DIY HVAC system over pressure device

So with all the vents open, my hvac unit trips the sensor at 250.

Can I assume that setting it between 260 and 280ish would be safe for my hvac unit?

250pa with all vents open seems high to me, however I’m not an HVAC engineer.
You might want to track down the technical specs for your unit from the manufacturer, the maximum static pressures should be listed in there.
If the pressures you’re seeing with all vents open are beyond those listed in the docs, then there’s something else up with your system since it probably isn’t operating at the efficiency it was designed for.

The only thing I’m finding is my CFM, and the dry to wet drop in pressure.

It’s 1600cfm with a drop across the A coil of 55pa wet (im assuming thats when the AC is running, which that’s about what its dropping. When the fan is running, it trips the sensor at just below 200.

The outside unit has a high pressure lalevel of 278psig, but I’m not sure how that ties into this equation…

I cant, for the life of me find the static pressure when AC is running.

Is this still the best setup for an alert for over pressure with my HVAC? I have a zoned system and I’m having trouble with a zone controller failing, so both zones are closed with the fan on.

I’d like to get an alert if this happens again to prevent damage.

Are there any other options?

Maybe something like this (I have no idea the pressure ranges needed)…

And this…

1 Like

sure, but why go through all that?, all one needs is a set point event trigger, the actual value as an attribute has little meaning in this context.

pressure safety switch hardwired to stop the fan, is the safe way to manage it.

https://www.grainger.com/category/duct-pressure-switches/hvac-controls-and-thermostats/hvac-and-refrigeration/ecatalog/N-yvc

I need a little help folks. I bought the exact linked items listed above in the OP

Tried to install today till the heat drove me out.

The door sensor always reports closed. Never ever changes

I used some misc wire to connect the left and right most terminals on the pressure gauge to the screw terminals inside the door sensor. Then as soon as I let smart things find the device and named and saved it , it immediately reported “closed”. Changing the dial from 60-300 made no difference.

I then tried removing the smart things device took the battery out. Disconnected one of the screw on wires on the door sensor and let it find the device again. Same thing. No change.

I grabbed the little magnet and rubbed it around the door sensor and nothing ever made it change to open. Heck then the battery fell out if it (can’t find the sucker in the dark up there) and it still reports closed!!

Do I have a bad sensor maybe? Something else I’m doing wrong?

Even now I’ve come down showered and the battery isn’t in the sensor and smart things still says it’s closed.

Instructions on the sensor said once paired the led would be blue but this led flashed yellow or green (I’m color challenged) during setup and then went solid once smart things found it. Never blue.

Help much appreciated

Very cool, Has anyone had any experience with the smart vents…
I was thinking of using doing some zone control and when some of rooms go un occupied have vents close down a little and when so many are closed it builds pressure in the plenum and tie the pressure switch to a relay that switches blower to low speed. Most residential units have 3 speed blower motors, A/C is usually high and heat is med. However to put smart vents in are not affordable from my pricing it would take a while for them to pay for themselves.

Happy 4th of July. Help appreciated on my above post. Thanks

Just touching base with this thread. Do I have to use special code to recognize the door sensor as something else? I’m not YET using Keennect, as I’m easing into this whole thing and wanted to see where my static pressure is first.

I ordered a new door sensor because I’m wondering if I got a bad one. The battery has been out of it for 20 hours now, and Smartthings still shows it as 99% battery and reporting as closed.

Yesterday I posted this…

The door sensor always reports closed. Never ever changes

I used some misc wire to connect the left and right most terminals on the pressure gauge to the screw terminals inside the door sensor. Then as soon as I let smart things find the device and named and saved it , it immediately reported “closed”. Changing the dial from 60-300 made no difference.

I then tried removing the smart things device took the battery out. Disconnected one of the screw on wires on the door sensor and let it find the device again. Same thing. No change.

I grabbed the little magnet and rubbed it around the door sensor and nothing ever made it change to open. Heck then the battery fell out if it (can’t find the sucker in the dark up there) and it still reports closed!!

Do I have a bad sensor maybe? Something else I’m doing wrong?

Even now I’ve come down showered and the battery isn’t in the sensor and smart things still says it’s closed.

Instructions on the sensor said once paired the led would be blue but this led flashed yellow or green (I’m color challenged) during setup and then went solid once smart things found it. Never blue.

Help much appreciated

So I ordered a second eco link sensor. This one paired right up. Connected the wires and works perfectly from what I can tel.

The odd thing is that with all my vents wide open the static pressure gauge trips at around 50 Pa.

With the 4 keen vents all closed the sensor trips at around 80 Pa.

Seems too low to believe. I’m not sure if that means I’m in no danger of causing an issue. Or if the pressure gauge is somehow yo working correctly.

Help appreciated.

Sounds like your pressure sensor ports are connected backwards, try swapping them…

I will go up in the morning and try swapping them. I never tried that but I did visually confirm twice that the + terminal was connected to my supply port etc. I’ll swap and see though.

Would I get a subtractive number with them backwards?

Something is backwards, the numbers you reported are not possible…

Agree it seems odd but does seem possible. This is a heat pump so all components are inside the air handler. I drilled a hole for the supply port in the duct plenum right off the supply end prob 5 inches from where the plenum seals against the air handler box and about half way (top to bottom wise)

I couldn’t go out far from the unit because it’s a classic spider web duct layout. Aka a plenum box with a bunch of flex takeoffs going to each room

The return side, I just drilled the same hole right there 5 inches from the back entrance of the air handler

So in summary the values I’m attempting to measure will be just ductwork and filter. I use the simplest mesh filters.

With vents open at 60 pa that’s like .25" wc. Certainly seems low but not just insanely low. Admittedly I was expecting about .35 all vents open and at least .5" with the 4 keens closed. (There are 5 other vents open with no keen vent installed.

Def seems off agreed. I’ll try the swap in the am but outside that I don’t know how to verify the number. Don’t have another pressure meter.

Wc = inches of water column, 5 minutes and a foot of clear hose and you can build one for calibration purposes.

You know Mike your post was a “duh” moment. I’m an engineer and I know what wc means but just using some hose somehow didn’t cross my mind. I’m disappointed in myself. Lol

Anyhow, I rechecked this morning. Reversing the tubes resulted in the switch not tripping at all

Rechecked with the tubes in the original setup and with all 4 keen vents open it trips at about 100-110 pa. And with all 4 keen vents closed it trips around 125-130 pa.

Higher than my original setup but still lower than I woulda thought.

That means with those 4 closed I’m at about .5 inches wc .4 wide open. Maybe Friday the coil was dryer? No sure that would affect the external number though.

I would have expected at least .6-.7 with the 4 vents closed.

I set the gauge to trip at .6 as my safety just in case but don’t think it will.

My original entire problem has been not cool enough upstairs in the summer.
When we first moved in last July it was 82 in my sons bedroom at 7 at night and ac couldn’t pull it down after running all day

I’ve since added 12 inches of cellulose insulation (helped some). This allowed his room to hold 77 at 92 outside.

I then added 4 keen vents to push air away from our cool (north facing) master bedroom and towards my sons south west facing bedroom This helped more but still would let his room reach 76 at 92 outside.

Then I had a powered roof exhaust installed 2 weeks ago which has helped more because my attic design has tons of soffit vent but only about two ft of ridge vent do the heat couldn’t escape

This helped further such that now his room will hold 74-75 at 95-96 outside.

I haven’t installed keenest yet. I’ve set time conditions such that our master bedroom 2 keen vents close at 9 am. Then a 4th bedroom over the garage has two keen vents. At 4 pm I close one of those which lets that room cruise up to 76 and my sons room holds 74.

Then at 9 pm they all reopen to make quiet to sleep

I put a smart switch on the attic fan to force it off at 9 also because the attic was so hot that its internal stat caused it to run all night.

We have not hit 100 here yet but we usually will have a dozen or so days over 102 temp (not heat index)

I was worried about over pressuring the ductwork.

In the afternoon when his room is 74-75 our master is 69 even with two keens closed. There’s two more vents in our master bath plus the leakage of the keens that are still allowing overcooling in there.

I was worried about the static pressure but based on what I’m seeing I think I’ll install two more keens in the master bath to force even less cooling into our bedroom.

With my static measuring this low it makes me wonder if my ducts have significant air leakage.

Also makes me ponder if my upstairs was originally a2.5 or 3 ton unit and somehow when it was replaced in 2010 if some moron contractor somehow convinced the owners that a new unit could fool more with less and installed a 2 ton which overall is too little tonnage but results in lower than typical duct static.

If these changes don’t prove to be enough my last step is to add an additional supply into each of the two southwest facing rooms to really blast those with cold air.

1 Like

Does anyone think these are more reasonable ranges or am I likely still way off somewhere?

Thanks