Lose as little heat in the greenhouse as possible

The selection and design of heating equipment depends on how much heat will be lost from the greenhouse, as well as the availability and cost of fuel. Heat loss is a function of greenhouse size, glazing, orientation, and local climate. In addition to fossil fuels and electricity, renewable energy sources, such as solar thermal for hot water production and photovoltaic panels and wind power for electricity generation, can help offset fuel costs.

Thermal screens and blankets
Heat loss from the greenhouse at night can be mitigated using thermal screens that are installed inside the greenhouse and suspended over the crop. Thermal screens are most effective when they are installed without gaps between the edges and walls of the greenhouse, to prevent heat from moving into the attic space above the screens.  Thermal screens with a reflective surface facing plants will reduce the amount of heat radiated from plants to the night sky. Screens with a reflective surface facing the outer greenhouse cover have low emissivity, which will prevent the screen from absorbing heat from inside the greenhouse and emitting it to the outside. Therefore, the ideal thermal screen will have reflective surfaces on both sides, usually in the form of aluminized fabric or aluminum foil that is applied to the blanket. Depending on the type of screen and how it is installed, it can reduce heat loss from the greenhouse by 20-60%, thereby reducing the amount of supplemental heating required.

Hot air (forced air) systems
These systems heat the air that will be delivered to the greenhouse using fans. Sometimes the hot air is introduced at one end of the greenhouse, relying on fans and/or temperature gradients to distribute the hot air to the other end of the greenhouse. In other cases, the hot air is distributed through overhead polyethylene ducts, floor-based tubing, or an underfloor plenum. 

Hot water (hydronic) systems
Hot water systems heat the greenhouse air with hot water that is pumped through bare pipes or finned tubes around the perimeter of the greenhouse, at the root zone, or buried under the floor. Generally, hydronic heating systems provide a more uniform distribution of heat. In areas where it snows, hydronic systems can also be used to heat the greenhouse gutters and melt snow.

Heat distribution
There are several means of distributing heat within the greenhouse. Hot air and hot water can delivered above the plants, at the ends or around the perimeter of the cropping zone, or at the root zone. Heat that is distributed overhead and around the greenhouse perimeter will produce hot air above and around the plant zone. Without the use of fans, hot air will naturally rise above the plant canopy and heat will be lost through the cover. Heat that is distributed near the floor and root zone of the plant will produce more uniform temperatures within the planting zone and require less energy for heat generation and distribution.