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The Woodfired Bottom-heated Greenhouse Bench

The Woodfired Bottom-heated Greenhouse Bench

Cultivating Questions Concerning the Bio-Extensive Market Garden

The Woodfired Bottom-heated Greenhouse Bench

by Anne and Eric Nordell of Trout Run, PA
illustrations by Ed Ochsner of Hohenwald, TN

Hi Anne and Eric,

This is Sam and Rich Wickham writing to you. We have been living and working on farms in the Finger Lakes for the past few seasons and, by way of our Amish neighbors and all the talented draft powered farmers in the area, we have been inspired to pursue farming with horses.

We have been passively gleaning knowledge from you through the SFJ, the world wide web, and your wonderful DVD and booklet which we got for Christmas this year. Your in depth, thoughtful coverage has been, far and away, the most valuable resource to us in our fledgling farming career. At first, we were just excited to find another couple who wanted to keep their farm a two person operation and how they went about that. However, it quickly became apparent that that knowledge was the very least we would stand to get out of poring over your old SFJ articles.

Anyway, we think we have exhausted our google search capacity in trying to find out more about your farm so we figured we had better sit down and write a letter to you both. We are quite curious about your wood stove greenhouse seedling set up. It has been difficult for us to determine the possible dimensions and arrangement of the system. Any insight into this would be much appreciated. Also, are you only heating up the stove at night or is it continuous throughout the day?

We are not prone to pestering but we are so intrigued by this idea that we just can’t help ourselves in this instance. We have been wanting to drop a line your way anyway to let you know that your work has elevated our understanding of farming (and appreciation of the craft) by leaps and bounds. Thank you for your generosity!

Take Care,
Sam and Rich
Caseville, MI

Before launching into the construction details of the bottom heated bench, we should explain why we chose this unusual method of heating the greenhouse in the first place. Looking for a low tech, low cost heat system which did not require electricity, we were intrigued with the nineteenth century greenhouse design in Peter Henderson’s Gardening for Profit. [see below – ed.] The flue from the wood stove of this renowned market gardener’s greenhouse ran underneath the length of the central bench and back again before joining the chimney built directly on top of the firebox. The heat from the stove created enough draft in the stack to pull smoke and heat through the long horizontal flue, warming the 20′ by 60′ greenhouse and providing bottom heat to most of the plants.

As much as this innovative heating system appealed to our minimalist outlook, we were concerned that this setup would be overkill for the 10′ by 16′ structure we intended to build in time for the 1988 greenhouse season. It seemed to us that a clean burning fire would make the small greenhouse too hot for the health of the plants and that moderating the greenhouse temperature with a smoldering fire would quickly lead to creosote buildup in the horizontal flue, a sure recipe for a chimney fire. We decided a safer, less polluting alternative would require thermal mass directly under the plants to capture the intense heat from a clean burning fire and release the warmth slowly for twelve or more hours.

The Woodfired Bottom-heated Greenhouse Bench

It took several incarnations to come up with a satisfactory design for the bottom heated greenhouse bench. In the final version we used two 55 gallon drums welded end-to-end for the firebox and a salvaged piece of 12” stainless steel chimney for the horizontal flue. (Metal culvert pipe or gasline casing might work just as well.) We learned the hard way that a large firebox and flue are necessary to dissipate the intense heat into the surrounding air chamber and to minimize heat stress on these components.

To shield the wooden greenhouse structure from the heat, we built the end walls of the bench with 4′ by 4′ pieces of sheet metal, cutting a large enough hole in one end for the door of the barrel stove and a smaller hole at the other end for the horizontal flue to exit and join the chimney outside of the greenhouse. A farmer that copied our design but neglected this safety feature had to call the fire department when the heat from the stove caught the greenhouse on fire.

The Woodfired Bottom-heated Greenhouse Bench

For the same reason, we built the side walls of the bench with concrete blocks. We placed salvaged grating from a sidewalk heat vent across the top of the block walls to support an 8″ layer of field stone and gravel, doubling the depth of this thermal mass over the barrel stove where the most heat is radiated. The weight of the stone on the grate keeps the side walls in place so mortar is not needed when laying the block.

The stove and flue rest on a bed of gravel sloped so that the pipe rises 12″ over the 16′ length of the bench. This modest slope along with the warmth retained by the thermal mass surrounding the flue produces a strong and dependable draft. When heat waves replace the smoke coming out of the chimney, we close the damper installed in the horizontal flue just outside of the greenhouse to hold as much heat inside the bench as possible. Managed this way, there is no creosote or ash buildup in the flue or chimney.

Since our goal is a brief, clean burning fire, we load the barrel stove with a couple of armloads of very dry wood. Scrap wood, tree trimmings, small pieces of seasoned hard wood all work well. For a number of years we used hemlock slabwood from a local sawmill. A half cord of slabwood was more than sufficient for the typical greenhouse heating season running from mid-March to mid-May.

The Woodfired Bottom-heated Greenhouse Bench

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