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 are the considerations for growing over winter?

Heating, interior tunnels, air circulation, humidity control

When someone is intending to grow in a plastic covered greenhouse over winter, the first order of business is to put a double layer of plastic with air between.

This cushion of air acts as insulation and will reduce heat loss by about 30%. A secondary benefit from this cushion of air is that it reduces condensation in the greenhouse dramatically.

Even if you are not going to heat the greenhouse, it makes sense to put the extra layer of cover with air between since it will provide a significant extra margin for cold.

The small fan which puts the air between the layers is made to work against pressure. It can not “over fill” the cavity.

When installing the second layer of plastic, it is not pulled tight. This allows the second layer to puff up to about 10 cm which is the optimum space.

It is the dead air space that creates the insulation value. If the air is moving, you will not have optimum efficiency. This is the reason it is important to be vigilant about patching any holes in the plastic when they occur.

Especially if your intention is to grow without heating, you could benefit from installing and using small interior tunnels. These tunnels would only be covered during severe weather. Since the air volume is small, even the heat from a few light bulbs will make a difference.

Interior air circulation is even more important in the winter time than it is in summer. In the winter time when things are more likely to be closed up, the probability of stagnant air is high.

It is important to monitor the humidity level in your greenhouse as well. Getting rid of excess humidity will cost you some heat, but you are creating a healthier environment for your plants.

If you are using a forced ventilation system for either humidity or heat control in the winter, it is important to have the thermostat close to the incoming air so that things will shut down quickly if the incoming air is extremely cold.

Any heating system which is used in a greenhouse should have a stainless steel heat exchanger and burner. The standard aluminized units simply will not stand up long term in the moist environment of a greenhouse.

We hope you have found value in this series of posts, helping you prepare for your new growing adventure! Reminder that the deadline is May 31st for Fall Delivery. We look forward to working with you for your projects!

What are the considerations for growing over the summer?

Ventilation, excess heat, making sure roll up sides are critter proof

One of the critically important things that each gardener/grower needs to know is the temperature threshold of each plant.

This is the temperature at which a plant will stop growing and an even higher temperature at which a plant will die.

When a plant has passed the threshold where it stops growing, it will take some time to recover and start growing again.

This is why proper ventilation is so vitally important. On a sunny day, proper ventilation is defined as one air change per minute. You must know the volume of air in your greenhouse and have your ventilation method capable of moving that much air.

A forced ventilation system is the simplest to set up and control since it is thermostatically controlled. The up front and operating cost are significant. Each exhaust fan has a cubic feet per minute (cfm) rating which must be equal or greater than the air volume of the building. A forced ventilation system also has a motorized louvre which will open each time the fan turns on.

On a shorter structure that is facing into the wind, there is the option of ventilating through the doors and windows. This is an economical system but the hardest to control.

Adding roll up sides to a structure has the capacity of moving a lot of air as long as there is air movement around the greenhouse.

This is why it is a good idea to install a window as high on each end as possible. Since warm air rises, this will create a “chimney” effect on a calm day. The down side of roll up sides is the potential for draft across the floor and the fact that you have to be there to open and close the system. You may need to put up mesh to deter uninvited guests.

In addition to changing the air in the greenhouse, it is also important to circulate the air horizontally. Stagnant air pockets invite disease problems. There is not a prescribed rate to move the air, it is just important to keep it moving.

A proper watering system would not be considered as a part of making a greenhouse more efficient but it must be remembered that a greenhouse will increase the water needs of all of the plants.

A final consideration for growing over the summer is the potential need for shade. Most vegetables are good for full sun, all the time, but it is important to remember that this does not apply to all plants.

Wishing you a productive growing season!

We hope you are finding value in this series of posts, helping you prepare for your new growing adventure! Reminder that the deadline is May 31st for Fall Delivery. We look forward to working with you for your projects!