It’s About More Than “The Box”
I recently received an email from a friend requesting me to perform a load calculation on her home. She and her husband have been in the home seven years now, and felt the initial HVAC contractor must have undersized the air conditioning system because the home has “never cooled” and always felt “sticky.” The installing contractor and another behind him checked out “the box,” meaning the condenser and air handler, and decided nothing was wrong. After performing a manual J, I determined that the size of the air conditioner was correct and a check of the system showed that indeed “the box” was operating normally. The problem was outside of the box.
Home performance looks beyond the heating and cooling system itself. Traditional HVAC contractors are masters at cleaning, repairing, and selling air conditioning systems. Home performance contractors also look into how the air conditioning system interacts with the rest of the home. The HVAC contractor is the first to be blamed when a home is uncomfortable. Sometimes the HVAC contractor is to blame, sometimes not. Without home performance we cannot always see the full picture.
In this particular home the fault belonged partially with the HVAC contractor, partially with the insulator, and partially with the roofing company. The occupants’ discomfort was a combination of a leaky duct system, spotty insulation, and an attic exhaust fan that the roofing company convinced her would solve the comfort issues.
Duct Leakage—Is It As Bad As It Seems? No, It’s Worse.
Duct leakage is a two-fold problem. The first, most obvious problem is that the dehumidified, conditioned air that is meant to be put into the home is being lost. The second problem is that supply duct leakage places a home into negative pressure, increasing infiltration.
Let’s say we have an air conditioning system that supplies 2,000 cfm. Suppose the duct system leaks 25% of design cfm, so only 1500 cfm is being delivered into the envelope. The return for the home still wants to pull 2000 cfm. Since there are only 1500 cfm being delivered into the envelope, the system has to make up the other 500 cfm from wherever it can in the form of infiltration. Studies show that on average only about 20% of the total infiltration comes from outside air through window and door frames. The other 80% comes from wall cavities, attics, crawl spaces, and basements. Infiltration introduces dust and other pollutants into a home and increases the home’s sensible and latent loads.
Likewise, an imbalanced duct system can also introduce infiltration. A common problem we see is bedrooms with no return air. When the door to the bedroom is closed, the bedroom goes into positive pressure while the main living area where the return is located goes into negative pressure, causing infiltration. Home performance contractors have the tools and knowledge to test for and correct duct and infiltration issues, as well as check for issues with the envelope itself, such as problems with insulation and attic ventilation. Health and safety concerns, such as proper combustion of gas appliances also fall under the scope of home performance contracting.
Following the easy way out of installing a larger system in my friend’s home as a band aid would have only aggravated her comfort issues. My friend would be out the costs for a larger system, would likely have a higher electric bill, and her home would probably have been just as humid as it was when we started, if not more. If you hired a plumber because you had a leak in a water pipe and his solution was to bring a bigger bucket to catch the water and ignore the leak, would you ever call him again? Home performance looks beyond the band aids and searches for the root of the problem. Since HVAC contractors are first on the list to be contacted when there is a comfort problem, it is a golden opportunity for us to be the home performance contractors to resolve their issues.
There are tons of resources available regarding home performance contracting, but perhaps the best resources are within our own industry. Did you know that ACCA in cooperation with RESNET has the first ANSI approved home performance standard for existing homes? Reading Standard 12 “Existing Homes evaluation and Performance Improvement” is a great starting place. Another standard that is a must read for all HVAC contractors is ACCA Standard 5, Quality Installations. Since both standards are available for free download on ACCA’s website, there is no reason for you to NOT start today.
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