A double fan design is safer. If you depend on only one fan and that fan fails the time to repair or replace it could allow extreme temperatures to ruin some plants. The resistance to airflow depends on not only the diameter and length of your earth tubing but the number and angle of any bends. Fluid dynamics is complicated enough that substituting rules of thumb is common. It wouldn't be difficult to measure the resistance to airflow by comparing the power input of a known fan and comparing the resulting airflow returning out of the system. The overall volume of the greenhouse is only one factor. Others include details on weather: Lowest expected temp is not the whole story. How long an extended period of extreme cold is another, direction and speed of prevailing winds, latitude and seasonal sun angles changes, R factor and shape of greenhouse walls all play into your heat loss. In other words a two week span of 20 degree F temps may slam your crops worse than a single night of below zero. Here in the overcast Great Lakes region the USDA Zone is 6 But the sun, clouds and winter temps combine to make the period from Thanksgiving to Valentine's Day a period of low yield even with high supplemental energy demands. The additional cost and effort to upsize a system to maintain 50 degrees year round is questionable for Ohio. Stick with just spinach and kale for those 10 weeks when sunlight is scarce. Ultimately farming is solar powered. Other crops can be grown in January but it makes for a big investment with crummy returns.
