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When you are checking a unit of any kind you should be keeping your eyes open for signs of arcing and melting at all of your wire connections and contact points. We find issues with melting terminals on contactors and in disconnects regularly, but rarely do we think about the relationship between circuit ampacity and wire size and the connections to our equipment.

First, consider the fact that a #10 wire doesn’t always have an ampacity of 30 amps, it has an ampacity of 30 amps with a 60° Celsius rated assembly at 30° Celsius ambient. 


Now, look carefully at the wire and the contactor at the start of the article.

The wire (conductor) is rated at 90° and the contactor is rated at 75° when torqued down to 22 in/lbs on screw type terminals and 40 in/lbs on lug type.

So the entire assembly is only as good as the weakest link and the weakest link is the terminals and the terminals are only as good as the contact they are making.

Conclusion: The termination points are usually the weakest point of the circuit

When sizing conductors don’t forget ALL of the termination points. From the breaker to the disconnect to the unit, every termination point should be properly connected and the rating checked if you intend to use any ampacity other than 60° Celsius. 

Check those terminations.

For more great info on this go HERE

— Bryan

I have spent the last few days checking run capacitors on various systems with several different meters and this is what I found.

#1 – Comparing Start wire amps against Run + Common under the clamp together is meaningless as a practical test.

I used this test on 3 different systems with 3 different meters and came to the same conclusion, whether the capacitor is way too large, way too small or the right size, made no repeatable difference in the reading no matter how we read it.
Even if this is a valid test (which I cannot confirm at this time) the difference is within the uncertainty tolerance of the meter so it’s not useful for field testing.

#2 – The under load test does work (If your meter works)

reading the amps at the herm (compressor start wire) terminal multiplying by 2652 and dividing by the start voltage (herm to c) on the capacitor does work consistently on the compressor and the fan motor however some meters are less accurate at lower amperage readings so that may make a slight difference.
#3 – Power Factor works as a test but it’s a small change
I tested several systems with the Testo 770-3 in power factor mode by installing too large and too small capacitors. The power factor did decrease in all cases when the incorrect size was installed but in some cases the difference was very slight (from 1 to .99 with a 15 mfd too small run capacitor in one case). This means that while it is a valid and useful test it may not be sensitive enough to act as verfication that a capacitor is slightly outside of allowable specs.
— Bryan

 

Let’s take a deeper dive into the magic that is gas defrost..

Most techs who are familiar with heat pumps understand the basics of a gas defrost but when we apply this strategy to a larger system where we’re only reversing a small part of the system while we need to add some controls and valves to get the job done optimally.

Since we’re already familiar with the basics of defrost systems and controls, I’m not going to dwell on things like frequency or duration of defrost but we will get into some unique terminations methods and defrost efficacy testing that only work with reverse cycle defrosts.

There are 2 basic types of gas defrosts.   Hot gas defrost where superheated discharge gas is directed into the evaporator and “Kool gas” a trademarked name for a defrost that directs saturated vapor from the top of the receiver unto the evaporator.    Each have advantages and disadvantages but both work essentially the same way.

So, defrost starts and a whole lot starts happening at once.   3 electrically actuated valves all have to work together to make this happen.

First, we need to create a pressure differential between the gas we’re sending into the evaporator and the liquid line.   This is to allow that gas to flow through the evaporator and back into the liquid line.  There are many different valves that are applied to do this and an in depth treatment of each valve isn’t really possible here, so we’ll just look at the 2 most common places they’re applied.

Discharge line

This is more common on hot gas defrost system as opposed to Kool gas systems.  A valve is installed in the discharge line that, when activated, creates a pressure differential.

Liquid line 

Same thing, really.   This valve will work for either but is really necessary for a Kool gas system.   A discharge differential won’t work for Kool gas.

 

Regardless of the location in the system, the valve is typically adjusted for an 18-20 PSI differential setting.   If your equipment is significantly higher than your evaporator this may need to be set even higher.  We’ll get into a method to test this and ensure that the defrost is working properly towards the end of the article.

Differential created, we now need to direct defrost gas to the evaporator.   To do this, we have 2 valves.   One that stops flow from the suction line into the compressors and one that directs gas into the suction line and back towards the evaporator.   At the same time the differential valve activates, both of these valves activate and start the defrost process.

 

Photo caption:  the grey bodied valve, installed in the vertical line stops refrigerant flow to the compressors.   The brass valve installed on the horizontal line opens to admit hot gas to the evaporator.

Out in the evaporator, we’ve got a check valve piped to bypass the TEV

 

 not visible in this photo is the actual check valve.   The line leaving the distributor allows condensed liquid to leave the coil, bypass the metering device and re-enter the liquid line through a check valve.

 

Last thing is that, with all this heat being forced into the evaporator we normally want to turn the evaporator fans off and sometimes turn on small heaters to prevent water running off the coil from freezing on a cold drain pan.   Using either a pressure switch that cycles

Let’s “follow the gas” and try to visualize what’s happening during this defrost.   So, we’re sending high pressure, superheated vapor into a cold suction line.   That gas immediately starts rejecting heat into the surrounding pipe and any frost or ice that’s in contact with it.  Remember, we’re going backwards, so we hit the outlet of the evaporator and we’re heating it up, melting that frost away and rejecting heat from the gas all the way.   As we continue to pass through the evaporator, we’re going to reach a point where we’ve rejected enough heat to condense and possibly to even subcooling as a liquid.  Eventually, we reach the metering device and are routed through a check valve that bypasses that and wind up in the liquid line.  With a Kool gas defrost, we aren’t starting with superheated vapor, but the concept remains the same.  Warm, saturated vapor is sent to the evaporator where it condenses and is subcooled and forced back into the liquid line.

As liquid is condensed and pushed through the check valve, more and more hot gas is allowed into the evaporator to provide more heat to completely defrost the coil.  Without the pressure differential, we wouldn’t be able to push the liquid out of the coil because a pressure differential is required for anything to flow.

Is one ‘better’ than the other?

One drawback to hot gas defrost is the expansion and contraction of refrigerant lines due to temperature swings can be extreme if the lines run far enough.  Remember that copper can expand over an inch per 100’ of pipe with a 100° change in temperature, so we have to consider the expansion and movement of the piping.

Using a Kool gas defrost helps with the pipe expansion problems but tends to have less heat available for defrost and, combined with a modern push to lower compression ratios for efficiencies sake, can have problems clearing the whole coil during colder weather.

So, what can go wrong??

Sounds like a great system.  We’re reusing heat that would ultimately be wasted to melt frost from a coil.   Economically and ecologically awesome, right?

As with any complex system, there are multiple points of failure.  If any of the 3 electrically activated valves fail to operate either because of a control system fault or a mechanical problem with the valve itself, we set ourselves up for trouble.

If the differential valve fails, we won’t have adequate flow of refrigerant to get enough heat for a complete defrost.  Similarly, if the solenoid valve that opens to allow defrost gas into the suction fails to completely open, we won’t have enough flow.

If the suction stop solenoid fails to close, we’ll can see a range of problems from inadequate defrost from the amount of bleed through to a complete failure to close that allows all of the defrost gas to flow straight into the compressors.   You can see this same problem if the hot gas solenoid fails to close properly after a defrost.

 

Testing defrost

 

I promised earlier that I’d give a method to test gas defrosts to ensure that they’re working properly.

For this test to work properly, we need a coil that is free of large ice buildup but that has a ‘normal’ frost on it.   If I’m troubleshooting a particularly difficult system, I’ll first clear all ice from the coil, then disable defrost overnight and return in the morning to ensure that I have the right conditions to test the defrost.

Now, I’ll connect a thermometer to the line that bypasses the TEV at the evaporator and allow that to stabilize.  I really like to use a thermometer that record Min/Max readings for this job. You can also take temperature on the line leaving the evaporator or really anywhere along the liquid line that is dedicated 100% to that circuit.   It that line runs all the way back to the compressor unit, you can test it there although the further from the evaporator you measure the temperature, the less accurate the test becomes.

Make a note of the temperature in your notebook and go start a defrost.   Monitor this temperature and a distinct pattern should emerge if defrost is functioning properly.   The temperature will hold stable for a couple minutes.  Typically this is already pretty cold because we’re in a refrigerated space, then it will start to drop.   I will normally see a start temperature in the low ‘teens’ here and expect within 2-4 minutes to see it dropping and it will hit a low of -2 to -6.  This is a rush of liquid that has condensed in the evaporator and has rejected so much heat that it is very subcooled.

This temperature will then start to rise as there is less and less frost to absorb heat from the gas.  Once all the frost is gone, this will start rising pretty rapidly.   Once it hits 65° on newer equipment and 75° or so on older equipment, you can be sure that there is no frost left on the equipment and that any further defrost is just wasting time and is detrimental to equipment operation and possibly to product shelf life.

Much of the timing depends on the length of the suction line and the amount of frost buildup on the coil.   A shorter suction line will result in a faster temperature drop while more frost on the coil will result in a slower but deeper dip in temperature before it starts back up.

This is also probably the best method to use to terminate this type of defrost.   Monitor that temperature using whatever means available to you and, once the liquid temperature rises above either a manufacturer’s predetermined setting or one that you’ve field determined through testing, you can end defrost.

–Jeremy Smith CMS

 

 

 

This is a quick article from the archives that got a big response 4 months ago. I also just did a Facebook Live video this morning baring my soul on the topic of flowing nitrogen in response to an Email.

Enjoy.

Why is it called single phase 240 when there are two opposing phases?

I wondered why two 120v opposing phases was called “Single phase 240” for years.

Then someone pointed out to me that a typical “single phase” pole transformer only has one power leg entering and two coming out.

This freaked me out. How can a transformer primary be one phase, a SINGLE sine wave and output two perfectly opposing sine phases?

It’s just two separate winding wraps in OPPOSITE directions on the secondary. Stupid simple, but I just never knew it.


So unlike a three phases services that uses all three power phases from the power supply, the single phase service only uses one. The second phase is “created” in the secondary of the distribution transformer itself and is the same “phase” but opposite.

Pretty cool.

— Bryan

This quiz was written by Benoît Mongeau

 

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This article was written by Senior Refrigeration tech Jeremy Smith. Big thanks to Jeremy for his contributions to HVAC School and the tech community. 


Having spent many years in the trade and many years reading posts from techs on forums and social media, a big issue that I see is that troubleshooting is something of a lost art.

Troubleshooting is where the rubber meets the road for a service technician. Nobody cares what certifications you have, what union you belong to or anything else. If you can’t find the problem and solve it in a timely fashion, your customer and employer are not going to be happy.

One of the things that I think most guys struggle with is the mental aspect of troubleshooting. I’ll relate this in the form of a recent call I was sent on to “clean up”. It was a no heat call in a small convenience store. Trane RTU on a zone sensor.

The tech called me and related that the unit had a call for heat at the unit but the ignition sequence didn’t start. We talked a little about the problem, he checked some limits and a few other things. He wound up ordering an Ignition board and limit sensors. These were replaced late that night and the unit still didn’t work.
I was sent the next morning. Now, we get into the mental part of troubleshooting. 

I met the tech so that he could communicate the basics of what he did. We talked for about 10 minutes and he went on to his job and I went to have a chat with the trouble unit.

20 minutes later, I had the problem solved. I found a failed RTRM board. Now, you guys that do Trane all the time probably aren’t surprised, but let’s analyze what went wrong and how this could have been handled on a “one stop” basis.

What did I do that the first Tech didn’t?
For starters, I took everything that I was told about the unit, what it was and wasn’t doing and what everybody and their brother thought was wrong with it and I threw it all out. Put it in a box in my head, closed the lid and locked it.

I dug out the basic Trane “Service Facts” book and started the troubleshooting procedure from Step 1 and followed it to the end.

Now, I can make these arrogant claims about how I’m a Billy Badass service guy and how I’m more awesome than anyone else, but the simple fact is that I’m not. I do things a little differently and think a little differently than many others  and that sets me apart.

What did the first Tech do wrong? While I’m not in his head, I think that he focused on why the heat didn’t work instead of taking the unit AS A WHOLE and diagnose it as a whole. Kind of like the guy who can’t figure out why the fridge is warm and spends an hour working on it only to find the plug pulled.

So, the the mental aspect of troubleshooting cannot be ignored. 
Start at the beginning, work the process and troubleshoot the entire system. Being willing to read the manufacturers troubleshooting info isn’t a newbie move, it shows maturity.

Work on the troubleshooting mindset, don’t be a parts changer.

— Jeremy

(Edited by Bryan Orr, any mistakes are my fault)

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