How do you size infrared when replacing existing forced air unit heaters?
An infrared heating system is always sized at a lower input capacity when compared to forced air (convection) heating. This is due to different modes of heat transfer (radiation vs. convection), thermal mass and minimal stratification between ceiling and floor temperatures. For retrofit purposes, provided the unit heaters are maintaining the desired inside design temperature at ASHRAe design conditions, the following reduction can be utilized when recommending an infrared heating system.
| Type | Thermal Efficiency | % Reduction in System Sizing |
|---|---|---|
| High Efficiency Unit Heater | 80% | 32% |
| Convectional Unit Heater | 62% | 48% |
Assuming that the ASHRAE heat loss for a building is 100,000 Btu/hr, then the heater selection for this building would be as follows:
| Infrared Heater | Unit Heater | |
|---|---|---|
| Building Heat Loss | 100,000 Btu/hr. | 100,000 Btu/hr. |
| Infrared Compensation Factor (for radiant heating) |
0.85 | – |
| Thermal Efficiency (for convection heating) |
– | 80% |
| Heater Input Required | 85,000 Btu/hr (100,000 x 0.85) |
125,000 Btu/hr (100,000 / 0.80) |
