I have received many inquiries about how to improve ventilation of high tunnels from growers with tunnels that have only roll-up sides. The issues they are facing tend to be either high temp, high humidity or both, leading to plant stress or disease. These situations tend to be in less than ideal sites for ventilation and/or temperature control. For example, crowded lots with trees or other significant wind breaks close to the tunnel, high southern exposure (which can be good of course), and/or simply calm sites that provide little ventilation.
Roll-up sides alone tend to work for tunnels on sites with generally good air flow. Diffusion between inside and outside does happen, of course, but is slow and unlikely to achieve good ventilation along the center of the tunnel, especially with dense vegetation later in plant maturity. But, I think of a tunnel in this instance a bit like a wood stove. Without a chimney-effect natural draft, you’re really only getting ventilation from the sides and only then if there is a decent breeze. Warmer air and, therefore, humidity will tend to collect in the canopy and peak.
This probably is OK in many sites for most crops. But not always. In many cases gable vents will improve ventilation by acting as outlets for warm humid air in warmer seasons and by allowing for low volume ventilation in colder weather. I recommend a simple 24″x24″ gable vent (for a 30’x96′ tunnel) on each end wall, with a thermostatic wax cylinder actuator like the ones made by J. Orbesen Teknik APS available from LittleGreenhouse.com., FarmTek, and Agricultural Solutions among others The actuators require no electricity, are relatively inexpensive and are passively controlled by the wax cylinder based on temperature.
At the very least, when building end-walls consider framing in a rough opening to accept a 24″x24″ in the end wall so that a future install is easier. If you want to skip the expense of a louvered, wax cylinder system, you can use a manually-controlled sheet of plywood to open and close the vent. If you go with a louvered vent, seek one that has a flanged seal it closes against. Keenan Meier, and Munters-Euroemme has such flanged, louvered dampers.
These have zero daylight when closed which results in a solid seal. Most others on the market that I have seen have no such closure seal.
Fans in greenhouses and high tunnels generally perform two tasks: (1) circulation / mixing / stirring and (2) ventilation.
Circulation / Mixing / Stirring – Sometimes referred to as horizontal air flow or “HAF” fans, these fans are generally hung from the inside horizontal structural tubing. They only mix the air. The benefit of this is consistent, well distributed growing conditions. It also ensures that your control sensors are seeing the “average” conditions of the space. Remember that HAF fans work to mix the space (circulate the air) but don’t significantly improve ventilation. HAF combined with roll up sides can do the trick, but the site is the key. There needs to be a steady cross breeze for any significant air exchange to occur.
Ventilation – Ventilation, or “exhaust” fans provide air exchange between the inside and outside. This is really important in controlling temperature (cooling) or humidity (drying). The only way to remove heat or humidity from a standard high tunnel or greenhouse is by actively removing air from the space and bringing in outside air. Ventilation (cooling) systems are covered very well by Bartok and Aldrich (p. 70). Basic rules of thumb for ventilation are 8 CFM/ft2 (of growing space) for summer cooling and 2 CFM/ft2 for cooler months.
“Will the 007 be enough?” is a common question in early spring as greenhouses around the region fire up and we do our best to keep seed trays and their cargo warm on the still-cool nights. My mind instantly goes to “which movie?” And then I crash back to earth and realize this is a question about pumps and I am not Q. Continue reading Pumps and Pipes
I am often asked by growers and processors to recommend a thermostat for a greenhouse, cooler, or postharvest process use. There are many to choose from and their specifications can be confusing. It is important to remember just what a thermostat does. It is essentially no different from the light switch on the wall with one very significant exception. Instead of depending on a person to switch it from ON to OFF, we use a temperature measurement. The accuracy of both the temperature setpoint (what you set) and the actual temperature (what the actual condition is) can be critical for production quality and energy efficiency. Continue reading Thermostats for Agriculture
UVM Extension and others supported the recent installation of a 341,200 BTU/hr (output) multi-fuel biomass boiler at the Vermont Farmers Food Center (VFFC) in Rutland, VT. The boiler heats the Farmer’s Hall building with the capability to use several alternative fuels to displace propane. The boiler was fueled primarily on wood pellets but was also able to feed and burn grass biomass pucks. This demonstration project carried a cost premium when compared to a typical propane heater installation. That premium is paid back over time due to recurring fuel cost savings. A simple payback period of 2.2 to 8.0 years is feasible against a cost premium of $51,255 for the boiler depending on the fuel used and the amount of use. For more details about the project and the economic performance please see the report.
I am often asked by growers to help estimate what size heater is needed for a greenhouse or what minimum temperature their high tunnel will reach at a certain outside temperature. Below are some tools to help you do this yourself. I have presented them in a range of complexity depending on how much you really want to get into the math. Enjoy.
2. LITTLE MORE COMPLEX – VirtualGrower – http://ars.usda.gov/services/software/download.htm?softwareid=309. This is a free software tool from USDA ARS that is a bit more complicated than the simple form above. But there is benefit to the complication. As with any analysis, the more you put into it, the more you get out of it. VirtualGrower allows easier management of multiple “what-if” scenarios, includes regional weather and light data automatically, and accounts for heating and ventilation systems. You may find it interesting and useful.
4. HEATING BENCHES, ROOT-ZONE or GROUND – When heating the root zone locally, different heater sizing approaches are needed. This is covered well by “Root Zone Heating Systems” in Wilkinson KM, Haase DL, Pinto JR, technical coordinators. National Proceedings: Forest and Conservation Nursery Associations—2013. Fort Collins (CO): USDA Forest Service, Rocky Mountain Research Station.Proceedings RMRS-P-72. 62-65. Available at: http://www.fs.fed.us/rm/pubs/rmrs_p072.htm.
As I’ve mentioned in several other posts, I think the continual monitoring of conditions in greenhouses and food storage spaces is incredibly important for quality and safety and insightful for any operation. There is a really clever design for a do-it-yourself temperature monitoring system called Fido, on the FarmHack site. It uses an Arduino control and electronics platform, a cheap cell phone, and a few other pretty inexpensive pieces to do the job.
“A farmer-built electronic tool that can monitor greenhouse temperature, record greenhouse data, and alert the farmer to problems in the greenhouse via cell phone text message. This tool will be much more affordable and useful than commercially available greenhouse alarms (which rely on landline connections or internet connections, which usually aren’t available in the greenhouse).“
I’ll be trying to add RH monitoring to this soon, and will update the post when that is complete.