Commissioning proves performance on day one; monitoring keeps it there for 15 years. The same four temperature sensors used at commissioning become permanent building-automation-system (BAS) points that compute live effectiveness and flag drift, turning a one-time test into a continuous alarm system that pays for itself the first time it catches a stuck damper that would otherwise waste energy all winter.
Computing effectiveness in the BAS
The controller evaluates the same formula used in the field, but every minute:
eff = (T_supply_out - T_outdoor) / (T_exhaust_in - T_outdoor)
Plot it against the rated curve and alarm when it falls more than 10% below the design band. A sudden drop in a cold snap usually means the frost bypass is open; a slow, steady decline points to fouling. Either way the fault is caught in days, not at the next annual audit when the energy is long gone and the budget has already been blown.
Pressure drop, fan energy and fault rules
Trend the supply and exhaust static pressures alongside fan kW. A 25% rise in pressure drop at constant flow means the core is loading up and the fan is spending extra energy pushing air through it. That trend is the trigger for a wash cycle and a direct input to the fan-energy model. Add CO2 or enthalpy sensors upstream and the BAS can practice demand-control ventilation, trimming outdoor air (and the recovery load) when the space is empty, which is where the biggest recurring savings hide. Set explicit fault rules: a soft alarm at 10% effectiveness loss (investigate, schedule a wash) and a hard alarm at 20% (lock out free-cooling, force maintenance).
| BAS point | Type | Use |
|---|---|---|
| T_outdoor, T_exhaust_in | AI | Effectiveness numerator / denominator |
| T_supply_out, T_exhaust_out | AI | Outlet confirmation |
| DP_supply, DP_exhaust | AI | Fouling and fan-energy trend |
| Bypass damper, wheel speed | AO / DO | Control and freeze protection |
Data resolution, reports and verification
Log at one-minute resolution for the first month, then roll up to 15-minute trend logs kept for at least three years in the historian so seasonal comparisons are possible. Most BAS platforms let you schedule a monthly effectiveness and pressure-drop report emailed to the facilities manager, so the recovery performance is reviewed even when nobody is watching the screen. Export the same data to the retrofit savings model so the claimed energy saving is reconciled against measured performance, closing the loop between the business case and reality. Where an incentive or a green-building credit depends on verified savings, this automated report is the documentation auditors ask for first, and it turns a vague "recovery is working" into a defensible number. Pair monitoring with the ERV design guide so the alarm thresholds match the duty, and review the data center recovery notes where 24/7 loads make continuous trending especially valuable.
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