As the contractor, this is a frustrating situation — a hard-earned reputation is on the line and they want to make sure everyone is happy. For anyone who has found themselves in this situation without a solution, Russ King laid out a series of calculated steps at the ACCA conference, recently held in Orlando, that contractors can take to identify and alleviate some common problems plaguing homeowners.

Airflow Expert

King has more than 10 years of experience designing production homes as a mechanical engineer, the blueprints of which have been used thousands of times, with 250 subdivisions of his design in Las Vegas alone.

His designs covered every aspect of the house in extreme detail, down to each individual register labeled with its expected CFM. When his guys would go to the homes he designed, they would take airflow measurements, which were then weighed against the engineered number.

“We go to the point where every single measurement was slightly higher than what our target was,” King said, adding static pressure was also coming in slightly lower than designed. “What that means is (ACCA’s) Manual D has some safety margin built into it, and if you design with Manual D, your system will work a little better than you expect it to.”

But while the things on paper can translate to real-world results, anyone who’s been in the industry long enough knows that’s not always the rule.

Through all of these experiences, King developed a sixth sense for diagnosing comfort problems — and more often than not, they aren’t due to design. King was so confident in this fact that when he would fly out to homes by a contractor that used his designs to diagnose comfort problems, he wouldn’t charge anything for his time, unless the issue was due to poor building practices.

“I never paid for one of those trips. Ever,” King said.

There’s one simple, two-part equation used to fix these comfort problems, but King said people are always just looking at one side — the HVAC side.

“When you’re a hammer, everything looks like a nail — when you’re an HVAC contractor, everything looks like an HVAC problem,” King said. “That’s only half the equation — the other half is the house. Address the house. Look at the house first and then address the HVAC."

These two parts need to be in harmony if any viable solution is going to be reached.

“Don’t design an HVAC system for a crappy house — just don’t,” King said. “Make the house better, then design the HVAC system for it. Everybody will be better off.”

But before getting into how to solve the problem, King said it’s important to first know exactly what it is.

How to Interview the Homeowners

King said it helps to have a set of questions prepared beforehand to help identify the symptoms of the problem before trying to identify the cause.

Ask the homeowner what seems to resolve their issue (sun going down, closing the drapes, closing off vents) and find out what is the most comfortable room in the home. Bonus tip — use precise terminology when asking how they feel (warm, cold, muggy, sticky, sweaty, dry nose/throat), because the cure for a hot house can be much different than one that’s too humid.

When it comes to a “comfort problem,” King defines that as the HVAC equipment running fine (properly charged, solid electrical supply), but the homeowner still isn’t happy.

Once you’ve eliminated any potential equipment problems from the equation, there are really only three things that will cause comfort problems:

• Poor occupant behavior — i.e., they’re turning the thermostat off during the day and are surprised when a hot system isn’t immediately cooling the house when they arrive home.
• The load on the house is higher than it should be, caused by things like missing insulation, windows not matching the design, etc.
• Bad design — the system wasn’t designed correctly, there wasn’t a load calculation done, they guessed on the equipment size, duct size was chosen by rule of thumb.

“In my experience, in 99% of the homes I diagnose, the No. 1 problem is oversizing equipment,” King said. “The No. 2 problem — undersized ducts. About 99% of those 99% had both. Those two things in combination are trouble.”

Do the Critical Calculations

Historically, the most common method of sizing equipment was through trial and error, which almost always led to oversized equipment and undersized ducts.

“Oversizing equipment will cause more comfort problems than undersizing equipment,” King said. “Oversizing will cause stratification, uneven temperatures, higher electric bills, and shortened equipment life, compared to undersized equipment that’s just not going to keep up on the hottest of days.”

In simple terms, King said the negative impacts of oversized equipment can be reduced by using multi-stage or variable-capacity units. The negative impacts of both oversized and undersized equipment can be reduced with good duct design and good system airflow.

“I like to say a good duct design and high airflow will forgive a lot of sins, including balance problems, where you put the registers, stuff like that,” King said.

A properly sized duct system on an overly sized system is also less likely to have problems than an oversized system with undersized ducts, King added.

Another potential problematic source is the thermostat, as temp in every room in the house is dictated by the temperature near the thermostat.

Good airflow is the best way to combat this — know what the room-by-room load is and what each house needs. Given these loads can change throughout the day, King said making sure all that air is mixing is key. The best way to keep air mixing is by making the system run for a longer period of time, which is done by not over-sizing it.

This can also be solved in some instances by zoning, which is needed in certain houses, but King said his only problem with that is it’s often just a Band-Aid for bad duct design.

The overall under-sizing of ducts, especially returns, will reduce total system fan flow (or the amount of air across the coil), which will reduce the capacity and efficiency of a system.

“It takes that three ton and makes it a 2.8 ton,” King said. “It reduces efficiency. It reduces your SEER — your SEER rating goes down when your airflow goes down.

“When you reduce airflow, it’s a double whammy,” he added.

From a whole-house perspective, return is more important than supply, but for a room-by-room temperature issue, supply is more important than air.

King also gave an important reminder: Equipment cannot be properly sized unless you know the load of the house (demand, found in Manual J) and equipment cannot be properly sized unless you can accurately determine the capacity at design conditions (supply, Manual S).

King said sizing equipment needs to be thought of like tailoring a suit — you can’t just use one number, like a man’s weight in this example, because a 250-pound man comes in a lot of different shapes and sizes, and the tailor providing a suit that will fit a 300-pound man just to be safe isn’t going to work, either.

Diagnostic Testing

When asked by a friend what would be an indication of a good HVAC contractor, King said one that has a flow hood and actually measures airflow, and also has a duct leakage tester and a blower door.

“If an HVAC contractor has a blower door, now I know that they’re looking at the other half of the equation — they’re looking at the house,” King said.

There are several ways to find these critical numbers. Some require fairly expensive equipment, but none are extremely difficult. Whatever method a contractor chooses, King said, diagnosing problems by trial and error will ultimately be more expensive and time-consuming in the long run.

• External static pressure (ESP) testing — one of the easiest to do, and for the effort, one of the most informative as it gives a good indication of duct sizing and system airflow.
• The blower door test — shows outside air infiltration and, inversely, where conditioned air escapes outside. To measure infiltration, induce a known pressure on the house using fans.
• Infrared cameras — used to find locations of air or water leaks and can highlight missing or poor insulation and poor attic ventilation as well. For best results, use during more extreme (hot or cold) conditions.
• Data loggers — small devices that record temp or humidity at specific intervals that can produce information that aids with the diagnosis of equipment size, air balance, and occupant behavior. These also should be used during extreme conditions for best results and should be placed in the following locations: outside the house (north side or in the shade), right next to the thermostat, on a supply register (to see when the system turns on and off), in at least one “comfortable” room, and in several “uncomfortable rooms.” Make sure the homeowner understands these can’t be tampered with and the thermostat is left at an agreed setpoint. Try to monitor for at least a week and input the data into good software that allows users to turn certain data on and off and change the time periods and scale of the graph.

Software to the Rescue

In a bygone era, the math derived from numbers gathered during diagnostic testing was done on small paper tablets, but things are much more streamlined now and far less tedious.

There are several good pieces of software out there, including Kwik Model, which King designed himself, but these aren’t going to do much good and seem too complicated unless the user is familiar with load calculations and has a Manual J, S, and P background first.

To learn how to do the load calculations, King recommended taking a Manual J class and learning how to use ACCA-certified load calculation software, which are two separate things.

King also highly recommends searching “LBNL 47382” online to learn more about supply measurements.