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

Final Report for ONE15-240

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)
Expand All

Project Information


In this project a heat recovery ventilator (HRV) was tested  in a winter production high tunnel in Pennsylvania to determine it’s effectiveness in controlling humidity and maintaining higher temperatures in the tunnel by not using traditional open vent (OV) or fan ventilation.  The project monitored relative humidity (RH) and temperatures in the HRV test tunnel and a OV control tunnel with the following results.  The RH in the the HRV test tunnel was on average almost 5% lower than the OV control tunnel.   The HRV tunnel average 94.8% RH while the OV tunnel averaged 99.5%.  Temperatures in the tunnels where also monitored and the average temperature in the HRV tunnel was slightly lower than the average temperature in the OV tunnel.  During the project the daily temperature in the HRV tunnel was 42.2° F, and the temperature in the OV tunnel was slightly higher at 42.6°F.   The same crops of Asian greens and spinach where grown in both tunnels and though some harvest records where incomplete, the test tunnel out produced the control tunnel by more than $4,000 in economic terms.  The cost to purchase the HRV equipment was $941 and the cost in electrical usage over five months was $34.


Growers in the northeast who use high tunnels to grow crops throughout the winter face the challenge of low temperatures and high humidity conditions.   Humidity control through ventilation is seen by most prominent winter growers in the northeast as the most important practice for controlling foliar diseases in winter crops, even if it means lowering temperatures in the greenhouse (Hartman, 2014).  At the same time it is known that average daily temperatures greatly affect the growth rate of crops.

This project proposed to test whether or not the use of a heat recovery ventilator could effectively reduce relative humidity (RH) and increase the average daily temperature in a high tunnel, and as a result, increase crop production and profitability. Two high tunnels of the same size were used in the project.  One tunnel was used to test the capabilities of the heat exchanger, and the other tunnel as a control.

NCAT worked with Field Manager Pearl Weatherall at New Morning Farm, a certified organic farm with experience in winter production, to test the use of this nontraditional ventilation system.

New Morning Farm, a 95-acre certified-organic vegetable farm in Hustontown, south-central Pennsylvania, is owned and operated by Jim and Moie Kimball Crawford.

Hartman, Ben. 2014. Eight tips for winter success. Growing for Market. October. p. 5.

Project Objectives:

The objectives of this project where to answer the following questions:

1) Can an air to air heat exchanger adequately ventilate  (reduce relative humidity below 85%) in winter production in a winter production 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?


Click linked name(s) to expand/collapse or show everyone's info
  • Pearl Wetherall


Materials and methods:

Two 20-foot by 96-foot high tunnels on New Morning Farm were selected for this project.  The heat recovery ventilation (HRV) equipment was installed in the Lower High Tunnel (LHT) on October 6, 2015, while the Upper High Tunnel (UHT) was used as a control.   Humidity control in the control tunnel could be done with open ventilation (OV) only, however, because of winter growing management practices at New Morning Farm,  during the project the control tunnel was rarely opened for ventilation.   Regardless, we will refer to the control tunnel as the OV tunnel and the subject tunnel as the HRV tunnel.  The original intent was to compare the HRV tunnel to a tunnel that was opened regularly to vent for humidity control, and in the Results and Discussion Section of this report we talk about what impact control tunnel management may have had on the results. 

To try to make soil conditions in each tunnel equal, soil samples were taken in each tunnel in early September and sent to the Penn State Analytical Labs for testing.  The soils were amended based on the sample to prepare for the winter crop of half Asian greens and half spinach planted in each tunnel the last week of October 2015.

The equipment used in the project included a 106 CFM Fantech Heat Recovery ventilator, supply air and exhaust hood, flexible duct work, and EL-USB 2 data loggers for temperature and humidity recording.  Most of the equipment was purchased online, but some of the materials for the installation were purchased from local supply stores.  The breakdown of what was shipped versus local purchases appears at the end of this section.

The tunnels were planted and closed for the winter during the last week of October and on October 29 the data loggers started to record information.  The data recording stopped on March 21, 2016, when the tunnels were opened and prepared for summer crops. 

 Equipment and Costs:

Purchased online

1) Heat recovery ventilator  (104 cfm)                                                                                                               $630

2) Programmable touch screen wall controller                                                                                                $100

3) Two vent hoods                                                                                                                                                   $29

4) Insulated flex duct   (25 feet)                                                                                                                            $27

Purchased locally

5) Uninstalled flex duct (75 feet)                                                                                                                        $105

6) UV-resistant zip ties                                                                                                                                           $20

7) Outdoor heavy-duty extension cord                                                                                                                $10

8) Two 2-foot by 2-foot sheets of ¾-inch plywood                                                                                           $12

9) Exterior Screws (1-1/2 to 2-inch)                                                                                                                       $8


                                                                                                                                                    TOTAL                 $941

Research results and discussion:

Humidity Analysis


To answer the first question and see if the heat exchanger controlled moisture in the tunnel, relative humidity (RH) data from the two tunnels was compared (See Graph 1).  The solid pink line in Graph 1 shows that the heat recovery ventilator was not able to keep the relative humidity in the LHT  below the 85% threshold. 

In fact, the relative humidity readings in both tunnels were higher than anticipated.  The average RH in the LHT over the period of the trial was 94.8%, compared to the upper high tunnel’s average RH of 99.5%.  All else equal, this indicates that the ventilator helped lower the humidity on average by almost 5% in the LHT.  Although it did not maintain RH below 85%, it did lower average RH over the winter. 

You can also see from Graph 1  that the RH in the LHT corresponds with the outside RH readings more closely than the UHT RH readings.  A possible and likely explanation for this is that the HRV in the LHT was pulling in cooler dry air from the outside thus lowering the RH inside the high tunnel.  With regard to RH, this data shows that the HRV was performing as anticipated.


Some RH readings in both tunnels were higher than 100%, which indicates calibration problems with the data collection equipment.  However, readings from all three sources (LHT, UHT, and OUT) correlate with one another, indicating that the data loggers were at least reading consistently.  In addition, the high tunnels were irrigated with overhead sprinklers prior to planting. This saturated the soil and provided a constant source of moisture for the duration of the project, contributing to the high humidity readings.


Temperature Analysis

To answer the second question of whether or not the ventilator increases the average daily temperature, data from both tunnels was compared.  In Graph 1, the lower set of lines indicates the temperature data points. The LHT temperatures crested above the UHT  temperatures several times during the winter.  However, on average, the daily temperature in the LHT was 42.2° F, and the temperature in the UHT was slightly higher  at 42.6°.  So we can see that the average daily temperature was not increased by the use of the heat recovery ventilator in the LHT.



Graphs 2,3,and 4 show direct comparisons of the two high tunnels, the UHT, which is the control, and the LHT with the HRV installed. The 1:1 relationship line on the graphs offers a reference: if the data points are on the line, then the high tunnels are performing exactly the same. When comparing the RH (graph 1 on the left) the LHT had reduced humidity, evidenced by the numerous data points above the 1:1 relationship line. This trend is consistent with the use of the HRV.

The comparison of temperature (graph 2 in the middle) reveals the trend that the UHT was cooler than the LHT, with numerous points below the 1:1 relationship line, with an exception at lower temperatures where the UHT is warmer than the LHT.  This result was unexpected, but considering  that the HRV system was controlled by an external thermostat that shut down if temperatures got below 35° F, in order to keep the condensation line from freezing, it is not surprising to see the HRV not performing at the lower temperatures.

Dew point (graph 3 on the right) followed a similar trend to temperature , in which the UHT had a higher dew point at lower temperatures and a slightly lower dew point at higher temperatures.


Dew Point


Dew point data was not something we thought to look at when originally setting up the project; however, the data loggers used in the project automatically calculate dew point.  It might be important to consider dew point because it is the temperature at which condensation will form on leaves, creating more ideal conditions for foliar disease.  In a winter high tunnel growing situation, the lower the dew point, the less time there is for condensation to form, because the temperature has to drop low enough to cool the plants to dew point. 

The average dew point in the LHT was 40.3° compared to 42.2° in the UHT.  This 2° difference is not much, but it reflects the lower relative humidity in the LHT and the effect it had on the dew point.  This means that on average, the crops in the lower high tunnel would have to be 2° cooler than crops in the UHT in order for condensation to form on their leaves.  Based on the small average temperature difference observed in the two tunnels and this dew point data, there would have been fewer opportunities for condensation to form on crops in the LHT.  Admittedly, this difference may have been too small to have had any significant impact on disease control.




Because the average temperatures were almost equal in both tunnels, we would not expect to see increased production in the LHT.  However the available harvest data showed that the LHT produced 211.6 pounds more spinach and 167.3 pounds more greens from the same amount of growing area.  (See Graph 6)

The spinach and greens were sold for a minimum of $12 a pound, so in economic terms,  the LHT (with the heat recovery ventilator) produced $4,548 more than the UHT.  If the increase in production in the LHT could be attributed to the lower humidity and lower dew point temperatures created by the ventilation system,  the HRV installation would pay for itself more than four times  in the first year.  


Energy Use

The HRV that was installed for this project uses a maximum of 1.4 amps at 120 volts.  So at the most, it uses 168 watts of energy when running.  The unit did not run all the time during the project.  It would shut down whenever the temperature dropped below 35°F.  If we figured energy use on the worst-case scenario, by assuming it ran half the time, then the energy usage in kilowatts for the five-month trial would be 282 Kw.  At .12 a Kw, it would cost $33.84 to run the unit for the duration of the project.

 The HRV system was fairly simple to buy and install—they are simple systems that use common materials. The overall price for all the materials and equipment was about $1,000. 



Looking at the data, the HRV did lower the RH and thus the dew point in the LHT.  However, the average daily temperature was not effectively increased.  Production was higher out of the lower tunnel, but it is inconclusive whether this can be attributed entirely to the ventilation equipment or not.  More testing of this system is needed to determine its effectiveness.  Through this project,  there have been lessons learned about what could have been done differently to increase the effectiveness of the HRV.

Originally, the intent of the project was to measure the impact of venting a winter production high tunnel with a HRV in comparison to a traditionally vented tunnel.  Traditional ventilation would mean that doors and shutters would be opened to allow for humidity control in the tunnel.  However, the management practice of New Morning Farm is to close up the tunnels in the fall when winter crops are planted and then let a thermostatically controlled fan ventilate the tunnels if the temperature goes above 75° F.  (This is how the two tunnels used in the project were managed over the subsequent 2015-2016 winter season.)  This means that during the unusually warm winter of this project, the traditional ventilation systems in both tunnels kicked in more than usual, which mitigated any effect the HRV system could have had during these periods.  For this reason, the mild winter could have had an influence on the data, and the results might have been much different in a colder year or if the project had been moved farther north.

Along similar lines, the HRV system was controlled by an external thermostat that shut it down if the temperature got below 35° F.  This was to keep the condensation line from freezing.  Arguably, the HRV could be the most beneficial in conditions when temperatures are colder than 35° but the sun is out and heating the tunnel.  In these conditions, the HRV could ventilate humidity without losing the heat gained by the sun.  Having the unit shut down at 35° may have reduced its effectiveness in this project.  There are HRV units with defrosters that would solve this problem, but they use more energy to operate, which was a consideration in this project. 

Another issue that may have impacted the results of this project is that the 106 cfm unit chosen for this project was almost certainly undersized for the job.  In looking at the size of the tunnel and aiming for a couple of air changes per hour, a 200 cfm unit or larger would have done a better job at exchanging the air in the tunnel and, for this reason, would have been a better choice.




Research conclusions:

This project has always been seen as a pilot project and more trials are needed before  wide adoption of this type of ventilation system could be expected or even recommended.   In light of that, there is likely very little impact at this point from the results of this project.    However, the results of this project show that there may be an economic incentive to install and use heat recovery equipment for winter growing in high tunnels.  That result warrants more testing of heat recovery ventilation for winter growing on working farms.   If that happens and positive results are seen in the form of increased production, then more farmer adoption can be expected and heat recovery ventilation for winter production may become the first real innovation for winter high tunnel growers in the northeast in a long time. 

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

Results were disseminated through a webinar that is archived on the ATTRA website and can be found here  The ATTRA site receives over 500,000 visitors annually.  A publication was written describing the project, the instillation process for the equipment, analysis of the collected data and project  results.   The publication is also available through the ATTRA website here .


Project Outcomes

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

Some of the results from this project show that a heat recovery ventilator may be useful in lowering humidity in winter high tunnel growing but it was inconclusive on the issue of increasing temperatures and production.  The scope of this project was limited to one tunnel on one farm over one growing season.   As interest in winter production in high tunnels increases it would be worth while to test the use of heat recovery ventilators on a larger scale and in a variety of conditions to fully understand if there is potential for these systems to be a real innovation for winter growers. 

Information Products

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.