Modern Aire PSL 600, damper or backdraft damper?

marti8aSeptember 13, 2012

I know I read a discussion here or on the kitchen forum about something that goes on the roof and makes a bend downward to keep air from making the damper flap when the wind blows. But I can't find it.

I already have the PSL 600, and the HVAC guy came today to see what was needed to put in the duct. I don't know how many, if any, of these he has done. He sure seemed to write a big supply list before he left: (2) 12" adapters (I think it was adapter he wrote), 10' of 8" single thickness duct, damper, roof housing, and flashing.

I asked him what kept the damper from flapping and he said he didn't know.

I asked him what kept water from condensing in the vent pipe and he said the blower would remove all the moisture so there wouldn't be any to run back in.

I asked him if the duct they installed would fit inside the collar on the liner, or on the outside and he said it didn't matter, we would just screw the two together whichever way it fit best.

Does all this seem right to those of you who have been through it?

This was the ONLY place I found who would install the ductwork, so I don't have a lot of options about who does it, but I can give them as much information as I can find.

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I found a roof vent like I remember from that thread, and it has a damper in it. roof vent

Will one like this keep the wind from making the damper flap or do I need a spring loaded one?

And if this is enough, do you know what other parts the HVAC company really needs?

    Bookmark   September 13, 2012 at 4:19PM
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I believe that if there are two dampers in series, such as the Fantech roof device you linked to and the damper at the hood, wind gusts across the roof will have a difficult time causing both to lift together. Further, the roof vent seems to be particularly resistant to wind flowing up the roof.

Spring loaded may be needed if there is significant occurrence of wind blowing in windows such as to pressurize the house higher than the area of the roof where the roof vent is located. This can happen in particularly gusty conditions when the wind faces windows on the opposite side of the house from the roof surface that the roof vent is mounted on, but then the windows might be expected to be closed.

The HVAC company seems a bit cavalier. I'm not a duct expert, but I believe the requirement for hood exhaust ducts is that any grease collected flow down and remain in the duct. This requires that the higher duct fit INTO the lower duct.

It is possible with a cold attic and long duct run to condense water on the duct walls. This water can drip back down into the hood. Running the fan for some time after boiling down syrup or whatever should clear the duct well enough, and the lower damper will keep humidified internal air from rising in the duct and condensing when the fan isn't running. Generally, condensation to the point of water flow should be very rare, and if the ducts are connected correctly, the worst that happens is that the water drips end up in the baffles.


    Bookmark   September 14, 2012 at 10:10AM
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My parents VAH was connected via 3.5x10" to the attic, converted to 8" uninsulated pipe, one 45* bend then straight out the roof. It has a similar 'cap' to the thing you showed but shorter. It had a damper in it but it was not spring loaded. The total run was maybe 12-15'.

In the 35yrs this existed they never had any noise nor condensation issues.


    Bookmark   September 14, 2012 at 2:38PM
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Thank you both.

Is it common to have two dampers, or is one usually enough?

I thought his attitude was from inexperience, and that worried me. Dh could do this, he would just rather have someone do it that does it all the time and can do it right.

I did call Modern Aire and they said the duct could attach to the vent collar either way, but the preferred way was inside the collar.

    Bookmark   September 14, 2012 at 9:47PM
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Vents that point into the wind such as the one at the link in the second message above, whether passive or having fans, should have their own damper. Symmetrical vents such as upblast fans or similar housings used as vents are less susceptible to wind induced damper opening, but not immune.


    Bookmark   September 15, 2012 at 7:49AM
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Can you post a link to a vent like that? I don't care what kind of vent it is as long as it doesn't flap constantly when the wind blows.

This is on the side of the house that faces west, btw and there will be an overall height of about 6' from the top of the vent liner to the roof.

    Bookmark   September 15, 2012 at 12:06PM
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Look on Greenheck's site ( for a variety of examples. Both up-blast and down-blast are candidates for consideration. There are some HVAC suppliers on the web who sell similar units.


    Bookmark   September 15, 2012 at 8:43PM
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Thank you. I think I finally have the idea now.

    Bookmark   September 16, 2012 at 1:01AM
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I just got the price from the HVAC company. $685 to put in the duct, damper, and roof cap! I guess this will be another diy for us.

    Bookmark   September 18, 2012 at 12:05PM
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