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National Comfort Institute's Knowledge Base

National Comfort Institute's Knowledge Base is a collection of questions and answers to help NCI members and industry professionals. The knowledge base is open to both NCI members and NCI Online users.

Access Levels
Each question has an Access Level, NCI Member Only and NCI Online User. NCI Members can view all questions and answers in the knowledge base. NCI Online Users can read NCI Member Only questions but cannot access the answer. NCI Online users can only access NCI Online User level questions. Both NCI Members and NCI Online Users can ask knowledge base questions.

Asking a Question
Even though Knowledge Base questions are answered by both National Comfort Institute and trusted Industry Professionals, all questions and answers must be approved by National comfort Institute.

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Question Category: Air Diagnostic	NCI Member Only
Question: How can I measure air volume on a system with a 12-inch air sock as the duct? The customer has gone through 2 heat exchangers in three years. - To view, please click on the question. Asked Date: 12/28/2009

Question Category: Air Diagnostic	NCI Member Only
Question: how do calculate the correction factor for grill? - To view, please click on the question. Asked Date: 07/27/2009

Question Category: Air Diagnostic	NCI Member Only
Question: Is all btu gain/loss on the the supply side of the duct system sensible btus or can there be latent btu gain/loss as well? Assuming no other components installed in the duct system? Will cfm leakage(supply) have an effect on air temperature on the supply side of the duct system? - To view, please click on the question. Asked Date: 10/04/2006

Question Category: Air Diagnostic	NCI Member Only
Question: Im looking for a Universal External Static Pressur Chart , is there such athing and where can i get it? - To view, please click on the question. Asked Date: 09/21/2006

Question Category: Air Diagnostic

NCI Member Only

Question: Joined the site yesterday. I can tell I'm gonna love it. I'm a service tech and the company I work for does not have a flow hood. I use my static pressure gauge to get ESP. I also have a rotary dial type anemometer from Davis Inst. In my market usually there is only 1 return. Is it ok to use the anemometer to establish cfm at return grill and compare it to fan curve reading to determine return leakage? I Can't think of any accurate means of measuring supply register cfm total. - To view, please click on the question.
Asked Date: 07/26/2006

Question Category: Air Diagnostic

NCI Member Only

Question: How short is a short cycling for a furnace or air conditioner? I know my A/C unit is working well for its age and may be a 1/2 ton oversized as I have made improvements in the ductwork, insulation and air sealing. Today it is 88 degrees out and I just cleaned my condenser coil. The unit is running 10 minutes on and 12 minutes off. The freon pressures look ok (72 psi suction and 250 psi liquid for an R-22 unit. The inside temp is set at 78 and the current RH is 50%. The temp drop is21.3 degrees at the coil. so is a 45% duty cycle too short for these conditions? - To view, please click on the question.
Asked Date: 07/19/2006

Question Category: Air Diagnostic	NCI Member Only
Question: Does it matter if you use the neg port on the digital Magnahelic to take reading for the supply SP. 0 is 0 and I just convert the neg reading to pos correct? - To view, please click on the question. Asked Date: 04/13/2006

Question Category: Air Diagnostic	NCI Member Only
Question: After I traversed a 8x8 heart and cooley ceiling diffuser which had a ak factor of .2, I recorded 228 cfm from the diffuser, at this point do I subtract the AK factor from the recorded CFM? - To view, please click on the question. Asked Date: 02/10/2006

Question Category: Air Diagnostic	NCI Member Only
Question: How do I test for TESP with a double drop? Was I daydreaming when Al covered this in class? - To view, please click on the question. Asked Date: 07/06/2005

Question Category: Air Diagnostic	NCI Member Only
Question: I am reading a Supply of 1100 CFM at .1 SP. The return is reading 805 CFM at .2 SP. The return is right at the AH. WE totally blocked off the return and measured 0 CFM and we measured 0 FPM at each register. Is my hood out of whack? - To view, please click on the question. Asked Date: 03/30/2005

Question Category: Air Diagnostic	NCI Member Only
Question: How do I determine air changes per hour without knowing required CFM? - To view, please click on the question. Asked Date: 10/27/2004

Question Category: Air Diagnostic	NCI Member Only
Question: Where is the best place to measure building static pressure? (Commercial) - To view, please click on the question. Asked Date: 09/24/2004

Question Category: Air Diagnostic	NCI Online User
Question: Can I measure a pressure drop of a central return? How do I measure this type of pressure drop? - To view, please click on the question. Asked Date: 08/16/2004

Question Category: Air Diagnostic

NCI Member Only

Question: Is it possible to have a significantly higher CFM on the supply side of a system (almost double) than the return side even if the return is right off the equipment and doesn't seem very leaky? I measured 1080 CFM return and over 1800 supply on a double-wide trailer home system with a 5 ton blower and a single return grille right on the return plenum. Static was .9 inches wc. - To view, please click on the question.
Asked Date: 08/10/2004

Question Category: Air Diagnostic	NCI Member Only
Question: Can I measure the percentage return duct leakage on a service call? - To view, please click on the question. Asked Date: 07/26/2004

Question Category: Air Diagnostic	NCI Member Only
Question: How do get an accurate average wet bulb reading in a typical cooling system? - To view, please click on the question. Asked Date: 07/19/2004

Question Category: Air Diagnostic	NCI Member Only
Question: I have just the right amount of airflow into an office, but it's not cooling properly, what's the problem? - To view, please click on the question. Asked Date: 07/19/2004

Question Category: Air Diagnostic	NCI Online User
Question: Does a duct blaster have any real diagnostic value to a performance- based contractor? Do you feel that they accurately measure duct leakage? If so then why? - To view, please click on the question. Asked Date: 1/18/2003

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