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When you say 'older furnace' I am assuming old enough to where there is no circuit board. They will usually have a thermally activated 'fan-limit' switch with a bi-metal sensing device. Most older furnaces had either a Honeywell or a White-Rodgers controller and they look almost identical. With the furnace door off, look towards the top of the furnace vestibule (above the flame area) for a rectangular box approximately 3 1/2" wide by 5" long. Tap the side of the box with a screwdriver and if the thermostat setpoint has been satisfied and the gas burners have been off for more than three minutes, the fan should then shut off. The switch contacts are either stuck together OR the bimetal sensing device is weakening from thousands of cycles.
If it continues to run, squeeze the sides of the 'lid' of the box and remove the cover. Inside you'll see a round moveable dial with three pointers, marked 'fan off, fan on, and limit'. The 'limit' should remain at or near the 200 degree setting, the 'fan on' can be anywhere around 140 and the 'fan off' can be anywhere near 100 degrees. It is the 'fan off' position thats giving you the grief.
Turn the thermostat up again and sit there and watch the dial do its thing and yo'll understand better how this control works. Watch it rotate past the 'fan on' setting and observe the fan turn on. Turn the thermostat below room temperature and watch the gas shut off and the dial rotate back towards the 'fan off' setting. If it hangs up again before twisting past the setpoint, you can slightly increase the setting to compensate for the weakening bi-metal element. But just a tiny bit, not a great amount or you will have the fan shutting off with a lot of residual heat still in the heat exchanger. The idea of the delay in shutting off the fan is to remove all that residual heat and transfer it to the house.
There are other brands that utilized 'timed on' fan controls, and there are a few other variances of controllers but what's described above should cover the vast majority of older furnaces.
Remember there is ELECTRICITY inside the fan control and any adjustments should be done with the power OFF !!!!
Hi, these Honeywell combination fan/limit controls are used for turning the fan on after the combustion changer reaches the factory setting on the round dial that you see when you remove the cover. The high limit will turn the fan on when it reaches this temperature, and off when the stat is satisfied. If the blower motor fails due to a burn out or short, it shuts the main burners down on the high limit setting. These controls can be removed and you can clean the probe which has a set of contacts at the end. Sometimes this will fix the problem. Check the settings on the dial to see if they are set right to come on, the time it takes between cycles, and the temperature they are set to shut off. This sounds as though you need to check your thermostats also. They could be a problem. You say electric heat, but then you are saying flames? If these limits have the manual fan button, make sure they are on auto. Long delays could also mean you have a dirty flame sensor located by the pilot. You can clean the metal tip with steel wool or fine sanding cloth. Short heating cycles may be your thermostats like I said before. You shouldn't have to replace the fan/limits as I told you what they control. These will cause a fan to run and not shut off after the stat is done, so check to see if they are dirty. Please keep me posted. Shastalaker7 A/C, & Heating Contractor
You are to be commended for noticing this sequencing of events-- Good observation!
You need to adjust the Plenum control switch. (Or fan control switch)
Usually at the top front of the furnace, is a Fan control switch-- It is called the Plenum switch-- and it senses the temperature of the Air being heated by the fuel-- (Is your furnace Oil or gas?-- if Gas-- is it LP or Natural?)
This switch Has a cover that can be removed-- inside is a round dial, with some adjusting screw, and Degrees stamped on the perimeter of this round dial. You can take the cover off, while the furnace is running-- (Just do not touch the screw terminals that have electric power to them-- You can rotate this dial, and it will turn the fan on-- (Or if the trip levers are mis-adjusted-- it might shut off the burner-- just rotate back the other way, and it will re-light etc.-- The purpose of this control is to make sure your fan is not blowing COLD air. So-- lower the 'ON' switch tripper, down lower, so the fan will turn on sooner. You may have to loosen a set-screw-- but usually they are just friction held-- and can be forced to slide (rotating CCW) to the lower temperature setting. (If you get it too low, it will be blowing cooler air than you like at the registers.)
However-- to get the most heat from your furnace energy source-- you want the fan to come on as early as you can tolerate-- That will be delivering HEAT to your occupied space sooner, and giving yo more of those dollars you are spending on fuel, in the orm of heat in your house!
If your pilot light is lit then I would first check your thermostat. Make sure it is on heat setting and fan is on auto, turn stat up to where it should be calling for heat. If you don't get burner to light remove front cover from thermostat and see if it has a mercury switch, if it does move temperature selector up and down and see if you can see a spark in mercury switch vial. If y ou cannot see a spark remove screws that attach t-stat to base.Remove carefully and disconnect wires that go to r and w terminals on stat base. Temporarily connect these two wires together. This should make burner light if you have power to thermostat. If burner does light disconnect wires and replace thermostat. Also if you have a volt meter check across these two wires and see if you have power there. Should be around 24 volts a.c. If your thermostat has a fan on setting try that. If fan runs you know you have power to furnace The gas valve needs 24 volts to operate and inside the electric control box in furnace there is a transformer to step down 110-120 supply voltage to 24 volts. If transformer is working you should be able to hear a low buzz or humming sound. You can also check across gas valve terminals when t-stat is set for heat, you should read about 24 volts. If you have 24 volts to gas valve and it is not opening then you need to replace gas valve. The main purpose of the limit switches is to turn blower on when temperature in heat exchanger reaches fan on temp,shut off blower after burner shuts off and heat in heat exchanger reaches fan off temperature, and limit if temperature exceedes set point without blower coming on. I hope this helps you. Thanks
First thing I would suggest just to be on the safe side before you began shut power off to unit.
Remove cover on combination fan and limit.
I would set the fan on at 130-140 and fan off at 90-100 degrees. When you adjust setting grasp the outside of the dial to stop it from rotating when you are moving set point indicators. Replace cover and turn power back on. I hope this helps you. Thanks.
Hi, Yes, you have the answer to you're problem. Now, I don't no if you have just a high limit, or a combination Limit control? I will go with the combination fan control as when they go bad, they stick in the closed position and they have normally open contacts. When the heat exchanger starts to heat on a call for heat, there is a dial on the fan control with temp. settings on it , and it will start to rotate as the heat gets higher. When it reaches its set fan on temp, it with start the fan.We no the fan works so the blower is good, so it shuts the burners down on you're high limit temp. which is factory set to about 250*F. When you turn The t-stat from auto to on in the fan side, that solves the problem temp, only. You will need to R & R the Fan control. If you need some direction on doing this please let me know and if it is a Honeywell auto/on type that is rectangular in looks, and if you remove the cover you will see the temp. dial with arrows on it. Thank you. Best of Luck, and Sincerely, Shastalaker7
I am providing some information, but I encourage you to have a furnace technician look at your furnace. The reason is; the limit switch trips when there is a problem that may present a fire or combustion hazard. I've seen these switches go bad, but not very often. Usually they are tripping because the plenum is sooted up and excessive heat builds in the combustion chamber. This is common in older furnaces that have pilot flames- the flame runs all year and soot deposits in the plenum, plugging the air flow through the combustion chamber after years of use.
The limit switch is a safety control switch located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment. Check the thermostat first. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs adjusting.
To adjust the switch, remove the control's cover. Under it is a toothed dial with one side marked LIMIT; don't touch this side. The other side of the control is marked FAN. There are two pointers on the fan side; the blower goes on at the upper pointer setting and turns off at the lower pointer setting. The pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit and the lower one at about 90 degrees Fahrenheit.
If you mean the ambient lockout switch it shpopuld be oin the outside unit and all that you will need to shut off is the outside condenser before you open the panel. ther is a disconnect that you will have to throw either right by the unit or inside of a main breaker panel.. Hope that this helps and Thanks for using Fixya