{
  "version": "1.0",
  "calculator": {
    "id": "hot-water-cylinder",
    "title": "Hot water demand and cylinder size",
    "summary": "Works out a household's daily hot water demand and the cylinder a heat pump needs to meet it.",
    "tier": "advanced",
    "description": "Work out the daily hot water demand of a dwelling and the size of cylinder a heat\npump needs, using the formula in MIS 3005-D 3.5.2 and the BS 6700 guidance reproduced in MGD 007.\n\nThe demand is 45 litres a day for each of N, where N is the greater of the number of bedrooms plus one\nand the number of people actually living there. For a new build, where occupancy is unknown, the\nbedroom count is used.\n\nThe cylinder does not have to hold a whole day's demand, because it reheats as it empties. What it must\ncover is the largest draw-off before the heat pump catches up, which is why a larger heat pump needs a\nsmaller cylinder: reheat time is volume times temperature rise, divided by 14.3 times the kW.\n\nThe result gives the BS 6700 minimum, a recommended size rounded to a volume cylinders are made in, and\nthe reheat time. Note that cylinder volume is not the whole story: a heat pump cylinder needs a much\nlarger coil than a boiler cylinder, because the water arrives cooler and there is less temperature\ndifference to drive heat across the surface.",
    "standard": "MIS 3005-D",
    "category": "heat-pumps",
    "webPath": "/calculators/hot-water-cylinder",
    "example": {
      "bedrooms": 3,
      "occupants": 4,
      "heatPumpOutputKw": 8,
      "bathrooms": 2,
      "storageTemp": 55
    },
    "inputSchema": {
      "type": "object",
      "properties": {
        "bedrooms": {
          "type": "integer",
          "description": "Number of bedrooms in the property. Required."
        },
        "occupants": {
          "type": "integer",
          "description": "Number of people who actually live there, if known. MIS 3005-D takes the greater of this and bedrooms plus one. Leave it out for a new build, where the occupancy is not yet known."
        },
        "heatPumpOutputKw": {
          "type": "number",
          "description": "Heat output of the heat pump in kW when it is heating the cylinder. A larger heat pump reheats faster, so it needs less stored volume. Defaults to 6."
        },
        "bathrooms": {
          "type": "integer",
          "description": "Number of bathrooms with a bath. Defaults to 1."
        },
        "coldWaterTemp": {
          "type": "number",
          "description": "Incoming mains temperature in Celsius. Defaults to 10."
        },
        "storageTemp": {
          "type": "number",
          "description": "Temperature the cylinder is stored at, in Celsius. Defaults to 55, which is a common heat pump setting. A weekly pasteurisation cycle above 60 is separate."
        }
      },
      "required": [
        "bedrooms"
      ],
      "additionalProperties": false
    },
    "outputSchema": {
      "type": "object",
      "properties": {
        "bedrooms": {
          "type": "integer"
        },
        "occupants": {
          "type": "integer"
        },
        "designOccupancy": {
          "type": "integer",
          "description": "N in the MIS 3005-D formula: the greater of bedrooms plus one and the known occupants."
        },
        "occupancyBasis": {
          "type": "string",
          "description": "Which of the two gave the larger figure."
        },
        "dailyDemandLitres": {
          "type": "number",
          "description": "Average daily hot water demand: 45 litres for each of N."
        },
        "recommendedCylinderLitres": {
          "type": "integer",
          "description": "Cylinder volume from the BS 6700 guidance in MGD 007, given the number of baths and the heat pump output."
        },
        "minimumCylinderLitres": {
          "type": "integer",
          "description": "The BS 6700 minimum for this number of bathrooms and this heat pump output, before the allowance MGD 007 adds for extra bathing capacity."
        },
        "reheatMinutes": {
          "type": "number",
          "description": "Roughly how long the heat pump takes to bring the whole cylinder up from cold, ignoring standing losses."
        },
        "energyPerDayKwh": {
          "type": "number",
          "description": "Heat needed each day to raise the daily demand from mains temperature to storage temperature."
        },
        "heatPumpOutputKw": {
          "type": "number"
        },
        "storageTemp": {
          "type": "number"
        },
        "coldWaterTemp": {
          "type": "number"
        },
        "bathrooms": {
          "type": "integer"
        },
        "method": {
          "type": "string"
        },
        "references": {
          "type": [
            "null",
            "array"
          ],
          "items": {
            "type": "object",
            "properties": {
              "document": {
                "type": "string"
              },
              "title": {
                "type": "string"
              },
              "url": {
                "type": "string"
              },
              "clause": {
                "type": "string"
              }
            },
            "required": [
              "document",
              "title",
              "url"
            ],
            "additionalProperties": false
          }
        },
        "notes": {
          "type": [
            "null",
            "array"
          ],
          "items": {
            "type": "string"
          }
        }
      },
      "required": [
        "bedrooms",
        "designOccupancy",
        "occupancyBasis",
        "dailyDemandLitres",
        "recommendedCylinderLitres",
        "minimumCylinderLitres",
        "reheatMinutes",
        "energyPerDayKwh",
        "heatPumpOutputKw",
        "storageTemp",
        "coldWaterTemp",
        "bathrooms",
        "method",
        "references",
        "notes"
      ],
      "additionalProperties": false
    }
  },
  "url": "https://ampworth.com/api/v1/calculators/hot-water-cylinder",
  "webUrl": "https://ampworth.com/calculators/hot-water-cylinder",
  "exampleUrl": "https://ampworth.com/api/v1/calculators/hot-water-cylinder?bedrooms=3\u0026occupants=4\u0026heatPumpOutputKw=8\u0026bathrooms=2\u0026storageTemp=55",
  "method": "POST",
  "contentType": "application/json",
  "notice": "These calculators follow the published MCS methods, for understanding a system and checking a quotation. Anything that gets installed still needs a design and a site survey from a certified installer."
}