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truckkerjoe
12-16-2009, 03:55 PM
Hello, i'm new to this forum. We had a HVAC contractor look at our furnace, and he wants replace it with an OLSEN. The specifications states: 91k btus " input 75k-output, max.CM @ .20WCSP-Dir 1741 and Max.CFM @.50WCSP-Dir.1476. The cold air duct work that goes up to the main trunk is 20 x 8. He said that this was too small.
Question: Can the new furnace grill get cut to 20 x 25? Wouldn't this solve the problem? We have a 14 seer with a 30k btu central air, and I believe it's a 3 ton coil. Olsen comes with a variable speed blower. The main trunk is 14 x 8 x35 ft in the center of the house. Question: do I need to increase the return drop, and if so, at what size? If the drop gets changed, what about the main trunk? The filter is 20 x 25 x 1".

Very much APPRECIATED!!! Keep-on-truckkin!!!
Is this the fourum I want?

fabrk8r
12-17-2009, 08:49 AM
The 20" X 8" return should be okay for the air the unit will be moving. The velocity may be a little fast, which might cause some air noise at the return registers, so if that would bother you the RA size could be increased.

The supply air duct is the problem I see. If the largest duct in your SA trunk line is 14" X 8" the air will be moving at about 2,500 FPM. Your SA registers will be whistling.

truckkerjoe
12-17-2009, 02:38 PM
ok,thanks,but the trunk size I talked about is the return side only.The CFM numbers I gave you,well will that return size be ok?or should I have a static-load done? Doesn't the cfm@.20 wcsp-dir1741 dictade the size (drop) return needed? or the supply side.We are mainly worry about excessive heat on the exchanger and proper cooling not noise. THANK-YOU!!! :

thehammer
12-17-2009, 05:31 PM
I'm thinking you're talking about a residential furnace, gas? elec? Supply is 14x8, Return is 20x8 right? The basic "Rule of Thumb" that most codes follow is that the return should be about 40% larger that the supply. Often this is arrived at by a long series of jargon and calculations (as u-r spewing)
These calculations might vary that amount a % or 2 but in the end your R/A size is fine for a residential install... unless if you've not sized the unit correctly or miscalculated the size of the supply.
Also..
I would really like to remind everybody out there installing furnaces, lack of return air can kill by carbon monoxide poisoning, especially on natural gas fired equip.

cactassdupree
12-18-2009, 04:14 PM
How big is the house or whatever you are heating? :?:

sheetmental
12-18-2009, 10:50 PM
i haven't really learned much about sizing the duct, but was often told as an apprentice that you could never have too much return.

truckkerjoe
12-19-2009, 10:23 AM
Thank-you,all!!!! size-house spilt-ranch 1140sqf heat load loss 50k? state-PA. oil> I'am thinking that the bigger return,the better.When somone changes the(drop) don't you have to change the supply-return trunck ALSO??Maybe I should place the furnace-IN with the OLD drop 20x8 THEN have a static-test done after? The specs. reads>filter-size 16x25x1"can I cut-out (furnace) or RACK to that opening??....and if I need it bigger later I can? ANY suggestions! merry-xmas!!!!!

james80031
05-16-2010, 08:03 AM
Your static will be way off the chart. I make the return drops as large as possible, and for a furnace that size, right in the manufacturers specs it will say right around 14x20 minimum. If there is space, we go much bigger and place the furnace on a filter box because airflow from a bottom return is far less restrictive than a side return. The newer furnaces really need to breathe, which often requires enlarging ductwork. If your equipment was sized correctly using a manual J load calculation, preparing the manual D duct size calcs should be no problem. -james

Bud
05-16-2010, 08:40 AM
Thank you James and welcome to the site!

mel
05-16-2010, 03:14 PM
A normal return air drop is about 12x23, to fit a 16x25 filter rack. But this does not matter, because it will depend on the number of floor grills and the size of them that you have in your house. You need 400 cfm per ton of cooling. Your 20x8 is on the small side.

#1tinner
11-07-2010, 06:01 AM
I`m confuzzzzed shouldn`t the total return air registers be calculated at the grill face in order to make the correct size duct work for the return? fin spacing @ 1/4 spacing .vs. 1/2 spacing////1/2 spacing would move more air. But drawing through wall plate with a 14h by 24v the register does not matter it the opening in the wall plate..... Or am i incorrect?

Rob
11-07-2010, 05:41 PM
I`m confuzzzzed shouldn`t the total return air registers be calculated at the grill face in order to make the correct size duct work for the return? fin spacing @ 1/4 spacing .vs. 1/2 spacing////1/2 spacing would move more air. But drawing through wall plate with a 14h by 24v the register does not matter it the opening in the wall plate..... Or am i incorrect?

You're correct.
No matter what size of grille is installed, the air flow is limited by the opening in the wall plate.

b.c.tinbasher
03-16-2011, 09:33 AM
If I understand truckkers first post, the maximum air volume available from his new Olsen is around 1,700 cfm. For heating and cooling in a home or office, I always design for low duct velocity (around 800 fpm) to keep the air noise down. For 1,700 cfm, a duct size of 25 x 12 will give a duct velocity of 817 fpm, and very low static pressure loss. The 14 x 8 main r/a trunk is on the small side, even if we assume that the drop is in the center of the 35' and there are equal numbers and sizes of openings into the duct on each side, so 850 cfm per side, the duct velocity would be 1,090 fpm. This of course will work, but you might hear some air noise. Assuming the r/a scenario is as I supposed, I would have a main r/a trunk sized at 18 x 8. I size the s/a the same way, for low velocity. I always install dampers too, even on the r/a if necessary.

Rob
03-16-2011, 07:16 PM
I would have a main r/a trunk sized at 18 x 8.

How would you know this without calculating the Total Equivalent Length?
This is the main reason why I avoid duct sizing from my keyboard.

b.c.tinbasher
03-17-2011, 01:16 AM
[QUOTE=Dad;12714]How would you know this without calculating the Total Equivalent Length?
This is the main reason why I avoid duct sizing from my keyboard.

You make a good point. We cannot properly size duct for somone's home without a lot more information.
If I understand T.E.L. correctly, you are sizing duct based on a calculation of total static pressure in all ducts, with factors for elbows and other fittings. Do you also factor in the outlets and outside air duct?

To be truthful, I usually just look at the volume of air-supply, or return, that is to be ducted and size accordingly.
X amount of volume, travelling at a velocity that I choose with low sound levels in mind. This method ensures low static pressure loss as well. Od course if there is a lot of ductwork i.e. a very large home, or one that requires a lot of fittings, the total static will have to be looked at. I also consider it equally important to select appropriate outlets, in terms of size (noise again) and performance characteristics.
Cheers.