Replies

1. 
   junkhound | Jan 22, 2005 06:41pm | #1

   Maximize air flow across exchanger, which means all the ducting you can hang onto the output - you may need to add dampers later to balance your rooms, etc.

   You can never have too many or too big of ducts in the heating equation, but there is a point of diminishing reutrns where the space taken is not worth the savings.

   You need to learn the  basics, here is a good place to start (best I saw of first 10 in google search),  there are a number of good web sites that have free calculation tables.

    

   edit whoops, first had pasted in local HD address vs. web address

   http://www.engr.uga.edu/people/faculty/lawrence/Duct%20systems%20flow%20analysis.ppt

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   Edited 1/22/2005 10:42 am ET by JUNKHOUND

   
   

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   Edited 1/22/2005 10:43 am ET by JUNKHOUND

1. 
   TMO | Jan 22, 2005 07:54pm | #2

   Thanks but the link you have there is giving me a file that I can't open. Is Cold air return sizing important?

1. 
   IMERC | Jan 23, 2005 09:31am | #11

   very important..

   proud member of the FOR/FOS club...

   Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming

   WOW!!!   What a Ride!

1. 
   TMO | Jan 24, 2005 03:42pm | #12

   And?

1. 
   MikeSmith | Jan 24, 2005 04:10pm | #13

   yup... return air is very important...

   it can be     slightly     undersized compared to to your supply ducts.., with the result that  the conditioned space will have a  slightly positive pressure..

   but not only the main return.. but the room returns have to be sized to get good circulationMike Smith Rhode Island : Design / Build / Repair / Restore

2. 
   TMO | Jan 25, 2005 03:21pm | #15

   Thanks.

   Is the basic calculation a square inch type deal?

   Pi multiplied radiaus squared= 6"duct so (9 x 3.14) times 5 ducts = 141.3 square inches

   16"*25"=400 square inches  If I'm looking at this correctly I can several ducts without worrying about the return size.

   Correct or no?

     

    

3. 
   MikeSmith | Jan 25, 2005 03:57pm | #16

   i'd have to go to a heating manual to find how to size duct.. but  the calc. can be simple or complicated  .. someone who does it all the time should advise you..

    i do it about once evey 5 years or so..

   or.. i don't do it at all.. i just deal with a reputable HVAC who sizes his duct work correctly..... it's the only way to get the correct comfort levels  thruout the conditioned spaceMike Smith Rhode Island : Design / Build / Repair / Restore

4. 
   TMO | Jan 26, 2005 02:52am | #18

   That, of course, is the easy way to go. Pay somebody else to do it. :-)

5. 
   DanH | Jan 25, 2005 05:36pm | #17

   The reason for "sizing" duct to less than outrageously large is (besides saving space/money for the ducts) to balance airflow to different parts of the structure. If you're not especially worried about balance then make everything on the large side.

6. 
   TMO | Jan 26, 2005 02:54am | #19

   " If you're not especially worried about balance then make everything on the large side."

   This, of course, is how I will do it. 

   Thanks everyone.  

7. 
   IMERC | Jan 24, 2005 07:58pm | #14

   you asked yis direct question......""Is Cold air return sizing important?"" and I gave ya direct answer....

   proud member of the FOR/FOS club...

   Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming

   WOW!!!   What a Ride!

2. 
   rjw | Jan 22, 2005 08:15pm | #3

   The file he linked to is an MS Powerpoint "show."

   I'm far from an expert on ductwork and sizing, but I question if the "the more" is "the better."

   What kind of furnace do you have? Natural draft? An 80+ - has a draft inducer (a smallish blower you can see when you take the front panel off) with metal and (possibly) masonry flue? Or a 90+, which has a draft inducer and plastic (or, possibly a stainless steel) flue. (A 90+ will also have condensate drainlines coming off of the flue system.)

   I can give you some performance parameters for temp rise and flue temps to shoot for.

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   Sojourners: Christians for Justice and Peace

1. 
   TMO | Jan 22, 2005 08:39pm | #4

    Metal chimney. B-vent. Bought  the furnace for $200+/- so I'm not expecting miracles but would like to make the best of it. No draft inducer(I think) so its a natural draft if I understand the terms right.

     

3. 
   DanH | Jan 22, 2005 10:30pm | #5

   To maximize heat output, other factors being equal, you need to have the fan start up early, run long, and move as much air as possible (thereby removing as much heat as possible from the flue gasses). So more, larger ducts is better, as is turning up the fan speed if you can. Adjusting the fan thermostat (if there is one) to turn the fan on and off at lower temps achieves the early/long objective.

   There is a point, of course, where you may cause the flue gasses to get too cool (in a non-condensing furnace) and turn it into a condensing furnace. (Not good -- acid eats the heck out of the heat exchanger and flue.) But this is likely difficult to cause on a furnace that isn't already "high efficiency".

   Of course, in most cases your efforts are better spent on insulating and reducing air infiltration. In many cases it's probably possible to halve the heating requirements for the house with careful attention to these details.

1. 
   TMO | Jan 22, 2005 10:42pm | #6

   Thank you. I'm on the insulating and reducing air infiltration kick right now. Old drafty house, in the last week I've gone through 8 cans of foam and 10 tubes of caulk. Just purchased 8 more cans and 8 more tubes.

   As for the furnace suggestions, I actually turned the fan down it was noisy and it  caused short cycling(I freaking can't stand that.) If I tighten this place up some I may turn it back up to explore your suggestion. I turned the fan thermostat down like you suggested.

   Anything about the return air duct size?

   Edited 1/22/2005 2:44 pm ET by TMO

1. 
   TMO | Jan 22, 2005 10:47pm | #7

   Another question, I have about 2400 sf with a fair amount of windows. The place is an old summer Camp that I "converted"(in process). Is the 75000 BTU unit anywhere close to the right size?

   I'm Insulating as I go but the place is still drafty.

   Edited 1/22/2005 2:48 pm ET by TMO

1. 
   DanH | Jan 22, 2005 11:06pm | #8

   Where do you live. A 75,000 BTU unit wouldn't hack it for that size drafty building in MN, but would probably be fine farther south.

1. 
   TMO | Jan 22, 2005 11:23pm | #9

   Upstate, NY.

2. 
   DanH | Jan 23, 2005 12:46am | #10

   75,000 BTU probably isn't big enough unless you can get the place really tight.