01 / Result states
Directional when the evidence is clear. Uncertain when it is not.
Evidence adds points toward repair or replacement. A three-point margin is required for a directional state. A closer result is reported as uncertain, without forcing a verdict.
02 / Financial frame
Compare only the costs that still change.
Prior repair spending informs the reliability signal, but it is not subtracted from replacement because that money is already spent.
installed cost − verified incentives(current repair + prior repairs) ÷ likely net replacementnet replacement − repair − PV operating savingslikely incremental cost ÷ annual operating savings03 / Evidence signals
The ledger rewards patterns, not a single magic number.
The full ledger also includes repair scope, current operating state, ownership horizon, comfort, and known operating-cost changes.
04 / Quote check
Compare entered quotes, not an invented market benchmark.
Quote Check v1 compares up to three cash prices and reviews nine documentation criteria: price, exact model, efficiency, removal, permits, electrical or controls work, commissioning, ductwork scope, and labor warranty. A quote receives one point for each documented item.
median of positive cash prices entered(quote − entered median) ÷ entered mediandocumented criteria ÷ 9Price position means only lower, near, or higher than the small set entered by the homeowner. It is not a local market-price judgment.
05 / Lifecycle comparisons
Net upfront cost plus the present value of annual costs.
The HVAC and water-heater comparison tools use the same lifecycle method. The homeowner supplies installed cost, confirmed incentives, annual energy cost, annual maintenance allowance, ownership horizon, and discount rate for every option.
installed cash price − verified incentivesannual energy + annual maintenancenet installed + PV of annual running costVersion 1 does not forecast fuel rates, weather, financing, tax effects, resale value, future equipment replacement, or load. The interface asks the homeowner to test uncertain annual costs directly.
06 / Equipment planner
Planning ages open a research window, never an expiration date.
The device-local planner reports Monitor below 80% of the published planning-age signal, Research from 80% to below the signal, and Prepare at or beyond the signal. The status does not predict failure or replace inspection, condition, maintenance, warranty, or safety.
07 / Safety behavior
Safety does not become another point in a financial score.
A reported concern involving gas, combustion, electricity, pressure, leaking equipment, refrigerant, or another hazard changes the result to professional safety review. The calculator does not diagnose it.
08 / Limits
What this version intentionally leaves out.
- Future failure probability
- Resale value and buyer preferences
- Financing and tax effects
- Work missing from a contractor quote
- Professional equipment sizing or diagnosis
- Guaranteed incentive or rebate eligibility
Planning ages come from ENERGY STAR replacement-consideration guidance and a U.S. Department of Energy water-heater field study. They are signals, not expiration dates.