Exhaust fan good idea or no

[dante2]

I have a 5 year old 24x36x10 carport style building with 3 garage doors on the 36' side with a man door on the opposite side. No windows with 2" rigid foam insulation and a mini split. In the summer I usually can't stay in it past about 2:30-3 when the OK heat really cranks up. Recently I've thought about adding an exhaust fan to one end (pretty late in the game I know). Would it be much help and could I use a 24" high CFM floor fan for make up air? Or does it need a vent on the opposite side? The mini split is closer to a glorified dehumidifier than A/C so not a lot of help there. Doors are usually only opened for vehicles or to let some airflow thru in cooler weather. TIA for advice and suggestions.

 

[gahrajmahal]

I would think if the fan is taking air from the shop “conditioned” space it will not help any. If, there is a separate attic space, then absolutely it would make a difference and should be combined with continuous soffit vents or opposite side openings for air replacement. Otherwise, the “attic” fan will **** out the cooler air through the cracks between it and the shop.

If your shop is all one space can you create a partitioned area smaller than the entire space to make it easier for your mini split to keep up? Then, an exhaust fan in the upper area would be cooling the unconditioned space some, making the conditioned space easier to cool. Think, box within a box.

 

[matt_i]

I think you need more AC "tonnage" aka heat transfer capacity to compensate for the heat.

 

[ALinCarolina]

I would add a second minisplit. How much insulation is in the ceiling?
I can't say for Oklahoma but here in the southeast an exhaust fan would only be usable for a short time in the spring and fall. For example, when I build our home I installed a whole house fan but quit using it because it drew so much humidity in the house got moldy and mildewed.

 

[545_days]

I'm south of Houston and run two window units in a stick built and insulated garage (including 1/2" foam on the overhead doors) slightly larger in size than yours. If I want to work in the garage on the weekend I turn the AC on the evening before.

One window unit is the biggest I could get on 110V, and the other is the smallest, as I use it to control humidity in a small shop partitioned off from the main garage and it runs continuously.

As others have stated, a fan won't help in the summer unless it is pulling heat from a separate attic space. If the fan is in the occupied space it would be nice in the spring and fall. In either case, the fan can't move much air if you don't have a vent or vents on the opposite end so you get air flow through the building. If it is in the attic space, soffit vents would be best

 

[pcmeiners]

What make model size is your present mini split? Is the ceiling/slab insulated ?.
My garage is 200 squ ft larger than yours, with a 12k Fujitsu heat /cools from -7° to 101 ° no problem, at a cost about 15-$20 per month. Why would you want just a fan?

"an exhaust fan would only be usable for a short time in the spring and fall."

In the spring /fall an exhaust fan is maybe useful but not in temperatures nearing the dew point unless you like rust patina, with the exception of a few extremely dry states.

 

[545_days]

I'm south of Houston on the coastal plain, and didn't have rust problems for the 25 years prior to installing the AC units. However, I religiously kept unpainted steel waxed or oiled in order to achieve that result.

It's rough in the cooler months when a warm front rolls in and the increase in humidity causes water to condenser on everything.

 

[niget2002]

I've been considering adding an exhaust fan too. My thinking is the AC in the shop is only set to 95 when I'm not out there. There's a lot of days were a semi-cool front will come in and lower the outside temp into the mid 80's but the shop is still in the low 90's. It'd be nice to have the exhaust fan kick on and pull the cool air in from outside.

I have the automation abilities to even have the fan kick on/off based on humidity so it doesn't just **** moisture in.

But then it also seems like it's more trouble than it's worth. Usually if a cool front like that blows through, I just go out and open up one of the big doors and let the shop air out on it's own.

 

[Dumber than lumber]

I think others have pretty well covered it. But I will say these things:
Are the garage doors insulated?
How much insulation in the attic?
Have you checked the mini-split for correct functioning? Is the m-s too small for the space.
The exhaust fan is not a good idea. But a fan inside a closed space like you seem to have can make it more comfortable if the humidity has been brought down.

 

[pcmeiners]

It's rough in the cooler months when a warm front rolls in and the increase in humidity causes water to condenser on everything

Ran a school building in NYC, with humidity as bad as Houston during certain times . When first occupied the building's 3 blower systems in the build were on 24/7. Until I put timers on the blower to turn off at night, in the spring/fall I would would open the building at 6 am and would have to vacuum up >25 gallons of condensation formed over night in the duct work (which was only 70 feet in length) when the air in the ducts went below the dew point .

 

[dante2]

If your shop is all one space can you create a partitioned area smaller than the entire space to make it easier for your mini split to keep up? Then, an exhaust fan in the upper area would be cooling the unconditioned space some, making the conditioned space easier to cool. Think, box within a box.

Unfortunately not enough space to partition it off very easily.

I would add a second minisplit. How much insulation is in the ceiling?
I can't say for Oklahoma but here in the southeast an exhaust fan would only be usable for a short time in the spring and fall. For example, when I build our home I installed a whole house fan but quit using it because it drew so much humidity in the house got moldy and mildewed.

2" in walls and ceiling.

What make model size is your present mini split? Is the ceiling/slab insulated ?.
My garage is 200 squ ft larger than yours, with a 12k Fujitsu heat /cools from -7° to 101 ° no problem, at a cost about 15-$20 per month. Why would you want just a fan?

"an exhaust fan would only be usable for a short time in the spring and fall."

In the spring /fall an exhaust fan is maybe useful but not in temperatures nearing the dew point unless you like rust patina, with the exception of a few extremely dry states.

Pioneer 24k I believe. Yes both are insulated.

 

[dante2]

I think others have pretty well covered it. But I will say these things:
Are the garage doors insulated?
How much insulation in the attic?
Have you checked the mini-split for correct functioning? Is the m-s too small for the space.
The exhaust fan is not a good idea. But a fan inside a closed space like you seem to have can make it more comfortable if the humidity has been brought down.

Doors are insulated. 2" all throughout. It does a great job keeping the humidity down no rust anywhere.

 

[dante2]

It was just a thought exercise and checking to see if it could have a net positive effect. It doesn't sound like the outcome will be good enough for a couple of extra hours per day puttering around.

 

[BurtEggley]

One of two things must happen, or a little of both. Either slow the penetration of heat so that the present system can keep up, or increase the cooling capacity to keep up with the heat entering. A lot of heat will come thru the ceiling and thru any places air can penetrate.

 

[gahrajmahal]

“ Unfortunately not enough space to partition it off very easily”

As I understand it now, there is no attic space. You could, as an experiment, partition off the ceiling space, maybe using silver reflective tarps, ventilate that with fans, and see if that lowers your temperature.

 

[bwringer]

Is the building and/or roof a dark color?

What's on the west side of the building? Is there any shade?

Perhaps there's a solar heating issue you can address.

My garage faces east. Insulating the old nasty dark brown door made a big difference, but installing a new white insulated door made a DRAMATIC difference. On hot days, it was just plain uninhabitable in there until afternoon. Point is, a color change might help a lot.

 

[PhantomEB]

I am putting one into my garage, kitty corner from the Man Door. Very well could see two just cause.

 

[racecougar]

Is there a ceiling or not? It sounds like there is 2" of foam on the underside of the roof, but the word ceiling has been thrown in as well.

As others have said, if there is a ceiling, proper venting of the attic space is absolutely worthwhile. If there is not a ceiling, adding an exhaust fan isn't going to help here. Additional insulation or additional energy consumption (via more HVAC) is needed. I'd push for more insulation, as 24K is already more than necessary for your footprint.

 

[HoosierBuddy]

I'm currently doing some experimenting in my 3 car oversized garage (6 inch insulated walls) which tends to get hot during the day and stay hot all the time because of the 90+ heat we've been having every day here in Indiana.

I put a in-window fan in each of 2 windows on opposite corners of the garage with one pulling air in and one taking air out.

The goal is to run the fans at night so the garage gets cooled down sufficiently, then shut them off in the morning and rely on the insulation to keep it livable during the day.

I'm only about a week into my tests, and the heat last week was as bad as we've had in a while, with night time temperatures only reaching the low 70's.

To this point, I couldn't recommend this approach. I'm turning the fans on about 11:00 PM with the garage 90-degrees and it's still 80 outside. Getting up in the morning with the garage 80 degrees and it's 74 outside. By 4 PM the garage is 90 again.

I'm going to continue to experiment with this. This week the highs are supposed to be more mid-80's and the lows into the 60's. For my plan to work, I really need it to get cool enough outside to get enough cool air blown through the garage to cool off not only the air inside but the walls and floor too.

To the OP....if your mini split had enough cooling capacity, it would keep up. But, the energy cost to do that might be prohibitive. I'm considering going to a mini-split in my garage as well, but I just got a $450 electric bill for the attached house and the bonus room above the garage which has it's own 3 ton AC. So adding an additional "meter spinner" is not something I'm wanting to do.

 

[Hohn]

You know, this reminds me of the perpetual debate in my family about the performance of some key players on our favorite sports team-- get rid of him or keep him. And of course, the "get rid of him" is only correct when there's a known better option at same or similar cost.

It's exactly like an exhaust fan. The only utility to sucking air out of a space is to the extent you can replace it with something you like better.

So if you're sucking out conditioned, dried air and replacing it with hot ambient outside air (at full humidity), it's quite likely it will make things worse rather than better.

Fans are primarily only useful for short cyclic type excursions and only on a certain time of that cycle. For example, you have a shop that was roasting all day in the sun and how there's cool air outside just after dusk. Perfect time to turn on a fan and **** out that hot air from your shop and replace with cooler outside air. And since all the interior surfaces are warmer than the outside, there's no condensation risk.

But that situation is reversed in the morning if you turn the fan on when your shop is cooler and drier than the warmer and wetter outside air. Turn on the fan now and your sucking in heat and ultimately mold and rust as well.

If the *timing* of the cool air is the problem, then add thermal mass (concrete, steel and cast iron) and a fan. But if it's the QUANTITY of cool air that's the problem-- i.e. it's just too hot too much of the time-- then there's just no workaround for more cooling capacity unless you can reduce the cooling load with better insulation and air sealing.