HVAC Airflow Basics

Air is what carries heating and cooling through a building. If not enough air moves across a coil or a heat exchanger, a perfectly healthy system will freeze up, overheat or trip a safety. Many problems blamed on refrigerant or gas are really airflow problems. That is why experienced technicians check air first.

What You Will Be Able to Do

• Estimate the airflow a system needs from its size in tons.
• Define static pressure and read it with a manometer.
• Measure temperature split in cooling and temperature rise in heating.
• Explain what a filter does, and what MERV rating to expect.
• Recognize the signs of low airflow.

Airflow in CFM

Airflow is measured in cubic feet per minute (CFM). A common target for residential cooling is about 400 CFM per ton. In very humid climates the airflow may be set lower so the coil removes more moisture, and in dry climates it may be set higher. Follow the manufacturer's table.

Worked example: a 3-ton system needs about 3 x 400 = 1,200 CFM across the evaporator.

Static Pressure

Static pressure is the resistance the blower pushes against as air moves through the duct system, including the filter, coil and registers. It is measured in inches of water column (in. w.c.), which is a small unit of pressure. A blower only moves its rated air if that resistance stays within limits.

Total external static pressure (TESP) adds the pressure on the supply side and the return side of the blower. Most residential equipment is designed for about 0.5 in. w.c. A common maximum rating for a residential air handler is about 0.8 in. w.c. Check the data plate. Above the rating, airflow drops and the motor works harder.

How to Measure TESP

1. Find or drill test ports in the return air before the filter and in the supply air after the coil. On a furnace, use the ports the manufacturer shows.
2. Connect a manometer and a static pressure tip. Point the tip into the airflow.
3. Run the blower at the speed being tested.
4. Add the two readings, ignoring the plus and minus signs. For example, 0.25 in. w.c. on the return and 0.40 in. w.c. on the supply gives 0.65 in. w.c.
5. Compare with the rating. Also compare the blower's fan table to estimate the CFM at that pressure.

Warning: Keep hands and tools clear of moving blowers and live wiring when you take readings. Replace test port plugs afterward so the ducts do not leak.

Temperature Split and Temperature Rise

Temperature split is the difference between return air and supply air on a cooling system. A normal split on a residential system is about 15 to 22°F, and about 17 to 22°F at a moderate load. A split above that range often means low airflow. A split below it often means weak capacity from low charge or high humidity.

Temperature rise is the same measurement on a furnace. It should fall inside the range on the rating plate, often somewhere from 35 to 70°F. Too high a rise means low airflow and an overheating heat exchanger. Too low a rise can mean too much airflow or a low firing rate.

Measure with a good thermometer in the return and in the supply, out of direct view of the coil or heat exchanger.

You can estimate CFM on a gas furnace from its rise:

CFM = output BTU/h divided by (1.08 x temperature rise)

Worked example: a furnace with an output of 72,000 BTU/h shows a rise of 60°F. CFM = 72,000 / (1.08 x 60) = about 1,110 CFM.

Filters and MERV

An air filter removes dust and other airborne particles from the return air. It protects the coil and blower, and it improves indoor air quality. MERV is the filter's rating. A higher MERV catches smaller particles, but it can also restrict air more.

MERV 8 is a common practical minimum for homes. Higher ratings can work well, as long as the filter has enough surface area and static pressure stays in range. A thick media cabinet handles a high-MERV filter far better than a thin 1-inch slot.

A dirty filter is the cheapest thing to check and one of the most common causes of a service call. It can cause frozen coils, tripped limits and high static pressure.

What Low Airflow Does

Common Airflow Restrictions

1. Dirty or wrong-size filter.
2. Dirty evaporator coil.
3. Dirty blower wheel. A coated wheel moves far less air than a clean one.
4. Blower set on the wrong speed.
5. Closed or blocked registers.
6. Crushed, kinked or disconnected duct.
7. Undersized return duct.
8. A failing blower motor or capacitor.

Zoning and Bypass

On a zone system, dampers close off parts of the duct system when those areas do not need air. If too many dampers close, static pressure jumps. A bypass damper or another pressure relief method lets the extra air escape, which protects the blower and limits noise.

Common Mistakes

Check Your Understanding

A 4-ton system should move about how much air? About 1,600 CFM, at 400 CFM per ton.

You read 0.45 in. w.c. on the return and 0.55 in. w.c. on the supply. What is the TESP, and is that a concern? 1.0 in. w.c. That is above a typical maximum of 0.8, so look for restrictions.

A cooling split is 28°F, and the coil has frost on it. What should you check first? The filter and airflow path. High split with ice points to low airflow.

A furnace rating plate shows a 40 to 70°F rise. You measure 85°F. What does that suggest? Low airflow. Check the filter, blower and ducts, because the heat exchanger is overheating.

Key Takeaways

• Residential cooling usually needs about 400 CFM per ton.
• Static pressure is the resistance to airflow, measured in inches of water column.
• Many residential systems are designed for 0.5 in. w.c. and top out around 0.8.
• Normal cooling split is about 15 to 22°F, and a furnace rise must stay on the rating plate.
• MERV 8 is a common practical minimum for home filters, as long as static pressure stays in range.
• Check airflow before you blame refrigerant or gas.