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2021 IECC Section C403

Lesson 6: Mechanical Systems, Part 1: Sizing, Efficiency, and Controls

IECC C403.1 to C403.4 · 14 practice questions

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Reading

Sections C403.1 through C403.4 cover the parts of an HVAC design that decide how much energy a building wastes before anyone touches a tool: how big the equipment is, how efficient it is, and what the controls do when nobody is home. Lesson 7 continues with economizers, ventilation, fans, and ducts. Local adoption governs which edition applies (Lesson 1).

Thermostat deadband and setback over one day A line chart of zone temperature setpoints from 4 a.m. to 10 p.m. The heating setpoint is 55 F overnight, ramps up from an automatic start at 6 a.m. to an example 70 F at 8 a.m., holds until 4 p.m., drops to 68 F under automatic stop, and falls back to 55 F at 5 p.m. The cooling setpoint is 85 F overnight, ramps down to an example 75 F at 8 a.m., holds until 4 p.m., rises to 77 F, and returns to 85 F at 5 p.m. The occupied gap between 70 F and 75 F is a 5 F deadband, the code minimum. Overnight the wide gap between 55 F and 85 F is the setback range. Thermostat deadband and setback over one day (C403.4.1.2, C403.4.2) Occupied schedule, 8 am to 5 pm 85 F 75 F 70 F 55 F 4 am 6 am 8 am 12 pm 4 pm 5 pm 10 pm Example setpoints: heat 70 F, cool 75 F Cooling setpoint Deadband: 5 F minimum Heating setpoint Unoccupied setback wide gap, 55 F to 85 F Automatic start Starts early (here 6 am) so the space is at setpoint by 8 am. Automatic stop At 4 pm setpoints drift 2 F or more toward setback (DDC zones). Setback range Controls hold zones no lower than 55 F or higher than 85 F. The 70 F and 75 F setpoints are examples. The code sets the minimum gap, not the setpoints.
While occupied, heating and cooling setpoints must sit at least 5 F apart, and the equipment stays off or minimal in between. When the building empties, the setpoints move far apart, and automatic start brings the space back before the next occupied period.

Loads and the scope of Section C403

Heating and cooling loads must be calculated under ANSI/ASHRAE/ACCA Standard 183 or an approved equivalent method, using the design conditions in Chapter 3 (Lesson 2). Loads may be reduced where the design uses energy recovery (C403.1.1). Data center systems are exempt from the controls and economizer sections C403.4 and C403.5 (C403.1) and follow their own rules.

Equipment sizing

Section C403.3.1 caps oversizing: the output capacity of heating and cooling equipment may be no greater than the smallest available equipment size that exceeds the calculated load. A single unit that both heats and cools must meet the rule for one function, and its capacity for the other function must be as small as available options allow. Two exceptions: standby equipment with controls that let it run only when the primary equipment is off, and multiple units of the same type whose combined capacity exceeds the load but are sequenced by load.

Worked example. A fellowship hall has a calculated cooling load of 38,000 Btu/h. The available rooftop unit sizes are 36,000, 42,000, 48,000, and 60,000 Btu/h. The 36,000 unit is too small, because the rule is the smallest size that exceeds the load. The 42,000 unit is the smallest that does, so it is the largest allowed. A 48,000 unit would be one size too big and fails. In the Gulf Coast climate an oversized unit also short-cycles and dehumidifies poorly, so the rule protects comfort as well as energy.

Efficiency: pointers to the tables

Equipment must meet the minimum efficiencies in Tables C403.3.2(1) through C403.3.2(16), tested and rated by the applicable procedure. Efficiency is verified by an approved certification program or, where none exists, by manufacturer data. Where a unit has several rating conditions, all must be met. When indoor and outdoor coils come from different manufacturers, the designer must show by calculation that the combination meets the requirement (C403.3.2). Inspectors do not memorize the tables; they compare the nameplate and submittal with the mechanical schedule. Two limits from this part of the code are worth knowing:

ItemRule
Hot gas bypass (Table C403.3.3)Allowed only with multiple unloading steps or continuous modulation; bypass capacity at most 50 percent for equipment up to 240,000 Btu/h and 25 percent above
Boiler turndown (Table C403.3.4)Systems over 1,000,000 Btu/h input: 3 to 1 up to 5,000,000; 4 to 1 up to 10,000,000; 5 to 1 above

System design: zone isolation, ventilation, and fault detection

Zone isolation (C403.2.1) applies to HVAC zones over 25,000 square feet or spanning more than one floor that are operated or occupied at different times. They must be divided into isolation areas with automatic controls that shut off conditioned air, outdoor air, and exhaust to an unused area, and central equipment must be able to run serving only the smallest area. Exceptions include outdoor air and exhaust connections on fan systems of 5,000 cfm or less, and areas that run continuously. Ventilation follows IMC Chapter 4, and a mechanical system must be able to cut outdoor air back to that minimum (C403.2.2). New buildings whose HVAC serves 100,000 square feet or more need a fault detection and diagnostics system that samples at least every 15 minutes and notifies staff; Group R-1 and R-2 are excepted (C403.2.3).

Thermostatic controls

Each zone needs its own thermostat that responds to the temperature in that zone, and each humidity control system needs at least one humidity device (C403.4.1). One exception allows perimeter systems that offset only envelope losses or gains, provided there is a thermostat zone for each exposure that faces one direction (within 45 degrees) for more than 50 contiguous feet and the thermostats sit in the zones they serve. Where a zone has a separate heating thermostat and cooling thermostat, a stop or programming must keep the heating setpoint from exceeding the cooling setpoint (C403.4.1.3).

Deadband

Where a zone thermostat controls both heating and cooling, it must be set up with a deadband of at least 5 F in which the supply of heating and cooling is shut off or reduced to a minimum (C403.4.1.2). Exceptions cover thermostats that need manual changeover between heating and cooling, and spaces that need precise temperature control as approved by the code official. The code sets the gap, not the temperatures. Worked example. A new programmable thermostat in a clinic is set to heat at 71 F and cool at 75 F. The gap is 75 - 71 = 4 F, which is under 5 F, so it fails. Setting heat to 70 F gives a 5 F gap and passes. The figure shows a passing 70 and 75 F pair.

Setback and off-hour controls

Each zone needs thermostatic setback controlled by a time clock or programmable controller (C403.4.2). Exceptions are zones that run continuously and zones whose full load is 6,800 Btu/h or less and that have an accessible manual shutoff. The setback controls must be able to hold zone temperatures down to 55 F in heating or up to 85 F in cooling (C403.4.2.1). The programmable controls must handle seven different daily schedules per week, keep their programming for at least 10 hours without power, and offer a manual override for up to 2 hours, a timer for up to 2 hours, or an occupancy sensor (C403.4.2.2). Automatic start is required for each HVAC system, adjusting the start time so the space reaches setpoint just as occupancy begins. Systems with direct digital control of individual zones also need automatic stop, which moves the setpoints by at least 2 F before the unoccupied period (C403.4.2.3).

Key numbers
  • Equipment capacity: no more than the smallest available size above the load (C403.3.1)
  • Zone isolation: zones over 25,000 sq ft or spanning floors (C403.2.1)
  • Fault detection: new buildings with HVAC serving 100,000 sq ft or more; sample every 15 minutes (C403.2.3)
  • Deadband: at least 5 F (C403.4.1.2)
  • Setback: hold no lower than 55 F or higher than 85 F (C403.4.2.1)
  • Time clock: 7 daily schedules, 10-hour memory, 2-hour override (C403.4.2.2)
  • Automatic stop: at least 2 F before unoccupied periods (C403.4.2.3)
  • Heated vestibule: heat off above 45 F outdoors; thermostat at most 60 F heating, at least 85 F cooling (C403.4.1.4)
  • Boiler turndown: 3, 4, or 5 to 1 by input over 1,000,000 Btu/h (Table C403.3.4)

Heat pump supplementary heat

A heat pump with electric resistance backup must have controls that allow the backup heat to run only when one of four things is true: the heat pump cannot meet the thermostat setting, the unit is in defrost mode, the compressor cycle has malfunctioned, or the thermostat has malfunctioned (C403.4.1.1). Without that lockout, resistance strips can run whenever the thermostat asks for a small temperature rise, which wastes several times the energy a heat pump would use.

In the field

A church fellowship hall gets a new rooftop heat pump and a programmable thermostat. At final, the inspector checks the things the drawings cannot show. Do the heating and cooling setpoints stay at least 5 F apart when she tries to move them together? Does the schedule cover seven days, and does the override return to the schedule within 2 hours? Is the setback able to reach 55 F in heating? Are the strips locked out except in defrost or when the compressor cannot keep up? A few minutes with the thermostat menu catches errors that would otherwise run for years on the utility bill.

Vestibules, hydronics, and other controls

Heated vestibules and air curtains with integral heat must shut off the heat when outdoor air is above 45 F, and a thermostat in the vestibule must limit heating to 60 F and cooling to 85 F or above (C403.4.1.4). Hot water boilers serving one- or two-pipe systems need outdoor temperature setback (C403.4.1.5). For hydronic systems, three-pipe systems that use a common return for hot and chilled water are prohibited, and a single boiler over 500,000 Btu/h needs a multistaged or modulating burner (C403.4.3). Water-loop heat pump systems need a loop temperature deadband of at least 20 F between heat rejection and heat addition unless an optimizing controller is installed (C403.4.3.3.1). Large hydronic systems of 300,000 Btu/h or more must reset supply water temperature by at least 25 percent of the design difference between supply and return (C403.4.4).

Practice questions

1. A fellowship hall in Climate Zone 2A has a calculated cooling load of 38,000 Btu/h. Available rooftop unit sizes are 36,000, 42,000, 48,000, and 60,000 Btu/h. Which is the largest unit allowed by IECC Section C403.3.1?

  1. 48,000 Btu/h, one size above the load for a safety margin
  2. 36,000 Btu/h, the size just under the load
  3. 60,000 Btu/h, because larger units last longer
  4. 42,000 Btu/h, the smallest available size that exceeds the load

2. A three-story Group B office building has one HVAC zone that spans all three floors, and the floors are occupied at different times. What does Section C403.2.1 require?

  1. Isolation areas with automatic devices that shut off conditioned air, outdoor air, and exhaust to an unused area
  2. A single thermostat on the middle floor that controls the supply air to all three floors at once, with no other devices
  3. Manual dampers at the air handler that staff close each night, with an hourly log kept by the building operator
  4. Nothing, because zone isolation applies only to zones over 100,000 square feet in floor area on a single level

3. A new 120,000 square foot Group B office building has HVAC serving the whole floor area. What does Section C403.2.3 require?

  1. A fault detection system that samples once each day and prints a monthly summary report for the building owner's files
  2. A fault detection and diagnostics system that samples at least every 15 minutes and notifies authorized personnel
  3. Nothing, because fault detection applies only to buildings over 500,000 square feet, where a central plant is used
  4. A commissioning agent who tests the HVAC system once each year on the anniversary of occupancy and files a report

4. A plan reviewer sees one thermostat on the plans controlling both an east conference room and a west storage room served by the same unit. What does Section C403.4.1 require?

  1. A single thermostat is enough if it is placed in the larger room and set to the average temperature
  2. A single thermostat is enough if the unit has two-speed fans that can slow down for the smaller room
  3. Individual thermostatic controls that respond to the temperature within each zone
  4. Thermostats are required only for zones over 25,000 square feet or for zones that span more than one floor

5. A new programmable thermostat in a Zone 2A clinic is set to heat at 71 F and cool at 75 F. What is the finding under Section C403.4.1.2?

  1. It fails, because the deadband is 4 F, less than the required 5 F
  2. It passes, because the setpoints are within 5 F of each other, which meets the intent of the rule
  3. It passes, because a deadband requirement applies only in Zones 4 through 8, not on the Gulf Coast
  4. It fails, because the cooling setpoint must be at least 78 F in every occupied commercial zone

6. Which of these is an exception to the 5 F deadband rule in Section C403.4.1.2?

  1. Thermostats that have a digital display and can be programmed for seven schedules per week
  2. Thermostats that require manual changeover between heating and cooling modes
  3. Zones smaller than 1,000 square feet that are served by a single packaged rooftop unit
  4. Zones served by a heat pump instead of a furnace, because the heat pump has its own controls

7. What temperature range must the setback controls of Section C403.4.2.1 be able to maintain for an unoccupied zone?

  1. Down to 65 F in heating or up to 78 F in cooling
  2. Down to 40 F in heating or up to 95 F in cooling
  3. Down to 55 F in heating or up to 85 F in cooling
  4. Down to 60 F in heating or up to 80 F in cooling

8. What must a programmable time clock provide under Section C403.4.2.2?

  1. Five daily schedules per week, and programming retained for 1 hour during a loss of power to the building
  2. Seven different daily schedules per week, and programming retained for at least 10 hours without power
  3. Seven schedules per week, and programming retained for 24 hours during a loss of power to the building
  4. One weekly schedule that repeats every day, and programming retained for 10 hours with a battery backup

9. A community center's programmable thermostat lets staff run the system for up to 2 hours after hours by pressing a button, after which it returns to the schedule. Does this satisfy Section C403.4.2.2?

  1. Yes, a manual override for up to 2 hours is an accepted option
  2. No, because an occupancy sensor is required
  3. No, because the override must last at least 4 hours
  4. Yes, but only if it also raises the cooling setpoint to 90 F

10. A Zone 2A office has direct digital control of individual zones. What must automatic stop do before the scheduled unoccupied period under Section C403.4.2.3?

  1. Shut off the outdoor air damper for the whole hour before the scheduled unoccupied period begins
  2. Lower the heating setpoint and raise the cooling setpoint by at least 2 F
  3. Raise the heating setpoint by 2 F so the space is warmer when the last people leave for the day
  4. Do nothing, because automatic stop is optional in Zone 2 and required only in colder climates

11. An inspector opens the controls on a rooftop heat pump at a Zone 2A clinic and finds the electric resistance strips run whenever the room is 1 F below setpoint. What is the finding under Section C403.4.1.1?

  1. Compliant, because any electric resistance heat is allowed as soon as the room temperature falls below the thermostat setpoint
  2. Compliant, because the strips are allowed whenever the outdoor temperature is below 65 F, a common lockout setting in the field
  3. Non-compliant only if the strips exceed 10 kilowatts of connected load, which is the size limit in the code
  4. Non-compliant: supplementary heat may run only when the compressor cannot meet the load, in defrost, or on a malfunction

12. A 400 square foot Group B office suite in Zone 2A is one zone with a full HVAC load of 6,000 Btu/h and an accessible manual shutoff switch. Does it need thermostatic setback under Section C403.4.2?

  1. No, because zones with a full load of 6,800 Btu/h or less and a manual shutoff are excepted
  2. Yes, because every zone needs setback controls regardless of load, size, or manual shutoff switches
  3. No, because setback applies only to zones over 25,000 square feet that span more than one floor
  4. Yes, unless the zone is located in Climate Zone 2, where setback is never required

13. A heated vestibule at a Zone 4A clinic uses a thermostat set to 68 F. What is the finding under Section C403.4.1.4?

  1. It complies, because vestibule heating is limited to 72 F, the usual occupied room setpoint
  2. It complies, because the vestibule limits in the section apply only to cooling and not to heating
  3. It exceeds the limit: vestibule heating may not be controlled above 60 F
  4. It exceeds the limit: the heating must be at least 75 F so people stay comfortable inside

14. A boiler system serving a Group I-2 building has a design input of 6,000,000 Btu/h. What minimum turndown ratio applies under Table C403.3.4?

  1. 3 to 1
  2. 5 to 1
  3. 2 to 1
  4. 4 to 1

Glossary

Deadband
The temperature range between the heating and cooling setpoints in which the system supplies no heating or cooling, at least 5 F under C403.4.1.2.
Setback
Lowering the heating setpoint or raising the cooling setpoint when the building is unoccupied to save energy.
Setpoint
The temperature a thermostat is programmed to hold in a zone.
Isolation area
A part of a large or multi-floor HVAC zone with devices that can shut off air to it when it is not in use.
Fault detection and diagnostics
Permanent sensors and software that monitor HVAC performance and automatically report faults.
Supplementary heat
Electric resistance heat that backs up a heat pump, limited to defrost, malfunction, or when the heat pump cannot meet the load.
Turndown ratio
The ratio of a boiler system's maximum to minimum firing rate, which lets it match a low load.
Automatic start
A control that learns how long the space needs and starts the system early to reach setpoint at occupancy.
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