The Variations of Passive Venting

In a nutshell, passive venting is creating an opening and letting the warm / hot air escape. The simplest form of this is opening a door or window. Just because it is simple does not mean that this method will be effective in cooling your greenhouse.

If your ends face into the prevailing winds, if you can make your doors big enough and if your greenhouse is short enough. That is a lot of “if’s” and you will still be doing a lot of running back and forth to have the correct amount of opening for the amount of ventilating you need to do.

We have already reviewed the most common form of passive ventilation in the roll up sides. It was noted that roll up sides work much better when paired with a high opening to create a chimney effect. It is a fact that hot air rises so the higher you can create an opening, the more effective it will be.

If you are relying solely on gable end windows, either motorized or manual, they will need to be quite large. You will also need to have the benefit of a regularly strong prevailing wind.

The most effective form of passive venting is a continuous roof vent. This will provide a continuous opening in the precise area where the air is the hottest. A roof vent should always be mounted down wind of the prevailing wind. The benefit of being down wind is that the wind creates a vacuum as it goes over the greenhouse and sucks the warm air out.  The air intake for a roof vent is often a roll up side.

A significant down side of a roof vent is the up-front cost. The cost is the same for a narrow and a wider structure. This is the reason they are almost exclusively put on wider buildings. The cost is simply spread out over a bigger area. The effectiveness of a roof vent still makes it appealing in spite of the cost.

Roof vents can be controlled manually with a chain fall opener or with a motorized gearbox. A motorized system can be a simple open / close controller that you set the limit switches or it can have a proportional controller that allows for pre-set stages. It is with an automatic controller that the benefit of the roof vent will really become apparent. Every couple of minutes it will react and adjust to the inside condition of the greenhouse.

All of the expenditures involved with your greenhouse need to be weighed as a cost versus benefit or cost versus return. This process is especially challenging when considering the options of ventilation. Many of the expenses are subtle and hard to measure. The returns are equally hard to quantify since weather is an additional variable. This makes it even more important to keep accurate records and be aware of your options for improvements.

The benefits and drawbacks of forced ventilation

By definition, forced ventilation included an electrically operated exhaust fan at one end of a building and intake louvre at the opposite end.

The biggest advantage of this system is how simply it can be automated. Running your power supply through a thermostat will allow the system to come on and shut off at the set point.

There are numerous things which need to be considered for forced ventilation to be effective.

The prime consideration is proper sizing of the system. Most greenhouses require one air change per minute to keep the temperature within a tolerable range for the plants. Fans are rated for the cubic feet of air they can move per minute (cfm). By calculating the air volume in a greenhouse, you can then get a fan that is appropriately sized.

There also is a formula to match the air intake to the exhaust. When the intake is too small, the capacity of the fan will be restricted but if the intake is too large, it also is undesirable. A properly sized intake will create some turbulence so that incoming air is mixed with the existing air before it moves through.

Consideration must also be given if the intake has netting over it to control incoming weed seeds or insects. This will restrict more air flow than is usually expected.

The orientation of fan / louvre combination is important. If the intake is on a south facing wall, the incoming air will always be warmer than average. This means you will be moving warm air rather than cooling.

If a fan is blowing into a prevailing wind, the fan capacity can be significantly decreased as well. If you do not have a choice but point that way, it would be wise to consider a hood or other deflector over the fan.

When you are deciding on what size fan to get, it is often suggested to get two fans with that combined capacity. It will cost more up front, but the extra payback will be on extra flexibility or options for ventilating. This gives you the option of “half capacity”.

When planning for a forced ventilation system, it is important to understand the requirements and cost of getting the electrical service to that area and also what the on going operating costs will be. In both cases, these expenses can be significant.

One last thing which needs to be pointed out for consideration has to do with irrigation. In a forced ventilation structure, things dry out more quickly and they will also dry out unevenly. Just something to be aware of when you are thinking everything will get watered the same.

In our next article we will explore the opposite end of this spectrum … some of the options of passive venting.

What is and isn’t “proper ventilation”?

Proper ventilation, in a nutshell, equals adequate air changes and movement while maintaining adequate light levels and humidity in a given area. Proper ventilation is about proper balance.

The challenging part of achieving this balance is that there is such a wide range of requirements for different plants. It is important to identify what is ideal for each variety that you are growing. It will not be feasible to have a perfect climate for each variety of plant, but you really should avoid grouping together plants with a wide difference in requirements. That would simply ensure that you are not doing very little properly.

Putting shade cloth over a greenhouse may lower the temperature to an acceptable level for your plants but that does not equal good ventilation. You may be depriving the plants of their critical light requirement plus you are not doing anything to enhance air movement.

Turning on an exhaust fan may create the required amount of air changes or movement, but with a temperature far below the plant optimum, your plants still will not achieve their potential. Fans also draw air in a straight line from intake to exhaust. This does not address “stagnant corners”.

Many times, light equals heat. They go up together and go down together. It is important to understand how and why they are controlled differently and where the overlap on that control is.

Generally speaking, proper air changes equals one air change per minute. Some plants require more and some can get by with less. For this, you will need to know what the air volume is for your building. It is a little trickier to calculate this on a gothic or quonset shaped structure. Using the length x width x 80% of peak height will give you a close enough number in either cubic feet or cubic metres.

With the next 4 posts we hope to create a more thorough understanding of how to achieve a good environment for your plants. This information will be helpful as to research options so that you can balance costs with benefits and return. Especially in the area of ventilation, many times expenses associated with proper ventilation will be offset by exponentially higher returns.

Coffee Time with Norm: Starting Soon!

Our weekly meet and greet chat is happening again this week, (STARTING in 15 min!) and we hope every week Wednesdays at 9am EST. We hope you can join us!

Today Norm will be covering Ventilation, and time permitting anchoring as well. You will be recorded if you join, but can choose to disable your camera if you are not comfortable having the recording posted on social media.

Join Zoom Meeting
https://us02web.zoom.us/j/83325248870?pwd=WkxKakRDakg4SzIyUVMzTlR2YXErQT09
Meeting ID: 833 2524 8870
Passcode: 5WBcJw

Join us online every Wednesday morning on Zoom to connect, chat, and find solutions to challenges you may have.
Since we aren’t able to get out to trade shows this year, we decided to host this quick weekly check in to connect and say hello!
Norm will be available to answer any questions you may have. You may also send them in advance to multisheltersales@gmail.com and we will answer them “live” on zoom.
We know you’re busy with your business too, so if you send in a question and can’t make it live, don’t worry, we will post a recording on Facebook and YouTube after the event.
Come out and connect, no matter which side of the country you’re on! this allows us to see everyone face to face that we may not normally get to because of distance. we look forward to having you join us in our new weekly special event.
Please note this is a time for connection, brainstorming solutions to your challenges, learning some information about structures and other questions. If you have a question about an order, or would like a quote, please contact the office separately and we would be happy to help you.
We are excited to have you join us for this new event and we look forward to your questions!
Join Zoom Meeting
https://us02web.zoom.us/j/83325248870?pwd=WkxKakRDakg4SzIyUVMzTlR2YXErQT09
Meeting ID: 833 2524 8870
Passcode: 5WBcJw
Coffee Time with Norm Videos

Featured Add-On: Center Pivoting Gable Vent

We wanted to remind everyone of the center pivoting gable vent that we offer as a simple solution to the challenges of venting a building.

For venting to be effective, it has to be as high as possible. This is a cost effective alternative to costly roof vents.

Part of the window swings in and part of it goes out. Because of this

  • rain is never an issue
  • the window is easy to control even in extreme winds.
  • Incoming air is always deflected up to mix with the warmest air.
  • The tubular steel construction makes it both lightweight and resist twisting over time.

A continuous rope goes from the top of the window to a pulley at the ridge, down to ground level and back up to another pulley and then to the bottom of the window. By tying the rope off to an eye bolt on a gable post, this allows for very simple control from ground level.

Please don’t hesitate to call us for more details or with any questions you have about this option for your building!

vent3Vent4

Properly Venting a Building

Properly venting a building is a critical consideration when planning your building.

Getting rid of the initial ground moisture, quickly, when you have erected your new shelter is something many people do not think of. Quality air changes for plants or animals is something that automatically comes to mind. Getting rid of moisture is equally as important for storing your valuables.

Vent4Since warm air holds moisture and warm air rises, it is important to have venting capacity as high as is possible.

Venting through the roof, with individual turbines or a continuous roof vent, is the most effective but also the most costly.

This is only really necessary when you are in a very protected spot and there is a real need to keep the temperature down in a long building.

vent3Most medium length buildings that have the ends facing into the prevailing winds, can be adequately vented with gable end peak vents.

  • Make sure that these openings are as big as possible and as high as possible.
  • It is also important to make sure your “windows” can withstand the winds in your area.

Our centre pivoting gable vent has filled this requirement very effectively.

  • With part of the window going in and part of it going out, the wind can never get hold of it.
  • With the top and bottom rope through a double pully on the ridge, it is easy to maintain precise control over the opening area

As with all the other considerations, please do not hesitate to contact us with your specific set of circumstances and challenges