Month: May 2018

Let’s first state the obvious. Most techs are intimidated by Psychrometric charts and Mollier diagrams, we JUST ARE. While there are some pretty complicated formulas that back up these diagrams, using them isn’t a big of a deal once you understand the different elements and then focus on one at a time.


Dew point is one example of a very useful measurement to understand, design for and test for in an HVAC/R system. Take an evaporator coil, do you know how to calculate the exact temperature at which that evaporator coil will start to condense moisture? can you tell the exact temperature at which a surface inside of a space will start to condensate and possibly grow mold? These are both cases where a basic understanding of a psychrometric diagram can help a technician.

While some of the elements on the chart are represented by curved or slanted lines, dew point temperature and humidity ratio / absolute moisture content are just straight lines horizontally across the chart.

So if we focus on a 65°F(18.33°C) dew point on the right side of the chart you will notice it crosses  over 92 grains (there are 7000 grains of moisture per lb) of moisture line and then goes all the way across until it intersects with the curved 100% humidity line on the left side. This shows us that at a 65°F(18.33°C) dew point the air always contains 92 grains of moisture per lb.. ALWAYS.

This also shows us that when the air is at 100% relative humidity the dew point, wet bulb and dry bulb temperatures are ALL THE SAME.

If we have a dew point of 55°F(12.77°C) the air contains 64 grains of moisture per lb. If the dew point is 30°F(-1.11°C) the air contains 24 grains… you get the point.

So now if you find the dry bulb temperature and the relative humidity you can easily calculate the dew point at which that same air will reach saturation and begin to form condensation.

Let’s say we have 75°F(23.88°C) dry bulb air at 50% relative humidity. We would simply draw a line up from the bottom at 75°F23.88°C) until we hit the curved 50% line. Then go right (or left) until you bump into the the grains of moisture and then the dew point scale. Now you know at what temperature that same air mass will start to condense water.

So we can see that this if this 75°F(23.88°C) dry bulb 50% relative humidity mass of air comes in contact with a surface that is 55.5°F(13.05°C) or less, it will begin to condense water. We also know that this air stream contains 65 grains of moisture per lb of air.

Forgive me for saying so, but I think this is pretty cool.

— Bryan

P.S. – If you want a good quality Psychrometric chart you can use THIS ONE

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 TypeResidentialCommercial / InstitutionalIndustrial
Main Ducts700 – 9001000 – 13001200 – 1800
Branch Ducts600 – 700600 – 900800 – 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

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