venting a dehumidifier

[oldjarhead100]

I know the theory behind a DH but in a grow room scenario if you vent the hot air out of the room will it still dehumidify the room enough , or does it need that hot air in the room to work properly

[DrPhoton]

No it doesnt need the hot air, it is a byproduct of the conditioning system required to create a cold radiator and condense moisture in the air. You may vent the heat if you wish.

[hycheese]

Chances are that the new 50 pint DH you bought has dual stacked coils and a single fan - the moist air is sucked through the cooler evaporator coil then immediately pulled through the warm condenser coil then the dehumidified warmer air is exhausted back into the room. Older units had separated coils and 2 individual fans and may have been possible to vent but even then the DH would slowly lower your room temperatures below what is needed or comfortable.

Recently I just pulled apart a 5000 btu window AC to see what was needed to split the coils to make a remote DH kinda like a mini-split heat pump. Even though I have all the equipment and HVAC tools to do it I decided to buy a second 30-pint dehumidifier for $100 instead.

One option you may have since it seems like you have quite a bit of ductwork is to make a bypass damper that can vent some of the DH to the outside and some of it to the inside. Basically, a wye connector pipe with a damper installed in the exhaust leg, when 100% open it should give a 50/50 blend and when closed it should send 100% back into the room. those dampers are common and cheap too. you could also try to force-feed the DH exhaust directly into the AC or as close to it as reasonable.

In the business, those dampers/vents are called enthalpy economizers which automatically adjusts the venting based on inside/outside conditions via remote sensors.

[DrPhoton]

Good post, have you worked in the air conditioning field ?

[hycheese]

40 years in industrial automation, all certs in refrigeration including large-scale ammonia R717 systems. My expertise is in PLC programming and instrumentation. I retired to a 3 hour/day part time job a few years ago because my brain was drying out.

[DrPhoton]

Ah yes i know exactly what you mean. I have a friend who is a machatronics engineer who is in the same field. I was looking into getring into control engineering myself, but never ended up there.

[Flockshot]

Run the dehumidifier outside the grow area and let the ventilation bring in the lower humidity air. Running the dehumidifier inside the grow area, and immediately pumping it out, is an expensive way to accomplish your goal

[Potted]

U opened up a big can of worms OJH I just said f#@k it and got a portable ac unit

[oldjarhead100]

apparently I'm not the only one who is trying to figure this out lol I did that too Potted I have both now DH and AC and still it gives me issues

[Toripony]

Been going round and round on this subject... my main problem is humidity being in the PacNW and my 168 cu. ft. room is in my 800 sq. ft. (8800 cu ft.!) garage; temperature is borderline issue. So, after a few circles I settled for the DH in the grow room... adds heat but keeps the RH a couple of % lower than if I tried to DH the whole garage. Gradually the garage RH lowers as my DH air from the grow room is vented into the garage. The heat is also vented into garage so the whole place heats up slowly as the day goes on (starts out at 65 or so each morning). Had decided against a portable A/C due to cost and the fact that I already have cool air (in the garage) if I could just circulate it at the right volume. Without raising the RH. Right. Could vent the grow room out the window (25ft. away; need ducting and bigger exhaust fans) to get rid of the heat and there goes all of my RH conditioned air out the window. AND, adding the bigger fan then I need bigger A/C and DH! Someone said (@Hycheese?) portable A/C was very inefficient, so I thought about a window unit... $50 on Craigslist... but not for me... no, I need 18K BTU for the whole garage running at 10 amps, just to cool my 168 cu. ft. box. Wayyy expensive and wasteful. Mount the window A/C in the grow room wall?... now it's heating up the garage, the source of my intake air.
How to balance the A/C, the DH, and the ventilation flow for a small box inside a big room?!?!? Wish there is a magic formula to figure all this out. Lacking one, I gave up... keep the DH unit set higher than I wish... it reduces the RH some without adding too much heat... it's a constant balancing act requiring me to change settings every day depending on the weather outdoors. I think the portable A/C located in the box is probably my best hope but I can't justify $200-400 when I only need it for maybe 6 weeks a year whereas I need DH reduction 10+ months. Yea, I know most portable A/C does both functions but is not as practical at DH as a dedicated DH unit. Arrrrgh!

[hycheese]

Don't forget you can cobble a $50 window unit into a makeshift portable by using a bit of foam board, duct tape, and a dryer vent hose.

[Toripony]

Omg, I shouldn't have read that right before bed... now my hillbilly engineering mind is racing! I like playing w/foam board... built my grow room with it... and duct tape, lol.

[hycheese]

There is nothing wrong with the portables except they tend to only come in the larger BTU sizes which will do very little dehumidification in a small room. When all else fails you must resort to the KISS principle and remove all variables until a cause and effect can be realized. Shut off all ventilation close all vents, set AC to 75F and DH to 50% and see what happens over a complete care cycle of a day or two.

[oldjarhead100]

I'll give that a shot

[hycheese]

Another thought, for those who work in HVAC probably know that when trying to charge an AC in low outside temperatures you either had to reduce the air flow by blocking the condenser fan with cardboard or install a low ambient kit on the fan motor.

The theory is to increase the head pressure which will also increase the suction pressure thereby raising the evaporator temperature, blocking off the right amount so as not to trip the compressor out on high head but enough to keep the evaporator below the dew point should allow for some cooling but even more dehumidification. On bigger central AC's we did this by oversizing the A-coils by a ton or so over the condenser tonnage.

On a portable unit, I could see reducing the duct size or installing a variable restriction could achieve the same effect, on a window AC just block off part of the back where the warm air blows out.

[hycheese]

No, you partially block the exhaust, it will simulate a higher outside temperature and reduce the capacity hopefully increasing the inside evaporator temp while dehumidifying more. only for the AC unit

[oldjarhead100]

ok thanks will try it

[Toripony]

hycheese, if I try ducting a window A/C to make it "portable", per your suggestion above, I would have to run that duct 25 feet to nearest window. Would that act as damping also, similar to blocking part of the exhaust? Or is that 25 ft. TOO much "damping"?

[hycheese]

if I try ducting a window A/C to make it "portable", per your suggestion above, I would have to run that duct 25 feet to nearest window. Would that act as damping also, similar to blocking part of the exhaust? Or is that 25 ft. TOO much "damping"?

25 feet of the corrugated duct can cause quite a bit of restriction, best to go with as large a diameter that is feasible the bigger the better. If the outside coil is 12" x 12" that equals 144 square inches, a 12" round duct is 113 square inches so that is already a restriction, an 8" duct would only be 50 square inches. It would be best to test bench it first with a cardboard box taped on the back (don't block side louvers) then roll a tube the size that you think will work and point it out the window. You can always use a cheap inline axial fan to downsize the duct but a free cardboard mockup would be the best approach before spending money.

Remember a window AC is divided into two separate compartments, the front end sits in the conditioned room while the back end is located outside a window, the backend also has side louvers that take outside air and blows it through the condenser exhausting through the back to cool it. however, if the whole unit is inside then it will use the air from whatever room the backend is located in essentially acting as a room vent. That is how a single vent portable AC works.

And now the caveat, most newer window units have a water slinger attached to the motor shaft that takes the condensed humidity and chucks it through the condenser to help cool it but also negates the need for a condensate drain. If you have a model like that you need to cut the slinger off and drill a drain hole or you will fill your vent pipe with water.

[Toripony]

So, if I understand this right, the best way for this to work in my case would be to mount the AC into the wall of my grow room so it would use the garage air to cool the condenser, exhausting that out the window instead of the conditioned air in the grow room? In that case, by what you said in your next post below, this might actually be a better solution than the single-vent portable AC? Wow, this is all great information, Hycheese, thanks so much for the education! Now the only part that scares me is that last sentence about cutting out parts and drilling; sounds like something I could screw up, lol.

[hycheese]

Think of a water slinger as a baby seal playing in a kids pool, just slapping the water around until it splashes out... take a wire cutter of sorts and cut it off - you'll know what it is when you see it because it dips down into the bottom condensate pan. pay attention to centrifugal forces and keep the fan in balance or your bearings will suffer.

When the vent is only a few feet it is probably not a problem as the high discharge heat keeps it in the vapor stage but at 25 feet it will re-condense and grow molds and such.

If you drill a hole low enough so that the water level never hits the slinger then you are good to go without further modifications.

[hycheese]

oldjarhead100 after our previous conversation I have verified my suspicions, here is a blurb I found on single vent portables;

"Unlike a window air conditioner, all the mechanical parts of a portable air conditioner are sitting in the room you’re trying to cool. It’s also a reason for the less-than-capable cooling: The portable unit uses conditioned air from the room to cool the condenser and exhausts the hot air out an exhaust hose. That creates negative pressure, causing unconditioned warm air from surrounding rooms or outdoors to be drawn into the room you’re trying keep cool."


Being an oldster I was only familiar with dual vent portables (I owned 2 of them up to a few years ago) didn't know single vents existed until just recently. Now that I read up on them I can see why they don't come in the smaller 5kbtu model like a window unit, it's because they are inefficient and scavenge conditioned room air for condenser cooling to reduce the manufacturing costs and ease of installation.

Single vent portables seem to meet the needs for simplified residential cooling with practicality overtaking efficiency, in a small dynamic grow room where a portable is exhausting 30% or more of your newly conditioned air can be a challenge, especially while trying to maintain a balanced pressure and not draw in crap air from elsewhere.

[hycheese]

Now to throw a wrench into your gears, I bought this hot water AC unit about 10 years ago. It is designed to sit on top of your water heater with a long copper tube that feeds into the tank, it heats your water while it cools your room. I never got around to installing it and ultimately bought an instantaneous tank free water heater.

Anyways the other day I ripped it apart to see if I could redneck a cheap dehumidifier out of it, nope bought a $100 unit instead. When looking at the picture you can see the outside condenser coil has been replaced with about 20 feet of copper tubing that should be placed in a cool water source. This unit can make 135F water all while cooling your house.

[oldjarhead100]

I have a space I can install a window unit ,I just cant get to it right now , I need to get this grow done first I think raising the head pressure is my best option right now or go back to watering every 3 days with no runoff and flushing every couple of weeks IDK