A possible greenhouse circuit diagram. Remember that in application, outdoor-rated cable and waterproof fittings will be used throughout.
IN THIS CHAPTER you’ll learn how to calculate how much electrical power your greenhouse requires and the basics for designing and installing a suitable electrical system. We’ll also discuss how to design and install various types of watering and heating systems. Even if you are hiring someone to build your greenhouse and install these systems, this chapter will help you ensure that the contractor is installing what will work best for you. For general information on electrical, heating, and watering systems and choosing which will work for your needs, refer to chapter 5, Controlling the Greenhouse Environment.
Before we begin any discussion of installing electricity in the greenhouse, it’s important to stress that a greenhouse is a humid, sometimes wet environment. The combination of electricity and water is a very dangerous one. To make sure your installation is as safe as possible, all electrical switches and fixtures should be installed in waterproof outdoor-rated housing to prevent the accidental flooding of any electrical outlet. All outlets should be waterproof as well and should be installed higher in the greenhouse, away from the floor, where they are most likely to get wet. I like to locate them under the bench tops or at roof level. For your safety, all electrical circuits in and around the greenhouse should be protected by ground fault interrupter breakers (see the specific information on these on here). If you’ve done electrical work before and choose to do this work in your greenhouse as well, you should still have a professional check the project as you go and before you turn on the electrical system. Don’t rely on the final check of a building inspector to assess whether your system is ready and safe to operate. The job of a building inspector is to determine that everything is done to code, not to see if you have connected everything properly. By contrast, an electrician can perform a continuity test on the circuit to see that your connections are safe and correct.
If you are going to do the work yourself, you’ll need more than a passing familiarity with the National Electrical Wiring Code (NEC) and you will be required to obtain wiring permits before you start designing and cutting and stapling wires. You can buy your own copy of the NEC for a little more than $100. If you’ve never done electrical work before, hire a professional electrician to do the job start to finish.
Before you (or an electrician) begin wiring the greenhouse, you should determine what kind of lighting you’ll need and how many outlets your setup requires for lights, fans, heating apparatus, and so forth. You’ll also need to determine the kind of lighting required for different areas of the structure (for example, overhead lights that illuminate the entire greenhouse and task lighting for specific areas where you’ll perform certain jobs). Overhead lighting can be a single 100-watt bulb or can consist of two or more waterproof fluorescents. Remember that all lighting should be housed in waterproof fixtures. For information on the kinds of lighting available for use in a greenhouse and how to determine your lighting needs for different areas of the structure, refer to chapter 5.
As you make decisions about your electrical system, you should incorporate them into a complete wiring layout (see the one in Deciding Where to Locate Lights) and a circuit diagram (see below). These will show the location of the breaker box, the thickness and type of each cable required, any switches or fuses that are in the circuit, and a load calculation to indicate how much power is passing through each circuit. Note that the layouts and diagrams here are for demonstration purposes only and are based on my 61⁄2 years of experience as an electrical engineer. You should not attempt to determine the particulars of an electrical design or install wiring in your greenhouse unless you have some experience with installing electricity.
The lights that illuminate the walkway and the remainder of the greenhouse when the growing lights are not on will be used more often than any others, so they should have their own switch at the door of the structure. For example, in an 8-foot-wide by 12-foot-long greenhouse, your general lighting might consist of two fluorescent fixtures with two 40-watt fluorescent tubes in each. To determine if these can be placed on the same circuit, see Calculating Your Electrical Needs and the circuit diagram below.
During the winter months, the overhead lights in the greenhouse may be on quite a bit, but these lights will be little used in summer, when daylight can last for 14 hours. Rather than turn lights on and off daily, you might install a simple timer that can do the job and be adjusted twice a year for daylight saving time.
After you’ve made your decisions and calculations for general lighting, incorporate all results into your wiring layout and diagram.
To determine your specific task and plant lighting needs, you’ll first have to define the area(s) you want to light. In general, for plant lighting you’ll need 30 to 40 watts per square foot of growing bed. When the plants are small, fluorescent lights may be located within 2 inches of the growing bed, but as the plants grow, the lights will have to be raised. Fixtures should be hung to allow for this adjustment. For larger plants in a greenhouse bed, you may want to use high-intensity discharge (HID) lights. But because these lights generate a great deal of heat and the light reflectors shade the plants from natural sunlight, you must be able to raise and lower these lights, too. This means you’ll have to consider carefully the location of the outlet boxes for the lights and the length of cable between them and the fixtures.
If you’ve never done electrical work before, hire a professional electrician to do the job start to finish.
For larger beds, you might want to install high-pressure sodium or metal halide lights (see High-Intesity Discharge Lights) to generate the high levels of light required. In this case as well the lights must be able to be raised and lowered and will need to be protected from water. Purchase fixtures outfitted with screens to help contain the glass should the lightbulb shatter.
Each of your task lights must be wired with an on/off switch. You can string several lights together on the same circuit, but be sure to determine amperage on the wire to avoid putting too many lights on one circuit. (See Calculating Your Electrical Needs.)
After you have made your decisions and calculations, incorporate all results into your layout and diagram.
Although there are no codes for placement of outlets in a greenhouse, at the minimum they should be located 18 inches above the floor, as the National Electric Wiring Code (NEC) requires, to help protect them from getting wet. As the sample wiring layout in Deciding Where to Locate Lights indicates, outlets can also be located directly under the top of a bench to help protect them from water. Because potting soil and pots may also be stored under the benches and block access to outlets, however, a single outlet may not do the job, so you might install several (the greenhouse in our layout has four outlets located at strategic positions under the bench tops). If you intend to set up a bench for propagating or growing seedlings, you’ll need an outlet near the bench for heating or germination mats and cables and perhaps additional task lighting. The layout shows an outlet on the left for this purpose. You should also include in your layout outlets for plant lighting and for any electric tools you’ll be using.
According to an electrician I know, a rough rule of thumb is to put no more than eight or nine lights on one 15-amp breaker.
In addition to lights, fans, electrical heaters, and humidifiers or misters will all need to be plugged in or hard-wired, usually on their own circuits and with their own on/off switches. A fan can be plugged into an outlet at bench level, but if it is to be located high in the greenhouse, you may need an outlet box closer to it. Along with its on/off switch, the fan wiring should include a thermostat in the circuit (see the diagram below). Each of your greenhouse electrical “appliances” and lights should be wired to a distribution panel with a ground fault interrupter breaker in the panel. This will cut the circuit as soon as the slightest amount of power goes to ground so that should you get water on an appliance, the power will be turned off before you can get an electric shock. (See Ground Fault Interrupter Breakers). The panel will have breakers to suit the amount of amperage flowing through each circuit in addition to a main breaker that turns off power to every greenhouse system at once.
Along with locating your lights, outlets, and panel, you and/or your electrician will have to determine how the circuits are to be laid out and calculate how much power each circuit must provide (see the diagram below).
Although an electrician calculates the amperage a circuit can handle, it’s useful to understand how he or she arrives at these figures. In calculating the amperage for lights, for instance, first add together the wattage of each light. For example:
ten 100-watt lights on a single circuit = 1,000 watts
Next, divide wattage by voltage to determine amperage:
1,000 watts ÷ 120 volts = 8.333 amps
On this circuit an electrician would use a 15-amp breaker to allow for fluctuations in the power and the possibility that you want to use a 150-watt light in one of the sockets.
Referring to the wiring layout in Deciding Where to Locate Lights, at 400 watts each, the four HID lights over the growing beds add up to 1,600 watts of power at 110 volts. Thus, the total amperage needed is:
1,600 ÷ 110 = 14.00 amps
This is quite a load for a 15-amp circuit breaker because AC power fluctuates between 100 and 120 volts. At 100 volts, the load would be 1,600 ÷ 100 = 16 amps, which would trip the breaker. You may also trip the breaker by plugging in a slightly larger lamp. There are two options in this case: (1) Put all the lights on one circuit with a 20-amp breaker to handle any power fluctuations and an automatic switch capable of handling the 20-amp load; or (2) use two circuits, each with a 15-amp breaker, hard-wiring the lights to ensure that no other power load can be plugged into the same circuit (although it might be possible to plug a larger lightbulb into the lamp and increase the power load.)
Referring again to the circuit diagram below, we can identify and calculate the electrical needs for each circuit.
Circuit A (top circuit in the diagram): The frequency with which the lights go on and off is controlled by an automatic timer. There are two 400-watt HID lights on this circuit:
400 × 2 = 800 watts
800 watts ÷ 110 volts = 7.27 amps
Because the total amperage for this circuit is 7.27, we can use a 10-amp breaker.
Circuit B: This circuit, too, has two 400-watt HID fluorescent lights, so it will also use a 10-amp breaker.
Circuit C: On this circuit are the fluorescent lights that illuminate the germination chamber — one 200-watt fluorescent light and two overhead 40-watt fluorescent lights, for a total of 280 watts. To determine amperage:
280 watts ÷ 110 volts = 2.5 amps
To be consistent, we can use a 10-amp breaker here too, with additional amperage available to put in more lights later.
Though the AC power might be formally 110 volts, the voltage can fluctuate, measuring as high as 120 volts or as low as 100 volts. This fluctuation should be taken into account when determining the amperage a breaker can handle.
Circuit C: This circuit has four outlets, typically rated at 15 amps each, and the heater circuit. Because the heater needs only milliamps to fire off the igniter, we don’t need to factor in its requirements. You never know, however, what will be plugged into the sockets, so a 15-amp breaker is installed here.
Additional Circuit: Marked “spare” on our circuit diagram, this circuit can also be installed from the house instead of the greenhouse. It is made up of a 3-wire circuit for the exterior greenhouse approach lights and allows the greenhouse user to turn lights on and off at both the greenhouse and at some point on the approach to the structure so that no one will have to walk to or leave the greenhouse in the dark. The circuit has three 150-watt outdoor path lights with a switch at each end (450 watts total).
450 watts ÷ 120 volts = 3.75 amps
Thus, we could use a 10-amp circuit breaker for this circuit, although a 15-amp breaker is more commonly used.
Each of these five circuits will go to its own breaker in the distribution box.
A possible greenhouse circuit diagram. Remember that in application, outdoor-rated cable and waterproof fittings will be used throughout.
In calculating the electricity called for in your greenhouse, you should determine the wire you will be using and the amperage it can safely carry. In almost all cases you will use either 12- or 14-gauge wire. If you will be installing a 220-volt heater (as opposed to a 110-volt model), however, you might require a larger wire, in which case you should hire an electrician to do the work. A 220-volt heater cannot be installed on a 110-volt circuit. A higher-voltage heater requires an entirely new circuit. Incidentally, a 220-volt heater can be an expensive form of heating a greenhouse in a climate like New England’s: A friend who uses one had a heating bill that topped $600 for the coldest month of the winter!
A 12-gauge wire can carry 2,300 watts but most electricians operate on the assumption that it can carry only 2,000. A 14-gauge wire can carry up to 1,700 watts, but for practical purposes, assume it can carry only 1,500. According to State Farm Insurance, a circuit with a 12-gauge wire should have a 20-amp breaker and a 14-gauge wire should have a 15-amp breaker. A 10-gauge wire can carry 3,400 watts (assume 3,000 watts) and should have a 30-amp breaker.
For all outdoor and interior greenhouse work, an electrician should install a ground fault interrupter (GFI) breaker, which turns off the electricity the moment ground fault, or contact with moisture, is detected. Conventional breakers may simply send power through the ground line if a fault is detected, whereas a GFI breaker turns off in a fraction of a second. If you are watering your greenhouse and accidentally spill water on a socket, instead of shorting out through you, the GFI breaker turns off the electricity immediately for additional safety.
Rough wiring consists of running wiring through the walls and installing outlet boxes. It’s done before the insulation and drywall are installed, if your greenhouse is to include these two materials. Rough wiring can be installed in several ways. The least expensive method is simply to staple the wires to the wooden frame of the greenhouse as they run from the breaker panel. Staples should be a maximum of 18 inches apart and no more than 6 inches from each outlet box.
If you have a custom greenhouse with insulated walls, holes can be drilled through wooden studs or supports and wire can then be run through the holes. Each hole must be a maximum of 3⁄8 inch in diameter. If you choose this option and plan to install drywall or some other interior wall covering, you should nail a metal plate over the stud at the same level as the hole to prevent drywall screws from entering the stud and wiring right where the hole is. (A screw that has contacted or penetrated the wiring is a fire hazard.) You should check your local codes to see what kind of installation is required in your area.
Unlike an exposed water pipe, which reveals any leaks, wires do not indicate whether an electrical circuit is leaking power to the ground. Consequently, every circuit should have a breaker in addition to an on/off switch.
In the house, breakers are located in an electrical panel located in the basement or in a cupboard under the stairs. All electrical power coming into a house passes through this main panel, and it should have its own circuit breaker (some have a separate circuit breaker between the main breaker panel and the incoming line) so that electrical power to the entire house can be cut off if necessary. For safety, before an electrician adds the wiring from your greenhouse panel to the main breaker panel, he or she will turn off the power to your house.
Finish wiring is the point at which wires are joined to the actual electrical sockets, light fixtures, hard-wired heaters, fans, and switches.
It is easiest and safest to begin finish wiring by installing the outlets farthest away from the main breaker panel and installing the breaker last. If the breaker panel is wired first, somebody could turn on the power while you or the electrician is still working on the system.
When all the sockets, light fixtures, heaters, fans, and switches have been wired and all circuits have been wired into the breaker panel, power can be brought from the main breaker board. After this, the wiring is ready to be checked. The electrician will use a multimeter to test the continuity or determine whether the circuit is complete. By turning the breaker on and off, the continuity between the black and white terminals in each circuit can be tested. There will be some resistance if there are lights or appliances in the circuit. If there is none — for example, the circuit is entirely outlet boxes — there will be no deflection (movement) of the needle on the meter, meaning that the circuit is open. If a light is plugged into a socket to close the circuit, there will be some deflection of the meter’s needle. After testing the circuit continuity, the continuity between the ground line and the black line in each circuit is checked with the circuit turned off and then turned on. In both instances, there should be no deflection of the needle (the meter’s needle shows no movement).
Electrical wiring should be run underground or on poles from your home’s main breaker panel to your greenhouse (or to the breaker box in the greenhouse). If underground, it should be buried deep enough so that it cannot be damaged by a digging tool. The National Electrical Code (NEC) stipulates that UF cable should be buried more than 24 inches and at the least should be protected with sand. A preferable method consists of protecting regular nonmetallic (NM) cable by installing it in conduit before burying it, which also makes it easier to replace the cable if it fails. Metal conduit is safest because plastic shatters easily, which can contribute to a cable’s damage or failure.
The cable should be duplex of a suitable size to carry the power load desired from your house to the greenhouse (either 8-, 10-, 12-, or 14-gauge two-wire cable — 8-2, 10-2, 12-2, or 14-2).
Electrical wire offers some resistance to the voltage that goes through it. If the cable is long enough, this resistance becomes significant enough to cause what is known as voltage drop. Because thinner wires offer more resistance than heavier-gauge wires, the solution for a longer run is to increase the wire thickness. Most electricians allow for a maximum 2 percent voltage drop along the wire and for a 14-gauge wire with a 15-amp breaker that should not run more than 75 feet. If you install a 12-gauge wire and a 20-amp breaker or a 10-gauge wire with a 30-amp breaker, the maximum distance the wire should run is 85 feet. If you need to go farther, you can install a 20-amp breaker on 10-gauge wire and travel 130 feet or you can put a 30-amp breaker on 8-gauge wire and cover 150 feet.
In our sample greenhouse circuit diagram, the total amperage is the total of all the breaker sizes or 45 amps. Thus, for this greenhouse, the cable should be able to carry at least that amount of load over a distance of 30 feet from the house to the greenhouse. Remember, though, that it’s unlikely that all the lights and heaters and outlets in the greenhouse will be in use at the same time. Also, the greenhouse represented here has a great deal of equipment in it; yours may not.
It’s important, too, that the breaker in the main house panel be at least 50 amps, large enough to support the entire load plus any surge loads. (Surge loads often occur when something is turned on from a dead stop. For example, an electrical motor may use 10 amps to run continuously, but when the motor starts from a dead stop, it might absorb 15 or 20 amps until it is up to speed.) Typically, a large greenhouse requires a 30- to 40-amp service, while most smaller, hobby greenhouses may require only a 20-amp service. Once the cable is wired to the breaker panels at both the greenhouse and the house ends and the circuits have been tested, the wiring can be turned on. With the breakers on this time, the electrician will turn on each circuit to make sure it works.
In most greenhouses, the plumbing consists of a water line running to a spigot in the greenhouse, but if you are a little more ambitious, perhaps you’ll want to install a sink and a drain.
If your garden is in an area that’s subject to frost and the greenhouse is unheated, the water line will have to be drained each fall or installed well below the frost line. Of course, the drain line will also need to be below the frost line, but where should it drain? Running it into your leach field is usually not a good idea because water that drains may contain a pesticide or herbicide spray. The best option is a dry well (see The Dry Well).
The first step in installing a water system is determining where the water line will enter the greenhouse, where the outlets will be, where the drain will be located, and where the dry well will be and then mapping these out much like a wiring diagram. As pointed out in chapter 8, The Foundation, if you are designing your greenhouse to sit on a slab, you need to think about these features before you pour the slab. It’s much easier to install water and drain lines before the concrete pour.
The following illustration shows possible locations for a water line and drain line in a greenhouse. Incorporated in this sample system are a simple sink, spigot, and a line for sprinklers or a drip irrigation system. The sink can be used for cleaning tools and rinsing vegetables before taking them indoors and the spigot can be used for filling watering cans and attaching the garden hose. If we look at this layout in conjunction with the wiring layout in Deciding Where to Locate Lights, we can see where potential conflicts between water and electricity outlets may occur. Such a comparison enables you to ensure that electrical lines are located where they cannot come in contact with water. In this case, the electrical outlet that powers the overhead light is situated high above the sink.
In some greenhouses, the spigot, sink, and sprinkler or misting systems all have their own outlets, but in this installation, there is a line by the door and a two- or three-way outlet attached to the spigot to allow a hose, sprinkler, drip irrigation line, or mister to be attached.
A single line attached to the sink drain near the spigot is also included on the diagram. This line empties into a dry well.
The water line comes from the house and is typically a 1-inch copper or plastic pipe (depending on what is required by the local zoning code), but if the distance is reasonably short and local code allows it, you can use 10-foot lengths of schedule 40 PVC pipe of the same diameter. When using short lengths of pipe, glue each joint carefully to ensure that it will not leak: Coat the end of pipe with pipe cement and push it into the connector until it is fully seated. Note that some PVC piping has been linked to the leaching of chemicals into the water line, so buy with care. Once you determine the type of pipe specified by local code, consult a plumber for the best option.
If you garden in a part of the country where the ground freezes and your greenhouse will be unheated, the water line must be located below the frost line. This means that you must dig a trench and bury the line 3 or 4 feet deep. When you backfill the trench, cover the pipe with sand or screened loam to ensure that it won’t be damaged by stones in the fill. Some experts recommend that you install PVC pipe inside heavier conduit such as schedule 40 pipe to ensure that it will not be crushed (see chapter 6), but this can nearly double the cost of the installation. The alternative to burying the line deep is to put a drain at the house end of the line and make sure that the line slopes from its uphill greenhouse end to this lower drain end so you can empty it completely each fall.
Whatever installation method you choose, the water line must have a valve at both ends so that, should it burst, you can isolate the line from the house. Because you may be spraying fertilizer or pesticides with a hose-end sprayer, it’s also important to install a nonreturn valve at the greenhouse end of the line to prevent these materials from being drawn back into the house plumbing.
Devising a plumbing layout helps you to see where everything will be located.
As indicated above, a water line to the greenhouse should have a valve at both ends. First, you must cut the line square with a pipe cutter: Clamp the tool onto the pipe, rotate it, tighten the cutting screw, and rotate the cutter again around the pipe. If this doesn’t cut the pipe, tighten the cutter blade a little more and rotate it until you cut through the material.
Before you solder or glue (depending on whether you’re using copper or PVC pipe), work steadily through the plumbing system and dryfit everything. Dry fitting first enables you to be sure that the entire system will fit together and that all the pieces are the right size.
How to cut a pipe using a pipe cutter
To connect copper lines, you’ll need a propane torch and a roll or stick of good-quality, rosin-cored solder. First, rub the end of the copper pipe with 320- or 400-grit sandpaper, emery cloth, or pipe sander about 2 inches in from the cut end until the copper is shiny. Then, heat the end of the copper pipe with the propane torch and apply solder to the outside of the pipe. Don’t hold the solder in the flame and let it drip on the pipe. Instead, push the solder directly onto the hot pipe; you’ll get a good joint only if the solder adheres to the pipe. Apply it all around the pipe and with an old rag, wipe the solder on the end of the pipe so that the first inch or two of the pipe is fully coated.
Next, take the other side of the connection (which might have a T, L, straight, or flared end) and wipe the inside of the end with sandpaper or emery cloth until it is shiny. Coat this area with solder as you did with the first pipe and then push together both pipes and heat them with the torch as you apply solder to the joint until the material runs out of it.
This method will produce a solid joint. Be extra careful if you are installing pipe near the wooden frame of your greenhouse. The propane torch can burn the greenhouse frame just as easily as it melts solder. Plumbers who do this work all the time often use metal or asbestos blankets to cover any wood in the immediate working area.
How to connect copper piping: Rub the end of the copper pipe with sandpaper, emery cloth, or a pipe sander (A). After heating the pipe, apply solder so it’s drawn between the joined pipes (B).
First, note that there may be code restrictions on using PVC pipe. Check your local code before installing any piping. If you are using PVC water pipe, you’ll need to clamp the pipe to its fittings with hose clamps. Some faucets are made for plastic pipes that have smooth ends; other fittings have a screw thread and require clamping a connector to the fitting before it can be inserted into a threaded hose pipe. Its best to use stainless-steel hose clamps and to use two clamps on each joint so that if one comes loose, the line won’t leak.
To connect PVC pipe, you’ll need PVC pipe cement (a small can costs about $5). The mixture comes with a small brush in the can lid. Simply wipe the end of the hose line with a dry rag, paint the surface with the glue using the brush, and push the pipe into the T-, straight, or elbow joint. Let the connection set before you move the joint.
Connecting PVC pipe using clamps and a T-joint
The best type of greenhouse sink is a deep, plastic, tub style that easily allows you to wash tools and gear — the kind of utility sink that many people install in basements. Stationary tubs are reasonably inexpensive and usually come with legs. Most have a single faucet with two on/off handles (one for hot water, one for cold), but you will be using only one of them. Installing a stationary tub is straightforward, involving fitting its legs, plumbing the faucet on the rear of the fixture, and connecting it to the main plumbing system.
To install the sink, first set it in position and connect the U-shaped drainpipe by bolting one end under the drain hole and the other to the outlet pipe.
Next, connect the faucet. While the method differs depending on the style of faucet you have, usually it requires that you coat the cold-water pipe with pipe compound, push the pipe into the existing socket of the faucet, and tighten the nut (which comes attached to the pipe) to hold everything in place. Repeat this procedure for the hot-water side and the job is finished.
Drain lines are usually made of 2-, 3- or 4-inch schedule 40 PVC pipe that’s joined as you would join the pipe for any line (see Connecting PVC Pipe). If a flush-mounted shower drain is to be set into the floor of the greenhouse, you should install rough plumbing with the drain at the correct height before the concrete greenhouse floor is poured (see Setting the Drainpipe). The top of the drainpipe should be cut to the right level for the flush-mounted floor drain, and when the floor is smoothed, the slab should be sloped toward the drain so that any water on the floor will drain easily. You may find it simpler to install the drain grate before the pour and then tape the top of it to prevent concrete from sealing it during the pour. If you choose not to install the grate before pouring concrete, make sure that you push this cover over the pipe while the concrete is still wet to ensure that the drain grate fits flush with the floor.
Rather than run the greenhouse drain into your septic system, where any pesticides you use may be harmful, run it into a dry well. A properly installed drywell can cost about as much as a greenhouse. It is basically an open-bottomed concrete tube or container filled with gravel and a sediment trap or filter. Water drains into it for dispersal. Before construction, check with the local zoning office to see what is allowed in your area. In some municipalities, you may have to obtain a permit or meet other requirements in order to install a dry well.
These systems are often used as laundry drains to ensure that the soapy water from the laundry does not enter a septic system. The problem with a laundry dry well is that it eventually becomes clogged with lint, and the problem with a greenhouse dry well is that it can eventually become clogged with plant residue or potting soil. To cut down on cleaning the dry well, install a sediment trap in the drain line and clean it regularly. You can also install a cover on top of the trap or on the well.
Installing a dry well requires digging a large pit and burying an open-bottomed tank or container in it. The container is then filled partially with gravel and the greenhouse drain line is run first into the sediment trap and then into the middle of the dry well. It is a very simple and effective system.
A dry well system
See chapter 5 for information on types of heating systems that can be used in a greenhouse and how to choose which one is best for your needs. Sources of greenhouse heat are gas, wood, and electric heaters. One location for a heater in the greenhouse is under a bench so that it can heat the cold air that is lower down in the structure, which then rises and circulates.
Gas heaters are usually stand-alone devices that are fed from a propane or natural gas source outside the greenhouse. A gas heater and the necessary gas line should be professionally installed and the line should be tested carefully for leaks before it is used.
Before deciding on gas heat, it’s important to check with the local building inspector to determine whether you need a permit to install such a heater in your greenhouse. Some municipalities require that all heating systems have a permit and that every system be put in by a licensed installer. Others may have special requirements about storing propane or natural gas. For example, you may not be allowed to stand a propane tank outside the greenhouse near the glazing, but you might be allowed to locate it against a solid house wall.
For a number of years in my warm greenhouse I used a Southern Burner propane heater — an open-flame, under-bench heater with its own pilot light and thermostat. It sends a great deal of hot air up toward the glass, where the air becomes cooler through its contact with the glazing and its movement throughout the greenhouse. When the air has cooled and sinks to the ground again, it gets sucked back toward the heater, setting up a circulation that is beneficial for plants.
Note that if you decide to install a propane heater, you must provide an air intake to the heater to prevent it from consuming all the oxygen inside the greenhouse, which could cause plants to die and jeopardize the health and well-being of you and others who work in the structure. To accomplish this, you (or a professional) can install a 2-inch schedule 40 PVC pipe in the wall to allow external air to be sucked into the greenhouse. The pipe can be cut to size and glued, if necessary (see Connecting PVC Pipe), and you can fit the exterior with wire mesh to keep rodents or pests out of the greenhouse.
After a professional installs the heater, you can install (or have installed) the thermostat, which is connected to the regulator block.
Before turning on the heating unit, it should be tested for leaks. The installer may accomplish this by brushing soapy water onto every joint in the copper pipe and then turning on the propane tanks. Any bubbles in the soapy water indicate a leak. If you or the installer see any bubbles, shut off the gas and take apart and remake the joint. When all the joints are bubble-free, the installer will light the pilot light and turn up the thermostat full blast to make sure that the heater engages. After the heater lights, you can turn down the thermostat to the desired temperature.
If you have a large greenhouse, a wood-burning stove is a reasonable option, especially if you have access to a woodlot. Such a stove does require a lot of maintenance, however, and you must fill it regularly to ensure that it keeps going. On a cold winter night, this can be a real chore.
Before any woodstove installation in the greenhouse, check with the local building inspector to see what’s allowed and how far the stove must be from other combustible materials. If you don’t have a building code for combustible surfaces, to prevent fire, assume that there should be no surfaces or plants within 24 to 30 inches of the stove. This distance is even greater for some of the more stringent fire and building codes. You’ll also need to determine how the chimney will exit your greenhouse and what the required setbacks are for combustible materials around the chimney.
To install a wood-burning stove, first locate it on the floor of the greenhouse. Ideally the floor should be concrete; if it’s not, you’ll need to install an insulated slab on which the stove will sit.
Next, install the stovepipe, which will extend through the greenhouse roof. Attempting to operate a woodstove without a stovepipe inside your greenhouse is extremely dangerous. Smoke will have no way of escaping the structure. Most pipes are 6- or 7-inch steel, fireproof tubing that radiates heat as the smoke passes through it. To install it, simply push the crimped end of one section inside the smooth end of another and fasten the pieces together with a couple of sheet-metal screws. Secure the pipe with strapping or brackets so that it’s at least 24 inches from any wooden surfaces. You can use perforated metal water-pipe strapping to hold the chimney if nothing else is available.
To cut down on heat loss during the winter, consider covering the glazing of your greenhouse with a layer of heat-shrink film, a transparent film that comes in various sizes in kits that include double-sided installation tape. Kits are available in most hardware stores for a few dollars.
To install the film, first press the double-sided tape around the window. Then cut the film to size and apply it to the tape. Finally, use a hair dryer to shrink the film so that it makes a tight, clear “window” inside the greenhouse glazing.
You can also install polyethylene sheeting over the inside of the windows, holding the plastic in place by nailing battens to the interior framing of the structure; or you can cover the entire outside of the greenhouse with polyethylene sheeting, holding it in place with battens nailed to the exterior framing; or install bubble wrap insulation over the inside greenhouse glazing, holding it in place with battens or tape.
If you own a conservatory or attached greenhouse, you may find that heat loss from your home through the glazing during a winter night is quite high. Rather than have unattractive polyethylene or bubble wrap stuck to the walls, you might prefer to add movable insulated roman shades or cellular shades (see Resources). Both are installed on tracks that are placed on either side (or at the top) of the window and completely block off the glass. Once quilted insulated window shades are pulled down, the insulation value of the window can approach R-11.