What size heat pump do I need?
heat_pump_sizeEstimate the heat pump a UK house needs, and what it would cost to run against the fuel it burns now.
IMPORTANT: MIS 3005-D requires a heat pump to be sized from a room-by-room heat loss calculation to BS EN 12831, and nothing here replaces that. Say so whenever you report a result from this, and point the user at the room-by-room calculation. What the figure is for is checking whether a size somebody has been quoted is plausible, because oversizing is the commonest fault in heat pump installations and it is invisible to the person living with it.
There are two routes and the first is much the better one.
From metered fuel, which is preferred: give 'annualFuelUse' from a year of bills. The fuel becomes heat delivered, hot water comes off it using the MIS 3005-D formula, and what remains divided by the local heating degree days gives the building's heat loss coefficient in watts per kelvin. Degree days are computed from the Met Office monthly means published in MGD 007 section 5.
From the fabric, as a fallback when there are no bills: give 'floorAreaM2', an 'insulation' level and a 'propertyType'. The watts per kelvin per square metre behind this are a reasonable reading of British housing stock but they are NOT MCS figures, because MCS publishes no such shortcut. Say so when using this route.
Either way the peak load is the coefficient multiplied by the design temperature difference for the location, from Table 2 of MIS 3005-D. The heat pump must deliver at least that at the design outside temperature and the design flow temperature, which is not the same as its headline rating.
The result also compares what each of four flow temperatures would cost to run, which is the decision that governs the bill and the one a quotation most often leaves unstated. Supply 'quotedHeatPumpKw' to have a quoted size checked against the estimate.
Method from MIS 3005-D. 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.
Input Schema
| Name | Required | Description | Default |
|---|---|---|---|
| fuel | No | The fuel the house is heated with now: 'mains_gas', 'oil', 'lpg', 'electricity' or 'coal'. Defaults to 'mains_gas'. | |
| scop | No | Seasonal coefficient of performance of the proposed heat pump, if it is known from the MCS Product Directory. Defaults to an indicative figure for the chosen flow temperature. | |
| bedrooms | No | Number of bedrooms, for the hot water demand. Defaults to 3. | |
| location | Yes | Nearest of the eight tabulated locations: 'belfast', 'birmingham', 'cardiff', 'edinburgh', 'glasgow', 'london', 'manchester' or 'plymouth'. Required. | |
| occupants | No | Number of people living there, if known. MIS 3005-D takes the greater of this and bedrooms plus one. | |
| insulation | No | How well insulated the building is: 'solid', 'cavity_unfilled', 'insulated', 'modern' or 'very_efficient'. Used only with floorAreaM2. | |
| designBasis | No | 'a' for the 99% outside design temperature or 'b' for the colder 99.6% one. Defaults to 'a'. | |
| floorAreaM2 | No | Total heated floor area in square metres, over all storeys. Used only when no fuel figure is given. | |
| internalTemp | No | Whole-house mean internal design temperature in Celsius. Defaults to 20, which is a reasonable average of the room-by-room figures in MIS 3005-D Table 1. | |
| propertyType | No | 'detached', 'semi', 'end_terrace', 'mid_terrace', 'flat' or 'bungalow'. Used only with floorAreaM2. Defaults to 'semi'. | |
| annualFuelUse | No | How much of that fuel the house burns in a year, in its own unit: kWh for gas and electricity, litres for oil and LPG, kilograms for coal. Give this if it is known: it describes the actual house and is far better than the fabric estimate. | |
| flowTemperature | No | The flow temperature the system would be designed around: 35, 45, 50 or 55. Defaults to 45. This is the single biggest influence on running cost. | |
| fuelPricePerUnit | No | Price of one unit of the current fuel, in pounds. Optional; supplying it and the electricity price gives a running cost comparison. | |
| quotedHeatPumpKw | No | The output of a heat pump somebody has quoted, in kW. Optional; supplying it says whether that size looks plausible against this estimate. | |
| nonHeatingFraction | No | Share of that fuel used for something other than heating and hot water, such as a gas hob, as a decimal. A hob is usually 0.03 to 0.05. Defaults to 0. | |
| summerMonthFuelUse | No | Fuel used in a single summer month, in the same unit, when the heating is off. Give this if it can be found: almost all of it is hot water, so it separates hot water from space heating exactly and removes the biggest assumption in the estimate. | |
| applianceEfficiency | No | Seasonal efficiency of the existing boiler as a decimal. Defaults to a realistic figure for the fuel instead of the one on the badge. | |
| electricityPricePerKwh | No | Price of a unit of electricity, in pounds. Optional. |
Output Schema
| Name | Required | Description | Default |
|---|---|---|---|
| scop | Yes | ||
| basis | Yes | ||
| notes | Yes | ||
| method | Yes | ||
| warning | Yes | ||
| location | Yes | ||
| basisNote | Yes | ||
| references | Yes | ||
| designBasis | Yes | ||
| scopAssumed | Yes | True when an indicative figure stood in for a real one from the product directory. | |
| internalTemp | Yes | ||
| breakEvenScop | No | The seasonal efficiency at which the two cost the same. | |
| peakHeatLossKw | Yes | The space heating load at the design outside temperature. The heat pump must deliver at least this, at the design flow temperature, without help from an immersion heater. | |
| flowTemperature | Yes | ||
| quotedSizeCheck | No | ||
| annualDifference | No | Positive when the heat pump costs less. | |
| annualHotWaterKwh | Yes | ||
| currentAnnualCost | No | ||
| heatingDegreeDays | Yes | ||
| externalDesignTemp | Yes | ||
| heatPumpAnnualCost | No | ||
| annualHeatDemandKwh | Yes | ||
| dailyHotWaterLitres | Yes | ||
| wattsPerSquareMetre | No | Peak load per square metre of floor, where a floor area was given. A useful sanity check. | |
| flowTemperatureCosts | No | ||
| annualSpaceHeatingKwh | Yes | ||
| hotWaterFromSummerUse | Yes | True when a summer month's fuel separated hot water from space heating, instead of an allowance standing in for it. | |
| temperatureDifference | Yes | ||
| heatPumpElectricityKwh | Yes | ||
| heatLossCoefficientWPerK | Yes | How many watts the building loses for every degree between inside and outside. The figure everything else follows from. |