Performing Manual J, S, & D On An Existing Home or Remodel

Date Published: August 4, 2023

HVAC mechanical design is a crucial part of the building process. The IECC requires that HVAC systems must be compliant with certain guidelines in order to meet code. The industry standard for calculating proper design is set forth in three important formulas: Manual J, Manual S, and Manual D.

As a company that specializes in mechanical design, we handle new builds, additions and renovations. In the case of new construction, the HVAC is starting from scratch, which makes the mechanical design process a little more straight forward.

But what happens when there is an add-on or remodel to an already existing property? With a whole HVAC system already in place in the existing property, it makes calculating HVAC design of the updated portions more complicated.

We get a lot of clients asking us about how to go about handling Manual J/S/D for remodels and add-ons. So we figured it’s about time to write a blog about it!

But first, let’s have a quick refresher on what Manual J, S and D actually are…

What are Manual J, S and D?

When calculating HVAC design, Manual J will be your first step. This step is used to determine the building’s cooling and heating load (i.e. the amount of energy it to takes to sufficiently heat and/or cool the space). Manual J analyzes a list of specific criteria (such as insulation levels, type and number of windows/doors, size and orientation of the building, among other items) that allow it to yield an accurate calculation.

Manual J Load Calculation Book

Manual S is the next step in the process. This step helps to determine what materials and equipment will be used to for the HVAC system installation. It works in conjunction with Manual J.

Manual D is only necessary when ducts are needed in the building. This process uses the findings from Manual J, Manual S and several other factors to find out the proper ductwork design for the project.

Preparing these reports requires specialized software tools and training.

For more in-depth information about Manual J/S/D, check out our article, Manual J, Manual S, and Manual D: What’s The Difference?

Manual J/S/D Reports for Remodels and Additions

Some counties still only require Manual J/S/D reports for new builds. However, more and more areas are also beginning to require them for additions and remodels.

This is primarily because municipalities want to ensure that the existing home and the addition will both receive sufficient heating/cooling from the HVAC system(s). As energy standards continue to tighten, we will likely continue to see more areas adopting the manual requirements for remodels/additions.

We often receive report requests from builders that intend to tie the new HVAC systems in with the pre-existing system, utilizing the pre-existing furnace and AC for the addition. While this can oftentimes be a viable route, making these calculations can get tricky…

The main issue with generating manual reports on these types of projects is that we usually do not have access to the design plans for the already existing ductwork. And we of course can’t see the ducts because they are hidden by the finishes of the home. Without knowing how the existing ducts are designed, we are unable to generate a Manual D (duct design) report. We are only able to prepare a Manual J/S report.

Even when we only need to do Manual J/S reports, we still see our fair share of obstacles. In many cases, the builders or architects will provide us with the plans for the prospective addition/renovation. However, homeowners often do not have access to other critical details about the existing home, such as insulation levels, types/sizes of windows etc.

Architectural plans used for Manual J preparations

What Should I Do For My Renovation Project?

We get a number of clients asking what route they should take with regards to HVAC design. We have been in the mechanical design business for quite some time, and have learned a thing or two along the way. So we are always glad to give our two cents.

It generally depends on a few factors. First, it depends how big the project is – are you adding a small fitness room onto the guest room, or are you building a 1,ooo square foot man cave on top of the garage?

It’s also depends on the information you have access to. Do you have the original plans and specs for the existing property?

It is also dependent on what is required for renovations in your jurisdiction. Some counties require J, S, and D; some just require J and S; and some don’t require any at all.

For Large Additions

When it comes to large additions, such as pop tops, it’s usually an easier route to install a whole new system in the remodeled section of the home. In this case, we are able to calculate all three reports – Manual J, S and D. This will yield results that are solely applicable to the renovated portion of the home.

But say you are aiming to utilize the pre-existing HVAC for the addition. In this scenario, if the architect is able to provide extensive details about the existing portion of the home, including architectural drawings, insulation levels, window size and specs, and the make/model of the existing furnace and AC units, then we will be able to generate a Manual J and S to verify if the existing equipment is sized properly to handle both the existing portion and new addition.

The Manual J will tell us the necessary furnace BTU and AC unit size for the given space. We then compare these numbers to the existing equipment to determine whether or not it will be able to handle the full load.

If the existing equipment will not suffice for the additional heating/cooling load, we inform our clients of the situation. At this point the client can either: A) Replace the existing furnace and/or AC unit with a new unit that can handle the full load. B) Install a separate unit for the new addition.

For Small Additions

For small additions, it generally makes more sense to use the existing HVAC in the home, if possible. We usually advise our clients to find out if their jurisdiction allows for Manual J/S reports without D. If a J/S report is sufficient for the county, that is the advisable route. Again, we will need a list of details from the existing home to produce accurate reports; and we will make sure the the HVAC system can handle the additional load of the renovation.

Scott Mechanical Design

As you can see, when it comes to HVAC design on remodels, it is not always “one-size-fits-all.” When finding the best plan of attack for a project, all while making sure to adhere to local code, things can get complicated fast!

Here at Scott Mechanical Design, we have a team of skilled specialists who are equipped with the knowledge, experience and tools to help make this part of your project move as smoothly as possible.

If you are in need of any Manual J/S/D reports for next project, do not hesitate to contact us for more information. You can also get started right away and apply for your Manual J/S/D with our online form. We look forward to earning your business!

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About the Author: Chris Kimmel

Chris Kimmel worked as an Associate Home Inspector for two years, handling numerous services including sewer scope inspections, pest inspections, mold air sample testing, radon testing, and water quality testing. Chris now works with Scott Home Inspection as a Content Writing Specialist.

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