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Now We Know What "Unvented" Heater Really Means

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Last week we had a really Bad day on the Farm. Pilot light went out on heater and temps went down way below freezing. If we didn't loose our entire crop (situation still evolving), it was at least a severe setback. Killed all of the foliage but maybe not the roots and the core of the chili plants.

Why'd the pilot light go out? Not enough O2, of course. The greenhouse film is a near perfect vapor (and gas) seal to retain heat. Also excludes exterior oxygen. We'd carefully closed the greenhouse ends to keep the cold out. Also did a great job of keeping out the fresh air. The heater, a Redstone Dual Fuel 30K BTU heater, states that no vent is required in an unconfined space. Manual gives calculations to determine if the space is confined, in which case ventilation is required to pull in external air. For our greenhouse, the calculation shows that we could use a heater with up to 38K BTU and not require ventilation. But ... this is a film greenhouse, virtually perfectly sealed from the exterior air ... not a drafty or even reasonably well sealed home. The Oxygen Depletion Sensor on the heater detected a reduced O2 level and shut down the pilot light. Safety feature. No pilot light, no heat; no heat, freezing temps; freezing temps, dead (or at least severely damaged) chili plants!

So word to the wise. If you use an unvented heater, which is certainly convenient, do provide some external air and exhaust the interior air. Guidelines from Unvented Greenhouse Heaters suggest that 960 ft3/hr are needed for every 100K BTU/hr of heat. That means, for our 30K BTU/hr heater, we need fresh air at a rate of about 300 ft3/hr. The energy required to heat this additional fresh air to greenhouse temperatures is less than 1% of the energy output from the heater. For the moment, we have just opened gaps in the film ends of the greenhouse. Pilot light has stayed on and the greenhouse has stayed warm. Longer term, we may add vents with forced air that is controlled using thermocouples to detect when the heater comes on. Are also thinking of some type of heat recovery ventilator. These are commercially available, e.g., Broan Heat Recovery Ventilator, or DIY, e.g., Heat Exchanger.

What ever you do ... make sure your greenhouse heater can breath. If not, the greenhouse won't be warm for long, and you'll be very unhappy with the result.

Epilogue: Here's a YouTube video describing the ventilation system we installed. This system allows us to seal the greenhouse well, which prevents unintended cold air incursions, while keeping the O2 levels up. Right now the ventilator runs 24/7. Later we will add a thermocouple switch to turn the ventilator on only when the heater cycles on. This will further increase our energy efficiency.


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