Desiccant vs Compressor dehumidifiers

Hi there, my wife and I opened a small organic perennial nursery last year near Portland, OR (Zn 8b). We have a 15’x 40’ hoophouse that is about 8’ tall. Im estimating about 3500 cubic ft of air space inside. Last year I had a jerry rigged hoophouse inside of a hoophouse set up that made it dual wall insulated which enabled me to keep it about 60 degrees with a couple space heaters. The problem was that it got very humid inside even with regular ventilation because the winter air here is so humid that ventilation alone doesn’t seem to dry it out enough. I used multiple box fans for air flow and to try to keep plant surfaces dry but it wasn’t good enough. While I still managed to grow many healthy plants, I definitely suffered some losses due to fungal diseases. This year I am making a few upgrades before winter including an inflation blower for a better dual wall system and I am looking into a dehumidifier to help keep the humidity down.

From my research it seems that desiccant dehumidifiers are preferable to compressor dehumidifiers in greenhouse applications because compressor styles can develop fungus inside them and blow spores into the greenhouse air. They also produce a little very welcome heat while I believe (but am not sure) compressor styles cool slightly. They also work better in a wider variety of temperatures and humidity levels. The main reason I am wondering if it is the best choice for me is that I am on a budget and cannot afford a more powerful commercial grade desiccant dehumidifier this year. At the $200ish price point I can get a 10-20 pint a day desiccant style dehumidifier which I really have no idea if that will be enough. Or for roughly the same cost I can get a 50-70 pint compressor style. Considering I will be heating the air to 60-70 degrees I would guess that the compressor style could operate at close to maximum capacity meaning much more bang for the buck in terms of water removal am I correct? However, if it does spread mold spores or cools the greenhouse too much the increased water removal may not be worth the other risks it poses. Does anyone have experience using either style of dehumidifier in their greenhouses and how do they seem to work?

One one more related question. I have heard that if I don’t get some outside air exchange there may not be enough co2 in the greenhouse. I will still open the greenhouse for cooling during our occasional sunny winter days. However, on cloudy days if I want to keep the greenhouse closed to hold in heat and air that has been dehumidified can I use a co2 tank to add supplemental co2 (I have a 20lb tank that is not being used) or is the co2 deficiency in a closed greenhouse too great to avoid regular outside air exchange?

Thanks in advance for your help getting my greenhouse microclimate dialed in.

I can't remember the name of the compound off the top of my head, but the same thing that is in damp rid is available in bulk from many farm supplies (and sidewalk deicer.) hang it in mesh bags above five gallon buckets.  $200 buys a lot

R Scott wrote:I can't remember the name of the compound off the top of my head, but the same thing that is in damp rid is available in bulk from many farm supplies (and sidewalk deicer.) hang it in mesh bags above five gallon buckets.  $200 buys a lot

It is Calcium chloride. I’ve read that it is a skin, eye and lung irritant and long term exposure in a closed environment like a greenhouse with lots of fans blowing particles around could lead to permanent lung damage.

I’m gonna go with a plug in dehumidifier but hoping to get some input from someone who has used them successfully in a small greenhouse before.

dont think you are going to like the price tag but search

Grozone SCO2 The Simple One

and

Titan Controls CO2 Regulator

i would get a burner though... they can be nat gas or propane

Thanks for the suggestion. I’ll likely get something like that or other commercial grade greenhouse controls in a few years when I build a permanent steel frame hoop house. I own a small landscaping business but am transitioning to solely design and growing the nursery on our land. Right now it’s in the very early stages so I’m just trying to get some semblance of climate control even if it isn’t perfect to get me by and to hopefully grow some plants to fund greenhouse upgrades.

I did more research and found a small propane heater with a low oxygen shut off and also got a carbon monoxide detector just in case. I may also get a basic co2 monitor and just to try to balance it manually. The two electric heaters I had didn’t quite seem to be able to keep temperatures where I wanted them so the added propane heat is a bonus in that regard.

As for the dehumidifier I’m still weighing the options and having a hard time committing without having much experience with either.

M. Phelps wrote:dont think you are going to like the price tag but search

Grozone SCO2 The Simple One

and

Titan Controls CO2 Regulator

i would get a burner though... they can be nat gas or propane

I have a silica gel dehumidifier it does around 2-3L per day so it's not a huge one it kicks out a fair bit of heat when running, enough to raise a small room in the house up by about 3C We have that type because our old house regularly dropped to low for condensing types to function. condensing types also produce heat but not very much, only that produced by the compressor and any fans. Think of the heat given off by your fridge, that's the level of heating you get from a condensing dehumidifier.

For any large application I would go with condensing if you can keep the area it is in warm enough, as the cost to run is much lower, however if you are heating with electric anyway then go for a silica gel model since it will help with the heating.

Condensing type work great in my experience. I could see mold spores being an issue but as long as you keep the intake and outlet clean you should be just fine

Unless it is outside the compressor style has to heat the building.  Converting water vapor to water makes heat.  It has to from physics.  And any mechanical system has to make waste heat.  So as long as it isn't dumping heat outside it has to heat the building as both output parts of the system make heat.

And you might want watch these videos if you are a DYI person.  If you use 2 of these stages and do the cooling heating your building.

Thanks everyone, I ended up getting a compressor style and it’s been working wonders in the greenhouse. Brought it down from about 80% RH and holding it around 60% RH. Hopefully no more botrytis like I had last winter. The added heat has also been welcome, it’s not much but I think it raises the temp at least a couple degrees.

One thing to concider with a propane heater is while it adds some co2 it will also be adding humidity another option for co2 adition is to take up home brewing and put your fermenters in the greenhouse.

Brian Karlsen wrote:One thing to concider with a propane heater is while it adds some co2 it will also be adding humidity another option for co2 adition is to take up home brewing and put your fermenters in the greenhouse.

I actually used to be a home brewer (and worked in a brewery for a few years) but alcohol gives me serious acid reflux and I have to take Prilosec daily to keep it from further damaging my esophagus so I avoid alcohol now. Fortunately a local store now carries some decent non-alcoholic craft ipa and stout so I don’t have to give up beer which I love! Maybe some day when I have more time I’ll try to make some N/A beer.

I have found that my greenhouse stays in the 700-900ppm range of co2 on its own. I’m thinking maybe it has extra co2 because of composting organic material under the floor since I rototilled and leveled a grassy area before putting down nursery ground cloth and constructing the greenhouse. I have just been running the propane for a few minutes several times a day because it bumps it up way above the desired 1000-1300ppm levels very quickly and therefore doesn’t seem to be a good option for heating my greenhouse. I’m now looking into a 240v electric heater since the greenhouse is near the previous property owners machine shop which is set up for lots of 240v machines. Right now I’m using two 120v heaters on separate breakers but it gets cooler than I would like on really cold nights.

A Booth wrote:

Brian Karlsen wrote:One thing to concider with a propane heater is while it adds some co2 it will also be adding humidity another option for co2 adition is to take up home brewing and put your fermenters in the greenhouse.

I actually used to be a home brewer (and worked in a brewery for a few years) but alcohol gives me serious acid reflux and I have to take Prilosec daily to keep it from further damaging my esophagus so I avoid alcohol now. Fortunately a local store now carries some decent non-alcoholic craft ipa and stout so I don’t have to give up beer which I love! Maybe some day when I have more time I’ll try to make some N/A beer.

Suggestion for reflux.  Try taking a tablespoon full of cheap yellow mustard for it.  Don't get the fancy stuff.  You want true yellow mustard.  I originally learned it from a Dr. Gott column.  For me it works.  It is slower than antacids but does a lot without doing any drugs.

for people looking for a "more natural" way of adding co2

one potential way could be to set up a bucket with calcite or other carbonate rocks and use an automated ph controller to dose vinegar(or other acid) into the bucket

i did a quick search to show the type of product i mean although i am sure there are less expensive options

https://www.amazon.ca/Bluelab-CONTPH-Controller-Monitoring-Solution/dp/B00O9CBYZ0

then using a meter for co2 one could dial in what ph to maintain in order to release the desired amount of co2

the calcite would have to be topped up occasionally as well as the vinegar reservoir