WHY IS A COOLING AND HEAT LOAD CALCULATION IMPORTANT?
When a customer requests an estimate to replace A/C equipment from an Air Conditioning contractor they are met with very different approaches. Some contractors will provide a price over the phone without inspecting the house, some will inspect the house and write up a quick estimate based on the equipment to be replaced, other will carefully inspect the old equipment and determine what additional items will be needed to meet or exceed the code requirements and the last will do the same things but will spend additional time looking at the house, measuring windows, doors, room sizes, ceiling heights, recording the insulation R-factors in the ceiling, floors and walls. His notes will include the cardinal direction of the house and windows. The last contractor is preparing to calculate the Heat and Cooling Load of your house. He will spend from one to four hours doing these calculations to determine what the industry standard is for sizing your new equipment. He will then prepare his estimate based on industry standards set by the Air Conditioning Contractors of America and the Manual J protocol.
In the past many contractors used a rule of thumb similar to one ton of air for every 400 sq. ft. of floor space. The Department of Energy states that the square footage per ton depending upon method of construction and insulation used for the residential houses falls between 250 to 1,250 square feet. How about the new contractor that does not use Cooling and Heat Load calculation? What are his chances of being wrong? Later in this article I will share a true story why over sizing or under sizing a system is important. The cities of Galveston and Pasadena will not let a contractor pull a permit to change out or install A/C system unless they turn in a Heating and Cooling Load calculations. The City of Houston for the first time now requires these calculations for "new construction" installations.
Using Cooling and Heat Load calculations to determine the correct size for equipment is the way to go. Air Conditioning Contractors of America (ACCA), Rheem, Carrier, Trane and Lennox recommends Manual J as the proper way to calculate cooling and heating loads. There are many different methods of calculating the heating cooling loads. When the energy companies (Reliant Energy) require heat loads, they don’t specify which method you must use but that you must use one. Good or bad using any heat load formula is better than none.
The U.S. Department of Energy states that only 14% of A/C contractors calculate heat loads on a regular basis for determining equipment size. I thought this figure must be a mistake, however, in class with licensed contractors, when polled "how many calculate heat loads on a regular basis?", only 10% admitted to using heat loads calculations. Some years back, I thought I didn’t have time to sit down and learn how to use that "thick" Manual J booklet or time to complete steps needed to determine equipment size. One of the manufacturers put on a heat load class that I attended and found Manual J did have a "short form" that was quick and easy to complete. This form was considered an "Envelope" heat load not a "Room by Room" heat load. After completing the first five, I knew what information I needed to gather at the job site and used a form I made up to gather the information each time I did an estimate. Then only 45 minutes were needed to complete the heat load and I had what I needed, which was better than not having any calculations. Later I bought a "Room by Room" software calculation program through Rheem manufacturing that gave me more information and was more exact.
I was able to see a difference in call backs and complaints immediately after using heat load calculations and duct designs. We have had no call backs because of duct size, air flow or the equipment size since.
To prove a point, one of my contractor friends had an ideal situation where he replaced a system in a house that had two zones which were a mirror image of each other. Both had a 5 ton system, same square footage and facing the same direction. He did the calculations and a 3 ton was called for but felt he needed to fudge a little and put a 3.5 ton system in just in case the heat load was wrong. A month after completing the work he went back and talked to the family and asked about the performance and comfort. The family stated they kept the old system set on 70 degrees as they had for years to stay comfortable. But the new system must have a problem with the thermostat because they are not comfortable with a setting less than 78 degrees. The new system feels great but the old system feels clammy. He realized that the new system was working as it was supposed to and was running longer, pulling out more humidity and provided comfort at a higher temperature for the family. He became a believer that day. This contractor did take humidity readings and found the new system area at 48% relative humidity and the old system area 72%. (Galveston)
Some of the new heat load calculation programs are simple and easy to use. There is a learning curve at the beginning for all of us but if we will stay the course and complete a few for practice before starting on customers’ projects, we will build our confidence in us and our companies.
On a positive note, our techs report when they are gathering heat load information for an estimate, the customer asks what they are doing. After explaining to the customer, the tech cannot determine the correct size of equipment without measuring the house for cooling and heat load calculations and what is considered in the calculations, the customer states "no one else has done this or spent this much time here." (music to our ears) What do you think that customer’s opinion is of our attention to detail and the accuracy of our company"s work on his project?
Patrick Clayton
Clayton Air
Galveston
Member Air Conditioning Contractors of America
