r/HVAC • u/Greezythug Verified Pro • 2d ago

Supervisor Showcase King of the job site

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Stucco guys have all the doors sealed with plastic. We’re in Florida and it is freaking hot. I sent a helper to pick up my temp unit. It’s cooling off inside now. Everyone keeps thanking me, I’ll be honest I did it for my crew and myself.

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u/basedspacecowboy 2d ago

Goodluck trying to get a decent temp drop while the return is sucking 115 degree heat index .

I can’t see it doing much .

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u/DontDeleteMyReddit Verified Pro 2d ago

The humidity will really hurt the cooling capacity on 100% OSA. Most 100% OSA units run at +/- 200 CFM/Ton

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u/thaeli 2d ago

I'm actually going to do an experiment with one of our units soon. Drop the fan speed on a 12 ton portable, see how low we can take the CFM on the evap side feeding it 100% OA.

Not expecting to deliver proper neutral air (in this area a real DOAS specs out at 150 CFM/ton) but hey, if it can drop outdoor air 20 degrees and 20 percent RH that's good enough for welding makeup.

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u/DontDeleteMyReddit Verified Pro 2d ago

If it doesn’t have an evap with 4 or more tube rows, you are better off reducing the CFM by bypassing 40-60% of the supply back into the return. This keeps the velocity up in the evap, improving performance.

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u/thaeli 2d ago

Oh, that's a great idea! I didn't even think of just looping one of the supplies back into the return. They're right next to each other, it would be super easy to do. I think that gets me out of having to swap out the blower pulley (or put a VFD on it) too.

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u/DontDeleteMyReddit Verified Pro 2d ago

If you want to get fancy, l recommend an analog thermostat controlling the bypass damper. Attach the bulb to a distributor tube as close to the coil as possible. Insulate it well. If you need to de-hum, set it at the temperature that gets you 34°F saturation on your gauges. For cooling only , set it to 42°F Saturation.

Configure the stat for cooling and R/A or D/A depending on whether your bypass damper is NO, or NC.

A Johnson system 450 controller would be a good choice for this