With more cities and states requiring heating systems with as much as 92% to 97% efficiency, the advances made in improving the effectiveness of forced-air unit heaters over the years mean they’re still a popular heating choice for growers.

Bent-tube heaters, a newer forced-air gas unit heater with thermal efficiency between 83% to 87%, provide an excellent compact solution for greenhouse heating.

They direct a flame through a tube (fire-tube) and use a fan or blower to push heat from the tube into the greenhouse.

Gas unit heaters have either standard efficiency (82%) or high-efficiency (92% to 97%) ratings.

Gas unit heaters with high-efficiency ratings convert a larger percentage of energy or fuel input into usable heat, which helps reduce energy costs by lowering energy consumption.

Gas unit heaters that fall in the high-efficiency category have two important features:

• Large tube surface area: Compared to unit heaters with standard efficiency ratings, high-efficiency heaters have more surface area inside their tubes where hot gases pass through, which allows more time for the exhaust gases to cool. As a result, the unit can extract more heat for use in the greenhouse.
• Condensing system with separated combustion, meaning it draws combustion air from outside of the greenhouse. This keeps exhaust gases separate from the air inside the greenhouse to prevent contamination of the growing environment.

Separated-combustion gas heaters have one intake vent for drawing in combustion air and a second vent for expelling exhaust. This design creates neutral air pressure inside the greenhouse.

In contrast, a power-exhausted gas heater has one flue vent that obtains combustion air from within the greenhouse and discharges it through a flue vent to the outdoors. This action creates a slight negative pressure on the greenhouse.

A gas unit heater with a high-efficiency rating typically offers better seasonal efficiency, meaning they provide more consistent and economical heating throughout the entire season than their standard-efficiency rated counterparts because do so well at optimizing fuel usage and minimizing waste.