Off-Design Conditions

In operation, the cooling or heating loads are only a part of the design loads. HVAC system needs to respond to this lower demand, and some of the strategies are:

• CAV-RH (Constant Air Volume)

• VAV-RH (Variable Air Volume)

• Face and bypass coil

• Economizer

• Variable T for the heat exchangers

Analyze these processes by the same methods as design conditions.

Problems for cooling systems: thermostat controls TDb, and therefore humidity correct only at design (in general). As a result, space humidity varies with loads, and may need to iterate to space conditions. This is not a problem for heating conditions.


• same flow rate and W as design conditions, variable TDb

• good control at part loads


• lower flow rate in proportion to the sensible loads

• same coil dewpoint temperature => less dehumidification

Face and bypass coil

• bypass fraction is proportional to sensible loads

• no dehumidification of bypassed air => supply humidity is proportional to bypass fract.



Off-Design Conditions



Face and Bypass Coil


• used in spring or fall

• supplies outdoor air without operating a cooling coil; potential humidity problems

• limit is 100% outdoor air; control humidity rise with reheat

Off-Design Conditions




C: RA Economizer is off => lost cooling opportunity D: RA Economizer is on => energy penalty results

No reliable, durable and inexpensive enthalpy sensor for the enthalpy economizer.

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning