Static pressure testing

[ch4man]

Please help me understand this. My understanding of restricted ductwork would mean LESS CFM airflow at the same static pressure, rather than an INCREASE in airflow. How is it your system is delivering more airflow with restricted ductwork than what the blower performance chart says it will move under perfect laboratory conditions?

answer??? i dont know, thats why im here asking. make no sence to me either. just seams more times than id think i would find blower curves to be off one way or the other.

BT asked if i had a straight supply truck to test, sadly no. so once again today i tested fan only on hi and low, then turned on heat for temp rise method

by tesp/blower curve= hi speed, 1.0"wc tesp ~850-900 cfm as im off the chart.
by 416 traverse in drop = high sp= 1038 cfm
by temp rise method = 1042 cfm

on low speed, tesp/ bc = 0.71"wc and 515 cfm
by 416 traverse in drop = 849 cfm
by temp rise method = 912.


my head hurts, if didnt have to go to work in an hour, id have a beer.


ps yes energy star i understand that NCI inst recognised. i have no horse in that race, im looking to learn diagostic for residention existing systems 'cause that where i live.

but pls if you have some insight that will help my understanding i am all ears. thank you.......

[tipsrfine]

i took NCIs air diagnostic and balance class last year and this past summer was living and breathing capacity checks.
it seemed to me that blower curve are more of a guideling than actual numbers. too much imterpolation ( although what does that say to a TAB guy who uses fan laws regularly?). way more times than i ever expected the capacity checks reveiled something amiss with my calculations and always pointed to the CFM number i plugged into the total and sensbile formulas,

my own furnace is a mid eff amana with a 3 ton blower package. horrible ductwork with the most restriction on the return ( yeah i know, and doctors make the worst patients).

the following numbers are approx as i didnt record them but close enough to make my point,

the factor blower curve claims on low speed tap at 0.88" tesp to deliver approx 650 CFM. ok sounds about right?. well a return drop traverse with a testo 416 claimed 1100 CFM ,, huh! a 3 ton blower on low at 0.88 tesp. cant be.

but the capacity check showed terrible cooling numbers with no dehumidifcation. and this was just after i did a clean and check of my AC. SH &SC numbers right on. if i use 650 CFM in the total heat formula made no sense, yet if i plugged in 1100 CFM my capacity check results fell into place..

this is only one example. most times wile doing rebate data collection cap checks show way less airflow than ecm dipswitchs lead you to believe..

Tips, you sound like a TAB guy, any thoughts?




you are correct, airflow is the most important check to do. but static presure doesnt alway mean airflow

It is more than likely that what BT said about not having a straight piece of duct to test. Based on everything I've read (although there is always disagreement) using a ductblaster as a active (powered) flow hood is the more accurate method for testing air flow in residential systems. The following study indicates they a vastly more accurate than other flow hoods. http://epb.lbl.gov/publications/pdf/lbnl-49697.pdf

[ch4man]

can i borrow your ductblaster?

yeah ive read about those too.

oh well, this is only one system. ive decided to forget about for a while. but am still looking for example from other techs and ideas/info from the TAB guys.

peace out

[DeHeatify]

So if my manual says this. Do I take the measurements before or after the filter?



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[davidr]

NCI classes are not recognized by the Air Balance Council. Not saying you need that, but if you want to be an actual independent balancer certified to submit reports to clients requested by the state, or a commercial project by a builder, NCI is meaningless. Too many classes and companies in our industry creating these certifications with the only goal being a new source of revenue for them.

Why would it matter if NCI is recognized by AABC? They are another balancing organization.

There are a lot of architects and engineers accepting NCI certification on their projects. On some projects it is actually being spec'd.

If they accept a submittal package for the NCI certification, then NCI certification is absolutely valid for that architect or engineer.

[davidr]

It is more than likely that what BT said about not having a straight piece of duct to test. Based on everything I've read (although there is always disagreement) using a ductblaster as a active (powered) flow hood is the more accurate method for testing air flow in residential systems. The following study indicates they a vastly more accurate than other flow hoods. http://epb.lbl.gov/publications/pdf/lbnl-49697.pdf

We did some through research of our own comparing real air balancing hoods to a powered capture hood.

Most readings on the average were within 5 CFM of each other.

If you really read that report in detail and don't take it for face value the results are skewed.

[beenthere]

So if my manual says this. Do I take the measurements before or after the filter?



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Between filter and furnace.

[DeHeatify]

Between filter and furnace.

Good. That's where I've been taking them.

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[energy star]

davidr

It is not the same.

[DeHeatify]

Between filter and furnace.

see this is where the problem with manufacture data..no standards. I have learned to take esp readings from after the filter and before the a/c coil. and was confirmed here....yet I download my service manual and it shows this:



aaaand I'm confused again......

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[rundawg]

see this is where the problem with manufacture data..no standards. I have learned to take esp readings from after the filter and before the a/c coil. and was confirmed here....yet I download my service manual and it shows this:
aaaand I'm confused again......

My G50UH service manual says this in the notes of blower performance data: Also see Filter Air Resistance table.

Which means if you place the probe before the filter as they show in your picture, you still need to account for the PD of the filter based on the Filter Air Resistance chart in the manual.

Either way works.

[beenthere]

see this is where the problem with manufacture data..no standards. I have learned to take esp readings from after the filter and before the a/c coil. and was confirmed here....yet I download my service manual and it shows this:



aaaand I'm confused again......

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Yeah, standardization would be nice.

[air1]

You should always take static pressure readings when you suspect airflow problems. Signs of airflow problems could be furnace tripping on high limit, low suction pressures, icing evaporator. Most furnaces will have the design Total External Static Pressure (TESP) listed on the nameplate. If your total TESP is more than what the manufacture specifies, your duct work it too restrictive and the fan will not deliver the designed airflow. Your TESP measurement should include filter pressure drop. Measure the discharge static in the supply plenum far enough downstream to minimize turbulence. Measure the return static downstream of the filter and before the fan.
TESP will not tell you how many CFMs the fan is delivering only that the duct resistance it too high to deliver the design CFM. Most situations you might find that the return is too restrictive. There are many causes of too much restriction in the duct but thats another discussion. You might also find that the pressure is too low. Too low of a TESP can cause problems with some equipment. Mainly, to low of a pressure can cause fans to over amp and with some units can cause fans not to perform.