Radiator Sizing Guide
Radiator Size and BTU Calculator Guide: Choosing the Right Heat Output
For homeowners choosing radiators in London, suitable output means meeting the room’s assessed heat loss at the planned water temperatures, with dimensions that fit. A radiator size BTU calculator supplies an estimate; this guide explains how to interpret it using a worksheet and worked example.
For an existing problem, start with our guide to a radiator cold at the bottom, top or not heating up. If you smell gas: leave, call the National Gas Emergency Service on 0800 111 999.
Call with your postcode, the boiler make and any fault code showing.
- Gas Safe registered: 648977
- Phone line open 24/7
- Same-day appointments available
Radiator Calculator Results: From Estimated BTU to a Suitable Radiator
A radiator calculator result becomes useful when its assumptions, room requirement and the candidate radiator’s temperature rating are clear. Complete these six checks before ordering. For missing information, write “unknown, confirm before ordering”.
Room and assumptions
Record the room, measurements, construction details and calculator used. Stop if important inputs were guessed.
Heat requirement
Record the assessed room total in watts and BTU/h. Stop if the units or meaning of the result are unclear.
Design temperatures
Record intended flow, return and room temperatures. Stop if these have not been established.
Candidate output
Record the radiator model and output at those temperatures. Stop if you only have a rating for different conditions.
Physical fit
Record dimensions, valve space, connections and installation clearances. Stop if the complete installation will not fit.
Outstanding checks
Record uncertainties about performance, mounting or system suitability. Resolve them with the installer before committing.
The worksheet connects an estimate to a purchasing decision; it does not replace a room assessment.
What Do BTU and Watts Mean for Radiator Output?
Radiator output is a rate of heat delivery, expressed in watts or BTU per hour. Product listings often shorten BTU/h to “BTU”.
Stelrad’s glossary gives the approximate conversion:
- Watts × 3.412 = BTU/h.
- BTU/h ÷ 3.412 = watts.
For example, 1,000 W is approximately 3,412 BTU/h. These are two expressions of the same output, not different radiator sizes.
Converting units does not calculate room heat loss or adjust a radiator’s output for different water temperatures. Compare products using both the same units and the same temperature conditions.
What Should a Heating Radiator Size Calculator Take Into Account?
A heating radiator size calculator needs information about heat escaping through the room’s surfaces and ventilation, together with the temperatures the design must maintain.
Gather:
- Room length, width and height.
- Exposed wall, floor and ceiling details.
- Window and door areas.
- Known insulation and glazing.
- Ventilation assumptions.
- Adjacent heated or unheated spaces.
- Intended room temperature and external design conditions.
MYSON’s sizing guidance includes surface areas, insulation and temperatures; Stelrad distinguishes heat loss through the building fabric from ventilation heat loss.
Two equally sized rooms can need different outputs. A room with more exposed surfaces and poorer insulation may lose more heat than a sheltered room surrounded by heated spaces. Record unknown construction details for assessment instead of relying on a universal watts-per-square-metre allowance.
Why Does the Same Radiator Have Different Outputs at ΔT50 and ΔT30?
The same radiator delivers less heat when the difference between its mean water temperature and the room temperature is smaller.
Worked Example: Does a 1,500 W Radiator Meet a 1,000 W Room Requirement?
A radiator rated at 1,500 W at ΔT50 may fall short of a 1,000 W room requirement at ΔT30.
Both figures here are hypothetical. Using Stelrad’s published illustrative ΔT30 factor of 0.513:
- Room requirement: 1,000 W at the chosen design conditions.
- Corrected output: 1,500 W × 0.513 = 769.5 W, approximately 770 W, below the requirement.
- Reverse selection: 1,000 W ÷ 0.513 ≈ 1,950 W at ΔT50, before checking an actual candidate.
The factor is not universal. Verify its applicability to the selected radiator and confirm that model’s output at the intended conditions.
Zehnder’s temperature-rating guidance explains radiator ΔT as:
ΔT = (flow temperature + return temperature) ÷ 2 − room temperature.
- ΔT50 example: (75°C + 65°C) ÷ 2 − 20°C = 50°C.
- ΔT30 example: (55°C + 45°C) ÷ 2 − 20°C = 30°C.
ΔT50 does not mean a 50°C boiler flow setting. It is also different from the temperature drop between flow and return. These are calculation examples, not instructions to change settings.
Use the selected manufacturer’s output table or applicable correction method.
How Do You Match Heat Output to Radiator Dimensions?
Once output is established, compare radiator height, width and depth with the additional space needed for valves, connections and clearances.
Type 11, Type 21 and Type 22: Compare Construction as Well as Width
Type 11, Type 21 and Type 22 describe different panel and convector arrangements, so matching height and width does not establish matching output.
Stelrad’s panel-radiator definitions distinguish:
- Type 11: one panel and one convector set.
- Type 21: two panels and one convector set.
- Type 22: two panels and two convector sets.
For each candidate, check declared output at the same ΔT and installed depth. Type 22 does not automatically mean twice the heat. Vertical and column radiators need their own product data.
Measure accessible, cool surfaces only. Record:
- Available wall height and width.
- Allowable projection into the room.
- Visible pipe centres, measured between pipe centre lines.
- TRV, lockshield valve and connection positions.
- Access restrictions, furniture, curtains and other obstructions.
The radiator body is only part of the installed footprint. Matching the old width proves neither adequate output nor a direct fit. Use the proposed model’s dimensioned installation drawing; leave wall suitability, brackets and connection compatibility to the installer. Do not loosen fittings or remove the radiator.
Before Upsizing, Check Whether the Existing Radiator Is Working Properly
A cold room does not by itself prove that its radiator is too small; heat delivery and controls also need checking.
- Always struggled: assess room heat loss, available output and whether heat reaches the radiator properly.
- Changed recently: investigate the change before assuming a larger radiator is needed.
Note the heating schedule, normal thermostat and TRV settings, furniture or curtain obstructions, and when the problem occurs. Report uneven heating, recurring air or suspected poor circulation for assessment. These observations do not establish sludge, undersizing or a failed component. Avoid touching hot radiators or pipes.
When Should You Call a Heating Engineer Before Buying?
Call a heating engineer before ordering if heat-loss assumptions, operating temperatures, radiator performance or connection compatibility remain uncertain.
Replacing an Existing Radiator: Confirm the Requirement and Fit
Before replacement, have the required output and proposed radiator checked against the room and existing connections.
Prepare:
- Your room worksheet.
- Current radiator dimensions and known model details.
- A description of heating performance.
Assessment establishes whether replacement is appropriate and what alterations are needed. Existing valves and pipework may need changes.
Radiator replacement in London
Adding or Moving a Radiator: Check the System and Position
Adding or moving a radiator requires checks of room demand, position and the system’s ability to supply the heat.
Prepare:
- Available space and intended room use.
- Known heating system type.
- Proposed radiator location.
Do not assume a flat has its own boiler. The installer assesses pipe routing, circulation, controls and mounting. Discuss radiator installation in London before ordering.
Your checks here stop at accessible measurements and normal user controls. Never open the boiler casing or work on gas. Leave draining, valve-body removal and wiring to a competent professional. Gas-boiler work requires appropriate Gas Safe registration and competence.
Boiler repair, servicing and gas work
Discuss Your Radiator Requirements in London
To discuss radiator replacement or installation in London, call All Service 4 U with your measurements and proposed change. Our Gas Safe registered engineers can assess the work required; our business registration is 648977.
Have your worksheet, current dimensions, known system type and intended location ready. Call us 24 hours a day, 7 days a week.
Discuss your radiator requirements: 020 3740 6785, or contact us about your radiator requirements.
020 3740 6785
Frequently Asked Questions
Why Do Two Radiator Calculators Give Different Results for My Room?
Two calculators can give different results when their inputs or assumptions differ. Compare:
- Dimensions, exposed surfaces and insulation.
- Ventilation assumptions and chosen temperatures.
- Units and the result’s description.
Check each tool’s documented method. An unexplained difference is a reason to review the calculation, rather than automatically choosing the highest number.
Is the Calculator’s BTU Result for Each Radiator or the Whole Room?
A result labelled “room heat requirement” is the room total, not automatically the requirement for each radiator. Check the tool’s wording first. Hypothetically, two radiators each delivering 600 W at the same design conditions provide 1,200 W together. Their placement and system suitability still need checking; this arithmetic is not a recommended layout.
Can I Use an Old Radiator Output Chart to Choose a Modern Replacement?
An old chart is useful only if you can identify the radiator and its rating conditions. Check:
- Range, construction and dimensions.
- Output units and stated ΔT.
- The replacement’s current datasheet.
If identification is uncertain, base the new specification on the room requirement. Do not assign an old radiator an output from its dimensions alone.
Can I Work Out Radiator Size From My Boiler’s kW Rating?
Your boiler’s kW rating alone cannot establish the radiator output needed in a particular room. Room heat loss determines that requirement, while the heating system also needs checking for suitable operation. Dividing boiler capacity by radiator count overlooks differences between rooms and does not provide a reliable radiator specification.
How Do I Check an Engineer Is Qualified if the Job Includes Gas-Boiler Work?
Check the engineer’s Gas Safe ID card and confirm that their current registration covers the proposed gas-boiler work.
- Verify the individual through the Gas Safe Register.
- Check the relevant work categories, not registration alone.
Wet radiator work is not itself restricted to Gas Safe registrants. The qualification check must match the work being undertaken.
Still have a question or need urgent assistance? Call 020 3740 6785, tell us what has happened and we will tell you what happens next.