I understand that ACCA Manual N gives the industry-accepted requirements for small commercial buildings heat/cooling load calculations. Is that correct? What does 'Small Commercial' building mean? Is there a category of building that is larger or more sturdy than Small Commercial? Air Conditioning Contractors of America since 1969! Mission: To lead America’s indoor environmental and energy professionals to business success.! Proactively represent the professional indoor environment and energy community with emphasis on legislative, technical, and regulatory issues.
The Manual N for commercial load calculation is designed to instruct contractors and others on how to meet the complicated new heating and air conditioning requirements for commercial buildings. This manual treats in detail all that you will need to know, including the latest developments in the commercial construction industry such as the use of new materials, assembly, operational requirements, and more. Being up to date on your commercial load calculation will ensure that you meet any possible building codes that may be in place, are keeping the occupants safe and comfortable, and are keeping the utility bills as low as possible.
Manual N for Commercial Load Calculation
As will eventually happen, when it comes time to replace your heating and air conditioning system in your commercial building, be sure to have a professional technician perform a commercial load calculation beforehand. A ‘load’ is the amount of heating or cooling that is required in order to maintain the temperature level comfortable enough for the occupants. A ‘commercial load calculation’ is the complicated process by which you come to determine the exact size of the load and therewith the size and power of the heating and cooling system that will be needed. Choosing the correct size of the system will be important for several reasons, as mentioned quickly above.
In general, if the size of your heating and cooling system is too small and too weak, it will, of course, not be able to properly affect the temperature of the rooms, resulting in an uneven spread of temperature regulation. Also, if they are too small, they will be working overtime in an attempt to regulate the temperature, which will result in the units breaking down prematurely. On the other hand, if the cooling and heating units are too big, too powerful, they will be producing too much heating and cooling, even when set to their lowest setting. This will result in indoors being too hot or too cold, and it will also result in accumulated amounts of humidity.
Therefore, having the perfect sized heating and cooling system unit for your commercial building is key, and using a commercial load calculation will help achieve just that. This calculation will be determined by inserting the physical and thermal characteristics of your building. These include things such as the size of the building, the numbers of doors and windows, the efficiency of the sealing, and the expected climate.
More about the Manual N
The manual N, which is approved by the ACCA, is the most widely accepted manual when it comes to commercial load calculations. Using this state of the art manual, technicians can conduct one of two types of calculations: either a basic load test, or a more detailed computer-based test. Because this manual N is so well trusted in the industry, you can rest assured that, by using it, you will not only keep the occupants safe, but will also have the most cost-effective heat and cooling system available.
Finally, while it is recommended to go with a professional technician when it comes time to perform a manual N commercial load calculation, it is possible to perform one yourself. There are online courses that one can take for several hundred dollars which will give you detailed instructions, but be ready to get very technical if you plan on going this route.
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Unidentified compressor terminals
Once in a while you will come across a compressor whose terminal markings are not clear or missing altogether. To identify the Common, Start, and Run terminals, the only tool you’ll need is an ohmmeter. Use a good, dedicated ohmmeter. Do not attempt this with the ohm scale offered on your clamp-on ammeter, for instance, it’s not precise enough.
First, assign one unidentified terminal as “A”, another as “B” and the third as “C”, as shown in figure 1, for example. Pull the disconnect and remove all wires from the compressor terminals. Now, whip out that old Simpson meter and by using the Rx1 scale, you can identify these terminals. Place your ohmmeter between each set of terminals and record the readings.
Let’s say you read the following:
A to B = 2.5 Ω
B to C = 3.0 Ω
A to C = 0.5 Ω
Here are the guidelines:
The pair with highest ohm reading is not Common.
The next highest pair is the Start winding.
The remaining terminal is the Run terminal.
Now, apply the guidelines. Which pair has the highest ohm reading? “B” to “C”, correct? Therefore, Common is our assigned terminal “A”. Next, what is the next highest pair? “A” to “B”, correct? Therefore, knowing that “A” is Common, the Start winding is between “A” and “B”, making “B” the Start terminal. That leaves our remaining terminal “C” as the Run terminal. Easy peasy.
This relationship is important to know because the terminals of a good compressor motor will always have the following relationship:
Common to Start + Common to Run = Run to Start
Our example followed that relationship perfectly, as it should:
C to S = 2.5 Ω
C to R = 0.5 Ω
R to S = 3.0 Ω
Looking at the internal wiring of a compressor (as in Figure 2) may help to solidify that point in your mind. Notice the thinner wire used in the Start winding with many turns. That will create a higher ohm reading. Also, notice the heavier wire used in the Run winding with fewer turns. This will result in a lower ohm reading. Also, when you’re reading from Run to Start, you are reading both windings in series, hence the highest ohm reading.
Open internal overload
To check for an open internal overload (OL), first allow the compressor to cool off. If the internal overload is open, it can take many hours to reset. You can speed this process up by dumping liquid refrigerant into the low side at the service valve. The heat in the compressor will be drawn to the cool refrigerant as it evaporates in the compressor shell.
This method is preferred to duct taping a garden hose to the compressor dome. Line voltage and water simply don’t mix. Don’t be afraid to dump liquid refrigerant into the system. Remember it was a lack of refrigerant that most likely caused the internal overload to open in the first place. It’s a thermal overload, and it opens in response to a high temperature.
Now, place your ohmmeter between Common and Run, and then between Common and Start. If the overload is open, then both readings will show infinity (ꝏ), or OL on a digital ohmmeter.
Short to ground
Checking for a short to ground is fairly simple. Place your ohmmeter between the Start terminal and ground, then between the Run terminal and ground. A short would be indicated by any reading of continuity.
If your compressor has a locked rotor, you can reverse the direction that the motor turns by reversing the Start and Run windings. Reversing the rotation of the motor will usually free up a locked rotor. This procedure is not a cure for a compressor that is failing but it can buy you a little time. Be sure to reinstall the wires on the proper terminals once the rotor is freed up.
What we have discussed here is relevant to single-phase compressors. For three-phase compressor motors, all windings should have the same resistance. Each winding has its own thermal overload (OL), or they sometimes share a common overload. You can reverse the rotation of a three-phase motor by interchanging any two leads.
Acca Hvac Training
These checks can be performed as needed during troubleshooting. They can also be employed as part of a spring PM and/or a fall PM for heat pumps.
- Compressor Motor Check-Out - September 1, 2020
- Outside Air - May 29, 2020
- Airflow Basics Part 2: The Lost Art of Choosing a Blower - May 27, 2019
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