Author Topic: maximum distance from range hood outlet for a back draft damper  (Read 549 times)

Offline Huck50

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Hi:
 Just bought a 30" range hood that comes with a 8 inch exhaust transition piece,but did not come with a back draft damper....so I will have to buy one,not sure whether to buy one that goes inside the ductwork or one that is already inside a short piece of ductwork??
How far away from the exhaust port should this backdraft damper be,assuming that I already have a damper at the chimney roof vent.
Motor blower is 750cfm,but really maybe around 600cfm. with all piping,etc.
Any dampers I should be looking at? Thanks,Huck50

Offline Admin

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Re: maximum distance from range hood outlet for a back draft damper
« Reply #1 on: August 11, 2017, 01:01:13 PM »
I have never seen a range hood that didn't have a built in damper in the exhaust outlet port on the fan housing.

Keep in mind OBC Article 9.32.3.12 refers to the exhaust outlet vent outside, not at the fan itself.

Quote
9.32.3.12. Outdoor Intake and Exhaust Openings
(9) Except for exhaust outlets serving heat recovery ventilators, exhaust outlets shall incorporate backdraft dampers.

In a previous post you mentioned the existing stove vent was 4".  Table 9.32.3.5 requires at least a 6" metal duct or 7" metal flex, but in no case shall it be smaller than what is recommended by the manufacturer.

Usually when you install a fan over 700 CFM you need to look at make up air as the home may become depressurized.  An HVAC designer would have to complete the CEC calculations, Critical Exhaust Condition, to ensure no make up air is required.