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How Does a Relay on a Boiler Work?

Heather Bliss

The relay on a boiler is the part of the boiler system which triggers the boiler to release heat when the thermostat calls for it. The thermostat is a device which measures the temperature of an area, and when the area gets too cold, it will send a message to the relay switch to release heat from the boiler. The relay on a boiler can sometimes be called a circulator relay because the relay triggers the action of a circulator pump. The process of a boiler's heating operation begins when the room that is being heated gets cold. At this point, the thermostat sends an electrical signal to the relay switch. The relay then triggers the boiler to turn on.

Chain of Action

Types of Relays

There are two basic types of relays: the solid-state relay and the electromagnetic relay, with several sub-categories of relay in both. An electromagnetic relay sends electricity to trigger an electromagnet which causes a part of the electromagnetic relay, called the armature, to swing and trigger the relay in various ways, depending on the type of electromagnetic relay it is. An electromagnetic relay is the original type of relay, and is the cheapest and least common to use. Because of the risk of a spark being created by the moving parts, most boilers use a solid-state relay, which has no moving parts.

Solid State Relay

A solid state relay switch can also be called an SSR or semiconductor switch. In a solid state relay switch, the relay is usually triggered by an LED light that activates a light-sensitive component of the boiler. Older boilers, particularly oil burning boilers, used a protective device called a stack relay switch, which is no longer installed in new boilers.

Zone Heating

While some buildings with a large shared boiler have one thermostat and relay to trigger heat to the entire building, some buildings have individual thermostats for areas called "zones." Each zone is given heat from the boiler from its own relay setup which triggers the boiler to release heat through a pipe that goes straight to the room which needs heat.