Normally Open vs. Normally Closed Switches

 Hey we're gonna talk about normally open switches versus normally closed switches when it comes to HVAC electrical pieces. I teach a lot of HVAC classes on low voltage and I, I think we could use a refresher. We could use some continuing education on the difference between normally open and normally.

Closed. So a normally open switch is a switch that is open for a reason, meaning it does not have contact. You have just. Very rough illustration, but you have two pieces over here and electricity is sitting on one end and it's not flowing through because it's O open, it takes a magnetic force to pull the two together in order to close the circuit.

So this is an open circuit, this is a closed circuit, meaning it can, electricity can flow through well, you have both types of switches. In the AC system. So if you're trying to determine whether a relay is bad, a sequencer is bad, a limit switch, a thermal link defrost thermostats, contactors, you wanna know if they're going bad.

You need to know whether or not it's supposed to be open or closed. So we'll go through, we'll go through all of them that are here. And we will talk about what they're supposed to be. So a fan relay, which is here, which controls your evaporator fan thermostat sends a signal, turns on your evaporator fan motor.

Your indoor fan motor 24 volts is required from the bottom in order to power and magnetize. Or to heat up a thermo disc, which sends power to the top terminals, which turns this on. So the bottom portion or the low voltage portion of the relay side will have continuity. So it's normally closed. It takes power through the bottom to send power to the top.

So the top portion of this relay would be open. It does not have power, and it won't have power until this coil heats up. So the top portion of this particular fan relay is open a heat sequencer. This right here the low voltage side of this will remain open until the thermostat calls for heat, electrical heat.

Then it will send a low voltage signal through the bottom terminal. So you can see I've got my leads here. I have no continuity. So this is a normally open switch. It takes the 24 volts to connect to this and order for it to close and send the power through the top to get your heat strips on. That is a heat sequencer so normally open.

The top portion of a relay is normally open. The bottom portion of the fan relay is normally closed. Let's get into our limit switch. Our limit switch here. This is a safety device that protects your heat system from heat. It is normally closed if heat builds up due to a dampener closing. Airflow issues fire.

Whatever, if this heats up, this will then go from a closed switch to an open switch to disconnect power. The limit switch is designed to protect you from heat. The fuse link over here, the heat terminal. This is also. Closed. It will allow current to flow through it under normal operating conditions.

The difference between a limit switch and a fuse link, the limit switch protects you from heat, like the actual heat, how hot something is. The fuse link protects you from current, so any over current, any surges in currents, anything that's pulling too much. This will cause the fuse leak in the back to melt and it will separate and it will also disconnect power.

So once this goes from close to open, you have to replace it. You cannot repair this. It has to be replaced. Let's go over to a a defrost thermostat. This goes outside. This is located in the condensing unit. Outside and these switches this one has continuity, which actually tells me that this is a bad switch.

It's not supposed to have continuity. This switch is supposed to be open. For Goodman specifically that is supposed to be an open switch. So this tells me I have a bad. I have a bad thermostat here, so this will need to be replaced. Normally this is a normally open switch, and then once the copper pipe or the line set in the condensing unit reaches four 40 degrees or less this will close, which will then cause the reversing valve to kick on and to run a heat cycle.

Or a cool cycle, or to reverse the flow of refrigerant outside so that it heats up and melts the ice. That's what the purpose of this is. But this switch is actually bad. Contactor, you have 24 volt terminals on the side.

Those are normally. Say it with me closed. The top portions of this are open. It's not until the plunger pulls in that closes the top. So this is just like a fan relay. And here's a tip. In the event of an emergency, can you use a contactor and replace of a relay? The answer is. Yes, you would put your 24 volt sides to the side of the contactor.

You would put your high voltage lines onto the top poles, and you have a makeshift fan reregulate. Now, I would not keep it there, but in a pinch it'll help you out in a jam. But hopefully this will help you understand the difference in the importance of having normally closed switches. Normally open switches and what their purpose is and how you go about testing them.