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Most programmable thermostats are "smart stats" which use a programmed algorithm base on a preprogrammed P (proportional) I (Integral) D (derivative) Loop. To make a long story short, the programmed temperature will make the room that temperature by the programmed time. This is based on how long it has taken it in the past to bring the room up or down to temperature. So if your setpoint is 70, and your set time is 7 AM, then the furnace or AC will bring the room to that temperature by the set time...not start at the set time.
if you have an electrical meter this is how to test for bad t-stat heating relay. (this is for the most typical furnace setup in a typical home) first you will need to find and do several things at the furnace. find the MAIN furnace switch and turn off. switch should be mounted outside of furnace on either side of furnace cabinet. remove both furnace doors. find the electrical circuit board, it will be the square thing that almost all wires go to or come from. find the 24v (t-stat) connection screws on the circuit board. they should a row of small phillips screws, and there should be a minimum of 5 screws in a row. the screws will be labeled as follows (not in certain order) R W Y G C . The screws you will be concerned with are R, W, and C. If there is not a C screw you will substitute the C screw for a unpainted part of the furnace cabinet in your testing. The last thing to find is the door switch which should be mounted behind the lower door. It is mounted so when the door is attached the switch is depressed. Use some tape or something to depress the switch without the door on. you will not have enough hand to do it manually during testing. get your electrical meter and your ready to test. always remember that if the Main furnace switch is turned on any of the electrical connections, motors, etc. inside the furnace could be energized at any time.
step 1 : with main furnace switch on, both furnace doors removed, and door switch depressed go to your thermostat and turn a minimum of 5 degrees higher than ambient. if t-stat isnt on then something you did is wrong. check switches.
2: with newer t-stats there can be a delay. so wait 5 minutes then go to furnace.
3: put electrical meter in AC and in correct range if you have a cheapo meter. you will be reading 24v so correct range is the closest to 24v but must be above 24v. take your leads and put one on circuit board screw labeled R, other lead on C. you should read 24v or similar. minimum voltage should be about 22v. anything less than 20v there is a problem far before your thermostat. if you read 24v move on.
4: take your leads and put them on the W screw (heat) and C screw (common24v). You also should read 24v. If you get a reading less than 20v or no reading at all then you either have a bad thermostat or bad thermostat wire.
You didn't include a model number, so it's hard to know for sure - but I'll take a stab at this one. Many of these t-stats have a programmable offset to reduce "hunting". This can be as little as 1 degree or more than 2 degrees.
This means if you set the offset to 1 degree and the room temperature to about 65 degrees, the furnace will heat the room until the t-stat registers a rising temperature of 66 degrees. The furnace shuts off and will not turn on again until the t-stat registers a falling temperature of 64 degrees. The average is 65 degrees, and prevents the furnace from starting and stopping numerous times instead of just once with the 1 degree offset.
You can further reduce the number of furnace start and stops by increasing the offset to 2 degrees. This would work like this when the t-stat is set to 65 degrees and the room is cool:
Room Temp Furnace 63 On 64 On 65On 66 On 67 On 68 Off
The room is now warm and heat or furnace is off. The room begins to cool like below:
67 Off 66 Off 65 Off 64 Off 63 Off 62 On
With a 1 degree offset, the room temperature varies +/-1 degree (about 2 degrees total). With a 2 degree offset, the temperature about varies +/ -2 degrees (about 4 degrees total).
Also, when the thermostat shut the heat off, the heat that was made is delivered to the space instead of wasted by sending outdoors - so the room will get a little warmer even though no fuel is being burned. The same holds true at the other end. When the t-stat turn heat on, fuel is burned, but there's little heat until the source has reached the optimum temperature and then the heat is delivered to the space.
I hope this helps & good luck! Pleas rate my reply - thanks!
Definitely a thermostat.It has 2 sections.One is a temperature reference section(what temp you want).The other is the temp reading section.If either one is bad,it will do just like you said.Sounds like the reference section is probably bad because it comes on for a couple minutes then shuts down.Since it was fine earlier in the day,I doubt there is anything wrong with the furnace because as you can see,it is temperature related.A furnace problem would be more of no operation at all,even if just dirty or whatever.The thermostat also would be a more common 'fail component' than the board.A suggestion also,both for a car and a house.During the summer,run the heat for 2-3 minutes,just to cycle it occasionally.In winter,run the A/C for 2-3 minutes.Do both perhaps once or twice a month.It helps keep the components,somewhat fresh I guess you would say,because cycling it occasionally helps to keep it going good.Also,if you encounter a problem,you may be able to get it fixed before you really need it.In the meantime,feel free to come on down to FL until it's fixed,we have room,lol.Although it's chilly now,the weekend will be 70's.(sorry)Good luck,Greg
if you set the thermostat on 70 and your house won't get ant cooler than 72, there is a problem with your system,not your stat. I'm thinking you are low on refrigerant if it is somewhat working. when the unit is running, if you can get your hand on the larger refrigerant line under the black insulation, it should be cold, about 55 degrees.
The Thermostat Wizard is an interactive tool that will guide you through wiring and troubleshooting your thermostat, as well as programming it to the most comfortable settings for you and your family. Simply answer the questions from the Thermostat Wizard to correctly connect, troubleshoot and/or program your Honeywell Thermostat. You can even print the instructions and bring them right to the thermostat to help make things easier. Go to: http://yourhome.honeywell.com/Consumer/Cultures/en-US/Support/Thermostat+Wizard/
However - the problem is probably not with your thermostat but with your furnace. It is most likely a dirty flame sensor - a maintenance issue. If you have not had your furnace serviced recently (more than 1 year) that is where you should start. Modern furnaces require regular maintenance.
if your furnace is on 24 hours straight, before it heats up your home, then either your cold weather and indoor temps are extreme, or your unit is grossly undersized. if however your furnace cycles on and off when the temp is still way colder than you have set it you have either a bad stat that thinks its hotter than it is, or a bad furnace problem and even though the furnace is getting a signal from the stat to heat it is failing to do so. you'll have to watch and think this through to isolate the problem. if you set the furnace to max heat and it runs and heats the house reasonably quick then its probably going to be a bad stat. if you set it to max heat and it shuts off way before it gets to setpoint then the problem is in the furnace. be sure filters are clean, blower wheel is clean , returns are no obstructed, etc. if you by chance have an old mercury bulb stat they had a thing in them called a heat anticipator which basically worked like this. a dial with a little pointer determined how soon it would be till the furnaced reached the desired temp and anticipated how soon to shut off the burners in anticipation of the cool down period so you dont overshoot the desired temp. if these are bad or moved to different setting they can short cycle the furnace on of on off etc. or shut down the burner almost as soon as it lights. setting is usually at .4 to .6 on most, but really a digital stat is only way to go. by the way check to be sure you don't have a hot air supply grill blowing directly on the stat or it will shut off as soon as stat gets hot and rest of house will be cold. usual location is 5 feet off floor directly over the main return air grill or under it as the case may be. good luck
Sounds like a relay sticking. Possibly in the thermostat or possibly the furnace relay at the furnace. The next time it happens, unplug the thermostat from the wall. If the furnace still runs then it's the furnace relay.