Aspirations for a Turbocharged Greenhouse

I'd long wanted to do something with the waste heat from our natural gas hot water service. Could this be teemed up with a greenhouse for improved results?

I had previously wondered whether, during summer, it would be possible to duct the exhaust through an absorption cooler to cool the interior of the house. Having done a bit of a search, however, I'd found nothing to suggest that this might be feasible, and the units used for refrigeration probably wouldn't work with the lower-temperature waste gas.

An different opportunity presented itself when we opened access to the flat section of our roof. It might be possible to locate a greenhouse on the roof and pipe waste gas from our adjacent hot water service. The appeal in doing this would be to add heat and carbon dioxide to the greenhouse, both beneficial to plant growth. The other input is water vapour, the other component of combustion.

My hope was that I might be able to be more successful in growing seedlings, something that I haven't had much success with in the past. Also perhaps be able to grow summer vegetables into the cooler months....

The greenhouse I bought was a small walk-in unit, approximately 2m x 1.8 x 0.7. The reason for the modest dimensions were due primarily to the difficulty of securing anything to the roof. Secondly, being primarily an R&D exercise, I didn't want to over-invest in what might be a green elephant.

For the turbo plumbing, I connected 100mm flex duct and suspended this with hose clamps off our bench structure.The final run of 4m tapered down from the inlet to roof height. The bottom of the duct in this run was punctured about every metre to allow condensation to escape. I discovered the need for this after the duct had sagged between the clamps, and found a substantial volume of water had collected.

I used a plastic drain unit as the vent into the greenhouse, and placed air bricks on the shelf above the outlet to act as thermal mass and help moderate temperature fluctuations.

Despite the condensation in the latter section of the ducting, there is still a lot of water vapour that finds its way into the greenhouse, so I added a low vent into the poly cover to allow some of the vapour to escape without losing all the heat.

At the business end, I connected the flex tube to the outlet of the hot water service and ran this up onto the roof and along to the greenhouse, a distance of about 8m. I wrapped the exhaust outlet with insulating tape before attaching the duct so as to thermally insulate the duct from the hot water structure - the duct obviously still gets hot when the burner is ignited, but the insulation means that the aluminium duct is not constantly drawing heat from the unit and losing this to the atmosphere along its length.

The energy injection into the greenhouse works well to keep it warm despite the wet wintry weather, and the water vapour keeps that the environment constantly humid. The combination works very well at germinating seeds, ensuring they don't dry out even if you forget to water for a few days. The celery seeds have just started to germinate so it will be interesting to see how these fare under the intense growing conditions.

I have found that thyme loves this environment, growing much more vigorously than I've managed in the garden during summer! The coriander grows very well although looks a little wan - perhaps too much growth and too little sunlight? The tomato plants don't do very well, the leaves being affected either by the high humidity or corrosiveness from the carbon dioxide. Having said that, the adult plant I transferred into the greenhouse is still doing a lot better than the one I left growing in the garden!

The system works well for seed-raising, as the moisture input ensures the seeds don't dry out during germination. Also the celery has grown exceptionally well, with good germination rates and strong growth. Tomato seeds I planted germinated well, but I have put these out on the roof because they don't like the aggressive medium of the greenhouse.