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Old 09-30-2008, 10:53 AM   #1
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Ive been doing my own repairs on the heat pump in our house for the past several years. Mostly, its consisted of logical thinking about what could be causing the problem, inspecting the parts involved, and then replacing items that were suspect. This approach has saved me a ton of cash on over the 3 years we have owned the home as the unit is 14 years old now and seems to need something done a couple of times a year.

What I'd like to do is learn more about the diagnosis of problems. How to properly hook up a gauge set for instance (I have the a set, just not sure what all to do with them), or to use an electrical tester to accurately detemine problems.

One of the problem areas I believe we have is very poorly designed ductwork, and so I'd like to find some information about how it should be designed to see if that is the case.

Can anyone offer advice on books, videos, or classes that could help me learn more in the areas of pressure testing, electical testing and ductwork design?

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Old 09-30-2008, 11:33 AM   #2
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I too have a poorly designed duct system and would love to know more. Ironically I likewise cannot find any good sources on these subject. If I was the conspiracy kind I'd say its is as if the industry is trying to keep them out of the hands of the public. ACCA Manual D is supposed to deal with heat duct design. Other than that I've had to scour my local library concerning books that deal with these subject.

One recent series is Audel HVAC fundamentals that may be descent. Get Volume 2 as it deals with Duct Design. My local library is getting that for me.

If you could share any resource you've encountered on this subject that would be appreciated.


Last edited by dac122; 09-30-2008 at 11:51 AM.
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Old 09-30-2008, 12:10 PM   #3
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Example: You need 400cfm of air per ton of cooling at 1000fpm. A 3 ton unit would be 1600cfm @1000fpm = a duct size of 240 sq/in. A24x10, 15x16, 8x30 anything that = 240sq/in.(assuming the duct only going one direction) 6 inch runs are good for about 200cfm @ 1000fpm, an 8" 340, a 10" 520. If your trunk is 24x10 and you take off 3- 6" runs you have removed 600 cfm from the total of 1600, so you would need to reduce the duct size. And I'm using a duct calculator!
Duct that is to big will not have the velocity to get the air out to the rooms and to small ducts drive up noise and drive down volume. Most instalitons will not have the duct reduced, but a good rule is after 20 ft, reduce it.
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Old 09-30-2008, 12:18 PM   #4
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Is the duct calculator you are using online?

Also in your calculation you say 400 cfm per ton, but then say 1600 for a 3 ton unit, did you mean a 4 ton unit? (400 * 4 = 1600?)

Next, you use 1000 fpm, is that the standard or what other factors might influence it?

Lastly, if a 4 ton unit can provide 1600cfm does that mean there can only be 8 6" ducts and after that you would need to have a larger unit to accomidate additional ducts?

Sorry to ask so many questions but this is exactly the type of info Im after
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Old 09-30-2008, 12:42 PM   #5
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Quote:
Originally Posted by IanWV View Post
Is the duct calculator you are using online?

Also in your calculation you say 400 cfm per ton, but then say 1600 for a 3 ton unit, did you mean a 4 ton unit? (400 * 4 = 1600?)

Next, you use 1000 fpm, is that the standard or what other factors might influence it?

Lastly, if a 4 ton unit can provide 1600cfm does that mean there can only be 8 6" ducts and after that you would need to have a larger unit to accomidate additional ducts?

Sorry to ask so many questions but this is exactly the type of info Im after
You are right I type slow and think way too fast 1600 would be correct. I have had this duct calculator for years its from the ACCA (air conditioning contractors of America) as you have guessed you just dial it in and poof it gives you sizes. And the 1000fpm the shop I worked in used that number for all residential jobs.
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Old 09-30-2008, 01:53 PM   #6
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Cub, could you tell us what tool you are using?
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Old 09-30-2008, 02:45 PM   #7
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See if this works




Now I dont know if ya can still buy one
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Old 09-30-2008, 02:49 PM   #8
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big pix

well that didn't work so well
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Old 09-30-2008, 02:52 PM   #9
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It looks like the small pic is all that has come through. Could you post the large pic somewhere else, or just tell use what it says?

Thanks
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Old 09-30-2008, 03:04 PM   #10
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this is almost the same
http://www.professionalequipment.com...n-calculators/
check it out!

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