01Four real causes
Supply below rated voltage
Output follows the square of voltage. At 340 V a 380 V rated heater delivers roughly 80% of its nameplate power.
Underestimated heat loss
Missing or thin insulation, plus steelwork bridging, can consume the whole design margin.
Flow away from design point
A pump or fan working somewhere else on its curve changes the duty: more medium flow, smaller temperature rise — it feels underpowered.
Fouling or a dead bank
Scale on the water side, carbon on the oil side, dust on the air side; or one control bank that never picks up.
02Voltage against output
P actual / P rated = (U actual / U rated)². End-of-line voltage drop, transformer tap position and a sagging summer grid can take 5%~10% off supply, which is 10%~19% off output. If the calculation used rated voltage while the site runs at 340~360 V, the shortfall is explained there — and with a long cable run, check the drop at the heater terminals, not at the panel.
On the loss side, thickness per the specification is not the end of it: scan the outer surface with infrared. Anywhere noticeably warmer than ambient is exporting heat. Steelwork, supports and nozzle penetrations often matter more than the insulation thickness itself.
03The four-step site order
Current
Clamp-meter each phase and compare with nameplate amps. One phase clearly low means that group is not energised.
Voltage on load
Record terminal voltage while running. No-load voltage proving fine tells you nothing.
ΔT and flow
Back-calculate the power absorbed by the medium from measured rise and flow, then compare with theoretical output.
Insulation
Walk an IR camera over the lagging and at brackets, nozzles and wall penetrations. Hot spots are heat escaping.
04Two things to re-check after the maths
Re-check two points: whether the design assumed continuous running while production is actually intermittent (cold starts need extra duty), and whether the control allows staged switching, so the plant is not paying for peak capacity at part load most of the time. Most underperforming sites end up fixed on run pattern and heat loss, not on the heater rating.