(Last Updated On: September 13, 2017) Definition of a Fan Limit Switch. How can i fix this problem ? The thermostat still turns the heat on and off but the fan doesn't go off when the heat goes off. 529 Pleasant Street, MS B19 (130º)?  To compensate, the mounting screw may need to be tightened only gingerly. This process is more likely to occur in very cold weather or in a very drafty building. This allows the blower fan to be set to run continuously. Click to Show or Hide Citations & References, L4064W Tradeline controller on an old gas furnace. But first, let’s take a quick look at how your furnace blower works in normal operation to determine whether you actually have a problem or not. Contact us online my issue is the exhaust fan blower on my gas furnace just keeps cycling when the thermostat is on the unit never fires the burners as the vent blower goes on and off every 15 seconds or so. 1. That's not a condition I had heard of before. So, if your air handler looks like the one below, try wiping the blades carefully with a dry towel or toothbrush. Other switch models include the 3F01-200, 3L01-180 and 3L01-200. You are not alone. Toll Free (800) 597-4575 ~ Voice (231) 946-7644 ~ Fax (231) 946-9332, Chatham Brass Company (this may be only a supplier of water heater controls that can replace other Therm-O-Disc control models) chathambrass.com/, Elmwood Sensors: Website: http://www.aaxico.com/aaxmfr/elmwood.html (now part of the Honeywell Corporation) USA office: AAXICO, 8881 NW 13th Terrace Replaced the flame sensor and checked the voltage & ground all of which is ok. Check whether your fan is set to On or Auto. But a simple error of mis-locating the bracket set screw can cause the screw to contact the bimetallic spring inside the switch - a dangerous condition that can cause the switch to fail to shut off properly in response to high temperature. I have an electric furnace that is bugging me. I have noticed that sometimes after the AC runs and the fan shuts off it comes right back on for about 30 seconds then shuts off. For a furnace that Cycles on and off during the call for heat, When this happens, I've tried jiggling the rocker arm that toggles the relay, but this doesn't help. Chuck I was hospitalized for 1 month & shut my thermostat off & when I came home & put the thermostat up to 63 where I usually keep it about, this is when all the trouble started with the furnace Not shutting down completely but it was Fine prior to that. But your advice that people try disassembling, cleaning, lubricating the switch makes me nervous. When you raise the thermostat to a warmer temperature, the furnace blower will turn on … When it is not functioning properly, it can cause your furnace to cycle on and off more frequently than is necessary. Thermodisc technologies is a division of Emerson Electric Co., now Emerson Climate Technology, and has been in business since 1947. So, if your air filter looks like the one to the right, replace it immediately and see if that fixes the problem. My furnace will come on, run it's whole cycle but where You think it's going to shut completely off, the fan or motor or whatever keeps running & within about 30 seconds the furnace goes thru another cycle where I hear the click to tell me the flames have come on. The solution to this problem is to replace the thermostat. The fan limit switch controls when the furnace blower fan turns on and off. Details are at FAN LIMIT CONTROL SETTINGS. Similar to a clogged filter, dirt on the blades of the air handler fan will decrease cold airflow over the heat exchangers, resulting in the same blower on/blower off pattern. °F., disconnect the thermostat reaching its set point and ceasing to call for heat and supply. Or warm air furnace fan cycles after HEAT-FAQs - questions & answers posted originally at website. 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