Garage Humidity Control

[ultgar]

Finally got the A/C running in the main workshop (2 1/2 ton Unico system through 12 air drops) and the system works great for controlling temperature. Unfortunately, humidity is the enemy to the cars and tools in our garages and with the heavy weather we've been having here in the Northeast, the A/C system will not maintain the desired humidity levels (45-55%). With a 72F setting, room humidity was 70% this morning, enough to allow mold and surface rust on bare metals to creep up in the shop.

As a stand-alone system, I installed a Therma-Stor Air Purifying Dehumidifier, mainly to bring fresh make-up air into the room, never really expecting that it would be needed to control humidity with an A/C system in place. The humidistat allows precise humidity control and in less than 10 minutes, the 70% humidity was down to 52% in a 20x32' room with 12' ceilings.

I'd highly recommend the use of a dehumidifier in a nice garage, even if its a small 30-40 pint unit available through the home centers. Air conditioning is not enough (unless you live in the Southwest with very dry conditions). See http://www.ultimategarage.com/UltGar2g.htm for some pics of the system I used.

 

[Tom Bryant]

That's a neat piece of equipment. I use a portable dehumidifier with good success. (about 950 square feet w/10 ft. ceiling) I don't keep drivers or mowers in my shop so there is rarely a reason to open the big door. My next step is central air. Since I use a conventional furmace and overhead ducts it will be an easy install and was planned for from the beginning.

Tom

 

[John Mcgraw]

While the de-humidifier will do the job, so will a properly designed A/C unit by itself. The de-humidifier controls the humidity in the same manner as an A/C unit. It condenses moisture off a cold coil which lowers the relative humidity when the air warms back up to room temp. The only difference is that a de-humidifier is usually designed to produce no cooling effect. It does this by heating the air back up after cooling it down to remove the moisture. This approach to humidity control while effective, is very energy inefficient. You expend energy cooling the coil and then expend more energy warming the air back up. There are units that utilize the waste heat from the condensor to warm the air, but they are usually pretty pricey.
The usual problems associated with poor humidity control on an A/C unit are either too warm an evaporator temp or an oversized unit. If an A/C unit has a 45 degree coil temp, has a duty cycle of being on at least 70% of the time on a warm day, it should be able to maintain a humidity of less then 45% in the conditioned space unless there is a lot of outside air being introduced into the space. Having an oversized unit will present problems since it has a very short cycle. There is a rule that is almost a mantra in our trade "A unit that is not running does not remove moisture".
I have a 1.5 ton unit in my 30 X 30 garage with 12 foot ceilings and the unit never shuts off on a 100 degree day, while maintaining a 40 - 45% relative humidity level in the garage.
In Central Texas a 100 degree day with relative humidity levels of 70% are not uncommon. The only days that an A/C unit will not be able to control humidity is when the outsude air temp is moderate and the himidity is very high. There is not enough demand for cooling to keep the unit running long enough. Obviously the winter time is a non-issue since even 100% humidity in 40 degree weather is a very low humidity when warmed to 75 degrees. You can plot out any set of conditions on a psychrometric chart. Sorry about rambling on, In addition to being a Corvette nerd, I am also an air conditioning nerd!
Regars, John McGraw

 

[ultgar]

Thanks for the detailed write-up John. I've only had my A/C system operational for a week and the outdoor humidity has been around 90% @ 75-80F all week here in the Northeast. My system at 2 1/2 tons is oversized for a 20x32' room which may help explain why it is not able to keep the humidity in check. However, since I'll be running cars at speed under load on the dyno, I had to take the btu's generated by the car into consideration.

 

[*89x2*]

so what would be a proper sized AC unit for a 34x26 garage w/ 10 ft ceilings and proper insulation??
TIA

 

[John Mcgraw]

You also have the disadvantage of cool mornings and high humidty. Here in Texas, the nighttime temp during the summer seldom drops below 80 degrees! we only have a couple of weeks out of the year where the A/C does not run enough to keep the humidity down, and from there we go right into heating.
Regards, John McGraw

 

[John Mcgraw]

89X2,
That is a loaded question. It depends on a lot of factors such as ammount of insulation in walls, insulation in ceilings, ammount of glass and orientation of the glass, color of the roof material, ect.
The best way to truly find the right sizing is to have a A/C contractor run an ASHRE Manual J calculation of your garage, but I would think that a 1.5 to 2 ton unit would be adequate for your climate. avoid the temptation to oversize the unit and keep in mind that most of the new generation of ultra-efficient units do not do a good job of humidity control. They run so much air over the coils in order to make them more efficient that the leaving air temp is fairly high and this leads to poor humidity control. In a tight house where the is little infiltration, this is not a problem, but in a garage, you need as low a leaving air temp as possible to control humidity. That being said, I just grabbed the first unit that I removed from a business that no longer needed it! THe unit is over 15 years old and has a very low SEER, but I will use it until it craps out since it was free!
Regards, John McGraw

 

[*89x2*]

John, Thanks! What would I need power wise for the Garage temp ctrls? I have a 60 amp circuit from the house out to the garage & it is divided up between all the plugs - 4 15 amp breakers...

I am just wondering if humidity control would be more important to address than the AC factor - I know AC removes humidity but the garage is alrady pretty cool as is...

Let me know what you think & again, Thanks

ps, blown insul up in attic of garage, cedar shake roof w/ ridge venting, r factor on walls, I don't recall - highest rating for inside 2x4 stud walls... One set of windows, one entry door and 3 bay doors... Hope this helps - Doors BTW are wood - will switch to iinsul if needed...

 

[ultgar]

You'll will typically need a 20-30a (220v) breaker for the compressor and 15-20a (220v) for the air handler. Humidity control is more important in my opinion for the garage. I'm doing a garage right now for a Seattle (Mercer Island) guy with an AC Cobra....no air conditioning but definitely has humidity control.

A competent A/C contractor can help you spec the right size unit. The first guy I brought in sized the AC system for my workshop and failed to take the equipment (dyno, 1700w audio, 3 plasma monitors, 3500w of lighting, etc) or a hot vehicle (I wonder what kind of btu's come off a big block Chevy or Viper motor after a few full throttle pulls in a closed garage) into consideration. If you have skylights or windows in your shop, especially with southern exposure, you'll need a bigger unit. (Note - I was talking to a BMW tuner 2 weeks ago with the same dyno as mine....he's got a 35,000 CFM fan pulling in outside air and directing it right at the front the front of his cars...pretty tough to control the room conditions with this kind volume).

Dehumidifiers (even the large one I'm using) typically only require a dedicated 15-20a (110v) circuit. SD

photo shows some of the 12 air registers throughout the shop. Still a lot to do here.

 

[*89x2*]

Thanks for the reply! I suppose now I would have to find a competent AC guy in the area... (Wash DC area)

I am really thinking I do not need an ac unit as I don't think temps get very high (85deg at most) as the back 1/2 of the garage is built into a hill... Humidity is my issue I think - it is not terrible, just noticeable.

Since I do not run the cars in the garage - no dyno (I wish ) or other forms of heat...

No skylights and the one set of windows face to the West w/ trees / woods casting a lot of shadows...

Would I put the dehumidifier up in the attic on the joists or, in the garage?? The dense air seems to be close to the floor - I guess it would "pull" it up though, right??

Thanks for the input - it is appreciated

 

[John Mcgraw]

Ultgar,
That is going to be a sweet garage! I wish I had a plasma screen Monitor in mine but there would be hell to pay at the house if my shop got one before the living room did! I will have to make do with the old 20" TV on a wallmount for a while yet. You are absolutely right about the BTU load of an engine, a small block puts out about 40K BTU or about 3.5 tons! I would think that a Big Block would be much more, and that does not even take into account the heat generated by the eddy Current brake on the dyno!
Regards, John McGraw

 

[ultgar]

Each of the dyno pods uses a Vickers pump and hydraulics for loading. The pods are water cooled and the hot water goes into my drywell or pond. Fortunately, most dyno runs only last 7-10 seconds after car reaches operating temps. Still, a lot of heat to deal with, especially on turbo cars running big boost. SD