Tag: relay

 

This is the tale of how I found myself stuck on a service call for over 12 hours on a weekend, due to my failure to re-diagnose an issue. I was working for a service company that had many accounts with local gas stations. These were large customers, and we did everything we could to keep them happy.

One Friday as I was gearing up for my on-call weekend, I was informed I must travel an hour and a half away early the next morning to a gas station where another technician had diagnosed a faulty X-13 blower motor. The technician didn’t have the right blower motor for the repair, so the system was still down. The catch was: no one knew what size blower motor was supposed to go in. No model numbers, no detailed notes, nothing. So I grab every size aftermarket X-13 motor I could find in the shop. I had up to ¾ HP. 

I arrived to find this location had two 5-ton package units mounted atop stands lifted 8 feet off the ground. After I setup the ladder and double-check the motor size, I realized it was 1 HP. I began calling all the parts houses in the area, hoping someone answered on a Saturday morning at 8am. No luck. I called parts houses in my local area and my co-workers to try and find a 1 HP X-13 motor that would work. Finally, I got in touch with one of my local suppliers. He had a motor that would fit the system I was working on, but he was an hour away from the supply house, and I was an additional hour and half away. Luckily, my employer at the time picked it up for me, and I had the part within a couple hours. I still had not re-diagnosed the system at that time.

Once I had the motor in hand, I quickly replaced it and had everything back together in a snap. I re-energize the system and….I curse loudly. The motor wouldn’t run. NOW I start re-diagnosing, a step I should have taken when I first arrived. Turns out, the original motor was just fine. The motor was not receiving 24v to the motor module, due to a faulty fan relay. I swapped out the 90-340 relay in the electrical compartment, restarted the system, and the blower ran beautifully. I hated myself.

I entertained the idea of packing up, walking away, and calling it complete, but I knew too well how that plays out. I began running complete system diagnostics, and found the system charge to be very low. I started leak searching the system with Big Blue from Refrigeration Technologies, and discovered a micro leak on the mechanical connection between the distributor tube and the TXV. No rubs outs were apparent, and it wasn’t a super loose connection, but it was clearly leaking. This was a package unit, remember, so I had to recover the entire system charge before I could make any repairs. 

 

Once recovered, I found the connection was just coming loose. A healthy dab of Nylogon the fitting connection and a torque wrench was all I needed to pass a nitrogen pressure test. Of course, the repair process was time-consuming, but eventually, I had the system evacuated, cleaned, recharged, and operating in peak condition under the current load.

I still would have needed to make the leak repair no matter what, but I could have easily saved 3.5 hours of time if i had re-diagnosed the system first when I arrived to the job. One could argue I was simply distracted by the chaos of the call, which would be true. However, a good technician should be able to follow the proper processes in spite of disorganization and frustration. I learned the importance of always checking behind yourself and others when you arrive to make a repair. Since, I have found the real causes for issues that were previously (either by me, or another technician) diagnosed as bad TXVs, reversing valves, motors, etc. 

 

ALWAYS double-check your work and other people’s work. You never know how valuable it is until you fail to do it, and it costs you time and money.

— Kaleb

 

 

In residential air handler/fan coils it is common to use a high voltage interlock between the blower and the electric heat strips to ensure that the blower comes on whenever the heat is on.

The problem is that it CANNOT work the other way around where the heat comes on with the blower.

Heat strips are generally going to draw 20+ amps depending on the voltage and KW rating which means you CANNOT power them through a typical relay like a blower relay or board which are generally rated for 15 amps or less.

The way the interlock is wired is really quite obvious but is easy to forget because it’s the reverse of what we are used to with a relay.

In short, we connect the blower to the “common” terminal on the relay, L1 power to the normally open (n.o.) terminal and the load (out) side of the heat strip contactor /relay/sequencer to the normally closed (n.c.) terminal.

This diagram from Carrier shows the blower connected on the common terminal and constant power coming in on black to the normally open terminal from the right side of the transformer primary.

Using this 90-340 relay as an example, the blower would connect to 1, power to 3 and the heat strips to 2.

I made a video on it as well if you need it. The result is that the blower runs with the heat but the heat doesn’t run with the blower.

— Bryan



Relays can be used for many different control applications including controlling fans, blowers, other relays or contactors, valves, dampers, pumps and much more. A 90-340 is a very common, versatile relay that many techs have on their truck so we will use it as the example.


A relay is just a remotely controlled switch that opens and closes using an electromagnet. The electromagnetic portion that provides the opening and/or closing force of the switch is called the coil. Relay coils can come in many different voltages depending on the application, but in residential and light commercial HVAC 24-volt coils are the most common.

The portion of the relay that opens and closes can be called the switch, contacts or points. These contacts can either be closed meaning there is an electrical path or open meaning there is no electrical path. Often this open or closed circuit will be described as “making” a circuit, meaning the switch is closed or “breaking” a circuit meaning the switch is open.


It is important when connecting a relay to distinguish which two relay points connect the coil. In the case of the 90-340, it is the bottom two terminals of the relay. Even though the coil is unmarked on most 90-340 relays, you can find it easily by locating the terminals with the small strands of wire connected. These two points connect together through the electromagnetic coil. When 24 volts of potential is applied across the coil the switch portion of the relay will switch from open to closed and closed to open depending on the terminal. Keep in mind that in a normal 24v circuit one side of the coil is connected to a 24v switch leg such as the thermostat “G” circuit for blower control, and the OTHER side of the coil is connected back to common.

The other six terminals are switch/contact terminals and the relay has a diagram embossed right on the top for easy reference. The way the circuit is drawn shows the de-energized state of the relay, meaning the state of the switches when no power is applied to the coil. When power is applied to the coil the points that were previously open (broken) now become closed (made) and the ones that were closed become open. When two points are closed when no power is applied to a relay coil we call them “normally closed” when they are open when no power is applied they are called “normally open”.


So based on this embossed diagram on the relay 1 to 3 and 4 to 6 are open (normally open) with no power to the coil and closed when power is applied. 1 to 2 and 4 to 5 are closed (normally closed) with no power and they open when the coil is energized. There is never a path between 2 & 3 or 5 & 6 because between them, at least one of them is always open. There is also no path or circuit between the top three terminals and the bottom three terminals or between the switch and coil portions of the 90-340 relay.

The data tag on a 90-340 shows both the coil voltage as well as the LRA (locked rotor amps) and FLA (full load amps) that the contacts can handle at various voltages for inductive (magnetic) loads like motors. It also lists the amp rating if the relay is controlling a RES (resistive) load like a heater or an incandescent light.


This relay can control a 39.6 LRA and 6.9 FLA Motor or a 15 amp heater at 240 volts based on the data tag.

— Bryan

Look closely at a contactor on your truck and you may find some interesting ratings you never noticed. Things like terminal ambient temperature ratings and torque specs. One reading you may overlook is the RES AMPACITY of the contactor or relay. The RES rating is the RESISTIVE LOAD AMPACITY (amperage capacity) or rating.

Remember, a contactor and relay is both a switch (contacts) and a load (coil) that controls the switch (contacts). The FLA, RES and LRA are the ratings of the switch / contacts / points portion of the relay / contactor, not the coil portion. The coil is just rated for voltage and cycle rate (hz).

You will notice that the RES (Resistive load) rating is higher than the  FLA (Full Load Amperage) rating and much lower than the LRA (Locked Rotor Amperage) rating. So the contactor above the contactor above would be sufficient to control/switch a 40 amp full load INDUCTIVE load and a 50 amp RESISTIVE load.

So what is the difference between an inductive and a resistive load?

An inductive load is a magnetic load that converts electrical energy into kinetic energy (motion) through electromagnetic force (magnetism). This would usually be motors and solenoids in HVAC/R, really anything where magnetism and motion are involved (Transformers and inductive rages being examples of induction WITHOUT motion for the purist).

An inductive load will experience a spike in current when voltage (potential) is first applied, this is called locked rotor amps (LRA) because it is the current a motor will draw when it starts up from a locked or stalled position.

A Resistive load is a load that converts electrical energy directly to light or heat as the electrons flow through a resistive conductive path. These would be things like heat strips, crankcase heaters and incandescent light bulbs. Resistive loads do not have any internal variation in load and the voltage and amperage are completely in phase. This means that when a contactor, switch or relay are controlling a resistive load there will be less load variation.

The conclusion is that often contactors and relays can handle a higher amperage across the contacts when the load is resistive (Light / Heat)  than when it is inductive (Motor / Motion).

— Bryan

If you do any larger commercial work you’ve probably seen a DIN rail without knowing what it is called. It is simply a mounting standard that originated in Germany in the 80’s and slowly worked its way over here.

DIN rails can be used to mount terminal blocks, relays, starters, breakers… just about anything electrically. They aren’t designed to conduct electricity like a busbar, though they are used in some cases as a grounding assembly.

The most common DIN rail type is the “top hat” or TS35 shown in these photos.

Components that attach to a DIN rail have little release clips so that they can be easily installed and removed. In addition to the “top hat” style there are also some less common, heavier DIN rails with a C and G style configuration.

So if you ever see one of these don’t be alarmed, you can just sound cool when you call your boss and tell him “yeah, it’s one of those DIN rail mounted relays” and just wait for him to say “What??”

— Bryan

P.S. – There are Two Incredible Giveaways going on right now that you should signup for. Air Oasis is giving away a bi-polar and a Nano air purifier and you can signup HERE. Also my buddy Corbett Lunsford from Home Diagnosis TV is launching a new building performance mastermind course and giving away a TON of HVAC tools HERE

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