It all started in January, when the cold really settled in. I started to notice that when the furnace was on, the airflow from the vents was inconsistent. In the early morning when it would start up (coming up to the daytime setting from the lower overnight setting), it would blow for a few minutes, but then stop. Then start. Then stop. Etc.
As with most things in my life that are not a dire emergency, I kind of ignored it for a while. But I eventually called in an “expert” — an HVAC contractor that was on the list of approved contractors from the Bosch web site. They were able to come by a few days later (it was not a “NO HEAT” situation, and there were people without heat). The very polite tech came in and made some changes to the thermostat’s settings, and went on his way. Basically, he changed things so it would use the aux heat (gas furnace) more often, and the heat pump less often. Well, I knew that wasn’t the end of it, because the problem was most evident (to me) when it was using the gas furnace! Sometimes, the furnace would lock out, requiring a circuit breaker reset to get it to try again.
They came out again about a week later, this time more willing to listen. And the technician was able to experience the problem when it happened, and he said, “Yup — the blower needs to be replaced.” which was what my guess was, too. However, what I didn’t expect was for it not to be covered by warranty (it was installed < 5 years prior), and for the price to be $1827. Yikes! I tried to see if I could get a hold of the original installer, but he was a small contractor doing this kind of work part-time (his full time job was an HVAC technician for M.I.T.), and it seems that he quit doing that kind of work shortly after installing our system. I also tried to find a “warranty friendlier” contractor, but came up empty there as well.
So I “phoned a friend” who has a lot of knowledge and expertise in this area, and he thought it unlikely that the blower was bad, and that it was more likely that one of the furnace safety switches had gone flaky. The Bosch furnace has a lovely single LED indicator that blinks out a numeric code if there’s a problem. I had only ever noticed it at 11, when it locked out. I told my friend, “If I were smart, I would hook up a webcam and some software to monitor the LED…” but it sounded like a hassle. However, it also seemed like the best way of getting some diagnostic data to get a better handle on what the problem was. If I failed to mention it, this unit is in an attic crawl space, so it’s a pain to get to.
I then “consulted” with ChatGPT about the idea. It suggested that the camera module (ESP32) I had was underpowered for doing that kind of image analysis work, but that an ESP32 module with a light sensor would be cheaper and the code would be easier. So I set off down that path. “We” were originally going to have 2 light detectors, and put filters over them so we could tell if the LED were lit in green or red. But the filters ended up cutting so much signal that all that was left was noise, and the detector was unreliable. So we ended up with a single detector, essentially attached to the LED via a small tube. And this worked great!
Meanwhile, I had also purchased and hooked up an airspeed sensor module, so we could monitor the airspeed through the main trunk duct (see the graph above). I was able to show that no matter whether the fan was on manually, or for the gas furnace, or the heat pump, it just ran very unreliably. It would run for a few minutes in a row when cold, but once it warmed up it became very erratic. Then the LED monitor would sometimes show we had gone to code 4, which indicated that the overtemp device kicked in, and had shut down the furnace. Often this would escalate to a code 11, where you would have to manually turn power off and on to get it to reset.
So all the evidence was pointing to a bad blower, and in fact, to the Electronic Control Module (ECM), which probably had a bad capacitor or something similar. But you can’t just replace the ECM — the motor and ECM are essentially a single unit. So I looked around and found a place to order the blower motor from, and ordered it. It arrived this past Thursday (19 March 2026), and I waited until today (Saturday) when Tyler would be available to help with the install.
It took just about 2 hours exactly to do the swap. After turning the power off, we had to disconnect a ton of wires,
and document what needed to be connected where. After that, the entire blower unit with the control board can slide out. Once we had the unit out, we were able to bring it to the basement and do the swap.
This involved loosening the squirrel cage from the motor shaft, then removing the suspension from the housing. Then getting the squirrel cage all the way off, then removing the suspension from the motor housing.
That got us to the halfway point, so it was time to turn around and head back — starting with attaching the suspension to the new motor, then the motor to the unit, then the squirrel cage to the shaft. Then we brought everything back upstairs, and reinstalled it into the furnace housing. Finally, we turned the power back on, and monitored and tested everything.
And we have been successful! Everything now works as expected, and the fan ran for 20+ minutes with the heat on, and turned off when it hit the set point. I then ran the fan manually for 90 minutes, and it ran consistently. So, all looks good! And instead of paying the pros $1827 for this service, we paid $752 for the blower and did the work ourselves, saving over $1000!
