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:
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:
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.