Preparing for Installation of Hyrdroponics System

After doing some reading, and speaking with some experts at internet sites that sell hydroponics equipment, I settled on a type of ebb and flow system called Multi Flow. Hydroponics is defined as growing plants without soil. In the Multi Flow system, plants are grown in a series of pots filled with Hydroton media (a pebble-sized growing medium made from clay). The plant pots are connected by flexible tubes to a larger control pot. This allows nutrient solution to flow from the control pot to the plant pots and back again. Several times a day, nutrient solution is pumped from a large 55-gallon reservoir into the control pot. The solution then flows by gravity into the pots containing the plants. The plants’ roots are allowed to be soaked for 15 minutes at a time; and then the solution is pumped out again from the control pot to the reservoir. The solution in the plant pots flows back into the control pot as the control pot is evacuated to the reservoir, in order to equalize pressure throughout the system. A timer controls how many such ebb and flow cycles occur daily.

All the nutrient solution in the plant pots and the solution in the control pot must be part of a single body of water, so that the levels of solution in the pots rise and fall together as a single fluid body. Since gravity is used as a means of distributing the nutrient solution to the pots, the platform supporting all the pots and the reservoir must be level, to ensure that all the plant pots have equal levels of solution at all times.

To set up a level platform, I constructed a frame from 2"x4" pressure-treated framing lumber, shown in the accompanying photo. The frame is 152" long and 42" wide. This was designed to just fit into the greenhouse through the rear window and be able to be set down on the floor while still clearing the heater that is mounted on the rear wall. (There isn’t a clear path into the greenhouse from the front, because the front door is too close to the adjacent tractor shed.) A square recessed area is built into one end of the frame to allow the reservoir and control pot to rest approximately 1" below the level of the platform upon which the plant pots are to be placed. This difference in height is designed to allow the nutrient solution in the plant pots to be completely evacuated, while about an inch of solution remains in the control pot. The control pot cannot be completely evacuated, but we do want the plant pots to be completely evacuated, since we don’t want the lowest portion of the roots to be permanently submerged.

To ensure that the frame would be as level as I could make it, I put down six foundation stones on the dirt floor of the greenhouse. These were 8"-square pavers with very flat surfaces that I purchased at Home Depot. I set them into the ground after carving out spaces in the hardened soil of the greenhouse floor, adjusting the height of the stones so that metal rods stretched from one stone to the other were all on the same level. I then filled in the spaces surrounding each paver with the small pebbles (i.e., pea gravel) that will be used as the greenhouse floor (i.e., the portion not covered by the frame). This was done to stabilize the pavers and discourage later changes in height of the platform due to settling. The second accompanying photo shows the frame placed on top of the pavers.
The remaining step was to cover the frame with a type of 3/4" plywood that is normally used as sub-flooring. I used Thompson’s WaterSeal brand Waterproof PLUS Clear Wood Protector on both sides of the plywood and on the edges, to prevent the plywood from becoming a breeding ground for mildew. I didn’t use this on the pressure-treated 2"x4"s of the frame.

Each of the plant pots measures 9" in diameter, while the control pot is 16" in diameter and the reservoir has a 24" diameter. I purchased a 30-pot system. Using a scale diagram, I figured out that I could just fit the 30 plant pots on the platform. Experience should tell me where there might be overcrowding, in which case I might not use all the plant pots. My plan is to have three rows on the platform. The row in the rear, closest to the side of the greenhouse, would be used for the vines (tomatoes, cucumbers, melons, pole beans, etc.). These will be allowed to grow upwards along a nylon trellis made for the purpose. The middle row would be used for medium sized plants that would not seriously overshadow the vines (sweet peppers, zucchini). The inner row, running down the center of the greenhouse, would be used for the shorter plants (lettuce, strawberries, bush bean). This arrangement would ensure that all the plants in the Multi Flow pots would have some exposure to both natural sunlight and the HID lighting that will be mounted and hung from the middle of the roof.

I also plan to grow some plants on the other side of the greenhouse, the side closer to the sun. These would be short plants, such as bush beans, lettuce, strawberries, radishes, etc. They will be grown conventionally in soil using special containers called the Grow Box by Garden Patch. This system uses soil, but has a built-in reservoir that keeps the plants watered from the bottom, as long as the reservoir is refilled occasionally.

I also need to save water from the dehumidifier for use in reconstituting the hydroponics nutrient mixture. (I understand that this has to be done every two or three weeks.) Rain- water or distilled water is required for use in the hydroponics reservoir in order to be able to control the pH of the nutrient solution. My well-water might be too "hard" to use for the nutrient solution. I don’t want to have to buy distilled water for the hydroponics, so I decided to collect and make use of the condensed water from the dehumidifier. I’ve collected some condensate water from the dehumidifier to make up some of the water needed for the nutrient solution, but it’s not clear if I will be able to collect enough every two or three weeks to replenish the nutrient solution on a continuing basis. It would also be nice if I were able to use the same water source to automatically replenish the reservoirs in the Garden Patch containers, but that also will require some experimentation.

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.

Determining Heating Requirements for the Greenhouse

Successfully heating the greenhouse means that you ensure the temperature inside the greenhouse does not fall below a specified level, despite the vagaries of winter weather. To achieve this, one has to calculate the expected heat loss (e.g., in BTU/hr) for a worst-case outdoor temperature, and then provide a heater than can keep adding heat to counteract this maximum heat loss on a continuous basis for as long as that worst-case temperature and/or other conditions (e.g., unusually high winds) persist.

Gas heaters are usually rated in BTU/hr, but electric heaters are generally rated in watts. Both ratings are units of “power,” or amount of energy used per unit of time. The standard conversion factor is: 3.41214 BTU/hr = 1 watt. One has to be careful to distinguish between heater ratings that specify the amount of energy used (i.e., input) versus the amount of heat created for use in the greenhouse (i.e., output). For electric resistance heaters, these amounts are very close, but for gas heaters, they can vary considerably, because some of the heat generated may be going up the chimney, if there is one.

To calculate the worst-case heat loss, I assumed that the temperature in my area would not fall below negative 10 degrees Fahrenheit (i.e., 10 degrees below zero). That is the worst-case outside temperature used by the U.S. Department of Agriculture for specifying plant hardiness zones. I am in Zone 6a (in Mercersburg, PA).

The heat loss depends on the difference between this worst-case temperature and the temperature desired to be maintained - in this case it is a temperature difference of 70 F degrees.

The heat loss also depends on the prevailing wind conditions, since heat exchange is more efficient at a barrier where the escaping heat is constantly removed; the construction of the barrier between the plants and the outside environment (e.g., the polycarbonate glazing); and details of the way the heat is provided (e.g., radiant vs convection heating). I used a formula developed by the Canadian Government, as discussed in detail in a separate document. The results of my calculations are just summarized here. To keep the inside temperature at 60 degrees F, my heater must continually replace heat energy lost at the rate of 3800 watts.

If we would assume an electric heater using mostly convection operates at 100% efficiency (which I understand is pretty much the case), this is the size of electric resistance heater we would need. (If a radiant, or infrared, heater, rather than a convection heater, were used, a different heating system factor would be used and the result might be slightly different. Convection heaters heat and circulate the air, while radiant heaters heat the objects in the greenhouse, but not the intervening air.) My calculation of heat loss includes an additional 20% as a safety margin, to account for possible errors, mis-approximations, etc., as well as the possibility that the temperature might actually drop somewhat below -10 degrees F.

If the greenhouse is to be heated with liquid propane (LP) gas, this wattage amount must be converted to BTU/hr, which gives us a heat loss value equal to approximately 13,000 BTU/hr heat output. If we assume our LP gas heater operates at only 70% efficiency, the heater input rating must be at least 18,600 BTU/hr.

Finally, Some Significant Progress

A lot has happened, and I’m sorry to say that I haven’t been keeping this blog updated accordingly. I was busy this summer learning golf, tending my outdoor vegetable garden, and training our white German Shepherd puppy, Ziva. Progress on my greenhouse was stalled for a number of reasons, one of them being lack of a decision on how to heat the greenhouse. Another was the questionable wisdom of investing a lot of money in the greenhouse when our funds were limited. And then, there was the fact that I didn’t really need the greenhouse to be functioning until the summer growing season was over.

Now, a lot of questions have been resolved, and much progress has been made.

I finally decided to heat the greenhouse with liquid propane (LP) gas. During the summer, we had the LP tank that is buried in our lawn refilled. We learned that we had not used very much gas over the period of about a year, through use of our gas fireplace and gas grill. We also would use LP gas to run our emergency generator, but we had no such emergencies since we moved into our new house more than a year ago. The refill amounted to only 154 gallons, roughly half of the tank’s capacity. Since we are obligated to pay for at least one full tank refill each year, as a condition of our “free” lease of the tank, we would have to pay for more than 150 gallons of gas each year that we would not use. Therefore, approximately, the first 150 gallons of LP gas that we would use to heat the greenhouse each year would be “free,” since we would have to pay for that gas anyway. This, added to the consideration that heating with LP gas would be more efficient than using electric power for heating the greenhouse, confirmed the decision to heat with LP gas.

The financial questions apparently have been resolved with the likelihood that I will soon begin part-time work with the agency I retired from in January 2007. The additional income will mean that I don’t have to pinch pennies getting my greenhouse set up properly. In anticipation of the added income, I went ahead and had an electrical contractor run electric lines to the greenhouse and neighboring shed, and had the LP gas contractor/supplier run a gas line to the greenhouse. The lines were initially just stubs, but will be hooked up to the proper equipment and sockets once the greenhouse construction is completed.

I also purchased a gas heater that will be hooked up by the gas contractor when the greenhouse construction is completed.

Another decision I made was to purchase tomato and cucumber seeds for use in the greenhouse this winter.

I’ll provide separate entries to discuss the details of the electric, gas, and vegetable seed issues, as well as my efforts (an those of others) to erect the greenhouse.

Progress, of Sorts

With just two days before the "crew" was to arrive to help me erect the greenhouse, I called it off. Too many things were not right. First, I had found out that the foundation I had prepared would not work. I had two days to fix the foundation, but the first of those days, I really didn't feel well and couldn't bring myself to work in the hot sun. I started, but then quickly began to feel really tired.

Then, I got to thinking about some other things that have gone wrong. The provider of my LP gas came back with a $1,300 proposal for running a gas line underground from my house to the greenhouse and modifying the gas pressure regulators to accommodate the additional demand. That didn't include the additional costs of purchasing an LP heater and having it professionally installed. There was still the issue of how to vent the heater - possibly having to replace one of the panes with sheet metal. At this point, for various reasons, I don't have the cash to purchase and install an LP heater.

The alternative is to heat the greenhouse with an electric resistance heater. To do that, I would prefer to run an underground electric line to the greenhouse. I can do that myself possibly, but I need a little more time to study the local code requirements and do the groundwork. It will be easier to run the line, a distance I estimate to be less than 30 feet, while the greenhouse foundation is still easy to move around. I learned that I can rent a power trenching machine for about $80 for 4 hours. The rental agent I spoke with said I should be able to finish the job in 4 hours. Together with the costs of electrical components, and purchase of an electric heater and the PVC conduit, the total cost should be much less than $1,300. That is, if I do the electrical work myself. I am also going to inquire about putting the outside circuit in my emergency generator system, so the greenhouse will continue to be heated in case of a winter power outage.

I originally preferred to heat the greenhouse with LP gas, but it just turned out to be too complicated to install a gas heater with proper venting. With electric heating, the energy costs might be greater, but the initial costs much less. It's something like the tradeoff I had to make when I bought my Toyota Highlander. If I had waited several months longer, I might have been able to purchase a hybrid version of the Highlander. I saw this as desirable from an environmental perspective, but when I learned that it would most likely cost me about $7,000 more than the comparable gasoline engine model, I thought I would have to save a lot of money on gas to make up this difference in initial cost.

The final reason for postponing the greenhouse raising was the local weather forecast. There was a 40% chance of thunderstorms. If the thunderstorms materialized with sufficiently high winds, it could prove to be a disaster. An incompletely constructed greenhouse could take off like a kite in a good wind.