove any dust or debris in or on the thermostat, particularly on its temperature sensing thermoistor or bimetallic spring.

2 Turn off heat - thus power to the thermostat, then remove the thermostat body or face, leaving its mounting plate on the wall; look where the thermostat wires come out of the wall: if there is anything but a very snug opening around the wires, seal it; I've used caulk, plaster, old newspaper, joint compound, even spray foam insulation (which will require some trimming back after it's dried).
Then replace the thermostat and turn heat back on. This step stops actual in-wall-cavity convection drafts from sending air right into the thermostat

3. Insulate the wall cavity - at least the stud bay on which the thermostat is mounted

4. relocate the thermostat to a more representative wall / location

On 2015-12-01 by Carol

Our thermostat is reading higher(80) than it is in here (66), the old one was doing the same so we purchased a new one. The wires for the thermostat run along the chimney in the wall, could it be reading that temperature?

Can we pull the thermostat off the wall and put some insulation right behind it. will that work?. I can raise the temperature in the house by setting the thermostat to 81 but then it gets too warm in the house. Any suggestions? We have two other thermostats and they seem to work fine.

On 2015-11-26 by (mod) -

I agree that's very odd, since nowhere in the system am I aware of a count-down timer. The problem may be on a control board in your new furnace. Have you called the installer and asked for a follow-up service call? What were you told about this abnormal operation?

On 2015-11-25 by jodi

we just put a new lennox gas furnace in and a thermostat the round heat and cool honey well the furnace runs good but comes on axactly every seven min why would that be

On 2015-11-21 by (mod) -

Lori if your AC is actually a heat pump it may not be working in heating mode.

Or your system is a heat pump but one that cannot heat the building when outdoor temperatures are quite low: there must be enough heat in outdoor air for the heat pump to work. When outdoor temps are too low then the heat system switches to a backup heating mode, often electric. That's probably what's not working in your building.

On 2015-11-20 by Lori

We have never had a problem with the heat working this past summer the ac went out. They replaced the compressor ac worked perfect all summer and now the heat isn't working correct. Works fine throughout the day and some nights but as soon as it gets 30 or below it doesn't work correct.

It isn't reading the right temperature and starts flashing a flame on the thermostat and the heat blowing out of the vents isn't as hot as it should be. Is it a compressor problem, a thermostat problem or something totally different?!?

On 2015-11-07 by (mod) -

I don't think it's the thermostat as yours seems to be giving an error message.

Be sure that the thermostat is set to HEAT and set above the room temperatures.

We don't know what kind of heat you have: heat pump, oil or gas furnace, boiler, or something else so it's a little tough to diagnose precisely.

If you take a look at https://inspectapedia.com/heat/No_Heat.php you'll see a sequence of things to check to figure out what's wrong.

On 2015-11-06 by Renee

My air conditioning works. My heat will turn on and bow air but is not hot air. Have a gas furnace. When the air conditioning is on the thermostat says cool. When heat is on it says heat but it blinks the word off and on. Wondering if something is wrong with thermostat

On 2015-10-08 by Ritzie

Compressor not turning on back again!!
I have a window type air conditioner used for cooling our room. I noticed that the AC compressor doesn't turn off. I instantly suspected that the thermostat is the problem. So, i have than some troubleshooting on the thermostat by adjusting the setting to a hotter side,

i was amazed the thermostat made a clicking sound and the compressor turned off. So, I doubt that the thermostat is the problem. But, then when i set the thermostat to colder side.

The compressor won't turn on, even when the thermostat made a clicking sound. I had to leave the AC shutdown(Breaker Off) for at least 20 mins to start the whole system running nrmally.

But the problem is having the compressor off(save electricity & let the compressor rest) and if i turn the compressor off (By adjusting the thermostat), i get a headache on starting the compressor again.
I appreciate your advise on this matter. Thanks!
blindeiz17@yahoo.com

On 2015-09-14 by (mod) -

Charlie I'd search InspectAPedia for AIR CONDITIONER ICING to find articles about frost and ice build up on AC coils - causes and cures.

On 2015-09-06 by Charlie

Thru wall AC will not cycle and eventually freezes up. To get AC compressor to shut off we have to increase temp on thermostat to extremes and it has continuously gotten worse (ie started at 76 now have to go to 94). Air is freezing cold coming out of vent when compressor is running. Unit is a Friedrich em24n34 with digital thermostat

On 2015-08-12 by (mod) -

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies,

...

Continue reading at THERMOSTAT DIAGNOSTIC GUIDE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see THERMOSTATS, DIAGNOSTIC FAQs-3 - more-recent thermostat troubleshooting questions and answers

Or see these

Recommended Articles

• THERMOSTAT WON'T TURN OFF
• THERMOSTAT WON'T TURN ON
• THERMOSTATS, HEATING / COOLING
• THERMOSTAT SETTING INSTRUCTIONS - Separate article - How to Set the Thermostat
• THERMOSTAT TROUBLESHOOTING - Separate article: What to check if a thermostat is just not working
  • HEAT ANTICIPATOR ADJUSTMENT
  • HEAT ANTICIPATOR ADJUSTMENT - T87
  • THERMOSTAT ERROR CODES - How to Translate the error codes on digital thermostats
• THERMOSTAT WIRE CONNECTIONS - how to wire up all types and brands of heating & cooling thermostats

Suggested citation for this web page

THERMOSTATS, DIAGNOSTIC FAQs-2 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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