Using heat recovery ventilation systems in high tunnels to increase winter crop production

Project Overview

Project Type: Partnership
Funds awarded in 2015: $9,782.00
Projected End Date: 12/31/2016
Region: Northeast
State: Pennsylvania
Project Leader:
Chris Lent
National Center for Appropriate Technology (NCAT)

Annual Reports

Information Products


  • Vegetables: greens (leafy)


  • Crop Production: high tunnels or hoop houses

    Proposal abstract:

    This pilot project proposes to test if the use of a heat recovery ventilator to ventilate high tunnels for winter crop production in Pennsylvania can effectively increase the average daily temperature in a high tunnel and, as a result, increase crop production and profitability. The National Center for Appropriate Technology (NCAT) will work with New Morning Farm, a certified organic farm with experience in winter production, to test run the use of this nontraditional ventilation system. Two tunnels of the same size will be used in the project. One tunnel will be used to test the capabilities of the heat exchanger and the other as a control. The Project lead will be Chris Lent, an NCAT Agricultural Specialist who will work with Pearl Wetherall, Field Manager for New Morning Farm, to run the project. A comparative analysis of temperature and relative humidity data collected in each high tunnel will be completed to determine if the new ventilation system can control humidity while increasing the average daily temperature of the tunnel. In addition, a crop trial will be run in each of the two tunnels to study the effect, if any, of the new ventilation system on production and profitability in winter growing. The findings of the project will be disseminated at a field day for winter growers at New Morning Farm, and through a webinar and short publication that will be targeted to northeast winter growers and archived on NCAT’s ATTRA project website.

    Project objectives from proposal:

    NCAT proposes to test the effectiveness of a heat recovery ventilator to ventilate high tunnels being used for winter growing. Heat exchangers are a tested, relatively low cost, and energy efficient technology that use simple systems to exchange inside air with outside air. Heat is transferred from the warm outgoing air to the cold incoming air, thus effectively ventilating a space without significantly lowering the temperature of that space and without losing the energy that was used to heat that space.  If a heat recovery ventilator can effectively increase the average daily temperature inside a high tunnel, crops will grow more rapidly and continue to grow during parts of the year they would otherwise be dormant. This would increase the amount of marketable product grown in these high tunnels. Likewise, if a heat recovery ventilator can be used in place of traditional ventilation to gain moisture control, foliar diseases will be reduced and condensation damage will be limited without wasting valuable heating fuel to make up for pulling in excessive amounts of cold winter air. The increase in saleable product and decrease in fuel costs would improve the profitability of winter growing in high tunnels in the Northeast.

    This project will test a new way to use heat recovery systems. It is designed so that if the results are positive, they can be duplicated and more vigorously tested in a larger trial throughout the Northeast.

    The proposed project will focus on the following four questions:
    1) Can an air to air heat exchanger adequately ventilate winter production in a high tunnel?
    2) Will a high tunnel being ventilated with a heat exchanger in cold weather have a higher average daily temperature compared to a traditionally vented tunnel?
    3) If the average daily temperature is increased by the use of a heat exchanger, is crop growth subsequently increased and to what degree?
    4) If crop growth is increased significantly, does the increased production justify the cost of a heat recovery ventilator?

    NCAT will work with a farm collaborator, New Morning Farm, who has extensive experience with winter growing in high tunnels. New Moring Farm has been selected as a participant in this project due to the fact that they have two high tunnels of the same size and type used for winter production on their farm. For this study, one tunnel will be used to test the effectiveness of the heat recovery unit and the other tunnel will be used as a control.

    An air to air heat exchanger (Fantech Flex100, or comparable heat exchanger capable of a minimum venting capacity of 100 cubic feet per minute (cfm)), will be installed in one of the high tunnels. These are approximately 45 lb. units that can hang from the framework of most high tunnels. The unit will pull air from outside the tunnel and pass it through an aluminum heat exchanger that will transfer heat from the warmer inside tunnel air to the incoming air. After the heat transfer takes place the now cold inside air is vented to the outside and the now heated fresh air is vented into the tunnel. Intake air from outside the tunnel will be supplied from close to the unit with insulated flex duct run to the outside through an end wall, and return air will be run to the opposite end of the tunnel to encourage good air circulation. This type of air to air heat exchanger technology was chosen for this project based on the fact that they have been used for decades in residential ventilation systems to reliably control moisture and save energy. These systems are well tested and proven technology that can be purchased ready to use “off the shelf”.

    When the ventilator is operating and cold air is brought in from the outside and warmed by the outgoing air, condensation can accumulate on the heat exchanger. For this reason, the unit will be run off of a thermostat to operate only when the inside tunnel temperature is at or above 33º F in order to keep the condensate line from freezing. Also a Kill-a-watt meter will be installed on the power supply side of this unit to measure the energy usage of the unit. A condensate line will be run from the unit to a 5 gallon lidded bucket to collect condensate from the unit.

    A Data Logger (Monarch Instruments Track-It HR/Temp) will be placed in each high tunnel and outside the tunnels. These will be set to record relative humidity (RH) inside the tunnels and temperature inside and outside the tunnels every half hour during the trial. Data will be downloaded from these devices into device software called Track-It where it can be charted or exported to a spreadsheet for manipulation.

    Comparative analysis of this temperature and humidity data will then be used to answer the first two questions: Can the new system ventilate adequately and will it increase the average daily temperature in the tunnel? A target relative humidity (RH) of 85% or lower will be used to determine the effectiveness of the heat exchanger to control humidity and ventilate adequately. This humidity threshold was chosen because Downey mildew, a major high tunnel foliar disease, is known to thrive in cool temperatures with 85% RH or above. Temperature data will be compared between the two high tunnels to determine if the average daily temperature was higher in the heat exchanger tunnel or the control tunnel over the trial period.

    To answer the third and fourth questions about increased crop growth and cost justification, a crop trial will be evaluated. Two 3’ X 90’ beds will be used for the crop trials in each high tunnel for a total of four beds. Soil tests will be performed on each bed in each tunnel. Soil amendments will be added based on the soil test results to give the test plots similar optimum growing conditions. Spinach will be planted in each of the two high tunnels. Identical watering schedules and cultivation practices will be used in these two test plots in order to eliminate any variables that might affect the growth of the crop. The farmer partner will keep planting, harvest, and sales records on the test plot crops for each tunnel. The farmer will also keep records on the level of crop quality (particularly disease control and control of condensation related crop damage) on a weekly basis. This damage will be determined by the estimated percentage of the crop lost or rendered unmarketable to both disease and condensation related damage.

    A final payback analysis will be performed based on crop sales records, crop loss records, the cost of the new ventilation equipment, and the energy costs to run the heat exchanger compared to the energy cost to run traditional ventilation (fans). The cost of the use of traditional ventilation will be based on an energy analysis of the traditional ventilation being used in the control tunnel during the six month trial period.

    This project will study aspects of winter high tunnel production in Pennsylvania. Preliminary work and set up will begin in the summer of 2015 in preparation for the 2015/2016 winter growing season. The crop trial and data collection will take place over that winter growing season. Analysis and information dissemination will follow in the spring and summer of 2016.

    The project will begin in July 2015 with the purchase of equipment and seeds. A 100 cfm air to air heat exchanger will be purchased and installation of heat exchanger and data collection devices will take place in September 2015. At the same time relative humidity and temperature data loggers will be set, tested, and installed at the farm. Soil testing of test plots will take place in September of 2015.

    Test plots in each tunnel will be amended and planted in early November and data collection will begin. Over the next five months the equipment will be maintained and data will be collected at regular weekly intervals. Records will be kept by the farm collaborator on crop plots over this six month period.

    A farm field day will be held at New Morning Farm in April, 2016 to share information from the project with other farmers. A Webinar and a short publication about the project will be developed and presented by project lead Lent and Wetherall by the end of May 2016.

    A detailed project timetable follows:
    July 2015 –Lent will research and purchase ventilation equipment, data loggers, and trial plot seeds for delivery to New Morning Farm.
    September 2015 –Lent will work with farm partner to install the high tunnel ventilation equipment. Data Loggers and electric meter will be set and installed. Soil samples will be taken on all test plots and submitted for analysis.
    October 2015 –Lent will travel to New Morning Farm to oversee trial crops planting and check on and start operation of the high tunnel ventilation equipment.
    November 2015 through March 2016 –Wetherall will keep planting, production, harvest and sales records on a weekly basis for all test plot crops. She will download relative humidity and temperature data into the Track-It computer software monthly. She will be responsible for basic maintenance of equipment, consisting of cleaning the air filter once a month.
    December 2015- Lent will submit annual progress report of the project to NESARE.
    January 2016- Lent will develop and implement the agenda and promotion of a farm field day at New Morning Farm.
    April through May 2016 –Lent will analyze collected data and along with NCAT staff will develop and promote a webinar on the project.
    April 2016 –Lent and New Morning Farm will present at the project field day event and collect participant evaluations. The field day will focus on dissemination of information and lessons learned up to that point and show how the ventilation system was installed.
    May 2016- Lent and Wetherall will present webinar on the project Webinar participants will be evaluated.  Lent will write short publication about the project and results.
    July 2016- Lent will submit a final project report to NESARE.

    The purpose of the field day is to show other farmers the ventilation equipment used in the project, the installation of the equipment, and to share data analysis and any effects the ventilation system had on crop growth. Project lead Chris Lent will develop promotional material and will access NCAT websites and social media outlets to promote the field day. The field day will also be promoted through agriculture organizations and list serves in the northeast.

    In addition to a field day, the results will be presented in a webinar targeting new, beginning, and current winter crop producers in the Northeast. Chris Lent will develop a webinar presentation in partnership with Pearl Wetherall of New Morning Farm that focuses on the outcomes of the project. The webinar presentation will share all analyzed data, lessons learned, and conclusions for the project as well as giving a practical lesson in installation of the equipment used for the project and answering specific questions from participants. Lent will also summarize the project results in a publication (1-4 pages) that will be available on the ATTRA website. The webinar will be archived on NCAT’s ATTRA Project website for future viewing. NCAT’s ATTRA Project is the National Sustainable Agriculture Information Service and provides technical assistance to farmers and educators throughout the country. The ATTRA website has over 500,000 unique visitors each year who access the over 400 publications, databases, and other technical information.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.