Boiler Module Validation

Stage 1 — Core boiler model per equipment type

Single-unit isolated boiler under variable hot-water demand. Three equipment classes (non-condensing gas, electric resistance, condensing gas), six part-load ratios, three nominal capacities, three supply temperatures, and two simulation periods (one-week steady + one-week sinusoidal variable). Each scenario runs in CogenS and EnergyPlus with identical performance curves pulled from the production coefficient database. Two FAIL scenarios are documented in the matrix: sub-minimum-cycling sinusoidal at PLR ≤ 0.10 on the 100 kW non-condensing boiler, where both engines hit the same operating-envelope edge.

Stage 1 is the foundational boiler validation. Does the boiler module predict the fuel consumption that EnergyPlus would predict, across the equipment classes a project will actually use, at the operating conditions a project will actually see? On 187 of 189 scenarios, the answer is yes — within the strict ASHRAE Guideline 14-2023 tolerance band on annual fuel energy, normalized mean bias error, and hourly variability.

What the matrix covers

The matrix sweeps three equipment classes (non-condensing gas with mechanical and natural draft, electric resistance, condensing gas), six part-load ratios (10%, 25%, 50%, 75%, 100% of nameplate plus the sub-minimum 5% / 10% bin), three nominal capacities (100 / 500 / 2,000 kW), three supply temperatures (60 / 82 / 95 °C), and two simulation periods (one-week steady and one-week sinusoidal variable). Each combination runs in EnergyPlus and CogenS with identical performance curves — same equipment, same heat demand, same outdoor weather. Then every output is scored against ASHRAE Guideline 14-2023.

Headline result

Strict-regime annual fuel deviation lands between -1.32% and +2.13% across all 144 strict scenarios — inside the +/-5% band. The 27 condensing scenarios sit between -6.86% and +11.09% under the documented +/-12% relaxed band (BiQuadratic FuelPLC 2D → 1D collapse rationale on the Cover sheet). The 18 sub-minimum-cycling sinusoidal scenarios sit between +0.06% and +16.59% under the +/-15% relaxed band; 16 of 18 PASS, with 2 documented FAIL at CAP100 / PLR10 / T82C where both engines hit operating-envelope edges. The CogenS boiler engine matches EnergyPlus at the level a lender's independent engineer expects.

Stage 1B — Full-year 8,760-hour realistic-load

Annual cross-validation where both CogenS and EnergyPlus run the same 8,760-hour weather-derived heating load on the same boiler equipment. Three equipment classes (non-condensing gas, electric, condensing gas at 60 °C in-design) each on the same 500 kW capacity with the same Chicago O'Hare TMY3-derived hourly load. Scored against ASHRAE Guideline 14-2023 at annual / monthly (12 bins) / hourly (8,760 points) / peak resolution.

Stage 1B is the full-year realistic-load test. A one-week synthetic period cannot prove a model behaves the way a real building does over a year of occupied operation. Stage 1B drives both engines with the same 8,760-hour load derived from real Chicago O'Hare TMY3 weather: a heating-degree seasonal envelope, a morning / evening DHW diurnal, weekday / weekend variation, clipped to PLR [0.20, 0.95].

Both engines see exactly the same load array. CogenS receives it as an explicit hourly array; EnergyPlus reads the identical CSV via Schedule:File. The two engines then run independently for 8,760 hours and the fuel use is scored at four resolutions: annual sum, monthly bins (12 months), hourly profile (8,760 points), and peak hour.

Headline result

Annual fuel energy +1.25% worst case across all three boiler types. Monthly NMBE at +1.25% worst case. Both inside ASHRAE Guideline 14-2023 strict (and the condensing case stays inside the relaxed band by 5× the safety margin). Hourly NMBE / CV(RMSE) and peak-hour rate all PASS strict on every case. This is the bankable evidence: a full year of real-building operation matches EnergyPlus inside the tolerance band an independent engineer expects on annual energy.

Stage 2 — Multi-unit identical staging

Multi-boiler plant where N identical units share a single hot-water load. Three equipment classes × three unit counts (N=2, N=3, N=4) × five system part-load ratios (10%, 25%, 50%, 75%, 100% of plant nameplate). Eight cases scoped out as documented control-law exclusions where the sub-minimum cycling regime is not validatable head-to-head. Tests how the staging rule (when to add the next unit) interacts with the per-unit performance curve.

Stage 2 validates the multi-unit identical staging philosophy. CogenS uses a band-fit rule (keep active units inside the optimum operating PLR range [0.44, 0.84]). EnergyPlus' 'Optimal' scheme prioritises loading each active unit to its maximum PLR. Both are valid engineering choices; this stage scores energy agreement under ASHRAE G14 plus a units-operating discrepancy gate.

All 44 in-scope scenarios PASS strict on annual fuel energy. The units-operating discrepancy is 0 in 32 / 44 cases (exact match) and 1 in 12 / 44 cases (the documented staging-philosophy difference).

Stage 3 — Multi-size (2-type) dispatch

Two-size boiler plant: 2 × 500 kW + 1 × 1,000 kW (= 2 MW plant) across five system PLRs and two equipment classes. The common topology where a smaller modulating boiler handles low-demand hours and a larger boiler stages in for peaks. CogenS' min-fuel proportional dispatch vs EP's 'Optimal' smallest-first scheme.

Stage 3 tests the two-size dispatch rule. CogenS' min-fuel proportional dispatch vs EP's 'Optimal' smallest-first. Both engines arrive at total system fuel within ASHRAE G14 strict and distribute the load across the two sizes under their respective control philosophies. The per-size split is reported as a Tier 2 diagnostic; the bankable energy total stays inside the strict band.

Stage 4 — Hot-water TES (standby decay)

Isolated stratified hot-water tank, heaters disabled, decaying from a uniform 90 °C across 168 hours. Two tank volumes (10 / 50 m³) × three U-values (0.3 / 0.5 / 1.0 W/m²K) = 6 cases. Validates the tank's standby physics against EP's WaterHeater:Stratified.

Stage 4 validates the TES tank's standby physics: an isolated tank decaying from a uniform 90 °C into a 20 °C ambient over a week, with both heaters disabled. Every transport term is identically zero in this regime, which is what makes it a clean test of the shell-loss physics on its own — and also why it cannot, by itself, tell you whether a tank puts energy in the right place. That is what the charge and discharge stages of the Thermal Energy Storage validation exist for.

These figures were re-scored against the current stratified-tank engine in July 2026. The tank-temperature trajectory PASSES strict ASHRAE Guideline 14 on all six cases, as it did before; the accuracy improves (CV(RMSE) roughly halves) but the verdict does not change, because the Guideline 14 bands are wide enough that a standby tank clears them on either engine.

The shell-loss diagnostic is the figure that did change, and it is worth being explicit about. It previously sat at -17.6% to -19.6% and was reported as a difference of convention between the two engines in how tank surface area is integrated. It was not a convention difference. For the tank geometry this sweep uses (height = 4 × radius) the cylindrical side wall is 8/10 of the total surface, so omitting the two end caps loses about a fifth of the shell — which is exactly the size of that column. The current engine integrates the full surface, including the caps, and the diagnostic is now -1.3% to -0.3%.

Stage 4b — Hot-water TES charge / discharge cycling

Cycling regime: 8 hours nightly charge (heater setpoint 82 °C) + 12 hours daily discharge (use-side LoadProfile:Plant draws 22 kW; 1.2 kW/m³ MOD or 2.2 kW/m³ DEEP) + 4 hours idle. Two tank volumes (10 / 50 m³) × two cycling depths (MOD, DEEP) = 4 cases, 168-hour week with 7 daily cycles each.

This stage has NOT been re-scored against the current stratified-tank engine — every figure below describes the engine that shipped before July 2026, and the charge-energy and shell-loss diagnostics still cite the Stage 4 “surface-area convention” rationale that the Stage 4 re-score has since retired: it was an under-count of the tank end caps, not a convention. The regime itself — hot-water charge and discharge cycling — is now cross-validated against EnergyPlus at far greater breadth by the Thermal Energy Storage module validation, which covers 36 hot-water scenarios spanning both duties against the 4 here.

Stage 4b validates the TES tank under deep cycling. The tank-temperature trajectory and cumulative useful discharge (the round-trip throughput that drives project economics) PASS strict on all 4 cases. Charge energy and shell-loss are reported as Tier 2 diagnostics against EP's full-surface accounting convention, not gates.

Stage 5 — Boiler + TES coupling

Coupled Boiler + TES plant: boiler maintains tank at setpoint while the load draws on the tank. Two equipment classes (non-condensing gas, electric) × two tank volumes (10 / 50 m³) × three system PLRs (30% / 50% / 70%) = 12 cases on the 1-week steady matrix, plus served annual TMY3-realistic canonicals (NONCOND gas + ELECTRIC variants) for full-year evidence, including a 20-year time-of-use financial layer on the Electric variant that gates cost of ownership with and without storage and the storage-incremental NPV against an analytical NIST HB135 present-value reference. (Internal rate of return and payback are reported alongside but are not gated — see the gate list.)

Stage 5 validates the coupled Boiler + TES plant: the boiler holds the tank at its setpoint while a downstream load draws it down, which is EnergyPlus's own IndirectHeatPrimarySetpoint dispatch. Two equipment classes (non-condensing gas, electric) × two tank volumes (10 / 50 m³) × three system part-load ratios (30% / 50% / 70%) = 12 cases over a one-week steady matrix. All 12 PASS strict Guideline 14 on boiler fuel, thermal output and tank-temperature trajectory.

These figures were re-scored against the current stratified-tank engine in July 2026. Because Stage 5 tests the composition of two engines, the re-score deliberately changed only one of them: the published harness was re-run unmodified with the storage engine swapped and the boiler engine left exactly as it was, so any movement is attributable to the tank rather than to two things at once. Re-running with the previous storage engine reproduces all four published gate ranges exactly, which is what makes the new figures a like-for-like replacement.

The verdict is unchanged at 12 / 12. The boiler fuel and thermal bands widen very slightly — by about two tenths of a percentage point at the worst case — and that is the expected direction: the tank's shell loss was previously under-counted by about a fifth (it integrated the cylindrical side wall but not the two end caps), so the boiler is now charged for heat it always should have been supplying. The shell-loss diagnostic moves correspondingly, from about -18% to about +2%.

The financial layer was re-scored as well. It runs on a separate case — a served annual TMY3 canonical for the Electric Boiler + TES variant — and applies an hourly time-of-use rate, so the storage engine's dispatch timing affects the result and not merely its annual totals. Twenty-year cost of ownership agrees with an analytical NIST HB135 present-value reference to within 1.84%, against a 5% band. Two clarifications on what that layer actually tests: internal rate of return and payback are computed and reported but are not gated, and on this canonical they are undefined because the storage NPV is negative, so there is no rate at which the cash flows break even. And the feasibility verdict itself is a diagnostic rather than a gate, because the two engines reach opposite conclusions on a near-zero NPV — which is expected for a control strategy that holds a tank at setpoint rather than shifting load into cheaper hours.

Stage 6 — Total Cost of Ownership and financial KPIs

Investment-grade financial outputs: total cost of ownership, net present value, internal rate of return, simple payback, and MACRS-200%-DB depreciation. Two cost bases (SMALL: 500 kW + 10 m³, LARGE: 2 MW + 50 m³) × six financial profiles (discount-rate sweep, inflation sweep, study-period sweep, base + MACRS tax treatment) = 12 cases.

Stage 6 validates the financial layer to the standard a lender's independent engineer requires. Operating present value runs against EnergyPlus LifeCycleCost (NIST Handbook 135 reference). The investment KPIs (NPV, IRR, simple payback) are validated against independent analytical references: polynomial-root IRR solver, direct NPV recompute, IRS Publication 946 MACRS GDS percentage tables.

All gates pass at exactly 0.0000% across all 12 scenarios. The financial engine is technology-agnostic so the boiler Stage 6 also serves as the foundation for the chiller, cooling-tower, and CHP financial validations.