Improve Low Fan Airflow

There are a lot of hidden issues that prevent HVAC systems from creating comfortable conditions. We will cover four of them in this article. The first is low fan airflow because it transfers cool air from the equipment to the living space. When equipment airflow is low, there is no hope for comfortable conditions.

A fan should typically move between 350 to 400 cfm (cubic feet per minute) per ton in cooling mode. If you’re testing a three-ton system, this means the fan should move 1,050 cfm (350 x 3 = 1050) to 1200 cfm (400 x 3 = 1200) across the indoor coil.

The easiest way to determine fan airflow is with a total external static pressure (TESP) test and a fan table. Use them to plot fan airflow as a starting point in your diagnostics. But before you test, inspect the air handling equipment for obvious defects, like a dirty blower wheel.

High TESP often accompanies low fan airflow. So, if you find it in the system, take some additional static pressure measurements to discover airflow restrictions hidden from your visual inspection. Common restrictions include:

• Restrictive air filter — even though it’s clean
• Restrictive indoor coil — even though it’s clean
• Undersized ductwork
• Restrictive duct fittings and transitions
• Poor design and installation (too many elbows, too long duct runs, too many restrictive turns)

You can measure static pressure drops across these components to find the highest resistance. You’ll find the largest airflow restriction where you measure the highest pressure drop. Next, you can recommend appropriate repairs to lower pressure and improve fan airflow.

Correct Improper Refrigerant Charge

Once airflow across the indoor coil is correct, you can focus on the refrigerant side of the system. Refrigerant charge is crucial to the equipment lowering the temperature and delivering its rated cooling capacity. If the refrigerant charge is low, delivered cooling capacity is also low.

You can measure the equipment’s dry-bulb temperature change (Δt) to see if you need to attach your refrigerant gauges. The Δt does change depending on airflow and humidity levels, so be careful — there is no one-size-fits-all number.

Many technicians assume equipment with a 16-20° Δt across the coil is doing its job. Unfortunately, this range makes a lot of assumptions about equipment operation. If you discover a low Δt across the coil, it’s safe to say you need additional readings to diagnose the system.

For your next step, you could measure the enthalpy change (Δh) and calculate the equipment’s delivered capacity with the total heat formula. But if you don’t know how to do this, gauge up instead and check the superheat and subcooling to find any refrigerant-side issues.

Once you identify and correct the issues, you can charge the equipment according to manufacturer recommendations. With airflow and proper refrigerant charge, you know the equipment will perform like it’s supposed to. But there is more to an HVAC system than the equipment. You should also consider the duct system because it determines comfort and efficiency more than the equipment.

Increase Duct System Performance

The duct system can prevent the best equipment from working right and create uncomfortable sleeping conditions. In fact, poor performing ducts cause right-sized equipment to operate like undersized equipment.

There are two main reasons a properly sized duct system cannot deliver the equipment’s cooling capacity into the living space. One is duct leakage, and the second is temperature gain. The duct system’s responsibility is to deliver the equipment’s capacity to the intended areas, not lose it to an attic or crawlspace. When the ducts lose air and temperature, they cannot perform as designed, and your customers won’t get the comfort they desire.

If you’re a salesperson on a sales call, a high-quality air balancing hood helps reveal a lot to customers about their sleeping environment. For example, when their bedroom should have 200 cfm of airflow and you measure only 75 cfm, that should turn on the light bulb. Once you discover this condition, you can continue to focus on solutions beyond the equipment.

You can also perform a quick temperature test across the supply register(s) and return grille in the problem bedroom. Four temperature readings let you assess equipment temperature lost through the duct system. If you’re interested in a procedure for performing this test, email me, and I’ll send you a one-page procedure.

The most appropriate repairs to increase duct system performance is to repair specific duct defects discovered from airflow and temperature testing. You don’t have to rip out the entire duct system and start from scratch. With the right measurements, you can pinpoint what doesn’t work and then focus on correcting that part of the duct system.

Identify Issues Beyond the HVAC System

Sometimes there are issues caused by poor building construction that the HVAC system tries to overcome. For example, no homeowner ever calls the insulation contractor or carpenter when their bedroom is too hot from the summer heat. Instead, they call you. So, you need to know how the work of others affects your system’s ability to provide comfortable conditions.

It’s challenging to control the comfort in a leaky and poorly insulated building. Not only are there temperature and humidity problems from infiltration that lead to discomfort, but you must also deal with the effect of surface temperatures on customer comfort.

A thermostat setting of 70°F will not feel like 70°F to customers if their bodies absorb extra heat from poorly insulated surfaces. In addition, they will sweat from excessive indoor humidity brought in by outside air and return duct leakage.

Many bedrooms in two-story homes are on the second floor. They are often the most uncomfortable rooms because of poor Btu delivery, heat naturally rising, and the reverse stack effect. These defects remain hidden from HVAC diagnostics but affect comfort. Understand them to help your customers improve their sleep and move beyond the HVAC system.

Personal Sleeping Preferences

It’s a good idea to consider variables beyond the HVAC system anytime you diagnose sleep environment issues. You can have the ideal HVAC installation and still not meet the sleeping needs of your customer. Remember, everyone sleeps different.

One of those differences are the mattress type and coverings your customers use. Some materials are synthetic and radiate body heat back to warm the skin as the night goes on. They act like an extra layer of insulation. Other materials are natural. They breathe easier and remove body heat to improve sleep quality.

Some customers also like air blowing on them while others don’t want to feel air movement at all. When you have 72°F air at a velocity of 30 feet per minute (fpm) across the head and neck, that can be uncomfortable for most people. Consider this principle as you look at supply register placement and selection. Anytime you see scoops on a register above the bed, it’s a visual clue your customers don’t want to feel air blowing on them.

The Way We’ve Always Done Things

Our industry must examine how we’ve always done things to provide people the sleeping conditions they desire. We have the knowledge and test instruments to check the contribution HVAC makes to quality sleep. But we have to use them and measure results instead of assuming them.

You can test your installations to see where they are instead of guessing about the results. You can have the confidence of a well-performing system, but do you? Can you say with certainty how well your systems perform?

What was the total external static pressure (TESP), fan airflow, and temperature change in the last system you installed or serviced? These readings are cornerstones of comfort. Not having the answers should make you feel a little uneasy. Without these measurements as a baseline, you don’t know how well the system can provide comfortable sleeping conditions.

I hope these articles help you see new opportunities to serve your customers. Consider their concerns. Ask them how comfortable their bedrooms are. With the proper training and test instruments, you can address these issues if you choose to.

Remember, HVAC is more than blowing cold and hot air — your systems affect the health of your customers. Wouldn’t you sleep better knowing your customers are also sleeping well?