{
  "schemaVersion": 1,
  "slug": "patio-heater-btu-calculator",
  "category": "household",
  "kind": "implementation",
  "independentValidation": false,
  "implementationHash": "8bb62d9ed549b5207234366df4e7e46e7792bebff99e681283ef2495ba416613",
  "provenance": "Generated from the calculation contract used by the published page. Publication is not external validation.",
  "sourceFiles": [
    "data/calculators/patio-heater-btu-calculator.json",
    "src/engine.js"
  ],
  "runtimeSource": "/sources/engine.txt",
  "runtimeHash": "5c888d8898afbdee36267bb9977829e1e2ac568f6677c8ee87469c139864645c",
  "scope": "The compute function records implemented equations, constants, lookup tables and custom rules. Inputs record defaults and allowed values; outputs record precision and units. Defaults are examples, not recommendations. General background references are not evidence for this specific algorithm.",
  "publishedFormula": {
    "formula": "Q = A h × 0.133 × dT × w; heaters = ceil(Q ÷ q0); coverage = A ÷ heaters",
    "vars": [
      [
        "A",
        "Patio area (sq ft)"
      ],
      [
        "h",
        "Ceiling height (ft)"
      ],
      [
        "dT",
        "Temperature rise (F)"
      ],
      [
        "w",
        "Wind factor (1 / 1.35 / 1.8)"
      ],
      [
        "q0",
        "Heater output (BTU/hr)"
      ]
    ],
    "note": "BTU/hr = area × ceiling height × 0.133 × temperature rise × wind factor (1.0 sheltered, 1.35 partial, 1.8 exposed). Fuel rate uses the selected heater output divided by 91,500 BTU/gal propane, 100,000 BTU/therm natural gas, or 3,412.142 BTU/kWh electricity. Seasonal cost assumes 26 weeks and sample unit prices unless overridden; model factors and prices are illustrative."
  },
  "externalEvidence": {
    "reviewedAt": "2026-09-26",
    "sources": [
      {
        "name": "U.S. Energy Information Administration — Energy Units and Calculators",
        "url": "https://www.eia.gov/energyexplained/units-and-calculators/"
      },
      {
        "name": "U.S. Energy Information Administration — Natural Gas Heat Content",
        "url": "https://www.eia.gov/tools/Faqs/faq.php?id=45&t=5"
      }
    ]
  },
  "contract": {
    "compute": "const windMult=wind==='sheltered'?1:wind==='partial'?1.35:1.8,volume=area*ceilingHeight,btu=volume*0.133*tempRise*windMult,heaters=heaterOutput>0?Math.ceil(btu/heaterOutput):0,fuelRate=heaterType==='propane'?heaterOutput/91500:heaterType==='natural'?heaterOutput/100000:heaterOutput/3412.142,defaultPrice=heaterType==='propane'?1.6:heaterType==='natural'?1.1:0.17,price=energyPricePerUnit>0?energyPricePerUnit:defaultPrice,unit=heaterType==='propane'?'gal/h':heaterType==='natural'?'therm/h':'kWh/h'; return {btuNeeded:btu,heatersNeeded:heaters,btuPerSqFt:area>0?btu/area:0,fuelRatePerHour:fuelRate,fuelUnit:unit,seasonCost:fuelRate*price*hoursPerWeek*26,coverage:heaters>0?area/heaters:0};",
    "inputs": [
      {
        "name": "area",
        "type": "number",
        "default": 300,
        "min": 10,
        "max": 20000,
        "step": 10,
        "slider": true
      },
      {
        "name": "ceilingHeight",
        "type": "number",
        "default": 9,
        "min": 6,
        "max": 30,
        "step": 0.5,
        "slider": true
      },
      {
        "name": "tempRise",
        "type": "number",
        "default": 15,
        "min": 1,
        "max": 60,
        "step": 1,
        "slider": true
      },
      {
        "name": "wind",
        "type": "select",
        "default": "sheltered",
        "options": [
          {
            "value": "sheltered"
          },
          {
            "value": "partial"
          },
          {
            "value": "exposed"
          }
        ]
      },
      {
        "name": "heaterType",
        "type": "select",
        "default": "propane",
        "options": [
          {
            "value": "propane"
          },
          {
            "value": "electric"
          },
          {
            "value": "natural"
          }
        ]
      },
      {
        "name": "heaterOutput",
        "type": "number",
        "default": 40000,
        "min": 1000,
        "max": 500000,
        "step": 1000
      },
      {
        "name": "hoursPerWeek",
        "type": "number",
        "default": 10,
        "min": 0,
        "max": 168,
        "step": 1,
        "slider": true
      },
      {
        "name": "energyPricePerUnit",
        "type": "number",
        "default": 0,
        "min": 0,
        "max": 1000,
        "step": 0.01
      }
    ],
    "outputs": [
      {
        "name": "btuNeeded",
        "precision": 0,
        "unit": "BTU/hr"
      },
      {
        "name": "heatersNeeded",
        "precision": 1,
        "unit": ""
      },
      {
        "name": "btuPerSqFt",
        "precision": 1,
        "unit": "BTU/sq ft"
      },
      {
        "name": "fuelRatePerHour",
        "precision": 3,
        "unit": ""
      },
      {
        "name": "fuelUnit",
        "precision": 0,
        "unit": "",
        "type": "text"
      },
      {
        "name": "seasonCost",
        "precision": 2,
        "unit": "$"
      },
      {
        "name": "coverage",
        "precision": 0,
        "unit": "sq ft"
      }
    ],
    "interpret": null
  }
}