CEM common mistakes
CEM · Understand the traps and how to avoid them.
Computing motor input power as nameplate kW divided by assumed efficiency only, then reporting demand savings without accounting for load factor, part-load efficiency, or the difference between nameplate and actual operating current.
Candidates memorize P = kW/η and apply it to the nameplate rating rather than measured operating data, because nameplate values are the easiest numbers to grab and appear authoritative.
What to do instead
Always derive input kW from measured voltage, current, and power factor (or a power meter reading) at the actual load point, then cross-check against the nameplate. State the load factor and the efficiency basis (nameplate vs. part-load curve) explicitly in the answer.
Treating simple payback as the sole economic decision criterion and ignoring lifecycle cost, escalation, discount rate, and the analysis period when comparing ECMs.
Payback is fast to compute and intuitive, so candidates default to it under time pressure and forget that it ignores cash flows after the payback point and the time value of money.
What to do instead
For any capital comparison, present simple payback as a screening metric only, then compute NPV and lifecycle cost using the stated discount rate, fuel escalation, and study period. If the exam supplies a hurdle rate or analysis period, use it explicitly.
Applying blended or average energy rates to savings calculations when the facility is on a demand-charge, time-of-use, or tiered tariff, thereby overstating or understating savings.
Candidates see a single $/kWh figure on the bill and multiply kWh savings by it, missing that demand charges, ratchets, power factor penalties, and TOU periods dominate the bill structure.
What to do instead
Decompose the bill into energy, demand, and ancillary components before calculating savings. Match each ECM's impact to the correct charge component: kWh savings to energy charges, kW reduction to demand charges (with ratchet and coincidence rules), and check power factor penalties separately.
Confusing EER, SEER, COP, and kW/ton and mixing IP and SI units in HVAC efficiency calculations, leading to wrong conversion or wrong baseline comparison.
These metrics are all ratios of cooling output to energy input but differ in test conditions and units; candidates convert between them with a single factor and ignore that SEER is seasonal and EER is at a single rating point.
What to do instead
Memorize the unit relationships: EER = 12/COP approximately, kW/ton = 12/EER, and 1 ton = 3.517 kW. Use EER for rating-point comparisons, SEER only for seasonal estimates, and always show the conversion explicitly so unit errors are visible.
Reading a single instantaneous spot measurement from a portable instrument and reporting it as the annual or typical value, without logging duration, interval, or accounting for duty cycling.
Spot readings are quick and the instrument displays a confident number, so candidates skip logging and trending, especially on equipment with varying load such as compressors or chillers.
What to do instead
Use logging instruments over a representative period (at least a full production or occupancy cycle), report the measurement interval and uncertainty, and state the extrapolation basis for annualizing. Note instrument accuracy class and calibration status.
Misidentifying the applicable code edition or standard section, or applying a standard's prescriptive path when the project follows the performance path (or vice versa).
Codes such as ASHRAE 90.1 and IECC have multiple compliance paths and edition-dependent requirements, and candidates recall the general intent but not which edition or path the question specifies.
What to do instead
Before answering, identify the exact standard, edition, and compliance path stated in the question. If the question is silent, state the assumption and answer consistently with that edition. Know that 90.1 and IECC differ in scope and that local amendments may override.
Confusing energy accounting identities: mixing site energy with source energy, or treating energy units (kWh, therms, MMBtu, Btu) as interchangeable without conversion.
Site and source energy both use the same units, so the distinction is easy to overlook, and candidates convert between kWh and Btu imprecisely under time pressure.
What to do instead
Always label whether energy is site or source, apply the site-to-source factor explicitly, and use exact conversions (1 kWh = 3,412 Btu; 1 therm = 100,000 Btu; 1 MMBtu = 1,000,000 Btu). Keep a consistent unit throughout the calculation.
Oversizing or misapplying energy storage by ignoring round-trip efficiency, depth of discharge, and the demand-charge or TOU arbitrage window when calculating savings.
Candidates size storage on nameplate kWh and assume full cycling, forgetting that usable capacity is reduced by depth of discharge and that round-trip losses cut the arbitrage value.
What to do instead
Compute usable capacity as nameplate kWh × depth of discharge, apply round-trip efficiency on the charge/discharge cycle, and match the dispatch window to the actual TOU or demand-charge period. Present both energy and demand savings components.
Treating boiler efficiency as a single number and ignoring stack losses, excess air, and the difference between combustion efficiency, thermal efficiency, and fuel-to-steam efficiency.
Efficiency is often quoted without qualification, and candidates apply one figure to all calculations without checking whether it is HHV or LHV basis or whether it reflects actual excess air.
What to do instead
State the efficiency basis explicitly (combustion, thermal, or fuel-to-steam; HHV or LHV). Use stack temperature and excess air (or O2) to estimate stack losses, and note that reducing excess air improves efficiency up to the point of incomplete combustion.
Assuming a lighting retrofit saves the full connected load without accounting for lamp-lumen depreciation, ballast factor, control savings, or reduced cooling load interaction.
Candidates multiply fixture wattage difference by operating hours and stop, missing ballast factor, LPD compliance checks, and the HVAC interaction that reduces net savings in cooling-dominated buildings.
What to do instead
Compute savings from actual fixture input watts including ballast factor, apply operating hours and any controls savings separately, and add the HVAC interaction term (reduced cooling load minus any added heating). Verify resulting LPD against the applicable code limit.