Furnace filter differential pressure sensing?

Has anyone seen or done a project to sense when a forced air furnace filter needs to be changed? I was thinking about putting a differential pressure across the filter - by using 3.5mm bulkheads.

But I don’t know if measuring the differential pressure across a filter is a good way of determining if the filter is dirty? It seems like it would be, but the details of exactly where to place the ports to avoid turbulence impacting measurement, fan speed potentially changing, etc can really side line your readings.

I’m also wondering what kind of hardware I can use to install the port connector on the ducts. Some kind of 3.5mm tubing bulkhead connector…but I haven’t found anything that seems easy to install.

Just wondering if someone else had gone down this road and can provide some advice.

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This is a good idea. I think the sensor read location would be experimental. I have done some testing using diff pressure sensors in aircraft installations mainly concerning pressure drop validation. We had good laminar airflow through our heatsink so location was not an issue. If there is turbulent air at the sensor the reading will be choppy / all over the place. With a filter I would guess it’s mostly laminar. I think I have a sensor at the shop and may try to hook it up to an ESP32 and see what happens.

I’m wondering with pressure sensors, do you need the tubing to be “facing” the air flow? Or can you just put a tubing into the aluminum ducting perpendicular to the flow? It seems like the way the tubing rests inside the duct would impact readings a lot.

I would try to place it away or perpendicular. That is based completely on a guess as I’m an electrical not mechanical eng.

I think you might get a Venturi effect at right angles. I would face it downstream. i.e. aligned with the air flow but not facing into it. I’m no fluid mechanics engineer though.

Since this was the most pressing issue in my mind for some crazy reason, I reached out to a former colleague. After what felt like a lifetime of physics lecturing, he finally answered and say’s away from the airflow. There was a caveat in there but I had already walked away as I couldn’t deal with another hour of theoretical lecture.

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Did anyone implement this? I think it’s the correct metric to track, but monitoring the blower fan current over time might be an alternative

I’m also interested in this; have the electronics all working, just need to know how to mount the sensors.
Did anybody do it?
BTW, I think Venturi would only apply if there was a change in duct diameter, but it also seems to me like similar things would happen; hence the question/hesitation.
Also, how should I connect some tube to this:
This might help:

Filtrete has some filters with a BLE sensor that emits data about filter lifespan and has a barometric pressure sensor. They cost about $3-$5 more than the non BT ones I’ve seen at stores that carry them.

Someone did a teardown, they’re attached via hot glue (looks to be easily removable and placeable on a new filter if you have a glue gun), and have replaceable coin cell batteries.The person was also nice enough to document some of the BLE output.

Since you could buy one filter and migrate the sensor over repeatedly with a little work, that might be the move if there were an integration.

That certainly sounds like a nice feature/integration to have.
I currently have no BT/BLE devices connected to HA because I worry about the limited distance most BT/BLE devices can communicate over. Do I worry too much about this?
I have some BeeWi lights I wouldn’t mind connecting if distance wasn’t a problem.
Long ago I bought a “Class 1” BT dongle but I think it was fake; never got “too far” with it. Now I never see the class mentioned anymore and while BT or BLE 5 is supposed to allow long distance connections, it seems that’s true only for some devices.
Anyway, The original question still remains, as an ESPHome device could monitor more things in the heating room and even control stuff. Plus it already exists.

ESP32s can be set up as a bluetooth proxy, effectively ending range concerns if you have devices that aren’t in-range of your main HA host’s BT dongle.

I’m just finishing up the design on this device

The sensor support should be merged into esphome soon.

I’ve just installed it on my furnace. I’m happy with the results.

The drawbacks,

  1. The values differ from my professional grade 100$ pressure meter
  2. It needs initial calibration for the offset value

However, the price of the sensor itself (9USD) is a huge savings. Name-brand differential sensors are 40$+ just for the sensor.

My hopes is that while the absolute accuracy might be in question, for the goal of monitoring a furnace filter over time to determine if it is getting dirty, it should be adequate.

I’ve asked the manufacturer of the sensor what sort of minimum order quantity would be required to get a better suited version.

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This is great to know; thank you!
I suppose I don’t even need a dongle if I do that. I wonder what happens if I add the BT proxy to ALL my ESP32 sensors…