A few lessons learned

A project log for Ventbot: warm side cool, cool side warm

A DIY register booster project to even out the temps around my home.

wjcarpenterWJCarpenter 06/03/2023 at 18:170 Comments

With all five of my Ventbots now installed and operational for about a week, I can make a few "real life" observations.

First, vent openings can be tricky things. They look all nice an regular when covered with a register grating, but look inside and you might possibly find all kind of bent metal and force-fitting operations. Each of the vent openings that got a Ventbot was a little bit different, and each required some cuss-based persuasion to get the register grate back in place after placing the Ventbot fans.

I made the thickness of my fan brackets 2mm for durability, which I thought was also pretty thin. I didn't count on the tight tolerance between some of the vent openings and the register grate sides. I probably would have made the brackets thinner, or maybe even added an OpenSCAD parameter for a different thickness for that part of the bracket (I will probably still do that for the benefit of others, and to stop the nagging in my brain), except that I had already printed all the brackets twice and had all the fans screwed in place. So, more cussing, and I squeezed things into place.

When I did my initial testing for temperature triggers, I observed quite a large swing. I defaulted the configurable triggers to the values shown here. The idea was that these temperature extremes would never be reached if the HVAC was not doing it (at least for a house with both heating and cooling HVAC).

  ## CONFIG ##
  # If the temperature is above the high trigger, the vent is blowing
  # heated air. If below the low trigger, the vent is blowing cooled
  # air. Boosting is desired for both cases.
  HIGH_TRIGGER: "85.1"
  LOW_TRIGGER:  "60.5"

Three of my five Ventbots had a "heat on" temperature significantly lower than that and did not turn on when the HVAC blew heat. I haven't observed it yet, but I imagine those same rooms will have "cooling on" temperatures significantly higher than that. I configured those Ventbots to have triggers that were closer together. In at least one case, the new triggers are kind of close to the comfort band temperatures for the thermostat, which means they might be reached without the HVAC being active. On a warm summer day or a chilly winter day, the Ventbot might turn on its fans to accelerate blowing warmer or chillier air into that room. I'm going to let this one ride for a while to see how it goes, but I think that I will most likely eventually change things so that the Ventbots ignore their local temperature sensors and get turned on and off by Home Assistant service calls triggered by events coming out of the smart thermostat.

One of the side-effects of a Ventbot having a lower HIGH_TRIGGER is that it takes longer for it to decide to turn the fans off. The neutral zone is narrower, so it takes longer to get there. Look at the purple lines in this chart. (Despite the Home Assistant optical illusion, that purple line is for my orange Ventbot. Not my fault.) The top half is tachometer readings for Fan 1. The bottom half is temperature sensor readings, which also illustrate my point about the different peak temperatures for "heat on".

Finally, when I bought the 12vdc power supply wall warts, the longest cords I could get were just short of 2 meters. That was right on the edge of being long enough for use with my prototype Ventbot, so I also bought a collection of 2 meter extensions with the same barrel connectors. In three of the Ventbot locations, the electrical outlet was pretty close to the vent; in two of those cases, it was on the wall right beside the vent. I could have skipped the extensions in those cases, but the power supplies themselves each had a ferrite bead that made it impossible to push the end through the register grates we have. Doh! I've ordered a few inexpensive register grates that have a different grillwork pattern that I hope resolves this issue, though having a wire passing through the grate is not an elegant look in any case.