Garage ventilation fan sizing

[diy96]

Looking to add a basic fan to my garage in order to remove some of the humid air that occurs when the cars come in during the winter. We knock most of the snow off but some still eventually melts inside and then squeegeed out.

I keep the temp at a constant 50 degrees with a radiant tube heater.

I keep going back between a large 300cfm fan and a smaller 100cfm unit. These are just basic bath fans. The space has 5700cf so the larger one will turn the air over in approximately 19min versus 57min for the smaller one.

I don't really see any advantage to turning the air over quicker. It's not like it will drop the humidity in 19 min with floors still wet. Leaning towards the small one and run it off a humidistat and forget about it.

School me.....

Thanks

 

[Champ128]

I would be hesitant to use basic bath fans, they rarely move anywhere near the advertised cfm. Their numbers usually are only achieved when tested without any duct attached to them. You almost need to see the fan curve to make a decision.

 

[diy96]

I would be hesitant to use basic bath fans, they rarely move anywhere near the advertised cfm. Their numbers usually are only achieved when tested without any duct attached to them. You almost need to see the fan curve to make a decision.

Thanks for the reply.....

I was leery as to the advertised CFM as well. But while doing some research I found that most of the big names are all certified to meet the HVI (Home Ventilating Institute) standards for sound and advertised CFM ratings. The advertised CFM is rated at 0.1 static pressure which equates to 10' of galvanized smooth duct of the proper size and 1 elbow. The CFM goes down from there as the static pressure goes up......longer run, more elbows etc.

So....I am fairly confident on the CFM rating. As well, my run is only going to be about 6' with one elbow.

 

[sands35]

I have to ask. Why do you think this is a problem? It is a garage, with cars in it, that will just get wet again.

Do you have machine tools in there that need rust control?

 

[diy96]

I have to ask. Why do you think this is a problem? It is a garage, with cars in it, that will just get wet again.

Do you have machine tools in there that need rust control?

No machine tools but I do have window condensation problems and I have noticed some small areas of mold growth. The humidity levels have been as high as 70%. Thought it would be a good idea if I could exchange the air through out the day to try and lower the humidity value.

 

[sands35]

That can be done, but you will pay for it in your heating bill. Something closer to a fan used in an over the oven vent. ~3-600 cfm. Put it on an timer or humidistat. Might be able to find a used one locally for close to free.

Put the fan in the crawl space above the garage and then ducted into the eave or gable. Just use common bathroom duct and fittings. Though a 600 CFM fan will need 6" to 8" ducts (depending on duct run length).

Might be better off buying a de-humidifier. You could also try a ceiling or utility fan to move air around to prevent condensation. Get a cheep one and wire it into the socket for the (typically) common ceiling fixture in the garage.

If you have mold, then spray or wash down the windows with a 1:10 bleach water mix once a year. Might need to paint and caulk the jamb once every couple years, but that is a lot cheaper than any other solution.

 

[diy96]

That can be done, but you will pay for it in your heating bill. Something closer to a fan used in an over the oven vent. ~3-600 cfm. Put it on an timer or humidistat. Might be able to find a used one locally for close to free.

Put the fan in the crawl space above the garage and then ducted into the eave or gable. Just use common bathroom duct and fittings. Though a 600 CFM fan will need 6" to 8" ducts (depending on duct run length).

Might be better off buying a de-humidifier. You could also try a ceiling or utility fan to move air around to prevent condensation. Get a cheep one and wire it into the socket for the (typically) common ceiling fixture in the garage.

If you have mold, then spray or wash down the windows with a 1:10 bleach water mix once a year. Might need to paint and caulk the jamb once every couple years, but that is a lot cheaper than any other solution.

Thanks

Was looking at a de-humidifier but the cost to run it was high. leaning towards a ceiling mount fan vented through the sidewall. Just have to decide on the size

 

[pseudorealityx]

A dehumidifer in a 50 degree garage in the winter will do basically NOTHING except put some additional heat in the space. It doesn't run the coil temps low enough.

 

[pseudorealityx]

Looking to add a basic fan to my garage in order to remove some of the humid air that occurs when the cars come in during the winter. We knock most of the snow off but some still eventually melts inside and then squeegeed out.

I keep the temp at a constant 50 degrees with a radiant tube heater.

I keep going back between a large 300cfm fan and a smaller 100cfm unit. These are just basic bath fans. The space has 5700cf so the larger one will turn the air over in approximately 19min versus 57min for the smaller one.

I don't really see any advantage to turning the air over quicker. It's not like it will drop the humidity in 19 min with floors still wet. Leaning towards the small one and run it off a humidistat and forget about it.

School me.....

Thanks

The larger fan will dry the space faster. You're putting cold dry air into the garage (making up for the exhaust). This air warms up, and therefore can carry additional moisture. It picks up that moisture from the space, and then is exhausted.

 

[diy96]

The larger fan will dry the space faster. You're putting cold dry air into the garage (making up for the exhaust). This air warms up, and therefore can carry additional moisture. It picks up that moisture from the space, and then is exhausted.

Thanks, never thought of it that way.

 

[diy96]

Well I think I've got the fan sizing down and how and where it will exit the building. Now I'm starting to think about a fresh air intake. My space is fairly tight but I'm sure there is some leakage. The radiant tube heater has a concentric vent drawing fresh air from the outside.

Was thinking of just using some 2" pvc to bring in fresh air. Any ideas? Would this be sufficient? How would you prevent the wind from coming through the pipe? Would a P trap routing work?

Thanks

 

[pseudorealityx]

Assuming this is an attached garage, if you only put a 2" intake to the outside, you'll likely just pull air from inside your house... not ideal.

As far as blocking wind, then you need a damper, either gravity (thick "reverse" dryer vent), or motorized. It would power open when the fan kicked on, and then close when the fan shut down.

Typically the intake is sized larger than the exhaust.