On the Fence

I'm now undecided about whether to install a direct-vented heater or a non-vented heater, based on new, but still incomplete, information I recently obtained. I must caution casual readers that the following represents my limited understanding of how greenhouse heating systems work, and some of what is described here may be incorrect. I'm still trying to sort this out.

My reasons for choosing a direct-vented heater were that: (1) a non-vented heater might release combustion products, other than carbon dioxide (CO2) - such as benzene (a carcinogen) - into the greenhouse atmosphere that could harm the plants and people; and (2) CO2 could build up to levels that would trigger the non-vented heater's human protection system to turn off the heater when it was needed most, thus causing catastrophic failure if the temperature fell too low.

My resolve to install a direct-vented heater was shaken when my contact at BackyardHQ (parent company of 4SeasonGreenhouse.com) drew my attention to some installation information about a heater made by Southern Burner Comany. I went to the Southern Burner web site (http://www.southernburner.com/index.htm) and read about the installation requirements for both their vented and non-vented heaters. The Southern Burner vented heater, I concluded, is not a direct-vented heater. Their web site says: "The Model A-1 Unit Heater is an enclosed flame heater that requires venting to the outside of the greenhouse. It also requires a fresh or combustible air intake from the outside of the greenhouse." The venting of the products of combustion is required to be accomplished through a chimney arrangement where the exhaust leaves the greenhouse only after rising in a pipe to the top level. Incoming air to support combustion is through a small pipe near the base of the greenhouse. Since you won't be successful in bringing in fresh air from outside the greenhouse if there is no place for the interior air to go, some venting of interior air is required to make room for the incoming fresh air. These ventilation requirements indicate the need for at least two penetrations of the greenhouse structure, in addition to venting through one of the greenhouse's windows at the upper level. Southern Burner Company recommends installation of their heaters by professionals.

In a direct-vented system, the combustion air enters through the annular region of a single concentrically separated vent pipe that penetrates the greenhouse at bench level. The exhaust goes out of the greenhouse through the same pipe, through the central or core region of the vent pipe. I'm not sure what are the design features that separate the exhaust from the incoming air, so that the exhaust is not pulled in instead of fresh air. I'll have to look that up. In the direct-vented system, the combustion air does not mix with the air in the interior of the greenhouse. Only one penetration is required, and there is no ventilation required at the top of the greenhouse to allow for incoming fresh air. Because there are fewer required penetrations and the upper level window does not have to be opened, there is less convective heat transfer in a greenhouse heated with a direct-vented system, so it is likely to be more energy efficient.

When using a non-vented heater in a greenhouse, there is no direct venting to the outside. The products of combustion are mixed with the interior air. If the greenhouse air remained sealed from the outside air, oxygen would be constantly depleted and CO2 (and some amount of carbon monoxide too, I suppose) would build up, to the point where it was unhealthy for human respiration. To prevent humans from expiring, I understand that non-vented heaters are generally equipped with automatic shut-off systems that are triggered by too high a level of CO2. In a small greenhouse, the CO2 could build up faster, resulting in the human protection system turning off the heater much too frequently.

So, why would I suddenly be reconsidering using a non-vented heater? Well, it is looking a lot less complicated and less expensive to use a non-vented heater. I still don't have an easy way to install a direct-vented heater. A Home Depot employee told me there are bits that can be used to make holes in sheet-metal up to 3" in diameter. Tin snips would be needed to make larger holes. That doesn't sound very easy or neat.

Secondly, the idea of maintaining the interior greenhouse air separate from the outside air may not be such a good idea. I read about the possible growth of mold that might develop if the interior air remained stagnant and the humidity went too high. Also, a stagnant greenhouse interior could result in imbalances in oxygen and CO2. I understand that plants need oxygen for cellular respiration, while producing CO2, just like animals, but they don't need as much oxygen as animals do. However, during sunlight hours, when photosynthesis is in progress, plants generate oxygen, while consuming CO2. If there is no exchange of air between the interior of the greenhouse and outside air,the balance between oxygen and CO2 might shift in a way that was not good for the plants. Allowing exchange of some air between the greenhouse and the outside environment could act to smooth out imbalances in oxygen and CO2 levels.

What I don't know is whether the amount of interchange of greenhouse air with outside air that would be necessary to maintain the health of the plants is comparable to the amount of air mixing that would be necessary to keep people breathing properly and keep the safety mechanisms from turning off the heater. If the amount of air admixture is much less than what would be required to keep people breathing and keep the non-vented heater from automatically turning off, then use of a direct-vented heater would be worth the extra trouble of installation. However, if the amount of admixture of inside and outside air needed to maintain plant health is comparable to that required to keep the plants healthy and keep the heater from turning off, one might as well just use a non-vented heater. I need to do a little research to find out what the appropriate amount of admixture of inside and outside air should be, even when it might be practical to separate inside and outside air.