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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteSize radiators room by room from each room’s calculated heat loss, then check that the chosen emitter can deliver that output at the system’s design flow and return temperatures. Size pipes for the water flow each circuit must carry, using the pipe’s actual internal bore and checking velocity and pressure loss along the complete route. Floor area, an old radiator’s dimensions and a pipe’s nominal outside diameter are not enough to establish a safe or adequate design.
What radiator and pipe sizing must achieve
A hot-water heating system has two linked sizing problems. Each room needs enough emitter output to meet its heat loss under the intended operating conditions. The distribution pipework then needs to carry the water flow required by those emitters without excessive resistance. Getting one part right does not guarantee the other: a correctly selected radiator may still be under-supplied by a restrictive circuit, while a generously sized pipe cannot make an undersized radiator meet the room load.
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The method below follows UK-oriented guidance from CIBSE and the Energy Saving Trust. It is a design process, not a ready-made radiator schedule or universal pipe-diameter chart. The current CIBSE Domestic Heating Design Guide 2026 is described by its publisher as having a July 2026 version that supersedes the earlier 2026 version; CIBSE also advises readers to consult the accompanying corrigendum. For a project outside the UK, use the applicable local design weather data, standards and code requirements.
Gather the information before choosing equipment
Start with a survey of the building and the proposed system. A room-by-room calculation depends on more than the room’s floor area, and pipe sizing depends on more than the pipe’s visible diameter. Record the design basis so the assumptions can be checked and retained with the final installation documents.
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Room and building inputs
- Room dimensions and floor area, plus the surfaces exposed to outside, ground, roof space or unheated areas.
- Construction details or U-values where available, and the rooms or spaces adjoining each boundary.
- The intended indoor design temperature for each room and the project location, which determines the relevant outdoor design condition.
- Air-change assumptions and the room’s exposure. Survey guidance from the Energy Saving Trust also calls out the conditions above and below a room.
- Existing emitter type and dimensions, available replacement space, and any physical constraints on wall or floor-mounted equipment.
System and pipework inputs
- Heat source and design strategy, including the intended heating-water flow and return temperatures.
- Pipe material, actual internal bore, route lengths, fittings, valves and other equipment in each circuit.
- The heat load served by each circuit, the design flow-return temperature difference and the pump’s available head.
- Existing pipe sizes and routes if the system is being altered rather than installed from scratch.
Do not turn a watts-per-square-metre shortcut into a design result. At most, a clearly identified rule of thumb can provide an early sense of scale. CIBSE’s domestic factsheet recommends a full room-by-room heat-loss assessment rather than a whole-house estimate, a rule of thumb or copying the equipment already present.
Calculate heat loss separately for every room
The heat loss for a room is the output its emitter arrangement must supply at the selected design condition. Keep individual room loads separate: a heat generator sized for the whole home does not reveal which room needs a larger radiator or a different emitter. Two rooms with the same floor area can have different losses because their exposed surfaces, construction, air movement, adjacent spaces and target temperatures differ.
Use a recognized room heat-loss method or have a qualified heating designer calculate it. The public guidance cited here identifies the necessary room-by-room approach and survey inputs, but does not establish a complete calculation method or universal set of assumptions. It would therefore be misleading to supply a made-up formula, fixed air-change rate or allowance and present the resulting watts as a project design.
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Why a whole-house total is not enough
Adding every room’s load can inform heat-source capacity, but it cannot be used to select every radiator by dividing the total across rooms. A corner room, an exposed bedroom and a room over a garage can have very different losses from an internal room of the same size. The circuit serving each group of emitters also has its own flow requirement. Keep room loads and circuit loads visible as separate entries in the schedule.
Select radiators at the system’s design temperatures
A radiator’s catalogue output is not automatically the output it will deliver in a particular system. The output depends on the product and on the heating-water and room conditions used for the rating. If the project’s flow and return temperatures differ from the catalogue rating conditions, use the manufacturer’s output data or correction method to establish the radiator’s output at the actual design conditions.
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Match the corrected output to the room load
- Set the system’s intended design flow and return temperatures in line with the heat source and design strategy.
- Choose a candidate radiator or emitter and obtain its manufacturer’s output information.
- Convert the published output to the project’s design conditions using the manufacturer’s data or correction method.
- Check that the resulting output meets the room’s calculated heat loss and that the emitter fits the available space.
- Repeat for every room; do not assume one radiator size or correction factor suits the entire property.
CIBSE’s 2026 heat-pump guide includes guidance on converting radiator catalogue output to actual output, selecting steel-panel radiators and assessing existing radiator output. The appropriate selection still depends on the product data and the project’s temperatures. The sources do not support one fixed oversizing factor for all radiators or systems.
Account for lower-temperature operation
Heat pumps commonly deliver lower heating-water temperatures than equivalent fossil-fuel systems, according to the Energy Saving Trust. At lower design temperatures, a radiator may need more emitting surface area to supply the same room output; another emitter arrangement, such as underfloor heating, may also be considered. The actual design temperature and emitter response depend on the heat source, product and project, so do not assume a single flow temperature or radiator enlargement will apply to every installation.
When comparing a replacement radiator with an existing one, compare output at identical design conditions, not just height, width or the number of panels. A physically similar radiator can have a different rated output, and an old radiator may have been selected for a different flow-return regime. If the room’s fabric or use has changed, recalculate the load rather than copying the old specification.
Calculate circuit flow, then check the pipework
Once the emitter loads and design temperature difference are established, determine the water flow required by each circuit. The Energy Saving Trust expresses the heat-transfer relationship as Q = m × SHC × Δt: heat transfer depends on mass flow, the fluid’s specific heat capacity and the flow-return temperature difference. For the same heat output, a smaller temperature difference or a fluid with lower heat capacity requires more flow.
Use that required flow as the starting point for hydraulic pipe sizing. Check the material-specific internal bore, velocity and pressure loss over the complete circuit, including fittings, valves and equipment. Also check the index circuit—the circuit with the greatest hydraulic resistance—against the pump’s available head. A pipe diameter that appears adequate near the radiator does not prove that the pump can deliver the required flow through the whole route.
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A practical flow-calculation sequence
- List the emitter output served by the circuit at the selected design condition.
- Set the design flow-return temperature difference for that system.
- Use the heat-transfer relationship to determine the required mass or equivalent volumetric flow, using the properties of the actual heating fluid.
- Select a candidate pipe based on its true internal bore and material-specific hydraulic data.
- Check velocity and pressure loss for every straight section, bend, valve, manifold and other fitting in the route.
- Repeat for the index circuit and compare the total resistance with the pump’s available head.
This sequence prevents a common error: choosing a pipe from the radiator connection size and assuming the entire route will work. The pipe must be able to carry the calculated flow at acceptable resistance, not merely connect physically to the emitter.
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Use actual internal bore, not just outside diameter
Pipe labels and outside dimensions do not give a complete hydraulic specification. The Energy Saving Trust’s heat-pump guidance warns that plastic and copper pipes with the same outside diameter can have different internal bores. Use flow and pressure-drop data for the actual material and pipe size rather than treating nominal outside diameter as interchangeable across materials.
That guidance says velocities in small-bore pipework should generally be below 1 m/s and ideally pressure drop should be below 300 Pa/m. It also describes mini-bore as generally unsuitable for heat-pump installations. These are recommendations from that heat-pump guidance, not a substitute for checking the actual project against applicable design references, pump limits and manufacturer instructions.
Understand the microbore example without generalizing it
An Energy Saving Trust worked example uses a 1,308 W emitter and a 5°C temperature difference to calculate a flow of 0.0625 L/s. In that particular example, a one-metre 10 mm section has a calculated velocity of about 1.04 m/s. The page notes that longer runs of narrow pipe can cause significant pressure-drop and flow effects.
Those numbers describe one example, not a typical room load, universal pipe requirement or approval limit. The separate heat-pump guidance’s velocity and pressure-drop recommendations must be considered in their stated context. The example is useful chiefly because it shows why both flow and run length matter: a short section and a long circuit made from pipe with the same nominal size do not present the same hydraulic problem.
Decide whether existing pipework can stay
Do not approve or reject an existing microbore run from the word “microbore” alone. Nor should it be retained just because the old system appeared to heat the room. Establish the material and internal bore, calculate the required circuit flow, trace the complete route and fittings, and assess velocity and pressure loss. Then check whether the pump and system can deliver that flow under the proposed design conditions.
- A short narrow tail serving an emitter is not hydraulically equivalent to a long narrow circuit.
- Changing the heat source or design temperature can change the flow the circuit must carry.
- A satisfactory flow in one branch does not demonstrate that the most demanding circuit will work.
- If the available flow or pump head is inadequate, a designer may need to consider changes to the route, pipework, emitter arrangement or system design.
The Energy Saving Trust describes mini-bore as generally unsuitable for heat-pump installations, while its worked example illustrates that a particular short 10 mm section can fall within the example’s velocity range. These points are not contradictory: one is broad heat-pump guidance and the other is a limited calculation, not a blanket pass for existing installations. Have the actual circuit assessed before deciding to retain or replace it.
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Common sizing mistakes and their consequences
- Sizing by floor area alone: this ignores exposure, fabric, ventilation assumptions and room temperature, so some rooms can be under-heated while others are over-supplied.
- Copying the old radiator: the existing unit may reflect a different building condition or a higher-temperature system and may not meet the new room load.
- Using catalogue output without correction: a published rating at one flow and return condition is not proof of output at another.
- Choosing pipe by nominal outside diameter: different materials can have different internal bores and therefore different resistance.
- Checking only the branch next to the radiator: fittings, valves and the full route can dominate pressure loss.
- Using a short-run example as a universal limit: the 1,308 W, 0.0625 L/s and 1.04 m/s figures belong to one stated example.
- Ignoring the index circuit: the most resistant route determines whether the pump can satisfy the design flow throughout the system.
Compare design options on performance, not labels
When choosing between feasible approaches, compare the numbers and constraints that govern the project. Neither a product category nor a pipe-size label can establish suitability by itself.
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|---|---|---|
| Radiator or other emitter | Output at actual flow and return temperatures; room heat-load coverage; available wall or floor space. | Manufacturer output data or correction method checked against the room’s calculated heat loss. |
| Pipe material and size | Internal bore; required circuit flow; route length and fittings; velocity and pressure loss. | Material-specific flow and pressure-drop data, with pump and heat-source requirements checked. |
| System design | Selected flow temperature; room comfort requirements; changes needed to emitters or pipes. | Verified design calculations and heat-source performance under the chosen conditions. |
A larger radiator may address an emitter-output shortfall, but it does not resolve inadequate circuit flow. A pipe replacement may reduce hydraulic resistance, but it does not establish that the radiator output meets the room load. Treat the emitter and distribution as connected parts of one design. Do not promise a particular energy saving from either choice without a supported project calculation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Commission and retain the design basis
Before work is finalized, ask for the room-by-room heat-loss calculation, the design flow and return temperatures, the emitter outputs used for selection, and the basis for pipe-flow and pressure-drop checks. Keep the pipe material and size, relevant circuit routes and the design criteria with the handover records. Those details allow later changes to be assessed without guessing from old radiator dimensions or pipe labels.
Commissioning should verify that the installed arrangement matches the design: emitters should be connected as specified, circuits should be balanced, and controls and the heat source should operate at the intended temperatures. If a room remains cold, check the design load, actual emitter output, valve setting, circuit flow and pump head rather than immediately increasing the boiler or heat-pump size.
The CIBSE Domestic Heating Design Guide 2026 is the current professional reference identified for domestic design in this guidance, and its publisher notes that the July 2026 version supersedes the earlier 2026 version and has an accompanying corrigendum. A designer or installer working from it should confirm which version and corrections apply to the project.
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Questions to ask a designer or installer
- What heat-loss method and design outdoor condition were used for each room?
- What indoor design temperature and air-change assumptions were applied?
- At what flow and return temperatures is each radiator’s output stated?
- How was catalogue output converted to the project’s operating conditions?
- What flow-return temperature difference was used for each circuit?
- What are the pipe material and actual internal bore, and which pressure-drop data support the selection?
- Which circuit is the index circuit, and does the pump have sufficient available head after fittings and valves are included?
- Will the completed heat-loss calculation and design criteria be included in the handover pack?
Frequently Asked Questions
Can I size a radiator from room floor area?
Floor area can provide only an early sense of scale. CIBSE and the Energy Saving Trust recommend a room-by-room heat-loss assessment that also considers exposed surfaces, construction, adjoining spaces, air changes, exposure and design temperatures.
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How do I use a radiator catalogue rating?
Obtain the manufacturer’s output data and convert it, using the manufacturer’s method, to the system’s actual flow and return temperatures. Select an emitter whose corrected output meets that room’s calculated heat loss.
Do lower-temperature heat pumps always need larger radiators?
Lower heating-water temperatures often reduce the output of a given radiator, so more surface area or another emitter arrangement may be needed. The required size is project-specific and must be checked against the heat-pump design temperatures and product data.
Is 10 mm microbore acceptable?
There is no universal yes or no. Check the actual internal bore, required flow, route length, fittings, velocity and pressure loss. The Energy Saving Trust gives a one-metre example with a 1,308 W emitter, 5°C temperature difference, 0.0625 L/s flow and about 1.04 m/s velocity; those figures do not approve every 10 mm installation.
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Not without checking the internal bore and material-specific flow and pressure-drop data. Equal outside dimensions do not guarantee equal hydraulic performance.
What does the index circuit mean?
It is the circuit with the greatest hydraulic resistance. Its complete route, fittings and valves must be checked against the pump’s available head because it is the circuit most likely to limit system flow.
Should existing pipework be replaced for a heat pump?
Assess it rather than deciding from its label. Mini-bore is described as generally unsuitable for heat-pump installations, but a particular short section may calculate acceptably. The full circuit and proposed operating conditions determine the decision.
What documents should I receive at handover?
Request the room-by-room heat-loss calculation, design flow and return temperatures, emitter outputs and correction basis, pipe material and sizing checks, circuit routes and design criteria. CIBSE describes retaining these decisions and the full heat-loss calculation in the handover information.
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The Bottom Line
Calculate each room’s heat loss, select every radiator for the actual design temperatures, and calculate each circuit’s required water flow. Then verify the pipe’s internal bore, velocity and pressure loss—including fittings and the index circuit—against the pump and heat-source requirements. Existing microbore may or may not be suitable; only a route-specific hydraulic check can establish that.
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