When to Consider J, D, and S


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Service technicians can make a good living using their troubleshooting skills. Troubleshooting is identifying problems and establishing their cause. Effective troubleshooting demands more than just trying to match symptoms with causes.  

When it comes to HVAC troubleshooting, we naturally look for the simplest, quickest explanation. However, the root cause may not be so simple. There are many mechanically skilled HVAC technicians in the trades. These technicians can quickly identify mechanical problems and repair them quickly.  

A skilled technician should recognize when the root cause of component failure is related to improper equipment sizing and airflow. At a minimum, there are three tools that service technicians should be familiar with that can help them solve some underlying issues. Those tools are ACCA Manual J®, Manual D®, and Manual S® 

First, Manual J® estimates heating and cooling loads for almost all types of residential buildings. However, Manual J® does not tell you which size equipment should go into the house; it only assesses the heat loss and gains on a home. Second, Manual S® provides procedures for selecting and sizing residential cooling and heating equipment based on Manual J® with the help of the OEM performance data. Finally, Manual D® is used to design residential duct systems. 

Before jumping into suspecting that incorrectly sized, selected, and ducted equipment is the culprit, do your due diligence by thoroughly inspecting the HVAC equipment.  

Let’s start with some common complaints by clients that could result from an improper load calculation. Occasionally, you may get a complaint from a customer. “It doesn’t feel comfortable to me in here. It feels hot and sticky, and I have my AC set to 68 degrees.” As you know, this is only one of many complaints that may lead you to conduct a load calculation. Here are other common complaints that lead you to do a Manual J®:

  • Clients are complaining of discomfort during design weather days 
  • The client is complaining of the inability to control indoor humidity 
  • The equipment is short cycling  
  • Repeated parts failures such as capacitors and compressors.  

It is vital to understand that Manual J® is the foundation. It is the first step in diagnosing a potential issue resulting from client complaints with comfort and system performance.  

Next, Manual S® is the procedure to ensure the home has the correctly sized equipment that will meet the sensible and latent loads of the house. Manual S® emphasizes the importance of using OEM performance data that connects sensible and latent cooling capacity with the loads that affect home comfort. As with incorrect load calculations and incorrect ducting, incorrectly selected equipment has its list of issues that can arise. A home with the wrong HVAC equipment can cause:

  • Inadequately maintaining the indoor temperature 
  • Insufficient indoor humidity (Optimum range for space humidity is 40% RH-60% RH for health and comfort) 
  • Short cycling (Creating wear and tear on the equipment) 
  • Sensible and latent loads are not met at design conditions 

Not only can incorrectly size and selected equipment lead to the above issues, but it can also contribute to health issues for the home’s occupants. For health and comfort, 50% RH-60% RH is best. When it comes to Manual S®, think of it as fitting the house for ‘shoes.’ You wouldn’t buy shoes that were too big or too small that create sores or poor comfort; you will buy shoes that fit just right. The same principle applies to selecting the correctly sized HVAC equipment for the home.  

Next are some common issues related to inadequate airflow and a poor duct design. For example, when a duct system is not sized to handle the airflow requirement of the system efficiently, your client’s equipment may experience: 

  • Excessively loud equipment (If your client must turn the TV up when the HVAC comes on, that’s called a clue!) 
  • High static pressure  
  • Premature equipment and parts failure 
  • Inadequate comfort  

Please, keep this in mind when considering diagnosing and solving ducting and airflow issues, do not use ‘rules of thumb.’ There are false ideas that the required CFM equals the floor area multiplied by some magic number or that a particular size duct always delivers a specific CFM. When in doubt, consult Manual D®. If you suspect airflow/ducting issues, Manual D® should be your next tool to diagnose the problem.  

When it comes to diagnosing, there are so many avenues that must be explored. However, load calculations, equipment selection, and duct designs are the foundation to ensure the home’s HVAC equipment will work correctly and efficiently.  

As a technician, you are called to work on the entire system, not an isolated part. All system components depend upon each other. In truth, it is difficult to examine any single aspect of system performance while ignoring the rest of the system. With that being said, the savvy technician can recognize when the equipment issues will not be resolved by throwing parts or making adjustments. Instead, the intelligent technician will articulate to their client the need to perform a Manual J®, Manual D®, and Manual S® to drill down to the root cause of their issues and provide meaningful and valid remedies and not Band-Aid fixes.  

If you are not comfortable with performing the Manual J®, D®, and S® calculations, it is okay to request help and send someone to your client’s house to execute these calculations. You may want to consider reading ACCA’s Bob’s House. Bob’s House is a case study that discusses the residential design process from start to finish, using Bob’s House as an example. Although it provides a comprehensive solution for this particular example, it does not cover every scenario or investigates every possible solution. 

Your job as a technician is to educate your clients while at the same time being thorough and honest in your diagnosis and recommendations. Be sure to lay out all the facts so that your client can make an informed decision on how they want to proceed to remedy their HVAC system issues.  

Matt Akins
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Posted In: ACCA Now, Customer Service, Training

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