We finally have an efficient way to transfer air in our attached greenhouses. Using two inline ultra efficient fans and two thermostats we used the warmth from the adjoining interior living space to keep the greenhouse above 40 degrees and during periods of sunshine the greenhouse was able to heat the interior living space. After a very cold winter the cost to heat this greenhouse has been around $300. The most exciting discovery was the fantastic plant growth. The extra CO2 being pumped into the greenhouse from the living space was quite noticeable. With lots of growth in this packed greenhouse we can only assume that the occupants in the living space benefited with increased levels of Oxygen and the plant filtered air.