Methods and apparatus for differential energy based airside economizer changeover

1. A method of controlling an economizer in a ventilation system, comprising:

measuring a moisture related property of a first input air source to determine if conditioning of the first input air source to a desired output air temperature results in a wet-coil or a dry-coil condition;

measuring a moisture related property of a second input air source to determine if conditioning of the second input air source to the desired output air temperature results in a wet-coil or a dry-coil condition;

determining a moisture related lockout value and a dry-bulb temperature lockout value based on a cooling load presented by the first input air source;

comparing the measured moisture related property of the second input air source to the moisture related lockout value if the conditioning of the second input air source results in a wet-coil condition;

comparing the measured dry-bulb temperature of the second air source to the dry-bulb temperature lockout value if the conditioning of the second air source results in a dry-coil condition; and

selectively enabling the economizer based upon the comparisons to the moisture related lockout value and the dry-bulb temperature lockout value.

2. The method according to claim 1 , wherein the first input air source corresponds to outside air and the second input air source corresponds to return air.

3. The method according to claim 1 , wherein the first input air source corresponds to return air and the second input air source corresponds to outside air.

4. The method according to claim 1 , wherein the moisture related property of the first and second input air source measured to determine if a wet-coil condition results includes dewpoint temperature.

5. The method according to claim 1 , wherein the moisture related lockout value is calculated as the enthalpy value that when compared to an estimate of the supply air enthalpy is indicative of the energy associated with conditioning the first input air source.

6. The method according to claim 5 , wherein the measured enthalpy of the second input air source is compared to the enthalpy lockout value, when it is determined that the conditioning of the second input air source results in a wet-coil condition.

7. The method according to claim 6 , wherein the first input air source is outside air and the second air source is return air, and the economizer is enabled when the return air enthalpy exceeds the enthalpy lockout value.

8. The method according to claim 1 , wherein the dry-bulb temperature related lockout value is calculated such that, when compared to an estimate of the supply air dry-bulb temperature, is indicative of the energy associated with conditioning the first input air source.

9. The method according to claim 8 , wherein the measured dry-bulb temperature of the second input air source is compared to the dry-bulb temperature lockout value, when the conditioning of the second input air source results in a thy-coil condition.

10. The method according to claim 9 , wherein the first air source is outside air and the second air source is return air, causing the economizer to be enabled when the return air dry-bulb temperature exceeds the dry-bulb temperature lockout value.