2021 IECC Section C403
Part 1 sized the equipment and set its controls. This lesson covers the air and water side: whether a unit must cool with outdoor air, how much energy is recovered from exhaust, how fan power is capped, and how ducts and pipes are insulated and sealed. It all sits in Section C403 of the 2021 IECC. The Gulf Coast is Climate Zone 2A (Table C301.1 lists Harris, Galveston, Fort Bend, Brazoria, Montgomery, and Chambers counties as 2A), and 2A has rules that surprise people who assume hot means exempt. Local adoption governs which edition is enforced (Lesson 1).
An economizer lets a cooling system use outdoor air, or cool condenser water, in place of the compressor when the weather allows. C403.5 requires an air or water economizer on the cooling systems below. An individual fan system means a rooftop unit, split system, or air handler with its own cooling coil. Units without an economizer may total up to 20 percent of the building's fan-cooling supply capacity, or 300,000 Btu/h (1,500,000 in Group R), whichever is larger.
| Cooling system | Economizer trigger (C403.5, Table C403.5(1)) |
|---|---|
| Individual fan system, building other than Group R | 54,000 Btu/h or more |
| Individual fan system, Group R | 270,000 Btu/h or more |
| Chilled-water, local water-cooled, Zone 2A | 960,000 Btu/h |
| Chilled-water, air-cooled or district, Zone 2A | 1,250,000 Btu/h |
The exceptions matter most in Zone 2A. Exception 1 exempts individual fan systems not served by chilled water only in Zones 0A, 0B, 1A, and 1B. Zone 2A is not on that list, so a Houston rooftop unit is not exempt because of climate. Other exceptions: efficiency at least 10 percent better than the minimum per Table C403.5(2) (15 percent in Zone 3A, 20 percent in 4A); a system expected to run under 20 hours per week; process humidification for over 25 percent of supply air; supermarket open casework; a VRF system with a dedicated outdoor air system; or condenser heat recovery under C403.10.5 (very large 24-hour facilities only).
A required economizer must work with the compressor. Under C403.5.1 it is integrated: it gives partial free cooling while mechanical cooling runs, the outdoor air damper stays fully open (100 percent) whenever mechanical cooling is on, and controls may not create a false load, such as hot gas bypass, except at the lowest stage. An air economizer (C403.5.3) must modulate its dampers to deliver up to 100 percent of design supply air as outdoor air, may not be controlled by mixed-air temperature alone (single-zone systems excepted), and must relieve excess air so the building is not over-pressurized.
Humid climates need a high limit that drops the economizer back to minimum outdoor air when the outside air would add cooling load. Table C403.5.3.3 sets fixed dry-bulb limits by zone: 65°F in Zones 0A, 1A, 2A, 3A, and 4A, and 75°F in the B zones and the cooler 3C, 4C, 5C, 7, and 8, with 70°F in 5A and 6A. In Zone 2A the economizer goes off when outdoor air is above 65°F. Differential dry-bulb control (outdoor versus return air temperature) is not a listed option for 2A. Enthalpy controls are allowed in every zone, and because they count moisture they suit a warm humid coast.
Air-cooled unitary direct expansion and VRF units with an economizer also need fault detection (C403.5.5): permanent outside, supply, and return air sensors accurate to 2°F, and a controller that reports a failed sensor, not economizing when it should, economizing when it should not, a stuck damper, and excess outdoor air. Whole-building fault detection (C403.2.3) applies to new buildings whose HVAC serves 100,000 square feet or more, except R-1 and R-2.
C403.2.2 sends the outdoor air amount to the mechanical code and asks only that the system be able to drop to that minimum. Demand control ventilation (C403.7.1) lets outdoor air follow the head count. It is required for single-zone systems that must have economizers, and for spaces over 500 square feet averaging 15 or more people per 1,000 square feet that are served by a system with an air-side economizer, a modulating outdoor air damper, or design outdoor air over 3,000 cfm. Systems with energy recovery are excepted. Enclosed parking garages (C403.7.2) need carbon monoxide and nitrogen dioxide detectors that stage or modulate fans to 50 percent of design or less.
Outdoor air intakes and exhaust openings need Class I motorized dampers that close when the system is off and during unoccupied warm-up and setback, with leakage of 4 cfm per square foot or less at 1.0 inch water gauge (C403.7.7). Gravity dampers may serve exhaust and relief openings in buildings under three stories, in a building of any height in Zones 0 through 3, or where design exhaust is 300 cfm or less. The Gulf Coast gets that gravity option, but intake dampers must still be motorized.
Where the supply airflow of a fan system exceeds the value in Table C403.7.4.2(1) or (2), the system needs an energy recovery system with an enthalpy recovery ratio of at least 50 percent at design conditions. It must include a bypass or controls so an air economizer can still operate. The value depends on hours of operation (Table 1 under 8,000 hours a year, Table 2 for 8,000 or more) and on the percent of outdoor air at full design airflow: more outdoor air means a smaller trigger. Selected Zone 2A rows (the code fills in the rest):
| Outdoor air at full design airflow | Under 8,000 hours a year (supply cfm) | 8,000 hours or more (supply cfm) |
|---|---|---|
| 20 to under 30 percent | 16,000 or more | 2,000 or more |
| 40 to under 50 percent | 4,500 or more | 500 or more |
| 80 percent or more | over 120 | 80 or more |
Exceptions include the heating-design ratio in Zones 1 and 2, kitchen grease hoods, systems under 20 hours a week, and systems whose largest single exhaust is under 75 percent of design outdoor air. Nontransient dwelling units follow C403.7.4.1 (50 percent at cooling design, 60 percent at heating design).
Worked example. A Group B health office in Houston (Zone 2A) has a rooftop unit with 5,000 cfm design supply and 2,250 cfm design outdoor air, running about 3,500 hours a year, so Table 1 applies. The outdoor air share is 2,250 / 5,000 = 45 percent, the 40 to under 50 percent row, with a trigger of 4,500 cfm. Because 5,000 exceeds 4,500, recovery is required at 50 percent or better, with an economizer bypass. With only 1,250 cfm of outdoor air the share is 25 percent, the trigger is 16,000 cfm, and no recovery is required.
C403.8.1 caps fan power on any HVAC system whose fan motors total more than 5 hp nameplate. Under Option 1 in Table C403.8.1(1), total nameplate horsepower may not exceed CFMs x 0.0011 for a constant volume system or CFMs x 0.0015 for variable volume, where CFMs is the maximum design supply airflow to conditioned spaces. Option 2 caps brake horsepower and credits extra pressure drop from items such as MERV 13 to 15 filters. Single-zone variable air volume uses the constant volume limit, and an exhaust fan of 1 hp or less is exempt. At 10,000 cfm constant volume, Option 1 gives 10,000 x 0.0011 = 11 hp, so 12 hp of motors fails; a variable volume system of the same size may carry 15 hp.
Fans need a fan energy index of at least 1.00 at the design point (0.95 for variable air volume), with many exceptions such as fans inside listed equipment (C403.8.3). Direct expansion cooling of 65,000 Btu/h or more needs two stages of fan control where the unit controls its cooling from space temperature, with low speed no higher than 66 percent of full speed and no more than 40 percent of full fan power. Units that instead modulate airflow need modulating fan control, down to 50 percent of full speed and 30 percent of full fan power (C403.8.6.1). Units with an economizer need at least two fan speeds in economizer operation.
Supply and return ducts and plenums (C403.12.1) need at least R-6 in unconditioned spaces and, outside the building, R-8 in Zones 0 through 4 (R-12 in Zones 5 through 8). A duct inside a building envelope assembly must be separated from the exterior or from unconditioned space by R-8 in Zones 0 through 4. Ducts within equipment, and ducts with a design temperature difference of 15°F or less, are excepted. Ducts, air handlers, and filter boxes must be sealed (C403.12.1), and the pressure class must appear on the construction documents (C403.12.2).
| Duct pressure class | What the inspector looks for (C403.12.2) |
|---|---|
| 2 in. w.g. or less (low) | Joints and seams fastened and sealed with welds, gaskets, mastics, or tapes used per the manufacturer |
| Over 2 and under 3 in. w.g. (medium) | Insulated and sealed as in C403.12.1 |
| 3 in. w.g. or more (high) | Insulated, sealed, and leak tested to a leakage class of 4.0 or less, on sections totaling at least 25 percent of duct area |
Piping in heating and cooling systems (C403.12.3, Table C403.12.3) is insulated by fluid temperature and pipe size. Piping insulation exposed to the weather must be protected from sunlight, moisture, and wind, and adhesive tape is not allowed (C403.12.3.1). Fluids between 60°F and 105°F are excepted. Selected minimum thicknesses, for insulation within the stated conductivity range:
| Fluid temperature | Pipe under 1.5 in. | Pipe 1.5 to under 4 in. |
|---|---|---|
| 141 to 200°F (heating hot water) | 1.5 in. | 2.0 in. |
| 105 to 140°F | 1.0 in. | 1.5 in. |
| 40 to 60°F (chilled water) | 0.5 in. | 1.0 in. |
In the field
A Houston community center is getting a new packaged rooftop unit, and the vendor quote leaves out the economizer as unnecessary in a humid climate. Plan review asks: is the unit 54,000 Btu/h or more, and if so, which exception is claimed? Zone 2A is not exempt, so the cut sheet must show something like the 10 percent efficiency improvement. If the economizer stays, the high limit must be 65°F dry bulb or an enthalpy type. In the field the inspector looks for the damper actuator, the outdoor air sensor, a Class I motorized intake damper, and rooftop duct with R-8 under a weather jacket rather than tape. Replacement rules for existing buildings come in Lesson 13.
1. A 20-ton (240,000 Btu/h) packaged rooftop unit will cool a Group B office in Climate Zone 2A, and no exception other than climate is in play. What does C403.5 require?
2. A Group B dental office in Zone 2A has a single 4-ton (48,000 Btu/h) split-system air handler serving the whole suite. Under C403.5, is an economizer required for it?
3. A Group R-2 apartment building in Zone 2A has a corridor makeup air unit with a 200,000 Btu/h direct expansion cooling coil. What does C403.5 require for that unit?
4. A designer wants to skip the economizer on a 15-ton rooftop unit for a Group B community center in Zone 2A by choosing a more efficient unit. By how much must its EER or IPLV beat the minimum, using Table C403.5(2)?
5. An air economizer controller in Houston (Zone 2A) uses a fixed dry-bulb high limit. At what outdoor air temperature must it return to minimum outdoor air under Table C403.5.3.3?
6. An inspector in Galveston reviews an air economizer submittal that lists differential dry bulb (outdoor versus return air temperature) as the only high-limit control. What is the problem under Table C403.5.3.3?
7. A rooftop unit with an air economizer is calling for mechanical cooling. What position must the outdoor air damper be in under C403.5.1?
8. A plans reviewer checks a Group B clinic in Zone 2A. The fan system runs about 4,000 hours a year, with 5,000 cfm design supply and 2,000 cfm design outdoor air at full airflow. What does C403.7.4.2 require?
9. A fan system in Zone 2A runs under 8,000 hours a year with 12,000 cfm design supply and 3,000 cfm design outdoor air. Is energy recovery required by C403.7.4.2?
10. A constant-volume single-duct system for a Group A-3 fellowship hall in Zone 2A has a maximum design supply of 8,000 cfm. Using Option 1 of Table C403.8.1(1), what is the largest total fan motor nameplate horsepower allowed?
11. An inspector measures a supply duct run that crosses a Houston community center roof, fully exposed to weather, and finds R-6 insulation. What minimum R-value does C403.12.1 require on ductwork located outside the building in Zone 2A?
12. A designer specifies a supply duct system operating at 3.5 in. w.g. in a Group B office. What must be done under C403.12.2.3 in addition to insulating and sealing it?
13. An inspector finds a four-story Group B office in Zone 2A with a nonmotorized gravity damper on a 600 cfm restroom exhaust and a manual balancing damper only on a 3,000 cfm outdoor air intake. Which statement follows from C403.7.7?
14. Heating hot-water piping in a Group R-2 mechanical room carries 160°F water in 2-inch nominal pipe. What minimum insulation thickness does Table C403.12.3 require, using insulation within the stated conductivity range?