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CEM: HVAC Systems and Building Envelope

CEM · Work through your exam, one topic at a time.

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HVAC Systems and Building Envelope

This domain tests HVAC performance metrics, psychrometrics, envelope effects, ventilation, and system interactions. Questions often require converting COP to kW/ton, estimating chiller energy, or identifying why an economizer is not saving energy.

Metrics and psychrometrics

  • COP = cooling output ÷ energy input in consistent units. EER = Btu output ÷ Wh input. kW/ton = 12 ÷ EER = 3.517 ÷ COP.
  • SEER and HSPF are seasonal ratings; IPLV/NPLV describe part-load performance. Full-load efficiency alone does not predict annual energy.
  • Psychrometric properties include dry-bulb temperature, wet-bulb temperature, dew point, relative humidity, humidity ratio, and enthalpy.
  • Envelope performance depends on U-factor, SHGC, airtightness, and thermal bridging. Ventilation rates come from ASHRAE 62.1; comfort criteria from ASHRAE 55.

Worked example

A 500-ton chiller operates at 0.60 kW/ton for 2,000 equivalent full-load hours per year. Annual energy = 500 × 0.60 × 2,000 = 600,000 kWh. At $0.10/kWh, operating cost = $60,000. If a retrofit improves performance to 0.55 kW/ton, annual energy = 500 × 0.55 × 2,000 = 550,000 kWh, saving 50,000 kWh or $5,000. The reasoning: equivalent full-load hours convert a part-load operating profile into an equivalent full-load period. A chiller with COP 5.5 has kW/ton = 3.517 ÷ 5.5 = 0.639, which is consistent with the example.

System interactions

Fan and pump energy can equal or exceed chiller energy in some systems. Economizers save only when outdoor air conditions and control sequences are correct; a stuck damper or failed sensor can waste energy. Reheat systems can simultaneously heat and cool. Duct leakage and envelope infiltration increase both load and fan energy. Right-sizing and reset schedules often save more than equipment replacement.

Traps that cost marks

  • Mixing EER, COP, SEER, and IPLV as if they were interchangeable.
  • Using full-load efficiency for a chiller that operates mostly at part load.
  • Ignoring fan, pump, and cooling-tower energy in system comparisons.
  • Assuming an economizer saves energy without checking changeover and damper operation.
  • Confusing ventilation code requirements with energy-code requirements.
  • Overlooking thermal bridging and air leakage in envelope calculations.