Electrical Service Design and Installation

During the process of designing our new house, I had made arrangements for an outside 50-amp 240-volt line to be made available to my shed and greenhouse area. The builder had ensured this capability by running an empty conduit from an electrical box just outside our house to a location just outside the concrete slab that later became the floor of our shed. Once a decision would be made to install the wiring in the shed and greenhouse, the needed wires would be pulled through the conduit using a vacuum device.

The first step in electrifying the greenhouse was to wire the shed, including a sub-panel that could later be used as a base for extension to the greenhouse, which is adjacent to the shed. Then, conduit was put underground, between the shed and the greenhouse to allow for extending the 240-volt service to the greenhouse.

The design for the electrical service in the greenhouse required knowledge of the different types of equipment I anticipated I would be utilizing in the greenhouse and which ones would be in service at the same time. To do this, I had to do some research regarding the hydroponics and grow lighting systems I would be installing. I will describe the hydroponics system and the grow lighting system separately, but at this point, I have simply listed the electrical requirements for the greenhouse in this table.

The result of my research indicated that I would need two 240-volt receptacles and about ten 120-volt receptacles. The contractor suggested that it would be easier to put in twelve 120-volt receptacles, since they come in gangs of four. However, a strange thing happened while I was away playing tennis. The contractor mistakenly installed 24 120-volt receptacles instead of the agreed upon twelve, as can be seen in the accompanying photo. Each bank of 12 120-volt receptacles is protected by a 20-amp circuit breaker. There are two more 20-amp 240-volt circuit breakers, one to control the receptacle for the grow light system, the other to control the receptacle for the backup electric resistance heater. This adds up to 80-amps, which is in excess of the total amperage available to the circuit serving both the greenhouse and the shed. More about this anomaly below.

All of these loads are protected by connection to my whole-house emergency generator. In the event of a power outage, the generator would automatically start up to keep the lighting and other loads in the greenhouse going. The LP gas heater would be unaffected by a power outage, except for its blower, which is non-essential. But, if the gas heater was unavailable for some reason (e.g., out of fuel, clogged fuel line, faulty thermostat, etc.), continued operation of the electric resistance heater would be essential, so putting it on a line supported by the generator is a good idea. In an emergency, use of the grow lights would not be essential. Therefore, in an emergency, the grow light system could be turned off to allow the electric heater to operate. That kind of tradeoff might have been necessary if I didn’t have as large a line as 50-amp.

Regardless of the number of receptacles provided, there is a limit to how much current can be run through them all at the same time. A minimal amount of 120-volt load is taken up by the lighting in the shed, maybe two or three amps. The total for all the anticipated loads in the greenhouse is just short of 40 amps. (Using a 240-volt heater and 240-volt ballasts for the grow light system halves the amount of current that would have been required if 120-volt versions of this equipment was used instead.) There is just enough capacity to support the intended load. However, if one were to plug in a vacuum cleaner or other appliance with a sizeable motor, it is conceivable that the 50-amp circuit breaker for the entire line could be tripped. Therefore, I will have to be vigilant not to exceed the design load just because there are enough receptacles to plug in more appliances.

One more wrinkle to deal with is the question of what kind of 240-volt receptacles need to be put in place for the electric heater and the grow light system. My research indicated that the electrical resistance heater requires a different style of receptacle than the grow light system. Our strategy is to wait until the equipment is delivered to make sure the correct styles of receptacle are installed.

LP Gas Heater Installation

I had the Empire DV-25-SG LP gas heater professionally installed by the same company that provides my propane. The installation of the heater needed to be preceded by the installation of a gas line to the greenhouse. We decided that this could be accomplished most simply by extending the LP gas line that already went to my rear deck, about 25 feet away from the greenhouse.

Code required that the line be buried underground, about 18 inches, I believe, for the distance covered from the deck to the greenhouse. Once inside the greenhouse, the line could be attached securely to the foundation frame above ground level.

The contractor also had determined that the pressure in each of the regulators for my other gas appliances (i.e., BBQ grill, gas fireplace, and emergency generator) needed to be increased to allow for an adequate flow of gas to all appliances, including the new heater.

The heater comes with a template to determine the location where the penetration needs to be made in the greenhouse’s glazing. I had the heater mounted on the rear wall, just below the rear window. The heater was attached to two 2'x4' pieces of wood that then were attached to the aluminum frame of the greenhouse. The 7" exhaust/intake pipe went through a hole the contractors made in the lower rear center panel. No insulation was necessary, because the exhaust travels through the center of the exhaust/intake pipe. I had purchased and installed the optional vent cap extension, so the exhaust is further separated from the polycarbonate before being mixed with the outside air. This protects the polycarbonate panel from being adversely affected by the exhaust heat before the exhaust cools in contact with the outside air. The spaces between the exhaust/intake pipe and the polycarbonate panel were filled with a silicone sealer.

The contractor demonstrated that the installation was safe by having me touch the back of the heater (closest to the polycarbonate panel), which I observed was not hot, even though the heater was in steady-state operation).

The heater comes with a “millivolt” thermostat. This provides for control of the heater to maintain a minimum “indoor” temperature, without the need for AC power or a battery (that could run out). The electric current used to control the heater is generated by a thermoelectric effect that is utilizes heat from the pilot light. We strung the thermostat wire from the base of the heater to a location midway down the length of the greenhouse, because we didn’t want the thermostat to be unduly influenced by the hot air directly given off by the heater, nor be unduly affected by cold air inflow that might occur near the front door. I may experiment with the placement of the thermostat some more before I finally settle on a permanent location.

LP Gas Heater Selection

For reasons explained previously, I decided that I wanted to heat my greenhouse with LP gas, rather than with an electric resistance heater.

Based on the previous greenhouse heating requirement analysis, I concluded that I needed an LP gas heater with an input rating of at least 18,600 BTU/hr. (A Btu, or British Thermal Unit, is the amount of energy required to raise one pound of water one degree Fahrenheit.) I also had decided that I wanted a vented heater. A vented heater requires a penetration through the greenhouse wall or roof to exhaust products of combustion, and another, that is usually smaller, to let in fresh air.

Some of the LP gas heaters available for small (hobby) greenhouses are non-vented heaters. They use the air in the greenhouse for combustion and exhaust carbon dioxide (CO2) and water vapor as products of combustion into the plants’ environment. Now, CO2 and water vapor are essential for photosynthesis in green plants, so having some excess of CO2 in the greenhouse environment is desirable. However, if these compounds are produced in direct relation to the amount of heating necessary, rather than to the amounts needed for photosynthesis, there is a good chance that the CO2 may become too abundant and become toxic to the plants. Also, too much water vapor can promote the proliferation of undesirable fungi (mold, mildew). Although some enrichment of CO2 may be desirable for plant growth, most of the CO2 that is produced by a non-vented heater accumulates during colder non-light conditions when photosynthesis is not occurring.

Another problem with vented heaters is that commercially available propane is likely to have some impurities and combustion is not always complete. As oxygen is depleted in a closed system due to combustion, the heater may eventually become starved for sufficient oxygen. Under ideal conditions, for example, the combustion products should be just CO2 and water, but when sufficient oxygen is no longer available, carbon monoxide may form instead of CO2. An academic publication I read warned, “Never allow gasses to remain inside the greenhouse, as tomato plants are very sensitive to certain pollutants found in fossil fuel exhaust. Especially with kerosene and propane space heaters, the potential exists to poison plants with toxic pollutants.” I tried unsuccessfully to find further information about what those pollutants might be. I guess I might have to ask the authors of the publication about that. Nonetheless, I made up my mind that I would want to have a vented gas heater.

After doing some internet shopping, I narrowed my choice to a direct-vented heater. This vendor’s web page explains how it works. There are many reasons for choosing this type of heater. First, it combines the air input pipe and combustion exhaust pipe into a single double-walled pipe so that only one penetration is required through the polycarbonate wall of the greenhouse. As cold air is brought in to support combustion, it is heated by the hot exhaust gases flowing out, reducing energy loss and increasing overall efficiency. Also, the exhaust gases flow through the inner section of pipe, with the incoming cold air in an annular region surrounding the exhaust pipe. This reduces the need for insulation as the exhaust pipe penetrates the polycarbonate wall. If a simple exhaust pipe were to be used to penetrate the polycarbonate glazing, there would have to be special insulation provided to keep the polycarbonate from melting. The direct venting is through a horizontal pipe, penetrating a wall, so that a chimney is not required, with a penetration through the ceiling (which could threaten a leak).

Another desirable feature of direct vented heaters is its sealed combustion chamber. Only the air pulled in through the vent is used for combustion, so oxygen in the greenhouse is not depleted; and no exhaust gasses are emitted into the greenhouse interior, so there is no uncontrolled buildup of carbon dioxide, water vapor, or other products of combustion (due to the presence of impurities in the supplied gas). (This separation of the combustion chamber from the heated environment is similar to the way nuclear reactors are designed. In a reactor, circulating reactor coolant (e.g., water or steam), that contains radioactive impurities, absorbs heat energy from the uranium fuel and then gives it up in a heat exchanger. In a secondary circuit, coolant receives heat energy from the heat exchanger and then flows through the turbines to produce power. This limits the extent to which the turbines become contaminated with radioactive impurities.)

This separation of the combustion chamber from the heated environment will permit easier conversion of my greenhouse heating system to nuclear power at some point in the future - NO - just kidding!

In the case of a conventionally vented greenhouse heater, I would have had to have two penetrations - one to permit combustion gasses to escape, and another to let in fresh air. The Riga greenhouse is said to be less expensive to heat, because it has tighter construction, with less air seeping through cracks between the glazing and the frame, etc. It would have been silly to thwart this advantage by purposely making a 2.5-inch hole in the side of the greenhouse to permanently permit cold air to come in all the time. If such a penetration was not made, and no window or door was left sufficiently ajar, a conventionally vented heater would soon be starved for oxygen and would shut down. I’m surprised that there is no advice printed about the Riga greenhouse saying that non-vented gas heaters or conventionally vented gas heaters (with separate air intakes and exhausts) are not recommended.

When I started looking for suitable direct-vented LP gas heaters for my particular greenhouse, I found that the choices were very limited. I found only two models, both manufactured by Empire Heating Systems. I purchased the DV-25-SG model for under $600, but a fancier Empire model DV-20E was available at more than $1,000. The main difference between these models appeared to be that the more expensive heater featured power venting. I chose the lower-priced heater because of the price and the fact that the lower-priced unit did not require electric power for its operation. (The DV-25-SG utilizes “millivolt” controls that required no battery or AC power, but ran on thermoelectric power generated from the pilot light.) Also, the output capacity of the more expensive DV-20E unit, at 16,300BTU/hr, was actually less than the DV-25-SG model, which was rated at 17,500 BTU/hr output. The power venting, plus whatever other refinements the more expensive model has, results in a claimed 81.5% efficiency, while the DV-25-SG has 70% efficiency, but I was not sure that the efficiency rating took into account the electrical energy used to power the venting function.

Although the DV-25-SG unit did not have power venting, I did purchase the optional blower unit for it, to better circulate the heated air throughout the greenhouse. It was clear in the specifications that the heater would continue to operate (burning gas) even if power was lost to the blower unit. I would think that use of the blower during heating operation would increase the efficiency of the unit. This would be due to the fact that the thermostat would more readily respond to the output of the heater, if the temperature of the air throughout the greenhouse were more uniform. Apparently, the blower unit does not operate while the heater is off, so it cannot serve a dual function of ventilation when heating is not required.

I purchased my DV-25-SG unit from ACF Greenhouses. This vendor supported a web page that contained some good explanations of different features important to any decision to purchase a greenhouse heater. This page also includes a link to the vendor’s Heater BTU Calculator. I found it strange that I could find no other greenhouse-oriented vendors on the web that offered Empire heaters, and there were really no other direct-vented gas heaters available for small greenhouses requiring less than 20,000 BTU/hr output. I did find numerous other sources for Empire direct-vented heaters, but they were all significantly more expensive for the same models. These vendors focused primarily on heating for homes.