Greenhouse build questions

[orion141]

Hi all,

So I am getting closer to deciding upon a potential greenhouse for orchids at my home in Connecticut. Based upon size and money constraints I have been considering the 8x12' gothic arch greenhouse in an 8mm triple wall polycarbonate. I have a few questions regarding heating and cooling.

Gothic arch sells a fan/exhaust system that I will likely purchase. I am also thinking of purchasing the Hot Dawg (modine) 30k BTU propane heater as the primary source of heat (Greenhouse Hot Dawg Heaters| Gothic Arch Greenhouses). Anyone have experience with this heater?

As I will be growing many intermediate temperature orchids I will need cooling for a few months of the year. I was thinking an evaporative cooler would be reasonable here in CT (it does get humid at times, but not like southern regions). I know that some of the folks who grow orchids around here use them successfully. I believe the greenhouse has two 12' inch square vents (one on each end, as per the picture below). Does anyone have thoughts about a relatively simple evaporative cooler that could be used here. Could I use some sort of evaporative cooling pad system with a small pad placed in front of the intake opening so that when the exhaust system kicks on in the summer due to high temperatures it would bring outside air across the cooling pad? For example, something like this: https://www.gothicarchgreenhouses.co...ad_systems.htm

Example 8x12' gothic arch GH from their website:

Thanks in advance!
Orion

[estación seca]

Your simplest choice might be a small portable evaporative cooler than can be connected to a garden hose. Remove one of the vents, replace with 1/4" mesh wire hardware cloth to keep out critters, and set the cooler outside, blowing into that opening. They have a float valve so you leave the hose open just a trickle to keep the reservoir full.

[Ray]

Heating is going to be, by far, the most expensive part of greenhouse operation, and propane ain’t the cheapest fuel, so I highly recommend you look into a version of the heater having separated combustion.

The standard Hot Dawg uses interior air for combustion, then vents it out of the structure. You already paid to heat and humidify that air, and now you’re throwing it away. A separated combustion unit has an air intake pipe in the center of the exhaust. That air is brought into the heater from outside, is preheated by the exhaust, increasing efficiency, used for combustion and exhausted without consuming any of the interior air.

When I switched from a standard Modine heater to a Hot Dawg style with separated combustion, my propane use dropped 40%.

I suspect that a swamp cooler blowing into one of the vents might be better than just having pads over the vents, but a lot depends upon the specifics. One factor is pad area and thickness, but you could certainly rig two 3’ square pads to adapt to the 2’ square openings, for example. Secondly, there is the volume of air flow.

If you exhaust fans pull too much air through the pads, they will exceed the cooling capacity, and you won’t get the net temperature reduction you plan on.

[Keysguy]

If I had an outdoor space with good shade available from 10 am to 4 pm (or better yet in a shade house), I'd have most all of my orchids outdoors for as much of the summer as possible.
Have the swamp cooler available for those few days on either end of the outdoor season only and save a lot on your electric bill and I bet your plants will be happier getting the rain water.

[orion141]

Originally Posted by Ray

Heating is going to be, by far, the most expensive part of greenhouse operation, and propane ain’t the cheapest fuel, so I highly recommend you look into a version of the heater having separated combustion.

The standard Hot Dawg uses interior air for combustion, then vents it out of the structure. You already paid to heat and humidify that air, and now you’re throwing it away. A separated combustion unit has an air intake pipe in the center of the exhaust. That air is brought into the heater from outside, is preheated by the exhaust, increasing efficiency, used for combustion and exhausted without consuming any of the interior air.

When I switched from a standard Modine heater to a Hot Dawg style with separated combustion, my propane use dropped 40%.

I suspect that a swamp cooler blowing into one of the vents might be better than just having pads over the vents, but a lot depends upon the specifics. One factor is pad area and thickness, but you could certainly rig two 3’ square pads to adapt to the 2’ square openings, for example. Secondly, there is the volume of air flow.

If you exhaust fans pull too much air through the pads, they will exceed the cooling capacity, and you won’t get the net temperature reduction you plan on.

Thanks Ray, good point regarding the separated combustion units. I believe modine has a 30k BTU separated combustion unit as well. I assume that I would mount this suspended from the ceiling (although due to convective heat rise it would probably make most sense to have it on the floor?).

Regarding the swamp cooler I have a few naive questions. I assume that I would try have the exhaust fan as the primary means of temperature control and then once it can't maintain a temperature that is acceptable the swamp cooler would kick on. My confusion (and again this may be naive) comes from the fact that there are only two vents (an intake and an exhaust vent) in the greenhouse. If the swamp cooler is installed would I then have to have both the exhaust and swamp cooler kick on simultaneously since the only intake vent is then connect to the swamp cooler. Or can the exhaust fan draw air through the swamp cooler even when the swamp cooler is not powered on? I ask this because I don't think a swamp cooler will really be required for months outside of July/August/early September.

Thanks!

[estación seca]

Portable evap coolers have separate switches for the pump and fan. You can use them as a fan without the evaporative cooling. I don't think you would get good flow via your exhaust fan through a cooler that is turned off. Instead of the small exhaust fan you could put the cooler on a thermostat and use it to ventilate, with the water pump off, during warm but humid weather.

You could also install a screen door, and leave the glazed door open for the warmer part of summer when you don't want to run the cooler. That way the exhaust fan would have plenty of intake even with the cooler occupying the air intake.

I'm guessing critter issues might be why you want an enclosure, otherwise I would agree with Keysguy that your plants might be happier out in the open during summer. You could consider building a screenhouse with quarter inch hardware cloth over wood supports to keep critters out.

There are several evaporative cooler brands available. Your greenhouse would be served by the smallest ones available. Look up Hessaire evaporative coolers.

[Ray]

Originally Posted by orion141

Thanks Ray, good point regarding the separated combustion units. I believe modine has a 30k BTU separated combustion unit as well. I assume that I would mount this suspended from the ceiling (although due to convective heat rise it would probably make most sense to have it on the floor?).

I have done both. As long as you have circulation fans in the greenhouse that keep the air tumbling, it probably doesn't matter much, although there will be growing "zones" of different temperatures in almost any structure. At one point I had my heater blowing lengthwise down the greenhouse, under the bench, making that bench ideal for the really warm-growing stuff without overheating the whole volume.

Regarding the swamp cooler I have a few naive questions. I assume that I would try have the exhaust fan as the primary means of temperature control and then once it can't maintain a temperature that is acceptable the swamp cooler would kick on. My confusion (and again this may be naive) comes from the fact that there are only two vents (an intake and an exhaust vent) in the greenhouse. If the swamp cooler is installed would I then have to have both the exhaust and swamp cooler kick on simultaneously since the only intake vent is then connect to the swamp cooler. Or can the exhaust fan draw air through the swamp cooler even when the swamp cooler is not powered on? I ask this because I don't think a swamp cooler will really be required for months outside of July/August/early September.

Thanks!

ES answered about the swamp cooler functionality.

There are any number of ways you can configure things.

One way would be to have the swamp cooler be the primary device, blowing dry air in initially, having the pump come on to add more cooling as needed, then the exhaust fan as the "last resort" if the interior temperature was still too warm. That would require three thermostats. If the cooler came on with both the pump and the fan together (that's what mine did), then you only need two.

My greenhouse in PA used a combo of passive and active ventilation - I had two input vents fairly high in the end wall. They were operated by "wax motor" solar vent openers, and were the primary ventilation. If it got too warm with just them open, a swamp cooler kicked in, blowing cooler air in from low in a wall, forcing the hot air up and out. If that still didn't do it, a large (10,000 cfm) exhaust fan kicked in. Over time, I did away with the swamp cooler, relying on the verts and fan alone. With Aluminet shade cloth, that exhaust fan rarely came on.

[Selmo]

Getting great advice, just don’t over think it. Your greenhouse sqft. isn’t that much. Don’t fill that space with complicated heaters and swamp coolers, unless you really need them. You can probably get by with couple of space heaters in the winter and in the summer, a silver shade cloth and some fans should keep it close to the outside temperature. Just put in a vent low on one side and high on the other side. Remember to run dedicated water and electric lines to the greenhouse.
Do your research, keep it simple and enjoy growing

[Bayard]

I use an evaporative cooler in summer here in NJ. That plus a fogger and 50% Aluminet shade keeps temps below 90F on the hottest days. I used to use this window unit with good results: CHAMPION WCM28 EVAPORATIVE COOLER, 400 - 600 SQ-FT, 2800 CFM.
If you can, go up to five wall polycarbonate to save on heat and go bigger. A larger air mass is more stable as far as temperature swings, especially with heating. Besides, you’ll run out of room fast!

[orion141]

Originally Posted by Bayard

I use an evaporative cooler in summer here in NJ. That plus a fogger and 50% Aluminet shade keeps temps below 90F on the hottest days. I used to use this window unit with good results: CHAMPION WCM28 EVAPORATIVE COOLER, 400 - 600 SQ-FT, 2800 CFM.
If you can, go up to five wall polycarbonate to save on heat and go bigger. A larger air mass is more stable as far as temperature swings, especially with heating. Besides, you’ll run out of room fast!

Hi Bayard, thanks for the link to the evaporative cooler. How did you have your greenhouse set up with regards to automatic temperature control? Can this evaporative cooler be configured to run in fan only mode as well as evaporative cooler mode? I was thinking of having the exhaust fan and fan only mode of the evaporative cooler kick in at some temperature around 77-80 and then if it can't maintain the temperature to below 83-84 have the evaporative cooling portion of the swamp cooler kick on. Not sure if this is feasible?

As general question to everyone who has replied so far. Have you all found that the exhaust needs to be run even in the cold portions of winter for air exchange? Or will passive air exchange due to gaps/cracks in combination with internal circulation be sufficient to prevent mold/fungus/etc?

thanks!