Engineers Get an Earful on Load Calculations
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) put the spotlight on contractor, OEM, and software points of view at its annual meeting. The goal of the seminar, "Practical Viewpoints on Residential Load Calculations," was to drive these concerns home to the engineers and hopefully drive research into better, more practical methods for use in the field.
"There has been a lot of controversy on residential load calculations," said Charlie Culp, P.E., Ph.D., the session's moderator and professor of Mechanical Engineering at Texas A&M University. "This seminar offers a field of viewpoints," he said.
CONTRACTOR'S VIEWPOINTGlenn Hourahan, engineering vice president of the Air Conditioning Contractors of America (ACCA), presented information prepared by contractor John Sedine, of Engineering Heating & Cooling, Walker, Mich. (Sedine, like many attendees, had to fight weather-related travel delays and cancellations.)
"Why are loads important to contractors?" asked Hourahan. Most national and state codes demand that when they are installing an HVAC system, contractors need to do a vigorous load calculation. "If you were to ask a group of contractors in a room, every hand would go up," Hourahan said. "When you talk to them individually, it's different." Still, load calcs play a critical role in overall comfort. Today's suburban homes - "McMansions" - are much larger and have more glass.
"Contractors know it's important," he continued. Performing load calculations can lead to contractor differentiation, pitting the good contractor against the not-so-good contractor. However, "Consumers buy on price," Hourahan said. "They need to be made aware that you may get a low price, but you may also get low value. What consumers really want is value, not price."
What the contractor really wants, he said, is a simple method of accurately calculating the load. "If it would fit on a 3 by 5 card, that would be even better." In addition, "Contractors are still hesitant to have the computer between the customer and them."
Simplified spreadsheets, Hourahan said, are embraced by contractors. Robustness of the calculation is important too, "especially for the McMansions," with their vaulted ceilings and perhaps a skylight or two.
WHY BOTHEROne of the biggest problems in getting contractors to run load calculations, Hourahan said, is their lack of incentive to do so, both from the market and enforcement officials. An HVAC load calc requirement is now included in most residential codes; however, building inspectors and code officials don't enforce it. "Good contractors want it enforced."
Market demand is lacking in general. "Builders don't appreciate it," he said. "The builder wants to put the maximum size unit into every single home," whether or not it's in the customer's best interest.
"Homeowners don't know to ask these questions [about load]. They think that because they buy an air conditioner, they get that load calculation automatically. Consumers don't want to pay extra for load calcs and by large, they don't get 'em."
To make matters worse, "Many contractors believe the old rules of thumb are still appropriate." Vigorous load calculations are seen as "an unnecessary step" for which they won't get paid. In short, he said, there is no incentive for a contractor to perform load calculations.
Unfortunately, "professional contractors are at a disadvantage when bidding against contractors who do not undertake proper design work," Hourahan said. "How do we tip the scales?"
WHAT IS PRACTICALCharles (Chip) Barnaby, of Wrightsoft, Lexington, Mass., pondered, "Is it practical to do load calculations at all? If so, how?"
"Is it practical," he stated. "Essentially, it's necessary. I don't see how you can size an HVAC system appropriately without a load calculation." Not sizing the system accurately, particularly oversizing the system, can result in short run cycles, lower efficiency, poor dehumidification, and a higher electrical demand. Spread over many consumers, this results in higher utility costs and actual electric service disruption.
"In short, you need the correct size," Barnaby said. "You can't overpower it; you have to design it." The key is to perform them fast and efficiently, "so work can proceed without undue costs." In general, the contractor "enumerates the [load] sources and adds them up. That's just the breaks.
"Curbside vision doesn't cut it," he said. "You've got to look at the building, in the attic, and at the various loads."
MATH ANXIETY?"Load calculations are necessary and you just have to deal with it," he said. "I don't see any reason, in this day and age, to do arithmetic by hand."
Contractors who work in a particular region, where a common type of construction technique is used, can set up common factors once and fill in the blanks for other projects down the road, he said. "It minimizes the arithmetic, but there still is some.
"The use of a spreadsheet suggests itself," Barnaby continued. A PDA or small tablet computer also can be used onsite without much effort. "This can be done on a very modest computer that you carry around in your pocket."
Using load calculation software gives a contractor the ability of playing "what if" for various system options, he said. "If you do a rigorous load calculation, it can be more than a half-ton system savings for the consumer. Who in their right mind would put 4.5 tons in when they know 2 tons are going into the attic?" In such a case, a contractor could sell a duct-repair package.
DON'T UNDERSIZE HEAT PUMPSUnlike central cooling systems, heat pump operation and efficiency could suffer from undersizing, according to Steve Hancock, senior principal engineer with Trane, Tyler, Texas. "Manufacturers hear that oversized equipment is very bad, but the complaints don't come back to us," he said.
"Of course we endorse load calculations and the more rigorous, the better," Hancock said. "However, undersizing also is a problem. It leads to less-efficient operation for heat pumps in the heating mode."
A place like Tyler, for example, has a hot and humid climate. "The customer perceives the undersized system as a broken system." There is an efficiency loss due to oversizing, but according to a test he performed on one home's system, the loss was not significant, he said.
He also studied the humidity gain for a 3-ton system vs. a 5-ton system during a one-week period. "When the larger system runs, it is evaporating more humidity," he said. "It's the use of the continuous fan that makes the difference" in the latent (humidity) load. If the fan is drawing in air but no humidity is removed from it (e.g., the system isn't cooling the air), and actual run times are shorter, the latent load can quickly get out of hand.
Without the use of a continuous fan, humidity control was indeed better for the 3-ton system. "The 5-ton system, though, is not a disaster," Hancock said. "There is more variance in temperature and rh for the 5-ton system; yes, the oversized system is worse, but it's not a lot worse."
The heating mode is where a homeowner could experience the downside of an undersized heat pump system. In short, the undersized heat pump requires more use of supplemental heat, which drives up the electric bill.
"Heat pumps really need to examine impact on the heating load," Hancock said. "However, having a properly sized system is the best case."
For more information, visit www.ashrae.org.
Publication date: 08/07/2006