Hi. My wife is a light sleeper, so we set our thermostat to "fan on" every night. The unit works non-stop all night and creates background noise, in addition to heating the house. Last night, five minutes after setting the thermostat to "fan on", the unit stopped. SILENCE!! Then, a few minutes later it started again. This continued every few minutes: system turnes off then on. Why doesn't my thermostat setting to "fan on" make it run all the time any more? Please help. Thank you very much.

You posted NO information and my crystal ball is broken.
Brand of furnace?
Exact model number of furnace?
Brand of thermostat?
Exact model number of thermostat?

The above info has to be EXACT or we cannot look up the equipment to see what the problem might be. Thank you.

Hi again. Here is the info about my system:
Thermostat - good 'ole Honeywell non-digital round thermostat with two levers. The left one is marked "Cool-Off-Auto" and the right one is marked "Fan On" and "Auto". The gas-fired warm air furnace is a single stage Goodman (GMS8/GHS8/GDS8). Also, there was a label on the bottom of Goodman with Model# CAPF3030B6BA and a Serial# 0802173342.
Again, when I used to set my thermostat on "Fan On" - it would run all night. Yesterday, it started stopping and starting. Although, the house did stay warm according to the thermostat setting but the unit kept stopping and starting in "Fan On" setting. Thank you very much.

The first thing I would do is when the motor stops working kill the power and feel the motor. If it is real hot then the motor OR capacitor for the motor could be defective. I would test the capacitor first if you have the proper toolt to do that and if not just buy a new capacitor of the same value and voltage. You can look on the back side of the blower access door for the wiring diagram. It is a kind a shiney looking silver metal object and it will be wired into the motor circuit wires. Do this first since it is the cheapest part to buy. Then report back with your findings.

OK. Removed all covers, set thermostat to "Fan On" and set the temperature to way above the ambient. Went up to the unit, pressed the door limit switch and sat there for 20 minutes observing the operation. The Circulator Blower turned on, then there was a click, then the Induced Draft Blower kicked-in, then the ignitor, then the flames... all working now. After a few minutes, the flames go off and then the Induced Draft lower goes off, leaving only the Circulator Blower running. I sat through 3 full cycles of this but the blower never went out. Kept running (as it should) through the whole flames on-flames-off deal three cycles. All seems fine. I touched the capacitor (I know what they are supposed to look like) and it was cold. I touched the motor housing - it was cold. I could not reach the actual motor to touch. Tomorrow, I can bring my capacitor tester from work and see if I measure the right mF reading. Other than that - I guess I need to stalk it longer and see when it DOES cut off and then go up and see what's happening. Hmmmmmmm

All seems fine with one exception. You had the blower access door open diring your test and this is known since you held the switch in with your finger during th duration of the test.

Here is probably the problem.
Your return air is somehow restricted or has a dirty filter etc. The reason I say this is because when you had the blower motor compartment open during the test the return air had a large free air opening to suck air from so the problem did not exist with the blower door off.

Retest with the blower access door properly installed since that is how the furnace normally operates. Your problem might show back up.

Excellent. Very good observation. I did not think of it. Tonight, I will test it in normal operation mode. If it starts stopping, I will check and replace the filters. Then test it again. If, after doing the filters, it still goes into stop/start operation - we will have to dig deeper to isolate the problem. For now - thank you so much for sticking with me. I will report later tonight. Thanks again

No problem and be sure to post back.

OK. Last night, I decided not to run the heater until I actually check the filters FIRST! I did. THEY WERE DIRTY. I have washable filters - I washed them well, shook the water out and placed back in. The system ran great - never shut down. I had no idea that dirty filters can cause the blower motor to cycle on/off... Wow. Thank you so much for sticking with me and seeing it through to the end. I really appreciate it. I am new to this type of on-line help. Let me know if there is a way to post a positive feedback for you. Thanks again.

Well I am glad you got it all sorted out. I have never been a fan of washable filters but I do like poleyester ones with available frames to fit the furnace. Fiberglass ones are not any good at all since the only thing they may stop is a quarter size dust bunny. LOL If you hit the scales in the uper right corner of my post you can leave fed back if you wish. As a note for you. There is probably 150-200 years of combined HVAC experience available to the posters here. We have many qualified helpers that try to untangle the web of mystery in the HVAC business.

<snip>Fiberglass ones are not any good at all since the only thing they may stop is a quarter size dust bunny. LOL<snip>

We call them "hog's hair filters" and I tell people that all they stop is mice and small birds. And the former only for a short time. Merv 6 or 8 pleated are my favorite unless the air velocity is too high.

Cool. Great advice. I will check mine and upgrade. Although, it may not be a bad idea to have food for dinner without even going hunting. I may keep my old filters until after 2012 - may need the birds and other animals to fly into my food net. :-)