My furnace is shutting off the jets of flame prematurely and therefore the forced air is only slightly warm.
Is there some sensor or thermometer that I should be looking for that might be easily replaceable?
There is a sensor that switches the fan on when the furnace becomes warm.
The ones I’ve seen are oval shaped, about the size of a quarter and have two wires.
Obviously there is also one that shuts the gas off if the fan doesn’t come on.
Maybe you should call a furnace guy.
The fan comes on and the gas shuts off correctly, just too soon.
Well, sure, that’s always an option.
Just covering my bases in case you did yourself in. 
I think that’s the limit switch failing. It turns the gas off if it gets too hot, in case the fan fails and can’t pull heat away. It’s the oval shaped switch I described.
I googled “furnance limit switch”, you might try that.
Actually, we don’t have enough information to conclude that. It could be a limit switch, flame sensor, or thermostat. (or even gas/solenoid valve)
Questions for Folly:
How soon is soon? After a few seconds? Or…a few degrees before the thermostat reaches “setpoint”?
How long do the burners burn on average?
What is the temperature in the house when the burners shut off? What is the setting on the thermostat when this happens? (looking to see the ‘differential’ between ‘setpoint’ on stat and the actual temp when burners shut off)
Where is your thermostat? Is it digital or mercury bulb?
Is this natural gas? (or LP/Oil?)
Do you have filters? Are they clean? How often do you change them?
Do you have a volt meter? (& know how to use it?) Do you have some alligator type jumper wires? Lifehomeowners insurance? 
One other thing…
It may be a limit switch doing it’s job.
I wouldn’t be so quick to condemn a switch. If the coil is clogged, filter clogged, restricted supply/return air registers or a host of other things, the furnace limit switch may be disabling the furnace periodically for good reason.
Folly, either call a furnace man, or give us some answers to the questions above and we’ll give you some direction.
Also
Brand of furnace?
Standing pilot? or, “Hot surface” (tuning fork type thing that glows cherry red) or,
“Spark” ignition? (Basically, how does the furnace achieve combustion?)
How old approximately?
Occasionally, everything seems to be working fine and the burners burn for approx. 20-30 secods before the fan kicks in. When it isn’t working so well, the burners burn for 4 or 5 seconds tops. I have 3 identical furnaces, so I have two to compare to that are working fine.
When things aren’t working correctly the house temperature maxes out at about 60 when the thermostat is set to 70.
Digital. Different room then the furnace.
natural gas
filters changed recently
All yes’s here.
Frigidaire.
A very interesting glowy thing, and,
I was told about about 2 years old.
Looking inside, I see the flame sensor. There is some sort of other sensor that looks very simple inside the air chamber. Just two wires going to a circular disk of some sort. That same wire connects to another circular disk that sits above the gas flames.
Thanks all for taking an interest.
It would appear that your flame sensor is failing. If it doesn’t sense flames within a few seconds of the main gas valve opening, it shuts off the gas. I would highly recommend getting a qualified HVAC service company to repair this for you. You really don’t want to mess around with this system unless you know what you are doing.
From what you are describing, it is likely that the flame sensor is “blind.” This is a very common thing. Over a few years an indiscernable film builds up on the sensor and it no longer can sense the flame. The sequence of operations on most furnaces are such that if the flame isn’t “proven” in 2-4 seconds or so the unit shuts down. (because in the absense of flame/combustion raw gas is flowing into your home)
This sensor will look like a small steel rod; most are stright, some have a 30° bend in them them. They are by design in the flame. (duh) Most often they are on the opposite side of the “very interesting glowy thing” (hot surface ignitor).
There will be one wire running to it. It can be taken out with a ¼ nut runner or thin blade screwdriver.
- Shut the power off to the furnace. (the 120V Supply)
- Take the flame sensor out and clean it up with emory cloth, or sand paper. Even your wife’s nail file will work. You need not go mongo on it. Simply sand it up a bit—you’re cleaning it, not sanding it down.
- Re-install, just the way you took it out.
If you have any gumption, and are as handy as a fairly intelligent chimp, you can do this.
Notes:
a) It’s not fragile
b) It’s very low voltage–as long as you turn the 120V power there’s little risk of you getting hurt.
c) If it is the flame sensor, the results will be immediate—problem corrected.
d) I take cash or checks.
The disks are limit switches and are in series with one another. Some sense heat, some “flame rollout.” Your problem probably doesn’t lie with them.
If the problem persists, (and I think it won’t) there are a couple thengs you can do with the volt meter and jumpers that will narrow the problem down a bit.
So far so good. I think the the raindog got it in one.
Would have felt pretty silly to pay someone to come out to spend 5 seconds with a piece of sand paper.
Hope that does it.
Of course, like any other knowedge based profession you’re paying as much for the technician’s education and knowledge than the time/sweat involved.
A service company [rightfully] will charge you 1 hour minumum–in Chicago maybe around $100-140 I’m guessing. An ethical, professional will ‘sand for 5 seconds’ but also check combustion, check other functions, change the filter and other ‘things’ that are preventive maintenance related. At least there’s more value in the equation that way.
Others, however, will work the 5 minutes, collect a check and be on their way. If it continues to act up, let us know.
True. And thanks again for sharing your knowledge.
There’s that old saw about the car mechanic that charges the customer 200 dollars after tapping the engine once with a hammer. That’s 5 dollars for the tap, and 195 dollars for knowing where!
You’re welcome!