oldjarhead100
New member
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
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venting a dehumidifier
- Thread starter oldjarhead100
- Start date
hycheese
New member
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.
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.
Toripony
New member
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!
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!
oldjarhead100
New member
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
hycheese
New member
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.
hycheese
New member
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.
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.
oldjarhead100
New member
I'll give that a shot
oldjarhead100
New member
you block the intake correct
oldjarhead100
New member
ok thanks will try it
hycheese
New member
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.
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.