What is a horizontal air flow (HAF) system in a greenhouse?

The horizontal air flow, or HAF, concept utilizes the principle that air moving in a coherent horizontal pattern in a building, like a greenhouse, needs only enough energy to overcome turbulence and friction loss to keep it moving. In other words, you just have to “kick it along.” Small, 1/10- to 1/15-horsepower circulating fans work well in providing the air movement needed. These are made to move air with little resistance as compared to exhaust fans that have blades designed …

Can I use the exhausted heat from a high-efficiency furnace vent to heat a greenhouse?

No. There are several reasons not to attempt heating a greenhouse with a high-efficiency furnace vent:
1) Carbon monoxide could be lethal to anyone entering a greenhouse heated by furnace exhaust.
2) Carbon monoxide interferes with photosynthesis in plants.
3) It would not be worth the investment.
You have already been environmentally responsible by investing in a high-efficiency furnace. You could run the exhaust pipe through a greenhouse, but there should be very little heat in the exhausted air from …