Heating A Greenhouse

Depending on where you live, winter can be a gentle experience with the occasional flurry sighting, or a harsh gulag lasting half the year.  While plenty can be done with an unheated greenhouse,  for many growers, a greenhouse heating system is a necessity during the winter months.

When planning a heating system for your greenhouse it helps to understand a little about how heat is both created, and lost.

A greenhouse loses heat in four ways :

Conduction : Heat moves through an object from one medium to another : from inside the greenhouse to the outside through the frame and glazing

Convection : flow of air caused by a difference in temperature : hot air rises from a heater because cold air is drawn in at the bottom.

Radiation : Heat transfers away from an object through space without the necessity of a medium. Greenhouse glazing can either absorb, store or reflect radiated heat.

Infiltration/exfiltration :  Cracks under doors and vent windows, and glazing seams allow cold air to enter (infiltration) , and warm air to escape (exfiltration). In a greenhouse, warm air escapes through cracks in the roof, drawing cold air in through openings near the floor.

How Much Heat Do I need?

Before investing in a heating system, the first step is to determine how much heat you need. While it’s possible to use formulas to calculate both the solar heat generated on a clear day, and the heat lost due to convection, conduction, radiation, and infiltration/exfiltration, there’s a  simpler formula to approximate  how much heat you’ll need to add:

(wall area + roof area) x difference in temperature between outside and inside the greenhouse x 1.1= Number of BTU’s required.

Now that you know how much heat is required to keep your greenhouse warm, it’s time to create a heating system. This may be passive heating system, and active heating system, or combination of the two.

PASSIVE HEATING SYSTEMS

Passive heating systems use water or stone to store solar energy during the day, and radiate it back into the greenhouse at night, while the hot bed method uses heat produced from the decomposition of organic matter.

Water storage tanks

Large tanks or drums filled with water, and painted black, soak up solar energy on sunny days and radiate it back into the greenhouse at night. The challenge this method is warming the areas in the greenhouse that are furthest from the water tank  ; even with circulating fans, the areas farthest from the tank can experience a dramatic temperature difference. Also, consider that tanks can freeze in winter after a few days without direct sunlight.  Thawing a frozen tank takes a lot of sunlight and heat , actually cooling the air around it as it does.

Placing the tank in the middle of the greenhouse where it is less likely to freeze is one way to address this, and, while sort of diminishing the goal of using passive solar energy, an electric aquarium heater can also be used to prevent the water in the tank from freezing.

A solar water heater can also be used to heat liquid that is then circulated through copper pipes to a heat exchanger inside the water storage tank, but again, the system only works when the sun is shining. On cloudy winter days, these systems are often supported by a wood or pellet fired stove or furnace.

Rock Heat Storage

These systems comprise insulated heating bins filled with large chunks of rock that absorb solar heat blown across the medium by fans during the day, and then circulate the warmth back into the greenhouse at night. While they require little to no maintenance once installed, rock heat storage systems do require a substantial amount of volume:  every square foot of greenhouse glazing, 3 cubic feet of rock is needed. While large wire baskets can be used to store the chunks of rock, a more efficient way is to place 3- 4 feet of rocks beneath an insulated floor.

Hot Beds

For greenhouses where growing beds are used, the energy created by decomposing manure can be used to warm the grow environment around the plants. Simply dig out the beds down to a depth of about 3’ and fill them with horse manure and other compostable material like leaves and grass clippings, back to about a foot below the top of the beds. Then top that off with garden soil. As the organic matter breaks down, the temperature will increase to about 160 degrees over the course of about 5 weeks, remaining there for about two months before gradually cooling down to about 80 degrees. 

ACTIVE HEATING SYSTEMS

Systems that draw on sources other than the sun, including coal, wood, wood pellets, gas, oil and electricity are referred to as active heating systems.  While certainly more expensive to operate than passive heating systems, active heating systems afford much greater control of the temperature in the greenhouse, and allow for much higher temperatures.  Insulating the north wall and opting for multi-wall glazing will help offset the cost of running an active system.

Electric Heating

Clean and simple to use, electric heaters of course require that your greenhouse is wired for electricity. While electric heaters themselves don’t have to cost a lot, the cost of the electricity they use can add up fast in winter, especially if the greenhouse is poorly insulated.  Costs can be controlled by maintaining a temperature between 40 and 50 degrees during winter, which is warm enough to keep most plants healthy, without producing sticker shock when you open the power bill.

Wood and Wood Pellet Burning Stoves

While larger greenhouses are commonly heated with 55 gallon drum stoves vented with a chimney, a standard wood burning stove is adequate to heat most hobby greenhouses.  Besides the need for constant feeding, which means frequently opening the door and allowing heat to escape, the logs can also introduce insect pests into the greenhouse.  A better, but more costly option is a wood burning furnace placed outside the greenhouse. This means no heat lost opening the door, or unwanted insect pests, and it also eliminates any risk of carbon monoxide accumulating inside the greenhouse.

Coal Stoves

While they share many of the same benefits of wood burning stoves, including the ability to generate a lot of heat, they must also be monitored and refilled.  Consider where you’ll store the coal and dispose of the coal ashes.

Gas Heaters

Usually designed to run on either propane (LPG) or Natural Gas (CNG), gas heaters are easy to use, and safe as long as they are operated properly.  If the unit doesn’t have adequate ventilation or air intake, the oxygen in the greenhouse can become depleted, creating a hazardous environment for people. Propane leaks can also damage your plants. Models that require no outside power source are ideal, especially as a back up to an electric heating system.