In commercial HVAC you will find several different types of multi-stage evaporator coils, intertwined (like shown above), horizontal face split (one coil on top of another), and vertical (side by side).

Pay attention when staging a horizontal evaporator to ensure that stage #1 is on the bottom and stage #2 is on the top. If stage #1 is on top you risk condensate being pulled off of the coil when the water runs down the wet fins and then hits the dry second stage on the bottom.

By keeping stage #1 on the bottom the moisture adhesion will stay consistent as condensate drops no matter if one or both stages are calling.

You can also have this same effect when stage #1 fails and stage #2 keeps running on a stacked horizontal coil.

— Bryan


Yesterday I walked up on one of our managers who was talking to a junior tech diagnosing an intermittent controls issue on a pool heat pump.
In the background, you could hear an EXTREMELY loud compressor.

The junior tech had just been moving some wires around and next thing he knew there was a clack and then the noisy compressor and equalized pressures.

Do you know what happened yet?

I asked him to shut it off and if it was a scroll compressor.

Sure enough, it was.

What happened was an instantaneous short cycle caused by the loose connector being moved. In that split second the high-pressure gas in the scroll forced the scroll plate the opposite direction ever so slightly and once the power came right back on it was running backward.
Now, this really shouldn’t happen but when it does happen it’s because of one of a few reasons.

Three phase miswiring 

When a new 3-phase building is constructed or when a new unit or compressor is installed it is possible to miswire the phases resulting in a compressor running backward. This is not a good thing but can be corrected by switching any two legs of incoming power.

Instantaneous short cycling

This is what happened in the case of my junior tech. In most cases, the time delay in a board, thermostat or controller will prevent this from occurring. Sometimes the cause is internal to the system due to loose connections etc…

Miswiring or Failed Capacitors

In single phase applications, the run capacitor applies a phase shifted potential that helps get the motor running and keep it running. If the capacitor is failed or the compressor miswired it can occasionally (rarely) result in it running backward.

Failed Discharge Check Valve

Most scroll compressors have either an internal or external check valve that prevents the discharge gas from forcing back through the compressor causing it to spin backward.

Occasionally you may find a scroll compressor that makes a loud whirring once it cycles off. This can usually be corrected by installing a discharge check valve or by replacing the compressor if you choose.

Finally, be aware that anytime a scroll runs backward it can do significant damage. If you find one that is, shut it off immediately and correct the cause.

— Bryan

Recommended Duct Velocities (FPM)

Duct Type Residential Commercial / Institutional Industrial
Main Ducts 700 – 900 1000 – 1300 1200 – 1800
Branch Ducts 600 – 700 600 – 900 800 – 1000

As a service technician, we are often expected to understand a bit about design to fully diagnose a problem. Duct velocity has many ramifications in a system including

  • High air velocity at supply registers and return grilles resulting in air noise
  • Low velocity in certain ducts resulting in unnecessary gains and losses
  • Low velocity at supply registers resulting in poor “throw” and therefore room temperature control
  • High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal
  • High TESP (Total External Static Pressure) due to high duct velocity

Duct FPM can be measured using a pitot tube and a sensitive manometer, induct vane anemometers like the Testo 416  or a hot wire anemometer like the Testo 425. Measuring grille/register face velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer

First, you must realize that residential, commercial and industrial spaces tend to run very different design duct velocities. If you have ever sat in a theater, mall or auditorium and been hit in the face with an airstream from a vent 20 feet away you have experienced HIGH designed velocity. When spaces are large, high face velocities are required to throw across greater distances and circulate the air properly.

In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 600 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses.

Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. This helps greatly reduce total system static pressure as well as return grille noise.

Supply grilles and diffusers should be sized for the appropriate CFM and throw based on the manufacturer’s grille specs like the ones from Hart & Cooley shown above. Keep in mind that the higher the FPM the further the air will throw but also the noisier the grille will be.

— Bryan

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.

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