Calculating Greenhouse and High Tunnel Heat Loss

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.

1. SIMPLEST – Online greenhouse heat load calculator. This online calculator allows you to enter the dimensions, construction material and temperatures you are interested in and it estimates the heat (and cooling) load.

2. LITTLE MORE COMPLEX – VirtualGrower – 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.

3. HEAVY LIFTING, but FULFILLING – Do the calculations yourself! The formulae behind all of the tools above are well described in “Greenhouse Engineering, NRAES-33” by R. A. Aldrich and J. W. Bartok. Available here as a PDF: See p. 65-71 specifically.

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:

LED Lights – Status, Cost/Benefit and Pro’s and Cons

I have been receiving several inquiries recently on supplemental lighting for greenhouse production. The most common question is “Should I install LED lights to support growing?”
I have found one report to be the most complete and current on this topic and wanted to share it here.

Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures by Jacob A. Nelson and Bruce Bugbee. Published: June 6, 2014. DOI: 10.1371/journal.pone.0099010. Erik Runkle at Michigan State University also summarizes some of this work in Greenhouse Product News here.

There are some industry responses to this including this one from Inda-Grow. And a recent USDA report is somewhat contradictory in its findings here.

There is a also a nice summary by Robert Morrow in Hort Science (HortScience December 2008 vol. 43 no. 7 1947-1950) available here.

Nelson and Bugbee conclude;

The most efficient HPS and LED fixtures have equal efficiencies, but the initial capital cost per photon delivered from LED fixtures is five to ten times higher than HPS fixtures. The high capital cost means that the five-year cost of LED fixtures is more than double that of HPS fixtures. If widely spaced benches are a necessary part of a production system, LED fixtures can provide precision delivery of photons and our data indicate that they can be a more cost effective option for supplemental greenhouse lighting.

Manufacturers are working to improve all types of lighting technologies and the cost per photon will likely continue to decrease as new technologies, reduced prices, and improved reliability become available.

My take-away from all of this; LED’s have a higher initial cost, can have lower recurring costs, can be more effective for specific physiological benefit, and can support certain production layouts.  But the cost/benefit does not seem to pencil out quite yet.

Simple DIY Outside Air Exchange

In colder climates winter storage crops have been stored in passive root cellars for centuries, striking the balance between outside conditions and storage conditions. Although modern refrigeration systems are generally used for large scale, longer-term storage of these crops some enterprises seek lower cost and high energy efficiency options. Outside air exchange systems use an exchange fan to draw colder outside air into a storage room to maintain a depressed temperature. The control of this requires monitoring outside temperature and inside temperature and only allowing air exchange when the outside air temperature is low enough to cool the room, but also only when the inside room requires cooling.

This control can be accomplished with two thermostats wired in series; one setup for heating (outside/colder air) and one setup for cooling (inside/room/warmer air). All thermostats are essentially a switch whose state (on or off) is controlled by a temperature sensor and a setpoint. They are either purchased or configurable for heating (turn on the load at or below a setpoint temperature) or cooling (turn on the load at or above a certain load). In our case, we are using a heating thermostat “cascaded” to a cooling thermostat so that our “load” (fan) only comes on at or below a certain oustide temperature and at or above a certain inside room temperature.

Outside Air Exchange Overview Pic
An overall view of a mocked up air exchange system. This article doesn’t address routing or the air ducting. When using a bathroom exhaust fan, the inlet to the fan is the grill shown and would generally be mounted high to exhaust the warmer air. The outlet should be ducted outside. A separate inlet duct should be routed from outside to supply make-up (exchange / cold) air.

In this system (a mockup used for cold storage workshop instruction) I used the following items:
QTY 1 – Standard Light Switch – $0.69
QTY 1 – Switch Box – $0.91
QTY 2 – Johnson Control A419 Thermostats w/ 6.5 ft probe (one setup for cooling, one setup for heating) – $58.95 each ($117.90 total)
QTY 1 – 70 CFM Bathroom exhaust fan – $29.97
Misc 3 and 4 conductor 14 AWG solid wire.

Total parts: $149.47.
Total time about 2 hours for first build.


Outside Air Exchange Schematic and Wiring Diagram
Schematic and wiring diagram for the system shown.

A note on thermostats. I like a digital thermostat with a remote probe and I tend to use the Johnson Control A419. It has a 1 degF setpoint resolution and down to 1 degF differential. Each one can be setup for heating or cooling by making the proper adjustment of dip switches inside the box (see p.7 of the manual). Ranco make a similar thermostat in their ETC line. I’m still on the lookout for a good, inexpensive delta thermostat (a thermostat that controls a load based on a true delta-T, or temperature difference, between two locations.) Most are designed for solar hot water systems and don’t seem to allow for control at lower temperatures. And they are fairly expensive.

I ran mainly 3 conductor wire in this setup, but did find the 4 conductor wire to be a clean way of getting an additional wire from the first thermostat to the second. This allows for both thermostats to be powered regardless of the output state of the first.  This lets the user see both measured temperatures and make setpoint adjustments since both thermostats are always powered whenever the main power switch is on.

Outside Air Exchange Wiring Pic
Detail showing the wiring at each thermostat. Thermostat probe wiring and jumper setting not shown, refer to manual for your thermostat.

The basic settings for each thermostat in this setup were:

Outside Air
Inside Air
Jumper 1 JUMPED (Heating) OPEN (Cooling)
Jumper 2 OPEN (Cut In at SP) OPEN (Cut in at SP)
SP (Set Point)* 35 degF 40 degF
Diff (Differential) 1 degF 1 degF
ASD (Anti Short Cyle Delay)
1 min 1 min
OFS (Offset for BIN) 0 – Not used 0 – Not used
SF (Sensor Failure Operation) 0 (De-energize) 0 (De-energize)

* – Note the outside air thermostat should always have a lower setpoint (SP) compared to the inside air (room) setpoint.  This is what ensures the fan only comes on when the outside air will actually provide cooling.

A note on sizing the fan. Moving air works out to about 1 BTU/hr per degF per CFM (or ft3/min).

Cp_air = 0.24 BTU/lb/F
Rho_air = 0.07 lb/ft3…

Q_dot {BTU/hr} = V_dot {ft3/min} x Rho_air {lb/ft3} x Cp_air {BTU/lb/F} x (Tin – Tout) {F} x 60 {min/hr}

which all works out to

Q {BTU/hr} = 1.008 {BTU/hr/CFM/F} x V_dot {ft3/min} x (Tin – Tout) {F}.

So with a 75 CFM fan and a 10 degF temperature difference, this system can cool at a rate of 750 BTU/hr. Large fans or multiple stages are even possible. Watch the amperage on the load compared to the thermostats, though. You may need an intermediate relay to handle the larger current in bigger systems.

Be careful of drying out whatever you’re cooling. When you bring air in from outside, especially when cold out, it will generally be more dry (lower relative humidity). So you’ll want to keep track of humidity and may need to add some to the air.

Cross posted on, join the discussion. Download a fact sheet on this tool.

Farm to Plate Energy Success Stories

Vermont’s Comprehensive Energy Plan calls for obtaining 90% of the state’s energy from renewable sources by 2050 and reduce greenhouse gas emissions 50% from a 1990 baseline. What role can Vermont’s food system play in advancing this goal?

The Energy Cross-cutting Team of the Farm to Plate Network has released seven Energy Success Stories that showcase farms, businesses, vendors, installers, and technical assistance providers that have made a difference with energy efficiency savings and renewable energy production.

The stories were prepared by JJ Vandette and staff at Efficiency Vermont, Chris Callahan from UVM Extension, Alex DePillis from the Agency of Agriculture, and Sarah Galbraith and Scott Sawyer at VSJF. Funding for the project was provided by the Northeast Dairy Sustainability Collaborative (Ben & Jerry’s, Cabot Creamery Cooperative, Organic Valley, Stonyfield, Vermont Agency of Agriculture, and the Sustainable Food Lab).

The seven Energy Success Stories are the first in a series of resources that will highlight farms and businesses throughout Vermont’s food system that have made significant progress in saving energy and producing renewable energy.

The first 7 Energy Success Stories can be found on the Atlas at these links:

They can also be downloaded as PDFs here:

Oilseed Economics Update 2014

Yesterday we held our annual Oilseed Producer’s Meeting.  At this meeting, I presented an economic overview of oilseeds in Vermont.  Ina nutshell, Vermont has an installed on-farm biodiesel capacity of 600,000 gal/yr (5 sites) with a normalized initial cost of $1/gal of capacity (better than national average). Fuel can be produced for an average cost of $2.13/gal, and meal can be produced at an average cost of $340/ton.  The greenhouse gas emissions associated with this model are 60-100% better than US avg oilseed production (net sink) while the average energy return on energy invested (EROEI) is 4 to 1 (i.e. 4 gallons produced for every gallon used in production.  The model is on-farm production for on-farm use; i.e. cost avoidance.

This study made use of the Vermont Oilseed Cost and Profit Calculator, a tool we have developed over the years to collect all the enterprise costs associated with an on-farm oilseed operation that may turn the crop into meal, oil, and/or biodiesel.  It helps growers and others interested in the topic arrive at specific product costs and compare those costs to market prices.  We also have summarized three different likely oilseed enterprise scenarios in e report titled Vermont On-Farm Oilseed Enterprises: Production Capacity and Breakeven Economics. This work has had strong support from the Vermont Bioenergy Initiative of the Vermont Sustainable Jobs Fund and has been accomplished in close cooperation with the UVM Extension Northwest Crops and Soils Program.

Vermont On-Farm Biodiesel – Costs of Production

I recently co-authored a summary of the economics of on-farm biodiesel with Netaka White from the Vermont Sustainable Jobs FundThis report collects the economic and logistic learning from the past seven years of the Vermont Bioenergy Initiative (VBI).  The VBI has supported a wide range of sustainable fuel related efforts in Vermont.  Along with agronomic research by Heather Darby’s Northwest Crops and Soils team funding has supported efforts to streamline on-farm processing and production systems, improve safety, expand storage capacity and ultimately to expand adoption of sustainable fueling practices.

We showcase five Vermont on-farm biodiesel operations that use a variety of equipment to reach their different biodiesel and feed production goals.  We use data from these farms, collected over several years, to run a detailed economic analysis of a hypothetical 100,000-gallon per year on-farm biodiesel facility. A second hypothetical case based on a 13,000-gallon per year facility is also reviewed. Finally, we explain in ten steps how to estimate breakeven and profitability in both cases. The information and steps are applicable to any farmer interested in fuel and feed self-sufficiency or generating additional farm income. The analysis was done using the Oilseed Cost and Profit Calculator I developed under this program.

Pressing Costs2
This figure illustrates how the cost of production is distributed when seed is pressed to meal and oil.

In the two examples above, one a 100,000 gallon per year commercial enterprise, and the other a 13,000 gallon per year operation, both used representative sunflower production data from five Vermont farms. Over a range of crop production costs between $100 and $200 per acre, and yields between 1,000 and 2,000 pounds per acre, at current market prices the combined worth of the meal and the oil, or the meal plus the biodiesel are shown to be profitable.

COP Summary Table

Both scales of operation can also see a payback on their investment within a reasonably short time frame when selling meal and biodiesel or selling meal and using biodiesel to reduce enterprise expenses; 100,000 gallon per year payback is 6.4 years & 13,000 gallon per year payback is 9.4 years. This is based on current, relatively low fuel costs. Should diesel prices reach $5 a gallon, simple payback could occur in less than 12 months for the 100,000 gallon per year case and less than 3 years for the 13,000 gallon per year case.

CoolBots(TM): Inexpensive Cold Storage

Demand for on-farm cold storage of produce and other Vermont agricultural products is increasing as local markets for these goods expand. I receive many inquiries regarding CoolBotsTM, an adaptation of a window air-conditioner to make a cooler out of an insulated space. This article is intended to collect related resources in one place and to also highlight some considerations adopters of CoolBots should be aware of.

In a nutshell:

A farmer-built cooler (photo from

These systems utilize a commercially available controller ($299) to allow the AC unit to run with a lower temperature than normal. Store-It-Cold, The manufacturer’s website has excellent resources and FAQ’s. They include a list of AC units that they have had positive experiences using. They are also very clear about who should consider NOT using a CoolBot. Applications for which the CoolBot is not well suited, according to the manufacture, include;

  • rapid “pull down” of temperature (e.g. high levels of field heat or frequent exchanges of product)
  • freezers – CoolBots perform best above 36 °F.
  • sites with many door openings per day (e.g. > 6 times per hour)
  • running through the winter – not a show stopper, but you need to be more careful about which AC unit you choose

Other things to be very aware of, according to the CoolBot controller manufacturer, include

A CoolBot installation (photo from

A report commissioned by NYSERDA summarizes the cost, energy efficiency, and greenhouse gas emission benefits of a CoolBot installation when compared to a conventional walk-in cooler system at certain conditions. The cost estimate of the CoolBot system (15,000 BTU/hr) is $750 installed compared to $4,400 for a conventional system (8’x10′ cooler box cost not included).

The authors conclude that a CoolBot system can result in approximately 230 kWhr/year of energy savings ($30/year at $0.13/kWhr VT average) when cooling 100 ft2 of cooler floor area to 35 °F (assumes Albany, NY conditions). It is important to note that this analysis highlights the main energy efficiency benefit of the CoolBot system comes from the reduced operating time of evaporator fans. High efficiency fans and improved controls exist for conventional walk-in systems and they are even supported by rebates from Efficiency Vermont. When the CoolBot system was compared with a conventional cooler that also had evaporator fan controls, the savings went the other way; i.e. the conventional walk-in system resulted in 74 kWhr/year savings.