Is it a digital thermostat?....if so, then the tech should be able to see the high limit dropping out and know there is a problem after replacing the limit. If it's analog then it could be a bad heat anticipator. Again, a tech worth his salt should be able to troubleshoot it.

Could be a potentially serious problem that needs to be fixed. As tin said above, you need a good tech to look at this. Or call the companies back and tell them they didn't fix it. Are the flames rolling out? Could be a gas problem, flue problem, limit problem, airflow problem....

Do you have an older mercury bulb thermostat? If so pull off the cover and observe that bulb while the furnace is running. If the bulb is tipped slightly in one direction and then all of a sudden it tips in the opposite direction during the time frame you described before the room gets up to temperature its a prety good bet you've got a t stat problem,,,more then likely as mentioned, heat anticipator. Its a simple test but a good start on the process of elimination.

I definitely think the furnace is getting too hot and that is why the limit switch is getting tripped. When I touch the furnace on the outer casing just under the burner it is very, very hot. But why it is getting too hot is another question. I am just wondering if the furnace could have been doing this for years without us knowing? Because not changes in duct work have been made. What else could cause the furnace to get to hot? I plan to call the tech on Monday, after I have wasted most of the weekend staring at the furnace and thinking about this. Thank you all for your insight it is great!!!!! Also I took the blower door off and the same results, after 5 minutes burner turned off, the blower ran for two and the burner came back on. So I also removed the filter and then the burner stayed on about 8 miuntes vs. five minutes before turning off but still cycled off. Then I noticed the diagnostics light and that flashed four time which means thermal protection devices open. Do the return ducts go to the outside? Or are they the cold air returns within the house? How do they get blocked?

A good tech with measuring devices for temp rise, gas pressure, and volt meter to determine which safety is opening will know what is wrong in about 15 minutes. Don't pay anyone that can't or won't do that.

The tech just left and this is what I was told. Our furnace a Trane XE 90 model:TDC080C942B1 is a 80,000 BTU furnace and the problem is the amount of duct work going out of the furnace. It is a down flow furnace and I looked below and saw we have 1 ten inch, 1 eight inch, and 5 five inch duct work going out of the furnace and the eight inch actual had a metal piece over it turning it into a five. So I was told the furnace needed 1200 CFM outflow based on the furnace sized and before we removed the metal from the eight we were at 725 CFM available ductwork. So that is why we are overheating? The fan was turned up to high and that did not help. Because our house is a slab and the ductwork is in concrete our options are to get a new furnace, bust out the floors to add duct work (not an option). The heat exchanger is not cracked, the blower wheel is clean, the secondary heat exchanger was clean and the coil was also clean and gas pressure was checked. I was shown all of this. What should we do? The furnace was installed in 1995. We are getting heat and donÂt have complaints about the heat. But we do live in North Dakota and I hate to wait for it to fail and it be 30 degrees below zero and a Saturday. Should I get a second opinion, given it will be about 100 bucks to get someone to come. Any thoughts would be great.

The tech just left and this is what I was told. Our furnace a Trane XE 90 model:TDC080C942B1 is a 80,000 BTU furnace and the problem is the amount of duct work going out of the furnace. It is a down flow furnace and I looked below and saw we have 1 ten inch, 1 eight inch, and 5 five inch duct work going out of the furnace and the eight inch actual had a metal piece over it turning it into a five. So I was told the furnace needed 1200 CFM outflow based on the furnace sized and before we removed the metal from the eight we were at 725 CFM available ductwork. So that is why we are overheating? The fan was turned up to high and that did not help. Because our house is a slab and the ductwork is in concrete our options are to get a new furnace, bust out the floors to add duct work (not an option). The heat exchanger is not cracked, the blower wheel is clean, the secondary heat exchanger was clean and the coil was also clean and gas pressure was checked. I was shown all of this. What should we do? The furnace was installed in 1995. We are getting heat and donÂt have complaints about the heat. But we do live in North Dakota and I hate to wait for it to fail and it be 30 degrees below zero and a Saturday. Should I get a second opinion, given it will be about 100 bucks to get someone to come. Any thoughts would be great.

A quick calc and I would say you are at least 200-300 cfm short at a minimum, which is enough to cause the furnace to ride up on limit.

It's too bad but based on ductwork; you most likely have too large of a unit. Bigger is not always better and in fact puts out less heat because it cycles. ItÂs kind of like sipping from a cocktail straw, lots of work and energy for little beverage, switch to a soda straw and it's much easier to quench the thirst.

One of the beauties of inshot burners is you can derate the burners by putting in smaller orifices or lowering the manifold gas pressure also. With Nat Gas, you can usually lower it from factory/field setting of 3.5" to 3.0". Manufacturers will allow up to a 10% leeway. This will lower the input and perhaps help your situation. Ask a good tech if they will perform this for you.

The tech did not want to turn down the gas pressure. Should I just bite the bullet and gat a correctly sized furnace?

Sounds like an oversize problem. I've been out of the business for awhile but i remember something about a dump zone. Is this something useful for you? Is it possible to add a register on the plenum and heat the area around the furnace? Wasted heat maybe but if it stops your unit from overheating, it might work.

A properly sized furnace is most efficient, more comfortable and probably quieter.

Besides a dump zone,or an extra takeoff in your situation, as mentioned, a bypass can help, which in your case would be a 8" round duct run between the supply and return plenum.

I just had the Trane tech over and what is told me is this: I am in a townhome and all the units have the same furnace. He said several of the units have had this problem and what they have been doing is putting in a limit switch with a high of 270 degrees vs. the 200 degrees. I asked if there is a problem with the size of the furnace for the house and he said not the problem is the duct work and I said ok so the furnace is too big for the duct work? I just got a blank stare.

My question is putting a limit switch of 270 degree a problem when the furnace clearly stated limit switch should be at 220 degrees? Again the furnace is 13 years old and he said his records showed the prior owners had had the Trane Tech over 5 -6 times for the same issue.

Oh my god don't let him do that!!!!!!!!!!

Furnace limits are chosen for a reason, the added stress to metals which will ultimately cause a failure and perhaps resulting in a dangerous and perhaps lethal could/will result.

How close are you to the local fire dept?
Missaplication of controls on your neighbors furnaces should be noted, as if there is a loss, they are sure to win that lawsuit.

I have been having the exact problem here. furnace worked for
14 years and now started this fan limit cutoff stuff.
Someone mentioned a ByPass. Would this be an 8 inch pipe between the hot air duct and the cold air return duct?

Mine cuts off about 2 times a day. Sometimes it goes for a week without cutting off. Also seems to make turning off the
power to the furnace and then back on necessary to reset.
I have no cellar either, and was limited to how much duct work I could run. How about a cold air return directly into the attic? nice cold air and the cold air return goes right thru the attic. any problems with that?

Thanks in Advance