Specification for Control Sequences 
EXECUTION 
HEAT EXCHANGERS (VERTICAL FLOODED TYPE) - CONTROL SEQUENCE 

A. There are two heat exchangers, each having 100% capacity for the initial building load with shell spaces filled.(If space is to be expanded: and approximately 2/3 of the ultimate capacity including future addition.) 

B. Hot water temperature shall be controlled by temperature controller with sensing element in leaving water line from each heat exchanger, modulating control valves of each heat exchanger in condensate return line set to maintain 180°F (FA) hot water. The control valve is (optional: and controller are) provided by the heat exchanger manufacturer. 

C. Water temperature set point shall be linearly reset by outdoor temperature according to the following reset schedule: 
1. 180°F (FA) at outdoor air temperature 0°F or below. 
2. 140°F (FA) at outdoor air temperature 40°F or above. 

D. (Optional) The heat exchanger manufacturer shall provide separate high limit steam control valve with temperature sensor in leaving water line. When supply water temperature reaches high limit set point of 210°F (FA), control valves shall close, and send alarm to BAS indicating high limit condition until water temperature falls below high limit setpoint. When supply water temperature is below 180°F (FA), control valve shall be released to control to water supply setpoint. When supply water temperature falls below low limit of 120°F, alarm shall be sent to BAS indicating low limit condition until water temperature risers above low limit setpoint. 

E. (If space is to be expanded) Lead-lag selector software switch of hot water pumps shall select lead heat exchanger through BAS (Refer to HW pumps controls sequence). Control valves of lead heat exchanger shall be modulated to maintain hot water supply temperature setpoint, while lag heat exchanger (stand-by) control valves shall remain closed. Both heat exchanger shall be activated when the future addition is built, by activating lag heat exchanger when lead heat exchanger can not maintain the hot water set point or second hot water pump is indexed to run, and 1/3 and 2/3 capacity control valves of both heat exchangers shall be controlled in parallel. 

F. Isolation valve in entering water line to each heat exchanger shall be fully open when the heat exchanger is indexed to operate and shall be fully closed when the convertor is indexed not to operate after 10 minutes (FA) time delay. Condensate control valve shall not be