Season Extension: Ventilation & Heating

To make your greenhouse more of a year round functioning entity, there are two main areas in which you have to make the structure more efficient-Heating and Ventilation.

1. Heating is your biggest expense for the winter so retaining heat is a priority.

The easiest way to retain heat in your greenhouse is to install a double poly cover with air in between. A small squirrel cage blower attached to the inside layer of plastic aids in maintaining air between layers.

The more dead air space between these layers you can create, the closer you will be to achieving a 30% reduction in heat loss. Holes will result in air movement and therefore less efficiency. 3″-5″ consistent space is ideal. It is a given that you would have less than that around the edges and over the ridge.

Double plastic will have a considerably longer life span. It is important to realize that as plastic gets older, the light transmission will be reduced which will reduce production.

Infrared plastic (IR poly) does further reduce heat loss and increase light diffusion so it can also be a consideration when looking for ways to reduce heating costs.

2. Ventilation is also one of your biggest considerations for the warmer times of the year. Ventilation can be done through forced or mechanical methods or passive through vents or roll up sides.

Vents are extremely effective since they can be placed higher up where the heat needs to be expelled. Mechanical ventilation is more costly both up front and to operate but it is easier to control since it is attached to a thermostat. For mechanical ventilation to be effective, it needs to be sized and located properly.

Roll up sides are less costly and simpler to install but are restricted by the fact that you have to be there to open and to close.

Climate control is especially challenging in the spring and the fall since most days you will have the need for both ventilating and heating.

One area that you need to be especially aware of is stagnant air. Without proper air movement, circulation and exchanging, stagnant air can cause many different types of diseases. It is important to understand what your plants require

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.

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.

Season Extension: Ventilating & Heating

To make your greenhouse more of a year round functioning entity, there are two main areas in which you have to make the structure more efficient-Heating and Ventilation.

1. Heating is your biggest expense for the winter so retaining heat is a priority.

The easiest way to retain heat in your greenhouse is to install a double poly cover with air in between. A small squirrel cage blower attached to the inside layer of plastic aids in maintaining air between layers.

The more dead air space between these layers you can create, the closer you will be to achieving a 30% reduction in heat loss. Holes will result in air movement and therefore less efficiency. 3″-5″ consistent space is ideal. It is a given that you would have less then that around the edges and over the ridge.

Double plastic will have a considerably longer life span. It is important to realize that as plastic gets older, the light transmission will be reduced which will reduce production.

Infrared plastic (IR poly) does further reduce heat loss and increase light diffusion so it can also be a consideration when looking for ways to reduce heating costs.

2. Ventilation is also one of your biggest considerations for the warmer times of the year. Ventilation can be done through forced or mechanical methods or passive through vents or roll up sides.

Vents are extremely effective since they can be placed higher up where the heat needs to be expelled. Mechanical ventilation is more costly both up front and to operate but it is easier to control since it is attached to a thermostat. For mechanical ventilation to be effective, it needs to be sized and located properly.

Roll up sides are less costly and simpler to install but are restricted by the fact that you have to be there to open and to close.

Climate control is especially challenging in the spring and the fall since most days you will have the need for both ventilating and heating.

One area that you need to be especially aware of is stagnant air. Without proper air movement, circulation and exchanging, stagnant air can cause many different types of diseases. It is important to understand what your plants require

Stay tuned for the third and final installment of our series about Season Extension and Your Greenhouse coming soon!