Enthalpy is the total heat content value in air, considering both dry bulb temperature and relative humidity level. If either value changes while the other is constant the enthalpy value has to change.
APS was hired to write a specification to restore an HVAC system that had enthalpy logic. The only pneumatic component sold on the market at that time did not work, so APS developed a circuit that performed very close to the values of the psychrometric chart when sensing normal temperature and humidity levels (enthalpy).
A few years later a critical logic relay in the first development was no longer available, so APS developed another enthalpy circuit using a more common logic relay.
We have seen the flaw in the manufacturer’s design that disallowed their enthalpy module to not function properly transferred into computer logic in a BAS specification.
Whether pneumatic or computerized owners may be wise to simulate varying humidity and temperature levels (Enthalpy) to their HVAC control systems and confirm that the damper positioning is logical.
