Dead HVAC fan repair

Couple of weeks ago, upon returning home from a short trip we found our house much too warm and after quick inspection found that central AC was not working. First suspecting blown fuse i went downstairs hoping for a quick resolution. Alas that was not the case. Furnace (serving as the blower) had power and AC was working as i could hear fan outside as well as the coolant running through the pipes.

After opening up the furnace i found a board with blinking lights repeating pattern 3 green 1 orange and infrequently changing to 3 green 3 orange. These codes translated to blower motor not running at the right speed or not running at all. Starting a search for the solution on the net I’ve stumbled on this long thread which suggested that this might be due to a failed thermistor and that repair could be as simple as replacing it. Taking the power supply off the motor i was happy to find a charred and broken apart thermistor.

The part number was SG348 and after further research i found that this was a Inrush Current Limiter – a thermistor with negative temperature coefficient (NTC) which means that it’s resistance decreases as it gets hotter.  The primary purpose of it is to limit starting current in motors and transformers. This particular ICL thermistor has a resistance of 1 Ohm when it’s cold and can sustain 20 Watt of power. It was made by RTI Electronics under the brand name of SURGE-GARD and was often used in GE motors. The SG348 is not longer in production, but SG100/SG301 can be used instead. In fact any NTC thermistor with room temperature resistance of 1Ohm and 20A maximum current (or 1Ohm*20A=20Watt maximum power). In fact  DigiKey had 3 parts satisfying that requirement with 570-1041-ND being the cheapest at $1.77

Finding the replacement part was easy – finding it locally proved impossible. So after calling all of the local electronics stores I had no choice but to order from DigiKey and wait for the part to arrive. In the mean while i started looking for a work-around. Some people on the forum suggested short-circuiting contacts where thermistor was but i found it too risky. I decided to place a fixed resistor in place, however finding one that would not melt down at 20A was also not easy. Luckily, a local Radio Shack still had some memories of it’s former glory and carried 1 Ohm 10W resistors. Putting them in parallel to distribute the load i had a temporary work around:


Workaround using Radio Shack's 1Ohm 10W power resistor


This solution was a better alternative to the short-circuit however it reduced the efficiency of the fan essentially by wasting energy in the resistor all the time rather than only during startup as in case of a ICL thermistor. The picture above is after fan has been working for a week or so and you can see those resistors heated up quite considerably and even melted down part of a rectifier nearby. There was also a discoloration on the end of one of the resistors.  so i can certainly can’t call it a perfect solution but it worked for me.

Here’s how the power supply looked like with the replacement parts in place:

Replacement Inrush Current Limiting Thermistor in place


At the end, the most impressive part for me was the power internet allowing free peer to peer sharing of knowledge. I was pleasantly surprised how easy it was to find this knowledge on the net and how valuable it was – was I call a HVAC technician it would cost me several hundred dollars in replacement power supply or even entire motor.



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20 Responses to “Dead HVAC fan repair”

  1. Kelly Says:

    Thanks for saving me some time on fixing my motor controller. Will need to wait a few days to get it from Digikey as we don’t seem to have a good parts house here in Minneapolis Minnesota either (at least the part should not take as long to get from central MN)….

  2. Chris Says:

    I’m an HVAC tech and I just had a bad motor module on a Carrier furnace yesterday. The thermistor that you speak of was bad. As a temporary fix, I tied the two conductors together (as if it were never there) to get them heat for the night. Nearly every company out there will not let their techs solder in a new thermistor because to replace the motor module will make more profit. Also, most techs don’t know how to use a soldering iron for electronics. It might create a problem when something on the furnace control board fails that us techs can’t begin to fix and they see that we fixed someone’s motor module. I think I may start carrying a few of these myself though. Anyhow… you made a good catch that many techs struggle with. Good job!

  3. Mr. Timewise Says:

    And another “Thank You from me!

    I replaced the RTI “Surge Gard” SG348 (1 ohm, 20Amp NTC Thermistor) with a 1 ohm 30Amp version from Ametherm (p/n SL32 1R030; Digikey p/n 570-1059-ND). The 30 Amp version has a higher energy rating and a slightly higher dissipation constant.

    Actually, since we have two identical HVAC systems (upstairs and downstairs) I bought an extra SL32 1R030 for an expected eventual failure of the SG348 still present in our other furnace.

    BTW…the Trane brand 115V blower motors (actually from GE) in each system were labeled as Trane Part # D340316P13; Replacement #MOT5432S. The furnaces were built in 1998/1999.

  4. Mr. Timewise Says:

    A correction to my earlier entry…

    I installed a 1 ohm – 20Amp – version from Ametherm (p/n SL22 1R020; Digikey p/n 570-1041-ND, not the 30 Amp version.

    I used the 20 Amp version instead of the 30 Amp version after some consideration regarding the normal operating current drawn by the 1/2 HP blower motor (8.0 Amps). The lower resistance (about 0.025 ohms) of the 20 Amp version of the thermistor during continuous running of the motor would dissipate less heat. The 30 Amp part would have a resistance of about 0.05 ohm with a current of 8 Amps running through it. That would mean the power loss in the 30 Amp thermistor would be about 3.2 watts instead of 1.6 watts in the 20 Amp version.

    • abieneman Says:

      I believe it’s little bit more complicated but ends up in favor of 20A thermistor anyway. Resistance is dependent on temperature rather than current and temperature depends on current but also how well component can dissipate heat. To give an extreme example, if you take your 20A thermistor and put it in an ice bath then it’s temperature is not going to raise by much and it is still going to be around 1 Ohm even at 8A steady current.

      According to formulas on wiki (, dissipated power P = I^2*R = K(T-Tr) where K is dissipation constant (from spec sheet) and Tr is room temperature. You can also approximate R goes linearly from 1 Ohm to Rmin when temperature changes from T0 to Tmax (from spec sheet): R = 1 Ohm – A * (T-T0), where A is how much resistance falls with each degree C. A = (1 Ohm – Rmin ) / ( Tmax – T0 ). Putting them together yields power, temperature and resistance as a function of current:

      T = ( K*Tr + I^2 *(1+A*T0) ) /( K + A*I^2 )
      R = 1 Ohm - A*(T - T0)
      P = I^2 * R

      So for steady current of 8A, room temperature of 25c this gives:

      20A thermistor: T = 176 c, R = 0.076 Ohm, P = 4.90 W
      30A thermistor: T = 193 c, R = 0.119 Ohm, P = 7.63 W

      However as the ambient temperature goes up so does thermistor’s temperature which decreases it’s resistance lowering power loss. So at 50c air temperature power loss is 4.15 and 6.63 W for 20A and 30A thermistors.

  5. Jim Haddock Says:

    Many thanks. Some of the detail got a bit over my head, but you helped me out… a LOT!

    I ordered by the thermistor from Digikey. I did call Radio Shack, but the person that answered the phone had never heard the word thermistor. I assumed it wasn’t worth the 10 mile round-trip to look at their shelves.

    I went ahead and jump’d it (wire – no resistor) while waiting for the new part. I reasoned that if I could get it started once and keep it running (fan forced ON at the thermostat) there wasn’t much risk operating without the inrush limiter. Thoughts?

    • abieneman Says:

      Good idea to keep fan running to avoid on/off switches. Some people say that they run it short-circuited for a while but better safe than sorry.

  6. Keith Says:

    Thanks – I agree on the power of the internet. In less than 30 minutes, this helped me diagnose, order the right part and be able to fix what would have cost me $1500! THANKS!!!

  7. Al Says:

    Is there any safety exposure to using the 30A thermistor over the 20A thermistor?

    • abieneman Says:

      I am afraid this goes beyond my level of expertise. I would think 30A would be less safe as it acts like a 30A fuse so it will permit constant current up to 30A to flow without burning up which in turn could burn up the motor. 20A thermistor will burn at 20A perhaps saving motor from overload sooner than 30A one.

      • Al Says:

        Thanks and I tend to agree with the protection side of it It turns out that I have the 1hp motor and that uses the SG379. In limited searching, I *think* I can conclude that the SG379 is a 30A limiter.

  8. Eric Says:

    Open question; I’m thinking of mounting an automotive fuse socket on the outside and making the thermistor quick change. I realize the first one took over 10 years to blow, but how nice would it be to just remove the compartment cover, yank the dead one and insert a fresh one. I’m wondering though if it has to be in its proper place to function properly.

  9. steinbaugh Says:

    Just had mine replaced, under warranty cost $207 for labor. Replaced entire motor assembly, but I will keep this in mind, for next time. Unit is six years old, and the motor itself looked fine, with no shaft play. Tech said there were bad batches of motors, and this seemed not unusual for him. Wish I could have kept the old one, to inspect closer.

  10. Mike Kolcun Says:

    I was hoping that this would be the same issue on my motor. Unfortunately upon opening the motor control module, the thermistor looks fine, but the molex connector that connects to the actual motor is all burnt out. It doesn’t seem possible to fix, as the majority of the components within the control module are encased in some sort of potting material.

    Thanks for the post!

  11. Rob Says:

    Thank you for your informative post about the (DEAD FAN REPAIR).
    I also had the same issue with the ECM on my motor on a 6 yr old furnace.
    After calling a tech to come to tell me it needed a new motor that would cost $1550.00 plus install and for a few dollars more $2000.00 to $2500.00 for a whole new furnace installed!!! This made me think, I spent that money 6yrs ago to buy a so called high efficiency furnace to save money. In 6yrs I don’t think I saved anywhere near that amount, so I turned to the internet and after hours of researching I came by this blog and was happy to find this post which gave me a sigh of relief. The repair will now cost me less than $20.00 and a few hours of my time.


  12. Martin Hamel Says:

    Thank you so much!! My furnace went dead over the weekend and I investigated what was wrong with it. After looking into it and online, I found that this was the issue. After looking for a fan motor online, and finding a thermistor, I saved about $698,50… It was a breeze to replace the thermistor. Two thumbs up for your write up!!

  13. Robert Kacmarynski Says:

    I had the same problem. Woke up to a cold house and diagnosed the problem as the blower not coming on. Since it was a Saturday I had to call the emergency service number and they told me it would be between $800 and $1000 to fix it. Thanks to you I found the part at Digikey for $1.64 plus shipping. Amazon also sells a compatible part for $6.46 and free 2-day shipping if you have Amazon Prime. (Ametherm SL22 1R020-B)
    While waiting for the part I did the same Radio Shack resistor fix you did, except I made a set of longer leads so the resistors could be outside the housing so any excess heat is dissipated in the air-stream (not causing overheating or melting of recitfier). Thanks so much you saved me $1000 right before Christmas….

  14. Eric Says:

    Just a thought; While i waited for parts, I installed an old multi speed squirrel cage, that, with very few mods, fit my case. the Carrier A/C didn’t mind a bit, I’d imagine the heat would be okay with it also. i just triggered it from the thermostat & included a 24v relay. i just selected an appropriate speed and wired it for that. I’m sure that would knock the efficiency down a bit, but 97% efficient isn’t worth much if you have to throw a grand or 2 at it every few years. I pulled it out when the new thermistor came, and it is back to normal, but if it ever gets to the point of actually needing a whole replacement motor, this will go back in, and see it through its old age.

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