A software company specializing in products for builders has released an online program that performs three important calculations used in the design of HVAC systems: Manual J, Manual D, and Manual S calculations.
The worksheets developed by the Air Conditioning Contractors of America (ACCA) are used to determine heating and cooling loads (Manual J), to size air ducts (Manual D), and to select equipment (Manual S). Contractors and designers have several software programs to choose from but Elite Software says that its Rhvac Online program is the first to combine all three.
Elite’s Rhvac Desktop is a Windows program — it won’t run on a Mac without CrossOver, Boot Camp, or something similar — but the online version can run on any Web-connected device, including smartphones and tablets. The College Station, Texas, company offers an online demo for users who register at Elite’s website.
Contractors can save individual projects in the cloud and import and export files to Elite’s Rhvac desktop software, according to the company. Elite offers specs and details of Rhvac Online at its website.
Rhvac Online is available in two versions, each of which requires a one-year subscription: users who already have the desktop software (Rhvac 9) will pay $199; those who don’t pay $239. In addition, the company offers subscriptions for just the Manual S ($49) and Manual D ($89) for those who are satisfied with another Manual J calculator, such as Cool Calc. In that case, Elite’s Manual J would operate in demo mode only.
Rhvac Online has all the major features of the desktop version, but it can be used only when the device is connected to the internet, and it won’t show as much information on one screen as the desktop version. Ideally, Elite says, a designer would have both the desktop and online versions.
Calculations are important, but often ignored
The HVAC calculations published by ACCA are cited by many designers and builders as essential tools for sizing heating and cooling equipment correctly, and are required by most building codes. Even so, many HVAC contractors reportedly rely on old rules of thumb for specifying air conditioners, boilers, and furnaces — a certain heating or cooling output based on square footage, for example — so specified equipment often has too much capacity.
As GBA senior editor Martin Holladay said in a blog on the topic, a U.S. Department of Energy website casually suggests that homeowners should be able to get these calculations done by HVAC contractors at little or no cost. Unfortunately, experience suggests that few HVAC contractors have the skills or the software to do this.
“It seems that few HVAC contractors understand how to prepare an accurate report, often oversizing systems out of habit rather than designing them for the actual house loads,” GBA contributor Carl Seville has written. “Also, while the reports are supposed to be delivered before the equipment is installed, they rarely are, and mistakes are usually identified well past the time to make any corrections.”
Wrote Allison Bailes: “HVAC contractors, at least here in the Southeast, don’t often do Manual J load calculations at all. They mostly use rules of thumb based on square footage (e.g., 500 square feet per ton of AC capacity). When they take the time to enter data and run a Manual J, they usually do it incorrectly.”
Elite President Bill Smith said by phone that the low cost of Rhvac Online might encourage more homeowners to run the calculations, especially if they take advantage of the free Cool Calc for Manual J. “But the primary person who’s going to buy into this is an air conditioning contractor,” he said.
“The one thing that’s encouraging them the most is the fact that the International Residential Code since 2009 has had the requirement of Manual J and Manual S,” Smith continued, “but all the different municipalities around the country were slow to adapt.
“Every day that goes by we see more and more places where it’s required. So what I’m seeing now is not so much that contractors themselves say they ought to become better at this, they’re being made to be better. The municipalities are saying you’ve got to submit a Manual J or a Manual S to get a permit.”
Get building science and energy efficiency advice, plus special offers, in your inbox.