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.
FREQUENTLY ASKED QUESTIONS
A room thermostat can control a hot water boiler by sending a signal to the boiler’s controller or burner to increase or decrease heat output based on the room temperature. This is typically done through a relay or a valve that opens or closes to allow more or less hot water to flow into the heating coils or radiators. The thermostat’s setpoint is usually adjustable, allowing the user to set a desired temperature for the space.
A three-way valve in hot water boiler control is used to divert flow between the boiler and the heating circuit. It allows the boiler to supply hot water to either the heating circuit or the return line, depending on the valve’s position. This valve is often used in conjunction with a zone control system to control the flow of hot water to different zones or areas of the building.
Indoor/outdoor control of a hot water boiler involves using an outdoor temperature sensor to adjust the boiler’s output based on the outdoor temperature. This is typically done through a reset schedule, where the boiler’s setpoint is adjusted based on the outdoor temperature. For example, on colder days, the boiler may produce hotter water, while on milder days, it may produce cooler water. This control strategy helps to optimize energy efficiency and maintain a consistent indoor temperature.
Zone control by circulator control involves using a circulator pump to control the flow of hot water to different zones or areas of the building. This approach is often used in larger buildings with multiple zones. Zone control by three-way valve control, on the other hand, involves using a three-way valve to divert flow between the boiler and the heating circuit. This approach is often used in smaller buildings or where there are fewer zones to control. Both approaches can be effective, but the choice of which to use depends on the specific application and system design.
Yes, a hot water boiler can be controlled through the return water temperature. This approach involves using a temperature sensor in the return line to monitor the temperature of the water returning to the boiler. The boiler’s controller can then adjust the burner firing rate or valve position to maintain a desired return water temperature. This control strategy can help to optimize energy efficiency and prevent overheating or underheating of the system.
