Room-by-room heat loss

A room-by-room heat loss calculation finds the heat output each space needs at the winter design condition. It is what allows the designer to choose a radiator, fan convector or underfloor circuit for each room instead of hoping that a whole-house total will distribute itself correctly.

The whole-building design load and the room loads have related but different jobs:

• the building load is used to select the heat pump
• each room load is used to select its emitter and required water flow
• the most demanding room can set the lowest workable system flow temperature

How a room load is built

Each room is represented with its own geometry, design temperature and adjoining conditions. Transmission loss is calculated for its walls, floor, ceiling or roof, windows and doors. The basic element calculation is:

heat loss in W = area × U-value × temperature difference

Openings are deducted from their parent walls so their area is not counted twice. Internal elements also matter because a room designed to be warmer than its neighbour can transfer heat into that neighbour, even though this transfer does not leave the building.

Ventilation loss is then calculated from the relevant room volume, intentional ventilation, infiltration and transfer-air treatment. The method uses the applicable loss rather than adding several overlapping ventilation assumptions together.

The result is the room’s design heat load in W. Specific heat loss in W/m² can help compare rooms, but emitter selection uses the total room heat load.

Room and building totals are not interchangeable

Adding the displayed room values may not reproduce the whole-building heat load exactly. Internal heat transfers redistribute heat between rooms, while ventilation and infiltration are treated at both room and building level to avoid a false whole-house total.

This is not an error to fix by scaling every room until the numbers match. The designer should use:

• the compliant whole-building result for heat-source selection
• the unscaled room result for each emitter

The report should explain any material difference and resolve warnings such as a heat-transfer imbalance. It should not hide the difference by reverting to an older or simplified calculation.

Choosing the room inputs

Good results depend on good inputs. Each room needs:

• correct net dimensions
• its intended use and design temperature
• the actual or properly evidenced construction of each element
• correct adjoining spaces and their temperatures
• suitable U-values and thermal-bridging treatment
• ventilation terminals, flues and mechanical ventilation
• the building airtightness basis
• any agreed intermittent-heating treatment or winter heat gains

As at 22 July 2026, the MCS Heat Load Calculator uses RdSAP 10 data for external default U-values and the CIBSE Domestic Heating Design Guide 2026 for internal values and room ventilation references. The calculation report should retain its evidence and assumptions because reference datasets and tools can change.

From heat loss to an emitter

An emitter must provide at least the room load at the proposed water and room temperatures. A radiator’s familiar catalogue rating is commonly declared at a much larger water-to-room temperature difference than a heat-pump circuit will use. It must be corrected using the manufacturer data or recognised method for the actual design condition.

For a water emitter:

mean water temperature = (flow temperature + return temperature) ÷ 2

mean temperature difference = mean water temperature − room temperature

That mean temperature difference is used with the appropriate output data. The relationship is not linear and should not be replaced with a generic percentage.

Underfloor heating is checked differently. Its output depends on active floor area, pipe layout, water temperatures, construction, floor covering and allowable surface temperature. Fan convectors need output data at the selected water temperatures and fan setting, together with their sound and electrical requirements.

The worst-performing room

The system design flow temperature is normally governed by the room that is hardest to heat with its proposed emitter. A small shortfall in one room should not automatically force every circuit to run hotter. The designer can compare:

• a larger or different emitter
• a fan-assisted emitter
• a change to the underfloor layout
• a specific fabric improvement
• a separate temperature circuit where it is technically justified

MIS 3005-D requires the customer to receive the design emitter temperature based on the worst-performing room, as well as the design flow temperature leaving the heat pump before any blending valve.

Allowance without hidden oversizing

An emitter may be selected in the next available size above the calculated room load. That transparent product-selection allowance is different from inflating dimensions, U-values, ventilation and room temperature so that several hidden margins accumulate.

Where a construction cannot be confirmed, the calculation should identify the assumption and show how a credible alternative affects the room and building result. Further inspection or a measured value is better than a blanket percentage.

Rooms that receive substantial incidental heat still need careful treatment. Solar gains, occupants and appliances vary and may not be available at the cold design condition. The formal method’s heat-gain input should be used cautiously rather than deducting an optimistic everyday average.

Flow and hydraulic design

Providing enough emitter surface is only half the job. Each circuit also needs the water flow that carries its heat:

heat transferred = mass flow × specific heat capacity × water temperature drop

The designer turns the emitter load and intended flow-to-return temperature difference into a required flow, then checks pipe resistance, valves, manifolds and pump head. A radiator with enough theoretical output can still leave a room cold if it receives too little water.

Closing room controls also changes system flow. The design must preserve the heat pump’s minimum flow and active water-volume requirements when thermostatic valves or underfloor actuators close.

What the emitter schedule should show

A clear room schedule records:

• room name and design temperature
• calculated design heat loss in W
• retained or proposed emitter type and dimensions
• declared emitter rating and corrected output at the design condition
• flow and return temperatures
• required circuit flow where applicable
• any shortfall, selection allowance or dependence on fabric work

The installed emitter should be checked against this schedule during commissioning. Substituting a radiator because its dimensions look similar is not enough if its verified output or hydraulic resistance differs.

Related entries

• Heat loss survey
• Design flow temperature
• Heat pump sizing margin
• Radiator emitter upgrades
• Underfloor heating compatibility
• Primary pipework sizing
• Commissioning flow rates