dual exhaust fans, single exit cap


  #1  
Old 03-07-15, 10:45 AM
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dual exhaust fans, single exit cap

The plan:

Island hood 1000cfm 6inch round duct With 2 x45 angles at 15 ft run.

OTR microwave 450cfm 2-3/4 x 10 duct (equivalent 7inch round) run 6 feet at 2-3/4 x 10 then 1 foot at 6inch round With 2 x 90 degree angles in 2 3/4.

Both connect to a 8x6x6 wye to exit though wall cap. Straight 2 feet.

The reasoning:

6 inch from hood as that is the spec.

2-3/4 x 10 from otr as spec.

2-3/4 x 10 to 6 inch to fit the wye.

8 inch out because 6inch and 2-3/4 x 10 in square inches are roughly equivilent to 8inch.

Wall cap because I prefer it.

Issues:

I know nothing about static pressure And am unsure if 8 inch is neccisary for the final short run.

I live in climate zone 7 and will need to insulate the galvanized Some how.

High velocity on the hood may hold OTR damper closed, this wye is designed to minimize this effect.

Other things:

Back draft dampers. I can not find local suppliers even though these seem incredibly important. Is there a code reason I cant find these in Saskatchewan? Amazon.ca has a few.

The attic is insulated at the roofline.

Makeup air will be addressed seperate from this post please.

Other yhings I missed.
 
  #2  
Old 03-07-15, 01:28 PM
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NEVER exhaust two fans through a single outlet.

Also, your calculations are a bit off. A six inch duct is woefully small for a 1,000 CFM range hood, regardless of what the manufacturer might state. Nor is a 2-3/4 x 10 inch rectangular duct even close to a 7 inch round duct. The former has a cross-sectional area of about 27 square inches while the latter is about 38 square inches.

If the range hood has a 6 inch outlet I would immediately increase it to 8 inch minimum before running the ductwork.
 
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Old 03-07-15, 03:25 PM
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Never as in end of the universe or never as in not code or never as in constant airflow issues?

The hood has poor documentation but is a 6 inch pipe. It seemed weird to me as 6 inch seems to small. I can transiton right after I clear the ceiling, that would give me 12 feet or so at 8inch.

If the 7in is too large all the better. 6in is roughly 28 so thats better suited to 2 3/4.

Any idea on insulation?
 
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Old 03-07-15, 08:23 PM
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Never as in airflow issues. The wall cap will be too small if both fans are in operation and if only one fan is being used there will be a backflow through the idle fan unless a good backflow preventing flapper is used. Unfortunately, a good backflow flapper will also add considerable resistance to the airflow and thereby reduce significantly the air being exhausted. The less resistance in the duct the better the airflow and usually the quieter the operation.

Any duct that runs through an unconditioned space should be insulated to minimize condensation inside the duct.
 
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Old 03-08-15, 01:22 AM
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I'll watch for the make-up air post. If you have naturally drafted appliances you will probably need a interconnected fan to match the combined flow of those two exhaust fans. That means more ducts and you seem to be wanting to combine ducts (not good as Furd posted) to reduce the penetrations.

Bud
 
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Old 03-08-15, 09:30 AM
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@Furd Ok, thanks for the info. I asked at several lumberyards and direct to a few plumbers and did not get a straight answer.

Is it common for hood suppliers to include incorectly sized ports and expect installers to know?

One of the reasons I went steel instead of flex was to decrease noise.

Your recomendation is:
-Increase the hood to 8 inch as soon as clearance permits
- seperate runs one 6 inch and one 8 inch with each their own cap
-insulation should not be require as attic is insulated and heated

@Bud I think my power vent waterheater uses a non sealed combustion chamber... it has vents on the sides. I will need to check the model number.

Sealed furnace no gas appliances.

I think code here stipulates makeup air test and possible remediation for cfm out over 15cfm/100 feet cubed or something.
 
  #7  
Old 03-10-15, 01:55 AM
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Is it common for hood suppliers to include incorectly sized ports and expect installers to know?
It is fairly common for the hood manufacturers to undersize the discharge opening and most installers don't understand airflow dynamics.


Your recomendation is:
-Increase the hood to 8 inch as soon as clearance permits
- seperate runs one 6 inch and one 8 inch with each their own cap
-insulation should not be require as attic is insulated and heated
That is correct.
 
 

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