This guide is intended to be a design manual for the design engineer who is interested in historical or typical ways to control commercial HVAC systems.
Hot Water Boiler Controls
This diagram shows typical controls on a low-pressure hot water boiler. There are two pressure controls- an operating one and a high-limit safety control. There is an “altitude gage” and a low-water cutoff. The boiler, unlike steam boilers, must be filled with water at all times.
Zone Controls
This diagram shows typical controls on a low-pressure hot water boiler being used with room zone controls. The room thermostats T-1 and T-2 operate zone valves V-1 and V-2 with the circulator controlled so that if any one valve opens, the circulator runs. The boiler is fired at a constant temperature by its aquastat.
Three-Way Valve Controls
This diagram shows typical controls on a low-pressure hot water boiler being used with room zone controls. The room thermostat T-1 controls a 3-way valve V-1 and the circulator runs all the time. The boiler is fired at a constant temperature by its aquastat.
Room Thermostat Controls
This diagram shows typical controls on a low-pressure hot water boiler being used with room controls.
The room thermostat T-1 controls the boiler through its aquastat directly. The circulator runs as long as
there is hot water sensed by a strapon on the boiler discharge line.
Room Thermostat Controlling Pumps
This diagram shows typical controls on a low-pressure hot water boiler being used with a indoor/outdoor controllers T-2 and T-3 to give a variable hot water temperature in accordance with outdoor air temperature. The room thermostat T-1 controls the circulator directly.
Indoor/Outdoor Control of Boiler
This diagram shows typical controls on a l o w pressure hot water boiler being used w i t h an indoor/outdoor controller T-2 operating through relay R-1 the 3-way valve V-1. The room thermostat T-1 modulates the 3-way valve.
Zone Control by Circulator Control
This diagram shows typical controls on a low-pressure hot water boiler being used with the zone room thermostats T-1 and T-2 controlling the two circulators directly. The aquastat T-3 fires the boiler at a constant temperature.
Three-way Valve Control through Indoor/Outdoor Stat
This diagram shows typical controls on a low-pressure hot water boiler being used with a master-submaster control system operating a 3-way valve V-1 on the hot water system. For clarity, room thermostats are not shown.
Control through Return Water
This diagram shows typical controls on a low-pressure hot water boiler being used with a master-submaster control system T-1 and T-2 operating two valves, V-1 and V-2, to bypass hot water when not needed. There is a differential pressure controller DP-1 operating a bypass valve V-3.
Indoor/Outdoor Controls
This diagram shows typical controls on a low-pressure hot water boiler being used with a master-submaster control system T-1 and T-2 operating two valves, V-1 and V-2, to bypass hot water when not needed. There are two separate circulators, and one of them near the boiler automatically bypasses the boiler when needed.
