A relay is just a switch. The purpose of any switch is to turn something on and off. A relay turns HVAC equipment on or off. It’s that easy.
A thermostat is also just a switch that turns HVAC equipment on and off. So why would we bother adding a relay to an HVAC system when we already have a thermostat?
A relay gets us around the requirement that every device in an electrical circuit has to be the same voltage. A thermostat is usually low voltage, 24V. The HVAC equipment that the thermostat controls is line voltage. Because they are different voltages, the thermostat and the equipment cannot be in the same circuit. It's as if they speak two different languages. A relay is the translator between the two. It can be in both a 24V circuit and a line voltage circuit.
A relay can have more that one voltage going through it because there is no electrical connection among the parts of a relay. A 24V thermostat circuit sends electricity to the relay coil. That coil changes the 24V into magnetism. Inside the relay, that magnetism flips the switches to turn line voltage HVAC equipment on and off.
A relay usually has multiple switches. All of them turn on and off when the magnetism from the coil switches them. But the only ones that do anything are the ones that you wire into another circuit. The others are like a wall switch lying on a table. Flip it as much as you want, but nothing happens until you wire it up.
Normally open, normally closed
Right out of the box, with no electricity going through the coil, the relay switches are designated as either open or closed. A relay can have some of each, and they are marked on the relay. Every relay switch is either normally open (N.O.) or normally closed (N.C.) before electricity is applied to the coil. No electricity is called normal. The switch position is marked on the relay with these designations or with an open or closed switch symbol, two parallel lines is normally open. That same symbol with a diagonal line across it is normally closed. It is important to remember that we are talking about electricity that is applied to the coil, not about electricity applied to the switch.
How relays work
If you want to turn on a blower only when the thermostat calls for heat, how do you set that up? The thermostat is 24V and the blower is line voltage. You wire the blower to a normally open (off) relay switch. The blower stays off unless the thermostat calls for heat. The call for heat closes (turns on) the thermostat switch and allows electricity to flow to the relay coil. The coil changes the electricity to magnetism. The magnetism changes the switch from open (off) to closed (on).
To wire a relay, let’s start with the coil. Look at the relay terminals. Two of them are connected to the ends of the wire that makes up the relay coil. Recall that a circuit is always made up of a power supply, a switch and a load. The relay coil is the load. A 24V thermostat is the switch, and a 24V transformer is the power supply — a very simple circuit.
The remaining relay terminals belong to switches. Each switch has two terminals. Markings on the relay show which terminals go together. Wire each switch into a circuit with the piece of equipment to be turned off and on, with whatever power supply that equipment requires. Staying with the idea that every circuit is made up of a power supply, a switch, and a load, the power supply is line voltage, the switch is the relay switch, and the load is the piece of equipment. Again, it is a very simple circuit.
Even though electricity to the coil is 24V, the voltage supplied to the switches can be —in fact usually is — line voltage. The purpose of the relay is to allow line voltage equipment to be controlled by a 24V thermostat circuit.
You can turn on or off as many pieces of equipment as you have switches on the relay.
Testing a relay
Since there are two parts of a relay, you need to troubleshoot both. To test the coil, remove power from the coil. Set your multi-meter to continuity or resistance (Ω). Put a meter probe on each of the coil terminals. The coil is good if there is continuity (the meter beeps) or resistance (there is a digital reading).
To test the switches, remove power from all parts of the relay. Notice if the switch you are about to test is marked normally open or normally closed. With the meter set on continuity or resistance, put a meter probe on each of the two switch terminals. If there is continuity or resistance, the switch is closed. If that is not what the switch is labeled, the switch is bad.
If the switch tests good with the coil unpowered, then power the coil. Powering the coil causes all of the switches to change position. Again test the switch. This time it should be in the opposite position from its label. That is, if the label says the switch is normally open, it should be closed when the relay coil is powered.
Many of us want to think of switches as being like valves or faucets, but that does not work. Think of a switch as a drawbridge, not as a valve. When the switch/drawbridge is closed, electricity flows over it. A closed switch turns the circuit on. When a switch or drawbridge is open, traffic stops. An open switch turns the circuit off.
A thermostat is just a switch. It turns heating and cooling on or off depending upon temperature change. For living space heating or cooling, the thermostat is almost always 24V because a 24V thermostat is more sensitive to temperature change. A line voltage thermostat can be used in an area where a 10 degree or more temperature swing is acceptable, such as in a warehouse.
A contactor is the same as a relay, but bigger, both in physical size and the electricity it can handle. It is more often used with air conditioning, whereas a relay is used with heating. Contactor switch terminals are often coated with silver, so that they can withstand switching higher current.
Perhaps you have heard the terms "switching relay" and “isolation relay.” They could apply to exactly the same relay, but describe different purposes. The description above is a switching relay. The relay allows a low voltage thermostat to control a line voltage load. An isolation relay protects one circuit from the electrical effects of another by providing the magnetism of the coil as an intermediary.
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