Hi, First, I let my filter stay dirty for too long. The furnace was running, but no heat was coming out. So I changed the filter. But nothing. I cleaned the limit switch with a cloth because I read the 4 blinking lights indicated something was wrong with it, but that didn't help. I turned the furnace back on, but, like always, the flames light up, theres the normal hum like it's about to start blowing air, but no hot air is being forcibly pushed out via the loud fan. I can only hear small sound with hot air barely seeping through my vents. Just checked, while keeping the door switched depressed, I fired up the heat and the blower fan(?) won't spin, even though while the flames ignite and everything. Could the blower fan be the problem? Or is the the limit switch (indicated by the blinking 4 times) [I]causing [I]the fan not to spin? Like, do you think I have to get a new limit switch and test that first. How do I determine what the issue is? Or should I just call someone to fix it, and how much am I looking at? I'm just in a serious bind because it's 5 degress, and I have two kids in the house. Although they have a heater in their room, and spend the day at school, it's quite cold in here. Thanks for you help in advance! [/I][/I]

International Comfort Products Model #NBF075F16A1 original equipment mounted in attic of 18 year old single story 1700sf home. It all started when I replaced my battery powered digital TS with a Z-wave TS that requires 24Vac to operate. I connected an unused wire in my TS cable at the furnace to the C terminal of the control board and connected the same wire at the TS to the C terminal. TS worked as intended until I noticed once when walking by that the display was blank. Upon investigation, I noticed that when the TS called for heat, the display would shut off after about 5 minutes like there was no power. After about 35-55 seconds it would power up again and the cycle would repeat. The entire time the blower is still running so everything SOUNDS normal. At first I assumed that the TS was faulty and replaced it. The new TS did the exact same thing. Then I hooked up a voltage meter to the R and C terminals at the TS while it was attached to the base and I noticed the voltage dropped from 26Vac to ZERO when the TS shut off. That explains the blank display. Now since my blower still runs when this happens, I couldn't tell you if I was getting a voltage drop at R before the new TS was installed because the battery operated one always had a display. Also, the house always heated to the set temperature before and it still does even with the new TS. So I went up into the attic to see why my voltage at the TS was dropping and this is when I discovered the heater isn't working properly. What I observed was normal start-up: When I called for heat, the furnace vent blower would turn on, a few seconds later the burners would ignite, and after a short while the fan blower would start and everything runs like it should. The furnace would run for about 5 minutes, but even though the set temperature has not been met, the burners would shut off. Followed shortly with the vent motor shutting down. However the fan blower continues to run. After about a minute, the vent blower would start, the burners would ignite and the furnace would again be working. Until about 5 minutes later and the cycle would repeat. I realized that when the furnace would shut down, so would my TS display. So the voltage loss was more than likely due to a safety feature shutting off power to the heater. Also, more than likely it was due to overheating and when it would cool down it would restart. If it was overheating, it was likely due to an airflow problem, so I began the trouble shooting: 1) I replaced the 6 month old clean filter with a new one (I even ran without a filter for a cycle) No change. 2) I checked the exhaust vent for blockage (went on the roof and removed the debris-free cap and could feel a good flow of hot air. I disconnected the lower vent pipe from the vent blower and found nothing but a few small dead insects and a small leaf) 3) I checked the inside of air intake duct from the ceiling all the way to the blower intake and it was incredibly clean with no obstructions (selfie stick on cellphone camera allowed me to see the entire distance up and over a rafter). 4) I verified that all 10 registers throughout the house are fully open 5) I cracked open the cover on the evaporator box and although I could not see the intake part of the coil, the outside was extremely clean. So after checking all the ventilation points, I thought I could rule out airflow. I began some more troubleshooting. 1) Shorted out the R and W terminal on the circuit board to call for continuous heat. No change 2) Switched out the fan blower motor speed lead for heat from LOW (red lead) to MEDIUM (blue lead) to increase airflow (furnace would run longer and cycle off shorter, but still did it) Switched it back to LOW. 3) Tested for continuity (good) and then bypassed the Pressure switch on the vent blower. No change. 4) Tested for continuity (good) and then bypassed the high limit switch on the front panel of the heat exchanger (THE FURNACE WOULD CONTINUE TO RUN) Not knowing if the limit switch was kicking in too soon or if indeed the temperature was too high, I only ran it for a few minutes longer than the usual cycle. Here's the kicker. I also taped closed the safety switch on the blower motor housing door and ran the furnace with all sensors intact. With the blower door open, the furnace would run and run and not trip any sensors. I could feel the blower getting more air from the front of the door opening even though the intake duct is almost 2 feet in diameter. I actually had to turn it off after about 10 minutes because it wouldn't shut off. So there is my problem. My furnace needs more AIR than my perfectly clean huge air intake duct can supply. How is that possible? Again, for all I know, this is the way my heater has been operating for 18 years. I always assumed the burners were on when I heard the blower motor running because I was always feeling heat and the house would always get to the set temperature. I only discovered this due to the new TS shutting off when the safety measures on the heater kick in, which leads me to believe it's been acting this way for a long time. I plan to replace the limit switch to see if that cures the problem. But the actual solution sounds like more air is needed to the fan blower. However, the cycling still occurred even at a higher fan speed, so I am stumped. Any other suggestions? (Other than calling a technician, please. This is a DIY forum)