I have a 75000 BTU hot air furnace. I’m seeking to maximze its heat output. How many heat runs can I take off it? I have 5 6″ inch insulated runs right now. The cold air return measures 16 x 25. What other info do I need to pay attention to?
I’ve done the work on it myself and without any heed to HVAC rules (because I don’t know em). Silly contractor doing it all myself
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Replies
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
Edited 1/22/2005 10:42 am ET by JUNKHOUND
Edited 1/22/2005 10:43 am ET by JUNKHOUND
Thanks but the link you have there is giving me a file that I can't open. Is Cold air return sizing important?
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
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And?
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
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?
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
That, of course, is the easy way to go. Pay somebody else to do it. :-)
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.
" 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.
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!
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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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.
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.
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
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
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.
Upstate, NY.
75,000 BTU probably isn't big enough unless you can get the place really tight.