HVAC carrier gas furnace question

Got an old carrier downdraft gas furnace, pilot light style. Runs normally but at the end of the heat cycle after the fan shuts down then the fan will very briefly try to startup a couple times. Everything works fine otherwise. It uses a timer heat fan relay circuit for cool down. I'm guessing that is where the problem is but it seems like a weird symptom. Any ideas? TIA
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The blower is controlled by a temperature sensor near the heat exchanger in the furnace. The reason for this temperature sensor is to let the furnace temperature rise before starting the blower so that cold air isn't blown into the living area. Similarly, when the thermostat signals the furnace to shut off the heat exchanger is still hot so the temperature sensor keeps the blower running for a few minutes to extract the remaining heat rather than loosing it to your basement or chimney. It also prolongs the life of the heat exchanger by not letting it overheat. The reason your blower is cycling after the furnace turns off is that the temperature sensor has failed (or more likely) needs to be adjusted so that the blower stays on a bit longer to extract more heat from the heat exchanger.
Boden
JJ wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi, Usually there is bimetal heat sensor which controls fan on, fan off and safety cut off. Fan comes on and goes off after a certain delay to prevent blowing cold air and to extract residual heat from heat exchanger after flame is off. Some sensors are adjustable with dial and set screws some are not. Maybe the sensor is just tired. Tony
JJ wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

normally
will
You didnt post enough information to know what your unit uses as a temp control, but if its old enough, it will use a fan limit switch that is adjustable, and since they dont get out of adjustment by themselves, it may be telling you something. If its got a true ICB on the unit, the board may be failing, as the ONLY sensor for that setup is an overheat sensor, and if THAT is the case, then besides board failure, it could be an airflow issue...you havent gone to those 3M style filters recently have you?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.