What is your methodology for troubleshooting an HVAC system that “can’t keep up” on the hottest days of the summer? Let me walk you through my process and hone in on one or two ideas that have been overlooked for too long in our industry. In this article, I’m assuming we have ruled out intermittent issues, like float switches or overheating motors. We are focusing more on heat gain and heat loss—and why the HVAC equipment isn’t able to maintain the desired temperature in the house. 

Some of this testing works better with wireless probes, advanced airflow testing equipment, and a precision manometer. But all of it is still doable without any of the fancy-schmancy tools; it just takes a little longer.

Start out by looking at the big picture. How does the house feel when you walk in? Take a humidity reading. If the system has been running nonstop for the past two days, what would you expect the relative humidity in the house to be? Maybe 45–50%. If the RH is higher than that, why is it higher? Does the system struggle to keep up all the time, or just during the hottest couple of hours in the middle of the afternoon? How has the house performed in previous summers? Has the house been remodeled? How far away from the set point are we? 1–2°F? Or 5–6°F? 

1. Evaluate the HVAC Equipment

By this, I mean check your refrigerant pressures, line temperatures, and temperature differential. Check static pressure and fan speed taps. Make sure the system is running at full capacity. MeasureQuick is a great tool for this because if you hook up or enter all the data from the system you’re working on, it will tell you the system’s delivered capacity. From there, it’s a simple matter of looking at the capacity and comparing it to what the system should be doing. 

Capacity: Delivered capacity and temperature differential should not be confused with the 20°F delta T rule of thumb that we have used in the industry for years. The temperature split for a system can vary wildly based on airflow settings, relative humidity, and temperature in the space. For more on what your delta T should be, check out this article by Genry Garcia.

Airflow: If the system you are working on is set to the proper fan speed and your static pressure is high, you have an airflow problem. How does cool air circulate around the house? It “rides” on the air. If you have a 5-ton system that is only moving 1200 CFM as opposed to 2000 CFM, would you expect it to perform properly? No, it will have reduced sensible capacity. The image below is from a TrueFlow grid report. Maybe you don’t have the fancy tools, but I bet you can measure static pressure. Is the static pressure in the right range? Is it WAY off? Close? This reading will give you the data you need to hone in on what the problem with this house COULD be. It will cross items off the list—or at least begin to help you build a case for what is actually wrong. 

2. Evaluate the Duct System

Is the duct system leaky? A lot of time, you can figure this out with a visual inspection. Did the installers use mastic to air-seal the system? Do you feel any drafts of cold air when you’re crawling around in the attic? In DFW, where I live, I know that newer homes have better-sealed duct systems. The older homes use panned wall returns, which are just the open cavities between the walls as returns. Usually, the duct tapped into the top of the wall is unsealed, and you can spot the duct leakage because the insulation is dirtier and discolored around the spot where air is being drawn in.

Interestingly, this duct leakage matters less on cooler days. The problem becomes worse the hotter it is outside, and therefore worse in the attic. If you have 10% return duct leakage, the attic air is 85°F, and your return air temperature is 75°F, then the mixed air temperature is 76°F. If the air from your 10% duct leak is 120°F and the return air temperature is 75°F, then the mixed air temperature is now 79.5°F. That’s a significant capacity loss. 

If you have multiple returns tapped onto a plenum in an attic, you may not measure a true return air temperature. Taking a return air temperature measurement on both sides of the inlet of the blower motor is the most accurate measurement of true return air temperature. Measure on both sides and average the readings.

3. Learn about MAD AIR

MAD AIR stands for “Mechanical Air Distribution And Interactive Relationships.” Here is how I think about it: your HVAC system is like a recirculating water fountain. In this picture, water flows down the fountain, and then a pump on the back pushes the same water back up to the top. We want to use the same water over and over again. Otherwise, you’re going to be out there all the time filling it up. You can think of the HVAC system the same way. We want to use the same air over and over again because we have already cooled it off. Introducing new air, or “adding water” to the system over and over, is highly inefficient. 

How does air get lost/wasted within the system? Imagine you have a bedroom that receives 100 CFM of air. Now that it has flowed down to the bottom of the fountain, the blower, or the “pump,” wants to suck it back in and up. But what if the bedroom door is closed? Instead of recirculating the cold air from the vents, it is “filled back up” with air from other places: outside. 

You can actually measure these pressures within a house using zonal pressure diagnostics, or ZPD. This process, which involves closing doors and measuring pressures in different rooms, will tell you if your air recirculation paths are sufficient or if your HVAC blower is “filling the pump back up” from somewhere it shouldn’t. To do this, you need a precision manometer that can measure in the Pascal scale. A Pascal is a very small pressure measurement, it’s equal to 0.004 in wc. 

It is nice to have the tools, but to some degree, we know that most houses don’t have enough return air in every bedroom. If you’re trying to solve a comfort problem and you think it may be related to MAD AIR, check the relative humidity in the house. Again, if the HVAC system has been running all day, then the RH in the house should be low. High RH with long runtimes is an indication of infiltration. Do the homeowners keep a lot of the doors closed during the day? Have them open the doors and try running the system for a day. See what changes. 

These are my go-to areas for solving comfort issues when a system isn’t keeping up. Check capacity, airflow, duct leakage, and MAD AIR. I hope that helps.

—Matt Bruner