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A heat-recovery system captures a building exhaust air and reuses some of the energy to precondition the make up air before supplying it to the building.

Air-to-air heat-recovery systems for comfort-to-comfort applications can in general be categorized as

• sensible heat (dry bulb only) systems
• total heat (wet bulb - sensible heat plus latent heat) systems

Sensible Heat-Recovery Systems

A sensible heat-recovery systems will transfer sensible (dry-bulb) heat from the exhaust air to the supply air entering the building. Rotary wheel heat exchangers and cross flow heat exchangers are some of the common designs used in typical comfort-to-comfort sensible heat-recovery applications.

The amount of heat transferred can be expressed as

H = 1.08 q (t₂ - t₁) η         (1)

where

H = heat transferred (Btu/h)

q = quantity of airflow (cfm)

1.08 = a constant for sensible heat equations

t₁ = air temperature of the exhaust air before the heat recovery unit (^oF)

t₂ = air temperature of outside air after the heat recovery unit (^oF)

η = heat-recover efficiency

Example - A Sensible Heat Recovery System

The outside air temperature is 90ºF and the exhaust air temperature is 75ºF. The system operates at 40,000 cfm at 73 percent efficiency.

The sensible heat-recovery can be expressed as

H = 1.08 (40,000 cfm) ((90 ^oF) - (75 ^oF)) 0.73

    = 473,040 Btu/h

A Sensible Heat Recovery Calculator

The calculator below can be used to calculate the sensible heat recovery where the air flow, outside and inside temperature and recovery efficiency are known. 

q - air flow (cfm)

t₁ - outside air temperature before the heat exchanger (^oF)

t₂ - outside air temperature after the heat exchanger (^oF)

η - heat recovery unit efficiency

Total Enthalpy Heat Recovery System

For total enthalpy heat-recovery systems the heat recovered can be expressed as

H = 4.5 q (h₂ - h₁) η         (2)

where

4.5 = a constant for total heat equations

h₁ = air enthalpy of the exhaust air before the heat recovery unit (Btu/lbºF)

h₂ = air enthalpy of outside air after the heat recovery unit (Btu/lbºF)

Example - Total Enthalpy Heat Recovery System

The outside air wet-bulb temperature is 70ºF (enthalpy 34.1 Btu/lbºF) and the exhaust-air wet-bulb temperature is 60ºF (enthalpy 26.5 Btu/lbºF). The system operates at 40,000 cfm at 73 percent efficiency.

The total heat-recovery can be expressed like

H = 4.5 (40,000 cfm) ((34.1 Btu/lbºF) - (26.5 Btu/lbºF)) 0. 73

    = 998,640 Btu/h

A Total Heat Recovery Calculator

The calculator below can be used to calculate the total (latent and sensible) heat recovery where the air flow, enthalpy of outside and inside air and the recovery efficiency are known. 

q - air flow (cfm)

h₂ - enthalpy outside air before the heat recovery unit (Btu/lbºF)

h₁ - enthalpy outside air after the heat recovery unit (Btu/lbºF)

η - heat recovery unit efficiency

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