Heating costs are a primary obstacle to year-round home and commercial greenhouse growing in colder climates. Most demonstration compost-heat greenhouses use manure mixed with shavings or other bedding material in a composting chamber within the north wall of the greenhouse. However, at least two methods of wood-chip composting have been demonstrated to successfully heat greenhouses: one by direct heat from compost piles, and the other indirectly through a piped-water heat exchanger.
Active Pile Composting
Stand three pallets on end. Secure with baling twine and place in a corner of your greenhouse. For smaller structures, use chicken wire, with or without metal fence posts, to create a compost-containment bin to suit your greenhouse size. Install one or more of these compost containment units for every 120 square feet of floorspace.
Fill each compost-containment bin with brewers waste. Use a minimum of one wheelbarrow full of brewers waste per bin to reach a thermophilic mass. Moisten the pile with water. Top the pile with a 6 inches layer of hardwood chips.
Monitor the piles. After one week--or sooner, if the pile shows sign of rapid decomposition--mix the wood chips and brewers waste with a spading fork, re-pile in the compost container, and top within another 6 inch layer of wood chips.
Adjust the number and size of the compost piles to address your climatic and seasonal needs. When decomposition slows, remove the pile and use it as horticultural mulch, and start a new compost pile in the greenhouse.
Wood Chip Heat Exchanger
Pile chipped hardwood brush into a cylinder shape 6' high by 12' diameter using shovel or bucketloader. Spiral flexible PVC pipe around the pile in ten to twelve ascending loops, leaving sufficient PVC pipe extending from the pile to reach your greenhouse comfortably. Water the pile thoroughly each day for a week.
Encircle the pile and pipe with more chipped hardwood brush, to the same height, and about 6' in width, yielding a total pile 6' tall and 24' in diameter. Take care not to lose the ends of your PVC pipe when adding the outer ring of wood chips. Water the completed pile daily for one week.
Cut the lower length of PVC pipe half way between the compost pile and greenhouse, using PVC flexible pipe cutters. Place a hose clamp over each cut end, then insert threaded connectors in each end and tighten the hose clamps over them with a screwdriver. Connect recirculating water pump tubing to the threaded connector ends, plug in the pump, and prime it according to manufacturer's directions.
Connect copper pipe lengths with corners on the floor of your greenhouse, placing the long lengths approximately one-quarter of the way in from the outer edges of the greenhouse. Run PVC pipe under the edge of plastic-sheeting greenhouses, or dig small tunnels under the edge of glass greenhouses.
Use one PVC-to-copper-pipe connectors to connect the upperr PVC pipe end to copper pipe. Fill system with water at the lower PVC pipe end then connect that end to copper pipe as well.
Turn on the recirculating water pump to move compost-heated water through pipes within the greenhouse. Monitor heat output; depending on the thermophilic efficiency of the brushwood used as well as atmospheric conditions, the wood chip compost pile for greenhouse heat will need to be replaced in 6 to 18 months. Use decomposed brushwood for horticultural purposes.