DIY HVAC system over pressure device


(Mike Maxwell) #1

If you’re interested in a for real way of integrating an HVAC system pressure alert/trigger into SmartThings, this is for you.
I came up with this idea a few weeks ago, after being unable to make any factual use of Keens built in pressure sensors while developing the Keenect vent control smartApp.

You can build this bad boy for $50 bucks.
What you need:
–an adjustable HVAC differential (dual ported) pressure switch, such as:

–an ST compatible contact sensor with internal aux trigger contacts, such as:


–Some pressure probes (the LEFOO unit comes with them, and some hose), or you can get the real deal units like these (you’ll need two)

OK, now what…

  • Connect the pressure switches NO contacts to the contact sensor aux contact inputs.
  • Install the probes in your HVAC ductwork.
  • Connect the probes to the sensor with some bits of hose, (+ sensor port goes in the main supply plenum, just after the furnace, the - port goes into the return plenum between the furnace and the return air filter.
  • Set the pressure setting, and you’re done.

This will be the supported pressure management system in the next version of Keenect, the vent control smartApp.

[RELEASE] Keenect V1.2.0, optional separate vo settings for cooling, vent obstruction auto clear
Keen Smart Home Vents
(Matt) #2

I made my own version after seeing what he was up too. Easy to build and cheap. To install (I have a package unit HVAC) I just ended up removing the screws from the AC to House tin cover and drilling a 1/4" hold in each duct connector right by the unit.


(Devesh Batra) #3

My order should be here on Friday

If i get it right,

  • the first probe needs to be AFTER the Air Filter and BEFORE the FAN

  • the second probe needs to be AFTER THE FAN and main unit in the duct going to the house

I did not order the pressure probes ? Are the ones in the package with the pressure switch no good ? If so i can order the other ones and hope they get here on time

(Matt) #4

you what them as close to the unit as possible. If you look at your AC there is a duct coming in from the return (filter side) and one feeding the vents. Put the probes right where the ducts hook to the AC unit. If you ordered the stuff from my post the bottom port will go on the side for the vents and the top goes on the side for the return. You can test it by setting the pressure low (say 60) and then turn on your fan. if it trips the switch you have it right

(Mike Maxwell) #5

The probes that come with the LEFOO are simple tubes, the other style that I’ve shown are not subject to to velocity pressure influence, so they are more accurate.
However, in this use case I’d venture it makes no difference what so ever.

(Matt) #6

This is what you get with the LEFOO it is all you need its just a tube to feed the static pressure to the LEFOO.

(Devesh Batra) #7

ordering the other tubes now…


(Matt) #8

did you get the one I built? If so you dont need tubes

(Eric) #9

this is a fine idea to pipe across the filter for replacement notification. It’s a moving target to decide what pressure represents “dirty filter”, but monitoring differential pressure is at least reality-based.

(Matt) #10

all it take is measure flow with a clean filter then measure again with one that is worn out. See what the difference is and you know where to set your sensor.

(Justin H) #11

I did a very similar thing to monitor my radon mitigation fan. Now, if the fan shuts off or the vacuum pressure drops, I’ll get a notification!

(Devesh Batra) #12


I have a HVAC unit (heat/cold)

My Amazon packages are scheduled to be delivered on Monday


(Matt) #13

HVAC unit? Thermostat???

(Kavvy) #14

Is there another pressure sensor that would work? That one is sold out.

(Mike Maxwell) #15

Amazon carries three other models from the same manufacture, these have different pressure ranges, but any of them would work, since you’re looking for about a 200 pa trip point.

(Matt) #16

here ya go

(Kavvy) #17

Thanks! Added it to my cart.

What section of Keenect has the configs for this pressure sensor? I searched last night and couldnt see where it would be configured or called.

Also, any specific device type for the DIY pressure sensor, or should I use the common device type for the contact sensor

(Matt) #18

its jsut a contact sensor.

its under Advanced. Labeled “Over pressure contact”

(Kavvy) #19

Hmm, guess I missed it. Thanks!

(Kavvy) #20

How much back pressure is too much?

Or, at what setting should the sensor be set to? I had mine set between 150 and 200, and it would trigger.

Changed it to 200, as the OP has shown, and opened all the vents and it’s still “closing” the contact sensor.