anyone have an idea about this ?

My furnace fan is running 24/7 and make no special sound.

But when the heat turn on the fan start vibrating and making noise in the house. If I remove the panel (allowing lots of air in) the fan stop shaking.

Yes the air filter was changed :)

The furnace is a Notron by Broan 21B25M.

Help would be very nice :)

Thank you

  • In my 25-year-old basic Carrier hvac gas furnace the blower fan runs at higher speed for a/c or when the the fan on is selected. I believe that at the heat setting the fan runs at a slower speed. If there is a resonance at the slower speed, there might be more noise then. What kind of filter are you using? Some people use a filter which is too restrictive and slows down the fan motor too much causing it to labor. If you run the fan without this filter in place, how does it sound? Jan 26, 2017 at 19:58
  • 1
    Does this furnace have an electric resistance heating element? That is, this furnace is not a fuel gas furnace right? If so forget my comments about a flue induction motor. If it is electric resistance and the electric supply is inadequate, maybe the fan gets noisy when the heat comes on because the voltage to the motor drops below an acceptable value for the motor to run properly. Jan 26, 2017 at 20:56
  • Fans usually required periodic lubrication to keep the bearings happy. Have you been keeping up on that? If not, you might fix the problem by giving it a good greasing, as per the manual, now. Jan 27, 2017 at 1:19
  • Thanks Jim and Wayfaring ! Jim I am not sure how to check that ! I guess I will need to call a tech.. Wayfaring I didn't think about that !!! I will look if I can lubricate it. Thank you Jan 27, 2017 at 4:11
  • Definitely get service from a qualified tech. Jan 27, 2017 at 12:48

1 Answer 1


The blower fan should not be running continuously. You must get that repaired. Is the switch on the thermostat set to auto or to run? The thermostat can be incorrected wired in a way that keeps the blower fan on continuously.

The filter below the fan may be too restrictive or the ducting leading to the filter is constricted. Put in a new filter of the proper type. Some filters are too restrictive for a given furnace.

But note that these fans must have a certain load on them supplied by suction on the back side and/or high pressure on the outlet side from the combined resistance of the heat exchanger, the evaporator coil, and the ducting. If they are run without sufficient load, the motor will over-speed and burn out.

The quieting when you open the access panel might be due to the fan speeding up and getting out of a resonance that is causing vibration. Also the bearings on the fan motor might be bad or need lubrication. Some of these fan motors used to have bearings which required oiling through channels but most nowadays are permanently lubricated.

  • See this about MERV rating of hvac filers ontimeairfilters.com/air-filter-merv-rating Jan 26, 2017 at 21:51
  • Hi Jim and thank you for your help !!! I keep the fan running 24/7 because it seems it's the only way I can get enough heat in my basement. I could try to put it back on automatic. Jan 27, 2017 at 4:01
  • Hi Jim and thank you for your help !!! I keep the fan running 24/7 because it seems it's the only way I can get enough heat in my basement. I could try to put it back on automatic. The filter is Filtrete 300 Dust Reduction Air and Furnace Filter. 20x20x1. When I remove the filter nothing change, I did shut down the furnace and made the fan spin with my hand and it was spinning freely and it look like the fan is not loose. I will try to see if I can lubricate something or maybe change the fan ?? Thanks again Jan 27, 2017 at 4:08
  • Don't change the fan or the motor without evidence that it is bad. Don't squirt lubricant in and around the fan unless you see a channel for oil down to the bearings. Some people think a shot of WD40 is the cure for most everthing and can't ever hurt. This is not true. Find the true source of the problem before you start doing things that can go wrong. Look up the MERV rating of this filter you are using. Jan 27, 2017 at 12:41
  • In our central HVAC each register (duct opening into a room) has a constriction adjustment. Do you have this and how are yours set? Can you open the constrictions in the basement and or slightly close all the ones in the rest of the house. This would give more heat to the basement. Jan 27, 2017 at 12:45

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