Domestic Hot Water Test Matrix

Reference engine: EnergyPlus 26.1.0 — WaterHeater:Mixed with an internal heater, a scheduled draw, and Site:WaterMainsTemperature (Correlation). EnergyPlus runs first and defines BOTH the draw and the mains water temperature, so the two engines are compared on fuel consumption against an identical thermal duty rather than against two separately-authored load assumptions. Scored against ASHRAE Guideline 14-2023.

Reference engine: EnergyPlus 26.1.0 — WaterHeater:Mixed with its source-side coil on a hot-water PlantLoop fed by DistrictHeating:Water, and Heater Maximum Capacity set to zero so every joule entering the water arrives through the coil. This is EnergyPlus's own 'Indirect Water Heater' topology. The heat source is an ideal district-heating supply rather than a modelled boiler, so the comparison isolates the tank instead of blending a second model's part-load behaviour into the residual. Scored against ASHRAE Guideline 14-2023.

Reference engine: EnergyPlus 26.1.0 — WaterHeater:HeatPump:WrappedCondenser and :PumpedCondenser coupled to a 12-node WaterHeater:Stratified tank, with the annual stage driven by the DOE / ASHRAE 90.1-2019 Large Hotel prototype's own use-side heat delivery. Published in full as its own report.

Timestep-sensitivity control test

A falsifiable prediction, tested on all seven direct-fired scenarios at all four supported timesteps against the same EnergyPlus reference. A part-load efficiency curve is evaluated at each step’s part-load ratio, so a coarser step averages the burner’s on-time into a different ratio and charges a different penalty — but only if there is a tank whose on-time can be averaged. Sensitivity should therefore require both a part-load curve and tank state, and any case missing either should be flat. That is a prediction which could have been wrong in three distinguishable ways.

The prediction holds, with no counter-example. Cycling control carries no curve, so efficiency is constant and the shift is −0.15% to −0.73%. Tankless modulating carries a curve but no tank state — its algebraic branch reads the instantaneous served load, so the part-load ratio is identical at every step and the shift is exactly 0.00%. Storage plus modulating carries both, and is the only material mover at −5.19%. It stays inside the ±5% annual band at every timestep (+1.98% at 60 minutes, −3.21% at 15), so the timestep changes where in the band it sits rather than whether it passes. Every gated figure published here is the 60-minute one, the engine’s default. This is a control test on the model’s own timestep choice; sub-hourly agreement against EnergyPlus is not itself validated, and is listed as a known gap in the report.

Why the three cycling fuels return identical numbers

Electric, natural-gas and oil cycling storage all score +0.34% on annual energy, and the table above shows them moving together across every timestep. That is not a copied row. At equal efficiency these are the same physics with a different meter, and EnergyPlus itself returns byte-identical energy for natural gas and electricity at equal efficiency — fuel type is routing, not thermodynamics. Reporting them as three independent passes would inflate the count, so they are named for what they are: one validated model exercised at three nameplates. The axes that do carry different physics — storage against tankless, cycling against modulating, and three levels of jacket insulation — are the ones that produce different numbers, and they do.