Heating System Efficiency

The thermal efficiency of standard heating systems or unit heaters is typically 70-80 percent, but the seasonal efficiency can be much lower due to use of heated air for combustion or heated air escaping up the flue when the heater is not firing. Unit heaters are very popular for greenhouses.

Unit Heaters

There are five types of unit heaters:

  • gravity-vented
  • power-vented
  • separated combustion
  • high-efficiency condensing
  • direct-vented

A gravity-vent unit heater has a flue pipe connected to the outside and relies on the buoyancy difference of hot versus cold air to move the products of combustion out of the greenhouse (Figure 7). Heated air from the greenhouse is used for combustion and can continuously escape up the flue pipe, which reduces the overall efficiency.

Power-vented unit heaters have a small fan that operates when the heater is firing to meter the proper amount of air for combustion (Figure 8). Heated air from the greenhouse is used for combustion. When the heater is not firing the fan blocks air movement and increases the seasonal efficiency by 13 percent compared to a gravity-vented heater.

Figure 8: Power Vented unit heater. Source: Scott Sanford

Figure 8: Power Vented unit heater. Source: Scott Sanford

Figure 7: Gravity-vented Unit Heater. Source: Scott Sanford

Figure 7: Gravity-vented Unit Heater. Source: Scott Sanford

 
 
 
 
 

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