Full Version: One Weird Thing Killing Furnaces and ACs - Duct Pressure

[@peteerickson9759]

This is great. Just had my heat pump installed yesterday (i am in Seattle) and when they were testing it, static pressure was coming in at 0.8. Your video helped me understand why that wasn't going to cut it. The macro problem in my system is the ducts are too small. Short-term fix was to reduce the static pressure / fan speed setting and add a return grille on the (too-small) air drop, but this seems sub-optimal to me. This contractor doesn't do much duct rehab, though; gotta find one who can get me a BAD and upsize the main supply trunk, I think

[@DaveDugdaleColorado]

Another option to an Ecobee is to buy a SensorPush for $49 and place it on one of the supply registers, download the app, connect via Bluetooth and watch your system cycle. The graph it provides will clearly show a saw-tooth line that is very easy to figure out. I just did this on a 93° afternoon here in Colorado at 3:30pm and my 5 ton AC system runs for an average of 6 minutes on and 9 minutes off. I am not sure what a "short cycle" is but I bet I fit that description. The hottest it gets here is 97°. I have been doing a lot of air sealing over the past two years which might be causing this.

[@NatetheHouseWhisperer]

Nice! Have a link to the product? That’s definitely short cycling! Dehumidification typically doesn’t begin until 10 minutes of runtime, so you can see an east coast issue too in that runtime…

[@DaveDugdaleColorado]

@@NatetheHouseWhisperer www.amazon.com/SensorPush-Wireless-Thermometer-Hygrometer-Android/dp/B01AEQ9X9I/ref=mp_s_a_1_3?adgrpid=64571734068&dchild=1&gclid=EAIaIQobChMI-sfYuvqB8gIVD4zICh2njAEmEAAYASAAEgJPDvD_BwE&hvadid=410006245862&hvdev=m&hvlocphy=9028728&hvnetw=g&hvqmt=e&hvrand=16791967031249541948&hvtargid=kwd-361736866077&hydadcr=13576_11413894&keywords=sensorpush&qid=1627344661&sr=8-3 the battery life is amazing and the Bluetooth range is fantastic. I wish it was $10 cheaper, but I have 4 now.

[@Balticblue93]

@@NatetheHouseWhispererWe don’t have any humidity to dehumidify. We run our whole house humidifiers all year. Scientist say Denver is more arid than Phoenix. To get to 45% indoors is a miracle and my Aprilaire 700 Automatic runs 24/7. I own an HVAC company that also does Home Performance. So I have certifications in my field.

[@shubinternet]

That looks like an Aprilaire filter, maybe a 213?

[@kennethdean4349]

PSC = Permanent Split Capacitor

[@NatetheHouseWhisperer]

Thank you Ken! I figured someone would comment! 🤓

[@GlenS123]

D.O.E. did not mandate manufactuers use ECM's. They gave them a rate they must meet, not how to get there. Only way to get there is with ECM's. Like your comparison to blood pressure!

[@afrocraft1]

So do you measure static pressure _before_ installing the heat pump or after? If you take it before -- with, say, a traditional furnace installed, doesn't the furnace air handler affect duct pressure? But if after, of what use is that?

[@NatetheHouseWhisperer]

Good question. Before. You can often take static pressure and reference the manual to get an idea of what the flow is. Typically when you move from an old school PSC motor to an ECM one, duct pressure goes up about 30%. So it lets you know if you need to downsize equipment or upsize ducts.

[@afrocraft1]

@@NatetheHouseWhisperer Interesting. Thanks -- big fan!

[@Balticblue93]

.3-.8 are fine with current Variable Speed blower fan, PSC fans are worthless, the more resistance slow down as they are cooled this way, so they WILL not ramp up. Variable Torque is more adjustable but not ideal but same perimeters, but having the fan speed set too high for your supply ducts just need adjustment more in heating mode. Be very cautious on which filtration system. A 1” filter system should run a Merv 5-8 depending. If you have any Aprilaire, Honeywell etc etc 4”-5” filtration systems, running a Merv 10-13, you usually should be used on all ducted systems, you will then have plenty of flow depending on your supply ducts.

[@2silica]

Did miss something? Pressure is measured at a given flow rate. Doesn't pressure change dramatically as CFM changes? If I'm switching to a new air handler with an ECM motor how do I know what the design flow rate of the new air handler is?

[@NatetheHouseWhisperer]

It’s in there. Yes pressure varies with flow. It’s a square function. I mentioned turning the airflow up 38% on one system, duct pressure went up 85%.

[@famousdude1239]

So EMC motors are Not Smart enough to stop @ .7 iw while the PCS motor does not have a problem with .7 or .10 correct. And it takes a "communicating" $500.00 thermostat to cycle off high static conditions. So, we have a 500.00 thermostat plus 800.00 emc motor. that can operate at 20 60 watts during their low load cooling hours. What the real cost are we "really" saving PER YEAR, this is 80 % silly buzz word fluff. Have your static pressure checked via, a real contractor, then make corrections. Forget the Happy, and Bad word games.

[@oneounceslugAPEX]

"because youre fat" hahahaha