by Luis Escobar
As equipment performance tolerances have tightened, home construction technologies have advanced, and new information on homes operating as a system becomes prevalent, it becomes more critical that HVAC contractors provide Manual J load calculations.
Cornerstone Of Design
Load calculations are the first step in properly designing an HVAC system that meets the heating and cooling loads for a home, thereby providing a comfortable interior throughout the year. Despite the primary importance of accounting for the total heat flows into and out of a home (i.e., load calculation), many contractors still don’t perform a load calculation on a regular basis.
There are a number of reasons why mechanical contractors don’t make load calculations a standard operating procedure of their design process. Some don’t want to invest the time it takes to do a full and complete survey. For others, it’s an added cost because it’s not already a part of their business model. Unfortunately, some just don’t know how to do one. Others have decided it’s not worth it because they see it as a guessing game.
No! It’s not a guessing game. Sure, not all of the variables within a home are known (like the installed insulation R-value or fenestration performance ratings), but there is a plethora of information and default guidance available to help you make informed assumptions available from ACCA, and from your own experience.
No Load Calculation – So What?
We all know that an old piece of HVAC equipment may not have been the proper size for the home when it was first installed way back when. It may be even more inappropriate for the home if there have been any substantial improvements in the meantime, like new windows (reduced load) or a new addition (increased load). Simply installing a new piece of equipment of the same tonnage will therefore be equally inappropriate.
If the equipment is undersized for the actual needs of the home, then the indoors won’t reach the thermostat set-point on a design day. If it’s oversized, then there is an increased likelihood of negative comfort, health, equipment, and economic impacts. These impacts are then exacerbated during part-load conditions - a.k.a. the majority of the year!
A Self-Imposed Disadvantage In A Changing World
The internet has revolutionized nearly every aspect of our daily lives, and its impact is becoming more pronounced in our industry. With each passing day, consumers are becoming well-versed about how their home actually works as a system, and ways in which they can be more comfortable while reducing their bills. There is a wealth of information available to them online from sources ranging from equipment manufacturers, to green advocates, to utilities, to government agencies. How will you credibly answer a consumer that asks you directly why you’re not doing a load calculation and can show you various respected online sources that make it clear that it is essential for a system design?
Value For Your Customers
The value of a properly-sized HVAC system (based on an accurate load calculation) is multifold.
- The homeowner will be more comfortable in their own home;
- Avoiding oversized equipment will result in better humidity control during part load conditions;
- The homeowner can save on initial costs;
- The equipment won’t cycle continually (which reduces equipment lifespan); and
- The homeowner’s energy usage will be lower.
Lower energy usage is a major selling point for many environmentally conscientious consumers. In 2014, the National Institute of Standards and Technology quantified the energy usage impact of oversizing HVAC equipment; the increased energy use can be as high as 35%, and that’s if the only fault is oversized equipment.
Position Your Company For The Future
Our industry is rapidly moving towards whole home performance; assessing the various construction aspects of the home (e.g., insulation, windows, envelope, etc.) as the components of a larger system. In undertaking an accurate load calculation for an existing home, a designer conducts a survey of the home in its current state. More likely than not, the information gathered during this survey will be entered into a software program that will perform the mathematical calculations and output the total heating and cooling loads.
Many contractors have found that by making slight adjustments to the input, they are able to show the homeowner, in real-time, how upgrading different parts of their home can reduce the heating and cooling loads, which can then lead to smaller equipment being installed. This places those progressive contractors in the home performance arena.
ACCA conducted a sensitivity analysis on the impact of thermal envelope improvements on a home’s load calculation. For more information on the results of this analysis, and an expansion of this topic, please see the March 2015 ACCA Town Hall titled, The Value of Load Calculations for Existing Homes at ie3media.com/ValueOfLoadCalc.
Author's Note: Luis Escobar is ACCA's Manager of Codes and Standards. Reach him at luis.escobar@acca.org or 703-824-8870.