Ventilation · HAF · exhaust · not a kit

High tunnel ventilation: HAF fans, exhaust fans, shutters, thermostats and auto vent openers

Growers search high tunnel ventilation, hoop house fan, HAF fan for high tunnel, greenhouse exhaust fan for hoop house, and high tunnel thermostat after the frame is up. This page is the active ventilation sibling — circulation fans, end-wall exhaust plus shutters, plug-in thermostats, wax-cylinder auto vent openers, and a digital min/max thermometer. Passive options stay on high-tunnel roll-up sides and high-tunnel end walls. It is not a commercially manufactured high-tunnel kit and not a USDA payment schedule.

Extension quotes copied 11 October 2026, about 03:15 UTC from UVM Callahan 2019 / 2017 and UNH Ebba 2026 slides. Featured Amazon prices copied the same night. A missing featured price stays an em dash. Ratings are not copied. EQIP language below is the structure-only and producer-expense wording already on what EQIP will not pay for. No USDA payment dollar is invented here. Confirm fans or electrical components with your NRCS district conservationist.

Kit index: commercially manufactured kits. Replacement-parts directory: high-tunnel replacement parts. Size map: high-tunnel sizes. Shade: high-tunnel shade cloth. Tomatoes: high-tunnel tomatoes. Winter lettuce: high-tunnel winter lettuce. Winter heat: how to heat a high tunnel. Inflation blower (not a HAF or exhaust fan): high-tunnel double-poly inflation. Insect / exclusion netting (heat caveat when sides and ends close): high-tunnel insect netting. County inspection: EQIP checklist.

Ventilation is exchange. HAF is mixing.

UVM Extension Ag Engineering treats the two air-flow jobs as different hardware. Roll-up sides, ridge vents, and exhaust fans exchange inside air for outside air. Horizontal air flow fans stir what is already inside.

Ventilation is the exchange of inside air for outside air. This results in exhausting warm, humid air and replenishing it with cooler, drier air. Ventilation strips the humidity and heat from the space using exhaust fans, ridge vents and/or roll-up sides.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019).

Circulation is the mixing or stirring of air within the structure. This results in relatively equal conditions being well-distributed throughout the space. Circulation “averages” conditions using HAF fans.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019).

If you circulate without ventilation, you run the risk of having conditions in the structure growing overly hot and humid leading to plant stress and disease.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019). Mixing without exchange is not a ventilation system.

Remember that HAF fans work to mix the space (circulate the air) but don’t significantly improve ventilation.

Source: UVM Extension Ag Engineering — Chris Callahan, Improved Ventilation for High Tunnels (24 March 2017).

Passive first: roll-up sides and end-wall vents

Most hoop-house kits start with roll-up sidewalls and some form of end-wall door or framed opening. Those pages already exist. This page does not rebuild kit inclusion tables.

Passive systems are often preferred over active systems due to their inherent lower initial cost and energy efficiency and because they offer greater flexibility in siting (e.g. electric utilities are not needed). However, passively ventilated tunnels depend on steady, gentle prevailing winds to assist with ventilation. Avoid siting these tunnels too close to large structures, dense tree lines, or other obstructions that will prevent ventilation.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019). Siting note, not a wind-load table.

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.

Source: UVM Extension Ag Engineering — Chris Callahan, Improved Ventilation for High Tunnels (24 March 2017).

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.

Source: UVM Extension Ag Engineering — Chris Callahan, Improved Ventilation for High Tunnels (24 March 2017). Vendor names on that 2017 page are Callahan's citations, not Amazon SKUs on this page.

Kit-by-kit roll-up inclusion stays on high-tunnel roll-up sides. End-wall doors, zipper kits, and framed openings stay on high-tunnel end walls.

Active: electricity at the site, then fans and a thermostat

Exhaust fans, motorized shutters, and HAF motors need a circuit. A still, shaded, or tree-lined lot can make passive exchange fail and make a generator or a long trench the real cost — that dollar is not invented here.

Active systems use electric motors to spin fans and move air. Active systems, therefore, require electricity to operate and siting of a high tunnel or greenhouse may impact the feasibility of this. An example of an active system includes two end wall exhaust fans, two inlet louvers on the opposite end wall and a thermostat that opens the louvers and turns the fans on when the temperature is above 85 °F.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019). The 85 °F setpoint is Callahan's example, not a crop recommendation we invent.

Inlet louvers should be sized with an area equal to the exhaust fans. Motorized louvers with a flange to ensure good sealing are recommended to improve energy efficiency and plant health by reducing drafts.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019). Gravity or motorized shutters are the inlet half of an exhaust pair.

The sensing probe of the thermostat should ideally be aspirated (air flowing over it), shielded from radiation gain (the increase in heat due to incident sunshine), and located to accurately measure the air temperature the plants are experiencing.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019).

Wax cylinders can be used to open and close louvers, peak and gable vents when electricity is not available.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019). Wax cylinders are the no-electric actuator for a hinged louver, peak, or gable vent.

Sourced sizing — only the figures the pages printed

Callahan (2019) attributes CFM-per-square-foot and HAF volume rules to Bartok and Aldrich, Greenhouse Engineering (NRAES-33). UNH Ebba's 2026 slides use a shorter 8-foot volume on the same 30×96 footprint. Both are copied as printed. Tunnel Spec does not average them into a third number.

For hot season growing use 8 CFM/ft2 of growing space for cooler season growing use 2 CFM/ft2. For a 30 ft x 96 ft tunnel the total growing area is approximately 2,900 ft2. The hot season ventilation requirement is, therefore 23,200 CFM and the cool season requirement is 5,800 CFM.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019), citing Bartok & Aldrich NRAES-33.

Guidance for circulation flow rates is 25% of the overall growing volume per minute (also from Bartok and Aldrich’s Greenhouse Engineering). For a 30 ft x 96 ft tunnel with 15 ft peak, the total volume is approximately 34,000 ft3. One quarter of that volume is approximately 8,500 ft3, so the total HAF flow rate required is 8,500 CFM (ft3/min).

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019). 15-foot peak volume, not the UNH 8-foot example.

To ensure well-distributed mixing flow throughout the growing area, it is generally recommended to place HAF fans every 50 feet along the length of flow. In practice, especially with dense vegetation in certain crops (e.g. tomato and cucumbers) a maximum distance between HAF fans of 20-30 feet is more appropriate. HAF fans are typically installed in a “race track” layout intended to stimulate flow all around the growing area.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019).

This 12″ HAF is a very commonly used option that can provide about 1000 CFM of circulation.

Source: UVM Extension Ag Engineering — Chris Callahan, Ventilation in Greenhouses and High Tunnels (14 August 2019). About 1,000 CFM is that 2019 12-inch HAF caption — not the Amazon 1139 CFM listing below.

air speed (70-200 fpm; (1-2mph)) CFM: ¼ volume of greenhouse 30’ x 96’ x 8’ = 23,000 cubic ft 23,000 cubic ft x ¼ = 6,000 cfm need (4) 1,500 cfm fans

Source: UNH Cooperative Extension — Jonathan Ebba, High Tunnel Ventilation & Airflow (UVM / VVBGA 2026 slides). Slide text, 8-foot height, four 1,500 CFM fans.

1 exchange per minute active ventilation · ¼ exchange per minute winterizing roll ups

Source: UNH Cooperative Extension — Jonathan Ebba, High Tunnel Ventilation & Airflow (UVM / VVBGA 2026 slides). One air change per minute is the active-ventilation slide; one-quarter is the winterizing roll-up slide.

Those figures are not a shopping list for the Amazon 960 CFM shutter fan or the 1139 CFM circulator. This page does not divide 23,200 by 960 or 8,500 by 1139.

EQIP pays for the structure, not the fans

Practice Standard 325 High Tunnel System language on electricity and venting is already collected on what EQIP will not pay for. Fan eligibility and any payment dollar stay with the county. Confirm with your NRCS district conservationist. Tunnel Spec does not invent either.

Under the High Tunnel System Initiative, financial assistance is provided only for the structure itself. Electricity, venting systems, lighting, etc. can be installed providing the additions meets manufacturer’s designs and specifications. However, the grower is responsible for those costs.

Source: USDA NRCS High Tunnel System Initiative FAQ (Updated November 2024; Oregon-branded).

Producers can install heating, ventilation, or electrical systems at their own expense.

Source: USDA NRCS Pennsylvania — Seasonal High Tunnel System for Crops. One state office page, not a national fan payment.

Rules vary; call your county NRCS office.

Sourced Amazon ventilation parts

Titles and featured prices were copied from live amazon.com product pages 11 October 2026, about 03:15 UTC. Kit frames stay on manufacturer product pages. Roll-up cranks stay on roll-up sides. End-wall polycarbonate and hinges stay on end walls.

What these parts are for

• Circulation. iLiving ILG8TF12-DC (B0D3GTPHY6) is titled 12-inch, 1139 CFM, greenhouse among uses, freestanding or wall mount. Callahan's hanging race-track HAF is a different mount. This listing is the live greenhouse-circulation fan that returned a featured price tonight.
• Shutter exhaust. iLIVING 12-inch (B0CVSN1KRV) is titled 960 CFM with an automatic shutter and temperature / humidity controls. Callahan's 30×96 hot-season figure is 23,200 CFM — this page does not invent how many 960 CFM fans that is.
• Plug-in thermostat. Inkbird ITC-308 (B011296704) is a 2-stage 1100W outlet controller. Cooling outlet for a plug-in fan; heating outlet if you already have a heater. It is not a hardwired two-stage house controller and not a humidity controller.
• Wax-cylinder auto vent. Palram Canopia HG1027 (B00M5J323G) is titled for Canopia resin greenhouses. The description uses natural wax and a spring, no electricity. A 30×96 gable or peak vent still needs a framed opening — see Callahan's 24×24-inch note and the end walls page.
• Min/max thermometer. TempPro TP50 (B01H1R0K68) records high and low temperature and humidity. Hang one at crop height, not on a sun-baked bow, if you want the reading Callahan asks a thermostat probe to see.

FAQ

What is the difference between ventilation and HAF circulation?

UVM Extension Ag Engineering (Callahan, 14 August 2019) treats them as different jobs. Ventilation exchanges inside air for outside air — exhaust fans, ridge vents, and/or roll-up sides. Circulation mixes air already inside the house with HAF fans. The same page says circulating without ventilation risks conditions growing overly hot and humid. The 2017 Callahan note adds that HAF fans mix the space but do not significantly improve ventilation.

Do I need electricity for high-tunnel fans?

Active fans and motorized shutters need power at the site. Callahan (2019) writes that active systems require electricity and that siting may impact feasibility. His example is two end-wall exhaust fans, two inlet louvers on the opposite end, and a thermostat that opens the louvers and turns the fans on above 85 °F. Wax-cylinder auto vent openers and roll-up sides do not need electricity. Confirm power access before you buy hardwired fans.

Does EQIP / NRCS practice 325 pay for fans or electrical parts?

Tunnel Spec does not invent eligibility or dollars. The High Tunnel System Initiative FAQ (Updated November 2024) says financial assistance is provided only for the structure itself, and that electricity, venting systems, lighting, etc. can be installed if they meet manufacturer designs — the grower is responsible for those costs. The Pennsylvania seasonal high tunnel page says producers can install heating, ventilation, or electrical systems at their own expense. Confirm fan or electrical-component eligibility with your NRCS district conservationist.

What sizing figures are sourced here?

Callahan (2019) copies Bartok & Aldrich (NRAES-33): 8 CFM/ft² hot season and 2 CFM/ft² cool season; a 30×96 ≈ 2,900 ft² house is 23,200 CFM hot / 5,800 CFM cool. Circulation is 25% of growing volume per minute; 30×96 with a 15 ft peak ≈ 34,000 ft³ → 8,500 CFM HAF. Place HAF every 50 feet, or 20–30 feet in dense tomato/cucumber. A 12″ HAF is about 1,000 CFM. UNH Ebba 2026 slides use a different volume: 30′×96′×8′ = 23,000 cu ft × ¼ = 6,000 cfm and (4) 1,500 cfm fans; 1 exchange per minute for active ventilation; ¼ exchange per minute for winterizing roll-ups. Those two 30×96 HAF totals are not the same volume assumption — they stay labeled, not averaged.

Which Amazon ventilation parts are on this page?

Five live listings fetched 11 October 2026, about 03:15 UTC: iLiving 12-inch turbo circulator 1139 CFM (B0D3GTPHY6, $74.56), iLIVING 12-inch shutter exhaust 960 CFM (B0CVSN1KRV, $87.50), Inkbird ITC-308 1100W plug-in thermostat (B011296704, $35.99), Palram Canopia wax-cylinder roof vent opener (B00M5J323G, $83.05), and TempPro TP50 digital min/max hygrometer (B01H1R0K68, $10.99). Bayliss MK7 B002EIMRH0 was opened and skipped — no featured offer. A missing featured price stays an em dash. Ratings are not copied. Kit frames stay on manufacturer pages.

Are roll-up sides and end-wall doors the same as fans?

No. Roll-up sides and end-wall doors or vents are the passive options. Kit inclusion for those parts stays on the roll-up sides page and the end walls page. This page is the active-parts sibling: HAF/circulation, shutter exhaust, thermostats, wax auto vents, and a min/max thermometer. Callahan (2017) says roll-up sides alone tend to work on sites with good airflow and recommends a 24×24-inch gable vent on each end of a 30×96 with a wax-cylinder actuator when the site is still.

Rules vary; call your county NRCS office. Tunnel Spec does not determine eligibility, payment rates, or inspection outcomes. Confirm fans or electrical components with your NRCS district conservationist. As an Amazon Associate, we earn from qualifying purchases.