Manual J Exaggerates Cooling Load
The HVAC industry-standard method for calculating the cooling load of a building overestimates the mark by 24 percent according to a study conducted by Proctor Engineering Group. Contractors use a procedure known as Manual J, which is published by the Air Conditioning Contractors of America, to calculate the amount of mechanical heating and cooling needed by a building. The load becomes the basis for sizing air conditioners and heat pumps.
"I'm not saying that contractors should stop using Manual J," says John Proctor, who supervised the study. "They should just use it intelligently."
By overestimating load, Manual J is already conservative. Then contractors sometimes add a "safety factor" to the load and then another safety factor to the equipment sizing calculation. The total effect is equipment that is significantly oversized. That adds unnecessary cost to the equipment. It also reduces the operating efficiency, because it runs most of the time at a small fraction of its capacity.
The results were based on 38 new homes in Phoenix, Arizona, and Las Vegas, Nevada, that were monitored from July through September, 1995. The houses showed typical construction practices right down to leaky ductwork. The data collected included air flow near the evaporator coil and the amount of time the compressor operated along with four measurements of air temperature (supply, return, indoor and outdoor). Based on these measurements, researchers calculated the actual sensible cooling delivered to the houses. The results showed that the actual cooling load was two-thirds of the Manual J estimate.
The graph shows the hourly readings for one typical house. The outdoor temperature at this house reached 116°F. Of the 1316 hours monitored during this extremely hot summer, only 3 hours exceeded the load estimated by Manual J. At the very least, this shows that there is no need to add safety factors to the Manual J estimates.
Going further, the study suggests that sizing an air conditioner to only two-thirds of the Manual J load would be sufficient. At that level, the equipment would have provided 100 percent of the cooling for all but 88 hours (7 percent) of the time even though outdoor temperature exceeds design conditions for 160 hours (12 percent).
"This study focused on a hot, dry climate," says Proctor. "While the results probably apply to other climates, that needs to be confirmed with additional studies."
This article appeared in Energy Source Builder #50 April 1997,