Look for leaks

With cool weather coming, it is important to tighten up the greenhouse. Leaky vents and fan shutters often contribute a significant amount of air to infiltration heat loss.

Vents

Greenhouses with roof and sidewall vents operate on the principle that heat is removed by a pressure difference created by wind and temperature gradients. Wind passing over the roof creates a vacuum and sucks the heated air out of the vent. Cold replacement air enters through sidewall vents, open doors and other openings.

Calculating vent heat loss is difficult, as there are many variables. These include vent location and the temperature difference between inside and outside. External factors include wind direction and speed, adjacent trees, buildings and other obstructions. An older ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) method for calculating infiltration losses is based on the linear feet of crack.

Here is an example of heat loss from a 100-foot vent. It assumes a quarter-inch crack, resulting in 2.08 square feet of opening. The average wind speed is 10 mph, and the inside temperature is 65°F, with an outside temperature of 20°F. This results in a heat loss of 28,000 Btu/hr or 672,000 Btu/day. This is equivalent to 6.5 gallons of fuel oil, 8.7 ccf of natural gas or 9.7 gallons of propane over a 24-hour period based on 75% heating system efficiency.

Maintenance measures

Roof and sidewall vents need to be adjusted so that they close even and tight. This involves lubricating bearings, rack and pinions, vent arm hinge points and checking fluid in gearbox drives. In houses with vents that don’t close tight, adding weather stripping may be the only way to stop excess heat loss.

Maximum and minimum vent position limit switches should be checked to see that they will stop vent travel at the correct position.

Air leaks with rollup vents can be a problem, especially on windy days. There are systems that reduce infiltration, including (a) installing plastic or a polycarbonate panel over the first frame on each end to form a seal, (b) attaching hook-and-loop fasteners to the outside frames and an adhesive-backed felt strip to the plastic, (c) inflating a polytube attached to the end frames and (d) installing a curtain pocket.

Shutters

Heat loss through a shutter occurs by conduction through the metal or plastic and by air exchange (infiltration) through the cracks between the blades. If the shutter closes tightly, then most of the loss is by conduction. If the blades are bent or if the hinges are sticky, then infiltration can be the greatest loss.

As with vents, calculating shutter losses is difficult, as there are many variables. In addition to the infiltration losses with shutters, the conduction loss must be added.

Here is an example of heat loss from a 48-inch square shutter. It assumes that there are 12 blades with an 1/8-inch crack and two 1/8-inch cracks along the sides where the hinges are located, totaling 56 linear feet. The average wind speed is 10 mph, and the inside temperature is 65°F, with an outside temperature of 20°F.

The heat loss is equal to about 0.10 gallon of fuel oil, 0.13 therms of natural gas or 0.15 gallons of propane per hour. The total for 24 hours amounts to 2.4 gallons of fuel oil, 3.1 therms of natural gas or 3.6 gallons of propane. If the crack is wider or wind speed is greater, then these values will be larger. Doubling the wind speed will about double the losses. Also, as aluminum shutters have greater conduction, the heat loss will be greater. PVC shutters have slightly less heat loss but are more prone to damage.

Maintenance measures

Check operation of the shutters before the winter heating season. Lubricate bearings, check that blades are straight and close evenly and caulk around the frame. And make sure that the shutter motors are working properly.

Shutters should also be motorized to keep them open when in use and closed when the fans are off. Wind pressure differences cause non-motorized shutters to open, resulting is significant air exchange and heat loss.

During the winter, all fans and shutters are not needed, and at least half of them can be closed off and insulated. Disconnect power to these fans and shutters. A simple method of reducing heat loss is to cover the shutter or fan with a layer of plastic held in place with furring strips and attached with deck screws or double-headed nails for easy removal.

A more permanent solution is to cut a piece of 1-inch insulation board and attach it over the shutter. This can be done with TEK screws with large heads, sheet metal brackets that allow the insulation to slide over the shutter, magnetic adhesive strips or hook-and-loop fasteners. It is best if the insulation board is faced with aluminum foil, as this will reflect the radiant part of the heat back into the greenhouse.

A more permanent solution is an insulated box with a split cover hinged in the middle so that it can be opened on days when ventilation is needed.

Commercially available shutter covers are available from sources such as Grainger. These have an insulating material with a tough fabric covering. They are flexible and easy to attach with hook-and-loop fasteners or magnetic strips.

With increasing heating costs, payback for installing covers is very short. As winter approaches, check all vents and shutters to see that they close tight.