First let's explain the differences in the various types of growing structures including: high tunnel, cold frame, and greenhouse.
Cold frames are the smallest and simplest structures for protecting plants. They are generally low to the ground and don't use any equipment other than having vents or a roll-up cover that can be operated manually. These structures are used for frost protection of plants, extending the growing season a few weeks, and/or assisting in hardening off crops.
A growing structure that integrates heating and cooling systems and can be used to grow plants throughout the year is known as a greenhouse or hothouse. The design and construction of these is more substantial because of permanent electrical, plumbing, heating, and cooling equipment that is needed for their use.
High tunnels fall into the middle. They are taller and larger than cold frames like greenhouses are, but they are designed for season extending like cold frames and generally do not have automated heating and cooling equipment. Most high tunnels come with manual roll-up sides for ventilation. This type of natural ventilation does a very good job of keeping greenhouse temperatures in check as well when outside temperatures are warm and the sides can just be left down most of the day. It is not ideal as the sole means of ventilation though. As many growers have found out, the main issue is that it is difficult to regulate temperature with this system in colder weather and staged automation can be quite expensive. This is where supplemental ventilation comes in. By adding a small exhaust fan system to the high tunnel, you can have automated ventilation and cooling in colder weather where rolling the sides down will bring the temperature down too quickly and assist with ventilation when opening the side vents is needed. The fan system can also help lower the humidity level in the high tunnel to eliminate condensation. Since the roll-up sides will handle majority of the ventilation needs as outside temperatures increase, the fan will only need to provide a fraction (Usually around 1/3 to 1/5) of the CFM air flow required if it were the only means of ventilation. A 24" or smaller exhaust fan will be ideal for most high tunnels. Add an intake shutter for the other end and a thermostat, and you have a completely automated system. We offer several fan system options with the most popular being our Basic Plug-in Exhaust Fan System which does not require wiring. We also offer premium plug-in and wired exhaust fan systems as well.
