I have the same HFGH. It came with 2 roof vents. When when the sun shines, even with the door wide open, it can reach 120F. I can't keep plants in it over the summer unless I remove some of the side panels. Both vents leak so be careful where you place your fans or heater.

I have the HFGH 10x12, which has four vents, and I'm in my first year too. I have to agree with Greenhouser...on sunny days, with four vents open AND the double doors open, it still soared even higher than 120F in there. A second vent may not be as effective in cooling as you'd think (I was surprised that four roof vents didn't help as much as I had expected.)

I use my GH year round, so I did two things...I added an exhaust fan, and I built screens for added ventilation. (I also keep two inexpensive oscillating fans in there running 24/7 just to be sure the air is always moving.)

In your climate, greenhouser's suggestion of just removing some side panels might be a good thing to try. I have some really energetic native birds here that love to poke holes in my succulents, so I built screens using those DIY screen components at Lowes or HD. You do have to think about keeping electrical safely covered if your GH will be more open to the weather, but that's doable.

I used Aluminet shade cloth as the screening material, but you could use regular screen too. I may even try making some screen panels out of rigid hardware cloth this summer, especially for the funny triangle-shaped panels in the gable ends. Lots of things to try! Here's a link to my thread showing how I built screens for the side panels, in case it gives you some ideas:
Adding Aluminet Screens to my HFGH

The number of vents is not important, the size of the opening provided is what counts. For effective passive ventilation, 20% of the floor area (6x8= 48 sq. ft), about 10 sq. ft. of top vent area is needed. As important, and not mentioned in your post, is the bottom vent area. This should be 10% of the floor area, or 5 sq. ft in the 6x8 GH. Simply adding more top vents, without the needed bottom vents, will not serve the purpose of helping to cool the greenhouse. All the hot air going out the top has to be replaced from somewhere, and the cooler bottom area is the prefered place. Otherwise, the top vents will just be recirculating hot air.

I have plenty of bottom ventilation, exhaust fan and plan to put screen panels in the sliding door. I'm trying to get as much natural convection and minimize electrical consumption by the fan.

I realize a second vent is not going to keep the greenhouse cool. I just want to increase the airflow without removing panels. I caulked all but two of my panels in.

Any suggestions for vent design would be appreciated.

Happy Gardening

I have a 6x8 greenhouse and the roof vents do not allow enough heat to escape. I have installed 2 fans from HD that turn on when the temperature reaches 27C and turn off at 24C. My greenhouse is always under 27C even on the sunniest days. You might also consider installing fans to circulate air within the greenhouse. Here is a pic of my fans installed.
{{gwi:295151}}

Nice setup banana fun. What kind of CFM's do your fans move and what kind of intake area do you have?

I see you have got lighting as well. I'd be interested to know what you electrical setup is like. Do you have a single or multipe circuits?

Thank you for your information and picture.

Kauai_Hawaii

What is the black stuff around the poly panels? This doesn't look like a 6X8' HFGH.

If I were to guess I would say the cfm is somewhere around 500-800 cfm. I have 2 exhaust intakes attached to the door which brings cool air through the center of the greenhouse and not directly onto the plants. There is one 20amp circuit that is dedicated to the greenhouse. The fans are activated by a 30amp relay attached to the climate/irrigation controller I built.

This is not a HFGH. I bought it from HD 2 years ago for $299 which was a great price considering they were initially selling them for $799. It is a great design, the greenhouse panels slide into extrusions to create a rigid structure.

Without the two fans installed I wouldn't be able to grow anything in the GH except for cactus.

I bought a 10x12 HFGH, but I haven't found time to assemble it yet. I bought a 2600cfm exhaust fan for that one ;)

Here is a picture of the climate controller. {{gwi:295152}}

banana fun

I'm very impressed. You must have a background in electronics. Besides control of the exhaust fan what irrigation functions do you have control of?

I'm interested in seeing more pictures if you have any you would like to share. Where are you located and what are you growing in the greenhouse?

Thanks

The climate controller will be hooked up to the 24vac irrigation solenoid that is attached to my irritrol irrigation timer. It will be wired so that when misting turns on all the GH fans turn off so the water is not exhausted by the fans. I also intend to have a high temperature alarm that will mist the greenhouse for 2 min when the temperature exceeds 33-35C. I do not intend to install irrigation in my 6x8 GH. I built the controller for my HFGH. The main reason for building the unit was to have absolute control over every aspect of heating and cooling within 1C and 0.1% error across the entire temperature range. Also, typical thermostats tend to overheat by about 7C which wastes alot of energy. My controller will heat the GH within 1C which will save HUGE on the energy bill.

I can also keep the GH at temperatures below freezing which you can not do with regular thermostats. The PID temperature controllers are fairly inexpensive ($30 on ebay) and easy to wire if you know how to connect relays.

I am located in Toronto, Canada and I mainly grow tropicals, but it is full of veggies right now for summer harvest. I will post some more pics of the GH tomorrow so stay tuned.

"Also, typical thermostats tend to overheat by about 7C which wastes alot of energy. My controller will heat the GH within 1C which will save HUGE on the energy bill."

As you may know, the reason that typical thermostats have a range of 1-2 deg. C. as a trigger position is to reduce the number of cycles that the heater experiences. Almost all thermostats can be reprogrammed to reduce the lag period, but the result is more on/off cycles, thus more wear on the system, and an actual increase in electrical/gas usage. Your 7 C figure is somewhat out of the standard, considering that the normal heating thermostat is programmed with a one degree C differential.

Thanks for your input cuestarable. I remember reading that typical thermostats overheat by 7C, but it may have been 7F? The pid temperature lag on my unit is fully adjustable. The relays I use have aprox. 40 000 000 cycle life so I am not concerned about wear. The problem with most electric heater thermostats is that the thermocouple is located within the unit. Before I built my controller the temperature in my GH would range from 6C to 16C during typical heating application at -10 outside temperature, but my target was 10C.

I don't agree with you that my controller consumes more energy by less heating overshoot due to an increase in cycles. There is a spike in energy used when the heater initially turns on, but I doubt that it lasts more then 1 second.

Hi
I'm curious as to why you'd want a GH in Hawaii?? Wouldn't an open shadehouse serve better unless your trying to keep the temps below ambient?? Trying to grow temperates?? gary

Gary:

I wish I was still in Hawaii. I lived there for 32 years before coming to Maryland. I'm in the Annapolis area of Maryland, Zone 7. I still use my Kauai_Hawaii username.

I'm still looking for a design idea for a 2nd vent if anyone has suggestions.

Thanks

I have a HFGH 10-12, and put all four vents on one side, as I was concerned about wind tearing the vents out on one side. I have two Bayliss MK7's and the other two are the original manual ones. I have had the GH running for almost a year now, and never use the two manual ones. The MK7's open and close like clockwork daily. Other additions I have made are adding an exhaust fan (over door) and motorized shutter (on opposite wall), Aluminet shade cloth (50%), various fans running nonstop and a humidifier which I have coming on when the humidity gets too low for my plants' preferences. I am in southern cal., where it can be quite dry. Without the shadecloth everything inside would fry, even with all the other mods. You might consider alternatives to a second vent, but if you still think it's necessary, one option would be to install a round marine vent, which is passive. You would just install it in one of the poly panels. They are rain proof and you could install it somewhere other than in the roof, which might improve the interior draft. Not sure what you would do about it when you are winterizing, however, but I think some of them come with an option to close manually.
Another idea is to modify a large panel by cutting it horizontally, line the cut edges with aluminum channel (or even aluminum tape)installing a piano hinge to re-join the cut edges, cut a bit off the bottom to compensate for the hinge, shave a hair off either side to clear the channels in the framework, and fashion a handle like the one which came with your gh. Doing this on a side panel would be better than another roof vent, since adding another roof vent just offers a partial airflow pattern. Unfortunately this would involve removing the caulking you applied.
Lastly, assuming you want to make something out of the non-caulked panel (in the roof, presumably), you could still try to use a piano or other hinge, perhaps attaching it to the crown frame. If you would be using it daily, you would need to add some framework for stability.
Ann

Gardnergal:

Thank you for your information and suggestions. I'd looked into the marine vents you mentioned but the ones I found were expensive.

I'm working on a design using a section of the polycarbonate panel with a stainless steel piano hinge. Just trying to fabricate a frame for the panel and opening. I want to use an opener like the Bayliss or similar.

Thanks again. Happy Gardening.

I had to modify the Bayliss hardware for my HFGH 10x12, and would assume you would need to as well. The HFGH framing just wasn't solid enough to support the vent operation. I posted here regarding that event, over a year ago, but think it is still available if you do a search for the Bayliss MK7.
In spite of the fact that installation was not a simple as I hoped, the quality of the vent itself is very good.

I was surprised to hear that you found the marine vents prohibitively expensive. It was about a year ago I looked into it, but a few models looked very comparable to the Bayliss vent as I recall. Maybe Google "solar marine vent" for more options??
Good luck!
Ann

gardnergal,
Thanks for your notes on the Bayliss MK7,solar marine vents and ideas for piano hinges for vents. I, too, have had good luck with the Bayless, having to replace just one piston tube after 4 years of duty, and the other original three are still doing fine. As the proud? owner of three solar marine vent units, however, the results have been somewhat less than spectacular. These solar powered vents do move some air, but not enough to really do much good. In checking the current models, they range from $80-120, and vent 700-1000 cubic feet per hour. Even a 6x8 greenhouse needs 17280 cu. ft. per hr. of ventilation for optimum conditions. I have mine near a few large plants to help improve the micro-climate, but the overall effect in the greenhouse is minimal. With roof vents, bottom vents and exhaust fans, the underlying principle of one complete volume air exchange per minute is the goal. This is simply length x width x height of the greenhouse, resulting in cubic feet of space. As mentioned earlier, simply adding more roof vents does little if an appropriate amount of bottom vents are not added as well.

Yes, Cuestaroble, I agree that having additional roof vents did not make a significant difference for me(hence I have 2 unused ones in place). I think other options are preferable, but the OP is asking specifically for ways to manufacture an additional roof vent.

In my opinion, the grower should not have to open the doors to obtain satisfactory conditions. Indeed, adding vents and keeping the doors open will significantly counteract the usefulness of exhaust fan and shutters.

I think the best choice to regulate your greenhouse temperature is an exhaust fan. You can buy inexpensive thermostats that are used to control attic fans from HD for under $20. The two fans I have installed only cost $30 for both.

The biggest mistake I made since I bought my GH was not installing fans earlier. I lost hundreds of dollars worth of plants due to overheating and they were mostly Yucca!

I checked out those Bayliss MK7 openers, they seem very expensive for what they are. Why not use electric actuators to operate vents? They are cheaper, likely more reliable, water proof and can lift snow loads(if applicable). The only downside would be that they require a thermostat. Sorry to make this matter more complicated :)

kauai hawaii,
You mention that you have an exhaust fan; do you have any shutters?
Secondly, if you are trying to cut down on the electrical use from the exhaust fan, shade cloth (preferably reflective, i.e., Aluminet) will really help.
Ann