Tag: commercial

I heard a great presentation by Ron Auvil on VAV systems and it got me thinking…

Can you size a commercial system / perform a block load by the number of occupants?

Yes! 

No, just kidding that’s crazy talk. There is way more too it than that.

However, in a commercial environment, while the perimeter of the building is affected by heat loss/heat gain to the outdoors, the internal zones are “cooling only” zones with the primary load usually being PEOPLE.

This is where the 500 btus per hour comes in. On average a sedentary worker in a building will add 500 btus per hour to ALL areas of the building whether it is hot or cold outside. This creates an issue in the winter when the perimeter of a building requires heating and the center of the building requires cooling.

Now, keep in mind, a sleeping person generates heat more in the neighborhood of 260 btu/ hr so if it’s a REALLY boring job where workers dose off at their computers it may be less.

Add in the internal electrical loads from lights, computers and other equipment and you start to realize that EXTERNAL loads are only part of the equation, especially in large commercial buildings with many occupants. In fact, in a busy commercial space the internal loads generally far outweigh the heat entering from the outside (external load).

This is where the concept of thermal diversity comes in. On a cold day there may be a need for heat at the perimeter of the building to offset heat losses to the outside while still requiring cooling in the center of the building to offset the internal loads.

In a good commercial design you must have some method of dealing with the thermal diversity between internal and perimeter zones along with maintaining appropriate ventilation / outdoor air.

Food for thought.

— Bryan

This tip was created by Jason Pinzak and originally posted on the HVAC Technician’s Facebook group. It is reposted here with permission from Jason. Thanks!


Contactors are useful in commercial and industrial applications, particularly for controlling large lighting loads and motors. One of their hallmarks is reliability. However, like any other device, they are not infallible. In most cases, the contactor does not simply wear out from normal use. Usually, the reason for contactor failure is misapplication. That’s why you need to understand the basics of contactors.

When someone uses a lighting contactor in a motor application, that’s a misapplication. The same is true when someone uses a “normal operation” motor contactor for motor jogging duty. Contactors have specific designs for specific purposes.


When selecting contactors, you’ll use one of two common standards: NEMA or IEC. Both match a contactor with the job it has to do, but they do so in different ways.
The NEMA selection process always results in a choice of a contactor you can use over a broad range of operating conditions. For example, you could use a NEMA Size 5 contactor to run a 50-hp motor operating at 230V or a 200-hp motor at 460V.


Using IEC standards, however, you can size contactors very close to their ultimate capabilities. In many cases, this precision allows you to predict how long they’ll last. For example, an IEC-rated contactor may run a motor that draws 40A at full load. In that duty, it should last for more than two million operations. But, if you used it for consistent jogging and plugging, you’d have to replace it after just a few thousand operations.

Since a contactor should last for years, don’t automatically replace one that fails with an identical unit. Instead, take a few moments to see if there is an obvious problem. A contactor really has only two basic parts: the contacts and the coil. The coil energizes the contactor, moving the contacts into position. The contacts transmit the current from the source to the load. Heat can destroy either of them, so take a good look at both.

Contacts will overheat if they transmit too much current, if they do not close quickly and firmly, or if they open too frequently. Any of these situations will cause significant deterioration of the contact surface and the shape of that surface. Erratic operation and failure will be quick. To check the contacts, just look at them. Some minor pitting (see photos) as well as a black oxide coating is normal, but severe pitting or any melting or deforming of the contact surface is a sure sign of misapplication. Replace contacts with such symptoms.

Coils can overheat if operating voltages are too low or too high; if the contacts fail to open or close because of dirt or misalignment; or if they have suffered physical damage or experienced an electrical short. Coil insulation degrades quickly when it gets too hot. When it degrades, it will short out (and blow a fuse) or just open and stop operating.

To check a coil, measure the ohms across the contactor coil. Infinite resistance means the coil is open. A shorted coil will still often register significant resistance and can be confused with a good coil . If you happen to have a matching contactor nearby, compare the two coils. The shorted coil will usually have significantly lower resistance than the good one but a compromised coil can alos have a higher resistance. If the difference is significant, replace it. Replacing the contacts or coil often means replacing the whole contactor. But no matter what you replace, compare the NEMA or IEC rating with the job the contactor will be doing. If you match it to the application, it should last a long time.

— Jason Pinzak

P.S. – Here is another good article on the difference between IEC and NEMA rated contactors

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

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