Let’s Talk Shorts⌗

I’ll keep this as simple as possible. From an HVAC mechanic who went from not knowing what a meter could do, to electrical diagnosis being the strongest skill I have (and the most fun in regards to troubleshooting); and literally being able to keep things running and people comfortable… I want to talk to you tech to tech, so first a quick introduction of who I am. I’m currently 29 with two beautiful supportive loving sons and a wife. I’m a field supervisor primarily responsible for training the next level of up and coming technicians and mechanics to our beautifully diverse field. Back to the topic at hand. We’re going to start with a KISS method here, anything past that, call HVA-Chicks Tech Support at 856-390-4055 and their lovely staff would love to help steer you in the right direction through our ever growing, and complicated field.

What’s a contactor?⌗

Overwhelming to many new technicians. That’s what! It’s simply used to “make” or “break” an electrical circuit.

What is “making” or “breaking” a circuit?⌗

Also simple, closing poles together: Say we have 240V (just a generic voltage. Much of the world’s equipment runs on different voltages. Science doesn’t change and you can apply this to anything) coming into our line side of the contactor (feed or input depending on your part of the country) and sending it to the equipment side. We’re eliminating the potential voltage so we can bring the world to life by engaging that 12 year old compressor/fan, or that ancient blower/indoor motor.

How does a contactor/relay work?⌗

The “line” side we just went over needs to connect to the “load” side. On the coil of the contactor/relay we have a coil that gets a “hot” and “common” that once engaged by its switching device (also controlled by a relay/s) sends voltage to the coil and produces a magnetic field. That magnetic field pulls the “poles” in ( the poles are what connects line to load also known as contacts) and allows your machine to come to life whether it be your furnace, air handler, condenser, door bell, or even your horn in your vehicle. Many relays and contactors work the same way to pull in contacts and create a closed circuit.

What if it isn’t working? What if my fuse is popping? Refer to your ohm’s law manual.

Here are a few scenarios to think about:

My low voltage won’t send a signal to close the contacts and complete the circuit:That’s a bad contactor. The poles” are bad from corrosion, wear and tear, or other underlying factors. But what if our contactor/relay isn’t putting out our low voltage required? Is the transformer putting out required? Is the fuse good? No? That brings us to our main topic of discussion. You can’t have your “hot” touching anything on your “common”. That involves anything that “hot” touches. Let’s go over a simple straight cool system thermostat wire. Your “R” is your “hot” leg. The thermostat engages other relays to apply voltage to the functions necessary for your equipment. If any of those components have a low voltage short your fuse/transformer are going to pop.

How do we check this?⌗

It’s time to break out your meter. One common misconception is just to listen for the buzz. We’re not going off the sound it makes (sound means little in this scenario when it comes to almost all models) What we need is readings!. If you’re not testing, you’re guessing. One way I like to hunt a short is as follows:

On a straight cool simple thermostat, set your meter to ohms (after verifying there is no power) and check between your “R” leg to anything else that should be powering a relay (make sure power is off completely before ever checking ohms) most thermostats are powered by a battery/s that will engage the relays internally. Down here in the south those “powered” designators are usually going to be R,Y, G, O/B or W (this is not a one size fits all scenario) and you are going to check them against “C” for resistance. If you get anything except for OL you have a short in your wire. The fun part is sourcing which wire has the short. If all wires are disconnected from their designators, and read anything but OL, when checking ohms, there is a short. This does not apply to coils. If all wires check “safe” as I like to call it, unless there is a rare circumstance of an intermittent short being moved around, the wire is safe.

Let’s move to the coils. The best and easiest practice is verifying what the ohms should be on a coil. If unfamiliar with the ohms please refer to ohms law and literature on your specific coil. Technology is abundant, and so is knowledge so use the resources and literature at your disposal. With these laws applied you should be able to diagnose most pesky systems out there giving you a hard time. Don’t forget to give the wires a wiggle and this page a follow!

Ohms Law:⌗

V=I(R)

Author: Jacob Mills