Furnace Filter Sensor

Hi Guys,

Just wanted to put out there my thoughts on something to tell me when my furnace filter needs to be changed.  I was thinking about a small pressure sensor in the air return right before the furnace.  You wouldn’t even really need to know exact values, just be able to measure the change in pressure over time.  My thinking here is as the filter clogs up, less air gets through, thus less pressure.

I like to research things for stuff like this, like Pitot Tubes and MEMS pressure sensors, but have no ability to do anything with them.  :slight_smile:

But in the end like like something with real data, this is the change in airflow, as opposed to just “hey it’s been x months, maybe you should change it?”

I like the concept. The data we would need is how much flow of air until a filter is depleted. I don’t know how to physics very much, but if the same amount of air were moving in a clogged (smaller) tube, wouldn’t the pressure and speed be higher? I’d assume pilot tubes are expensive as well.

If you have a thermostat, you could count the time the furnace is on and estimate from there. So its not a general time frame, but a far better estimate.

After a little reading…

What you really want is the pressure difference between the two sides of the filter.

Most residential furnaces are rated to handle a total pressure drop of about 0.5 inches of water - about 0.018 psi.  The filter alone has a pressure drop around 1/5 of that, so maybe 0.0036 psi.  These numbers are apparently pretty unreliable - for example many systems have more than their rated pressure drop even without a filter, due to restrictions in the ducting, across cooling coils, and so on.  Also, filters are often undersized, leading to more pressure drop that the system is really supposed to have.

The bottom line is that you’ll be looking at very small pressure differences and making some non-obvious decisions.

If you want to get serious about optimal filter change times, you might try this: http://www.nafahq.org/pressure-drop-considerations-in-air-filtration/

I really like the idea, but any solution is going need either:

  1. A way for the user to tweak the settings based on whether they think the filter looks “dirty enough” when they are told to change it.  Not optimal at all, but the customer gets what they want.

  2. A serious analysis which considers the pressure rating of the furnace blower, the type of filter media in use, and goals.  There are tradeoffs between air cleanliness, filter durability, and system energy efficiency.

I think I’d be ok with option 1.  I realize nothing is going to be perfect (at least in v1.0) but combining some hard-ish pressure data with counting the number of hours that the fan is active might be able to tease out some interesting results.

Ultimately, I’d like to know how things like pet hair or pollen from open windows in the spring changes the life of a filter.  I imagine that after a year or so you could say “Well, we’re only at ### hours but the air speed has dropped off, must be clogged with something!”

V2.0 will solve all the worlds problems. :slight_smile:


As far as Wikipedia will tell me a Pitot tube is just a bent pipe, http://en.wikipedia.org/wiki/Pitot_tube.  As the pressure sensors like in a Fitbit come in varieties sensitive enough to tell not what floor you are on a building, but what step, based on air pressure.

All that’s required is MAGIC.

Another possible way to go here would be a light and a light sensor.  You put a small light on one side of the filter and a light sensor on the other.  Measure the output when the filter is brand new and then as the filter “fills up” the light diminishes.

The advantage here would be being able to test any time, not just when the furnace is running.  You could even, with a little tweaking, get this running with existing z-wave products.  I’m pretty sure there is a sensor that will just say if it sees light or not.  So… when your filter is dirty next time, setup your equipment and adjust the brightness of the light such that the sensor can NOT see light when the the light is on.  Now replace the filter and the sensor should see light.

If all goes well you should be able to send a zwave command to turn on the light and the light sensor.  Then the sensor should report back either: I see light or I don’t see light.  If it reports that it doesn’t see light… then you need to change the filter (or the light :slight_smile:

Well, if any tinkers are looking for a project, I just got more interested in an moving air sensor.

Dryer vent was collecting lint on it from the plastic aging and cracking. Pulled out a plug of the stuff and I don’t know how long that’s been happening.

That and in the past my furnace fresh air intake has blocked with leaves.

A little SmartThing that could tell me when something has changed in the airflow would be nice.

All good things come to those who wait:

Now we just need them to add in a Zigbee radio or something. But I’d prefer if the measurement was being done at the filter itself, of course.

Hah, looks like the link didn’t work: http://www.kickstarter.com/projects/333435704/filterwatch-low-cost-easy-to-use-air-filter-monito

I like your idea, but I have a different use for a sensor like this. I have a vacation home that has the heat on in winter time, set to 50 degrees to keep the pipes from freezing. I have a Raspberry Pi running Z-Wave that does temperature and humidity monitoring and I’d like to be able to determine when and for how long the furnace runs. By doing so, I can calculate fuel usage and know when to call the gas service for a refill.

Any solutions for this come about?