Ok, I will try and describe the problem with my 16 year old gas high eff Tempstar Furnace (NUGS075BG04).

We keep the stat at about 70 degrees. Here in Michigan the air is getting quite cold outside. We have been noticing that the furnace is having a hard time keeping the house warm. The furnace runs almost non-stop and the actual temp on our digital stat is only reading 68.

So here is where I begin troubleshooting. I discovered using a thermometer that the supply air coming out of register is only 71 degrees. I began playing with the honeywell limit switch. The settings were 55, 80, 250. I notice that the timer turns very jumpy. Also noted that the dial will cycle from one side to the other every minute or so.

I believe this is hitting the 250 hi-temp limit and then a small blower motor turns on to cool the heat exchanger by drawing air through the PVC pipe going outside. Also during this time the gas valve clicks.(Which I believe to be shutting off). The dial moves slowing again and the small blower shuts off, gas clicks again and the cycle will repeat itself.

I really don't think the furnace is overheating as the air coming from the register is not very hot.

The main blower motor was on the highest (Fastest)setting. I changed it to a slower setting to try and raise the temp going to the register, but this had no effect.

Am I correct in thinking that this Honeywell switch is faulty, or maybe something is dirty or?? I don't want to spend $80 on a switch if its not bad. I have not been able to see the burners at all, as I am not sure where they are located.

I did adjust the settings on the switch to 130, 160, 250 and they air coming from the register is now about 76 degrees.

Help please!!

If the high limit control is cycling the system, the furnace is likely overheating, even if the supply temp at the registers seems low. Check the temperature rise by measuring the return air temp near the filter, then the supply air temp near the furnace. The difference is the rise, and the rating plate should give you values for the acceptable range. If the rise is high, the furnace is overheating for some reason, and decreasing the blower speed will further complicate this condition. An excessive rise is usually the result of insufficient airflow or excessive gas input. If the air filter is not clogged, the air conditioning coil may be (if so equipped). You will likely require the services of a professional to properly correct this problem.

I will try measuring the temps tomorrow. The unit is not equipped with AC. I have the service manual and the specs call for a 35-55 acceptable rise. The air filter is clean. I do not believe I can check the incoming gas pressure without a (Manometer) lol whatever that is.

If you don't have air conditioning, and your furnace is a condensing-type (90% or higher efficiency), the secondary heat exchanger fin pack may be clogged (viewable by removing the blower assembly). This would produce the same effect as a clogged air filter or a/c coil.

If you have observed that the burners cycle on and off repeatedly while the blower continues to run, you may have a high-limit cycling condition-indicating a furnace that is overheating. Airflow restrictions (clogged filter and/or air conditioning coil, etc.) will result in very inefficient operation and high operating cost. Any heat produced by the furnace that is not delivered to the living space is going out the flue system. What you have described sounds very consistent with an overheating condition.

Hi KC13,
Here is a snip of what you posted on another topic. This is exactly what problems I am having. I also am experiencing very high utility bills($220) monthly. 1800 sq.ft. home.

I know the filter is clean and also had ductwork professionallly cleaned last year. I inspected the bottom of the furnace where the blower motor is and it was very clean. How can I figure out why else the system may be overheating?

Thanks in advance.