Hot Water Boilers Control

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.

Boiler plant room

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.

Hot Water Boiler Controls

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.

Zone Controls

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.

Three-Way Valve Controls

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 Controls

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.

Room Thermostat Controlling Pumps

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.

Indoor/Outdoor Control of Boiler

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.

Zone Control by Circulator Control

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.

Three-way Valve Control through Indoor/Outdoor Stat

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.

Control through Return Water

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.

Indoor/Outdoor Controls