Month: May 2018

Grounding and bonding is important and it’s a fairly broad topic.

In HVAC/R we generally just need to make sure that we properly connect the conductors (wires) provided to the proper connection points in the equipment and occasionally the disconnects.

These connection lugs and / or green screws are rated for grounding so long as the system is “listed” by UL or another listing agency recognized by the NEC (National Electrical Code).

In other words, of you follow manufacture specs and diagrams you are in good shape.

There are some cases where we are tasked with making proper grounding connections when making repairs and when adding in accessories.

I’m going to encourage you to take more time when grounding rather than simply wrapping a wire around a random equipment screw and calling it a day.

There are practical and code reasons for this, but one of the most compelling reasons is you don’t want to have been party to an injury caused by poor grounding.

Now for the code reasons.

The National electrical code wants us using grounding connections listed and designed for that purpose or making sure that we use screws that have

At least two threads engaged or a nut connected on the opposite side

And this can be tricky to accomplish when connecting to sheet metal.

Let’s take a look at exactly what the code (NEC NFPA 70® 2017) has to say –

250.8 Connection of Grounding and Bonding Equipment.

250.8(A) Permitted Methods.

Equipment grounding conductors, grounding electrode conductors, and bonding jumpers shall be connected by one or more of the following means:

250.8(A)(1)

Listed pressure connectors

250.8(A)(2)

Terminal bars

250.8(A)(3)

Pressure connectors listed as grounding and bonding equipment

250.8(A)(4)

Exothermic welding process

250.8(A)(5)

Machine screw-type fasteners that engage not less than two threads or are secured with a nut

250.8(A)(6)

Thread-forming machine screws that engage not less than two threads in the enclosure

250.8(A)(7)

Connections that are part of a listed assembly

250.8(A)(8)

Other listed means

One of the most common disputes about grounding involves the type of screws used so let’s take a quick side path to address that.

Take a look at the threads on each of these screw types. You will notice that the thread cutting (forming) and traditional machine screws have a fine thread with a less aggressive pitch while the drilling and tapping screws are designed to be “driven” into metal.

The NEC mentions a “machine screw” but the definition of what exactly is and isn’t a machine screw can vary so we won’t use that alone to exclude self drilling, self tapping and equipment screws.

We know that traditionally when someone says a machine screw they are talking about the finer threaded screw that can accept a nut and obviously there is no way to put a backing nut on any “drilling” type or typical HVAC equipment screws.

It all comes down to the thickness of the the metal and how many threads are engaged. This is why many equipment grounding screws are set into an extrusion or dimple in the metal and the screw is tapped into it.

When you are in a pinch you can always use a machine screw and a backing nut and the NEC says you are fine. You will notice in section 250.8 there is nothing about the screw needing to be green… so at least you can save on green paint.

— Bryan

Addendum

It was pointed out to me after I wrote the initial article that there IS a section about the equipment grounding screw needing to be green or marked. Thanks to Michael Labanz for pointing out this section

250.126 Identification of Wiring Device Terminals.

The terminal for the connection of the equipment grounding conductor shall be identified by one of the following:

250.126(1)

A green, not readily removable terminal screw with a hexagonal head.

250.126(2)

A green, hexagonal, not readily removable terminal nut.

250.126(3)

A green pressure wire connector. If the terminal for the equipment grounding conductor is not visible, the conductor entrance hole shall be marked with the word green or ground, the letters G or GR, a grounding symbol, or otherwise identified by a distinctive green color. If the terminal for the equipment grounding conductor is readily removable, the area adjacent to the terminal shall be similarly marked.

Informational Note Figure 250.126 One Example of a Symbol Used to Identify the Grounding Termination Point for an Equipment Grounding Conductor.

I remember it like it was yesterday… It was my first day of work as a trainee at my first technician job, just a wet behind the ears kid fresh out of trade school.

It was a Monday morning and technicians and I were standing in the dusty warehouse surrounded by stacks and stacks of brand new condensing units drinking the nasty warehouse coffee…

and I was LOVING IT

Finally, I had made it, one of the guys, listening to the war stories and well-natured ribbing and getting a caffeine fix for the day.

One of the senior techs was telling a story of low suction pressure and he said “So I figured it has to be the wrong sized piston” and he stopped and looked over at me and said “you know what a piston is….. RIGHT”

It seemed like an eternity passed as the whole group stared at me, I mumbled “a piston sure” and gave a weak nod hoping that “LIAR” wasn’t emblazoned on my forehead for All to see.

The tech turned and finished his story and my mind raced….

Of course, I knew what a piston was in an ENGINE or even a reciprocating compressor but I had no clue that the little hunk of brass with a hole in it that we called a “fixed orifice” in school was called a piston.

Later I learned all there was to know about sizing and replacing pistons. The installers I worked with often forgot to put in the correct size.

In case you are like I was, a piston is a fixed orifice metering device used in systems for many years. They are especially in residential heat pumps and straight cool systems. Even now that TXVs and EEVs are becoming more popular you will still see pistons in many new Carrier models being used outside as the heat mode metering device.

Piston Facts

There are three common piston designs I see regularly and while different manufacturers may use them I will group them by the manufacturers I know them by.

Lennox / Rheem Type

The piston shown above is the Lennox / Rheem style. It is directional, meaning it can only be installed one way with the cone (tapered side) pointed at the evaporator and the other side pointed at the liquid line. This type uses seals toward the end of the cone to help prevent refrigerant bypass and it also uses an o-ring to seal the “chatleff” style housing.

Carrier Type

Carrier used to call their pistons “accurators” and maybe still do although I haven’t heard that term for years. These pistons can be installed in either direction but still use the same “chatleff” style housing as Lennox

Trane Type

The Trane style has a much smaller size and is directional. The Trane housings do not use o-rings.

Piston Size

The physical exterior dimensions of the piston must be the same as all the others for that brand/series otherwise it will not fit properly. It is only the internal bore diameter that changes.

Pistons are sized in decimals of an inch like a gas orifice, usually from the 40’s up to the low 100’s. When a piston is described as being a “65 piston” that means it is 0.065 of an inch and a “104” would be 0.104 of an inch.

Check Flow Operation

In a heat pump system, every metering device needs some method of bypassing the metering device when the refrigerant flows in the opposite direction. This is done in TXVs by means of an internal or external check valve but with a piston, the piston itself is allowed to slide in the housing allowing restricted flow in one direction and unrestricted flow in the other.

This is actually where a piston gets its name, because like a piston in an engine it is a cylinder within a cylinder that can slide back and forth.

Any carbon, wax or other solid material that gets into the piston housing can cause one of three undesired conditions

Piston Restriction in the Desired Mode

If something gets into or covers the orifice bored into the piston it can cause a restriction resulting in low evaporator pressure, low suction, high superheat and normal to high subcool. When a piston is restricted and the system is a heat pump with a liquid line filter/drier properly installed, we will often alternate the system into cool and heat and see if that will break free the contaminants and catch it in the line drier. Otherwise, the piston should be removed, inspected and cleaned or replaced and a new line drier installed.

Keep in mind that some systems have a screen built into the piston housing inlet that can also block up. Look for this once the piston housing is disassembled.

Piston Bypassing (Overfeeding)

If the piston fails to seat properly it can overfeed the evaporator in the same way it would if the system had a larger bore size than it should. This will result in high suction pressure, low superheat and low subcooling. In these cases, the piston should be removed and inspected for proper bore size and signs of contamination around the outside or near the seal surfaces of the piston and the housing.

Opposite Mode Piston Restriction 

In some cases, a heat pump piston may fail to fully unseat in the opposite mode. This will result in a pressure drop and an undesired restriction similar to a clogged liquid line filter drier.  In this case there will be a clear temperature drop across that piston when there should be little to none.

For example, if you are running a system in cooling and you notice frost starting to form on the liquid line side of the outdoor, heat mode piston housing, you can be sure it is restricting in the opposite direction. Sometimes this can be resolved by switching back and forth from heat to cool a few times but often it will require disassembly and inspection.

This condition is similar to what happens when an external TXV check valve fails.

In Closing

A piston is a simple little hunk of brass, it drives me nuts when a tech incompletes a call so that someone can “replace a failed piston”. A piston doesn’t just fail, if one does have an issue it’s either the wrong size or something got into it and got stuck in it or caused it to stop seating properly. Many of these issues lead back to improper vacuum, failing to flow nitrogen, getting copper shavings or sand in the system etc…

Every good residential tech should have a little plastic container with various brands and sizes of piston in it in case you find one that is the wrong size or worn down from improper seating. I may be a little late to the game here since pistons are a dying breed but they are simple enough that a return trip for a “failed piston” seems like a huge waste.

— Bryan

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