Ventilation condensation

meg10yr · #1 2011-01-27, 5:27pm

I am in the process of moving my studio into our former garage. The plan was to take the ventilation through the attic and out. Today my husband expressed concern that this would create condensation and put moisture into the blown-in insulation. The room will be kept around 60F and the attic is the same as the outdoors. During the winter that can be anywhere from -05F to 35F. Has anyone had this happen? Any thoughts would be appreciated.

Thanks, PegC

cheng076 · #2 2011-01-27, 9:04pm

Since the vent ducts will contain air warmer than the attic air there will be no condensation into the attic insulation. There may be some inside the ducts as you then have warm air with moisture from combustion and a cool outside the duct temp. Condensation occurs when warm moist air is cooled. The warm air flowing so fast thru the duct should evaporate the moisture. JMHO
PJ

meg10yr · #3 2011-01-28, 8:31am

Thanks PJ.

RSimmons · #4 2011-01-28, 8:41am

One question to clarify: You intend to exhaust through a duct that goes through the roof and to the outside and you are concerned about condensation on the outside of the duct dripping into the insulation, not exhausting into the attic space so that the material is carried away by your normal attic ventilation. Correct?

Robert

meg10yr · #5 2011-01-28, 9:21am

Robert, that is correct. The duct will have 1 90deg bend and travel 25' to the exsisting gable vent fan.

PegC

laserglass · #6 2011-01-28, 9:59am

Quote:

Originally Posted by meg10yr

Robert, that is correct. The duct will have 1 90deg bend and travel 25' to the exsisting gable vent fan.

PegC

you need a much much shorter, direct path to the outside and a squirrel cage fan for that duct only. and you also need another short direct path to the outside for make up air that is not located near where your ventilation duct vents or where your outside propane tanks are located. you cannot just vent your ventilation duct near where your attic ventilation fan is located.

meg10yr · #7 2011-01-28, 10:51am

I have a natural gas hookup. There is 640 CFM in the hood as well as the attic fan at the end of the run. I plan to put baffles on the hood similar to a barley box. I will do a smoke test at various stages of installation. if there is still a need for it I will box in the attic fan and attach the ducting to the box. There are two double hung windows on the wall next to me and a man door and overhead door that can both be open as necessary. All make up air sources are well away from the vent fan.

Thanks for the input!

cheng076 · #8 2011-01-28, 11:20am

Sorry to jack the thread but....
Hey Mark, where you been? Haven't seen you post in some time.

RSimmons · #9 2011-01-28, 11:23am

I'm pretty sure that a 640 CFM fan is going to be seriously underpowered for your length of duct as the static pressure is going to be too high for effective ventilation. An extra fan at the end will just complicate matters. You'll be better served to get a much bigger squirrel cage blower, build it into a vacuum plenum and run your exhaust through that directly to the outdoors.

Robert

laserglass · #10 2011-01-28, 1:36pm

Quote:

Originally Posted by cheng076

Sorry to jack the thread but....
Hey Mark, where you been? Haven't seen you post in some time.

i don't post much any more. i got tired of arguing with people who were ignorant of facts.

Dale M. · #11 2011-01-28, 2:19pm

Quote:

Originally Posted by RSimmons

I'm pretty sure that a 640 CFM fan is going to be seriously underpowered for your length of duct as the static pressure is going to be too high for effective ventilation. An extra fan at the end will just complicate matters. You'll be better served to get a much bigger squirrel cage blower, build it into a vacuum plenum and run your exhaust through that directly to the outdoors.

Robert

I agree with this thought completely...

you are building a patch work system that will cause more grief than pleasure...

Dale

Dale M. · #12 2011-01-28, 2:20pm

Quote:

Originally Posted by laserglass

i don't post much any more. i got tired of arguing with people who were ignorant of facts.

I hear that....

Dale

meg10yr · #13 2011-01-28, 6:13pm

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Thanks guys!

RSimmons · #14 2011-01-28, 6:32pm

Quote:

Originally Posted by meg10yr

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Safety in general and ventilation in particular are things that some people simply choose to ignore for a host of reasons. I'm glad that you are taking it seriously and have sought advice here before you spent time and money on your setup.

Robert

meg10yr · #15 2011-01-28, 7:09pm

Thank you Robert. This has been my husband's project and he will do it his way. He has agreed that if I can show him it is inadequate we can do it the way I suggested - with a squirrel cage fan at the 90deg angle. For the past couple of years I have been in an uninsulated 5x7 shed with a direct vent for the hood through the roof. It is going to be great to have this new, comfortable environment to work in but I will do everything I can to ensure it is also safe.

PegC

Dale M. · #16 2011-01-28, 10:25pm

Through all this 25 feet of duct and fans and whatnot I have yet to see any calculations... What is hood (fume cabinet) opening size... What is duct diameter.... Has duct velocity been calculated, has static pressure been calculated....

IT may be your husbands project, but there is whole set of calculation that are proven to work, and if you follow then you almost have fool proof system... There is even a spread sheet available to use to help you....

Dale

laserglass · #17 2011-01-29, 9:23am

if you have a fan that can move 650 CFM through a duct that is 3 feet long, that same fan will move 65 CFM through a duct that is 10 times as long or 30 feet. and that is neglecting things like duct roughness, static loss due to elbows in the duct. you need to cut a hole in your home, that is the reality. i would suggest a sidewall rather than the roof, so you don't have to deal with rain/snow. alternatively, construct a baffle to pass your vent duct out through an existing window. but you cannot also use the window for make up air.