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FOR INSTALLERS · EMITTER SIZING
Radiator Sizing Service for Heat Pump Installers
Turn room heat-loss figures into an installable radiator schedule.
- Last updated: September 2026
- Heat Loss Hub · For Installers
Heat Loss Hub can take your room-by-room heat-loss figures and turn them into clear emitter decisions for each room.
We can assess the existing radiators, calculate their output at the intended heat-pump operating temperature and identify what can stay, what needs replacing, what needs supplementing, what output each room requires and which room is limiting the design temperature.
You can outsource just the radiator-sizing stage without handing over the rest of the heat-pump design.
KEY TAKEAWAY
You provide the room loads. We turn them into room-by-room emitter decisions.
GET IN TOUCH
Send us your room loads and existing radiator details and we’ll return a clear, room-by-room action list.
Radiator Sizing Only, if That Is All You Need
Not every installer needs a complete outsourced design. You may already have room-by-room heat-loss calculations, your own preferred heat-pump manufacturer, internal hydraulic design capability, your own pipe-sizing process and your own final design workflow.
You may simply need someone to turn the room loads into an emitter schedule. Heat Loss Hub can stop at that stage. This can suit:
- MCS installers with their own design process
- Internal designers needing extra capacity
- Small installers without dedicated emitter-design resource
- Businesses experiencing temporary backlog
- Installers wanting an independent second pair of eyes
You do not need to outsource the whole heat-pump design to outsource the emitter schedule.
What Radiator Sizing Actually Needs
Radiator sizing should start with the room requirement. The main inputs are:
- Room-by-room heat loss
- Room design temperature
- Intended operating temperature
- Existing radiator dimensions
- Radiator type
- Available wall space
- Customer preferences
- Other emitters in the room
The objective is not simply to find a radiator that looks large enough. It is to check whether the emitter can deliver the required output at the actual design condition.
KEY TAKEAWAY
Radiator sizing starts with the room load, not the radiator catalogue.
Existing Radiator Assessment
A heat-pump installation does not automatically mean replacing every radiator. Some existing radiators may already be large enough. For each room, we can assess:
- Radiator width
- Radiator height
- Panel type
- Convector arrangement
- Existing output
- Corrected output at the proposed design condition
- Room heat loss
- Whether the radiator can remain
Each emitter can then be given a clear status such as:
Retain
The existing radiator can meet the room requirement at the intended design condition.
Replace
The existing radiator cannot provide enough output.
Supplement
The room may need an additional emitter rather than a complete replacement.
Review
More information is needed before a reliable decision can be made.
This gives the installer a practical room-by-room action list.
Corrected Output at Heat-Pump Temperatures
Radiator outputs are usually published at a standard test condition. That figure is not necessarily the output available in a low-temperature heat-pump system.
Actual output depends on flow temperature, return temperature, room temperature, radiator size and radiator type. As water temperature falls, radiator output falls with it.
KEY TAKEAWAY
A radiator that works at boiler temperatures may not work at heat-pump temperatures. The existing emitter therefore needs to be assessed at the design condition expected from the heat-pump system. That is much more useful than simply comparing its catalogue wattage with the room heat loss.
What Happens When an Existing Radiator Is Too Small?
“Replace the radiator” is not always the only answer. Depending on the room, possible options may include:
- Wider radiator
- Taller radiator
- Higher-output panel type
- Type 22
- Type 33
- Additional radiator
- Fan-assisted emitter
- Fabric improvement
- Higher design temperature where justified
The best solution depends on required output, available wall space, existing pipe positions, customer preference and overall system design.
A technically ideal radiator still has to fit the property.
Which Room Sets the Design Temperature?
This is one of the most important parts of emitter design. Most rooms may work comfortably at a lower water temperature. One room may not. For example:
- Lounge works at the target temperature
- Bedrooms work
- Hall works
- Bathroom towel rail does not
The easy solution is to increase the design temperature for the whole system. But that may reduce heat-pump efficiency everywhere just to make one weak emitter work.
Another option may be to improve that single room — a larger radiator, different emitter type, additional emitter or fabric improvement.
KEY TAKEAWAY
One undersized radiator should not quietly set the operating temperature for the whole house. A good emitter schedule should make the limiting room obvious.
Wall-Space and Practical Installation Constraints
Radiator selection is not done on paper alone. Real rooms have constraints. These may include doors, furniture, kitchens, bay windows, curtains, built-in cupboards, low window sills, existing pipe positions, customer preferences, maximum acceptable width and maximum acceptable height.
If the technically ideal radiator will not fit, the design needs another solution. That is why available wall space should be considered as part of the emitter-sizing process.
Towel Rails and Bathrooms
Bathrooms often become problem rooms in low-temperature systems. A towel rail may look large but still provide relatively little useful heat output at lower water temperatures. The bathroom may also have a high target room temperature, external walls, large windows and small available wall area.
This can make the towel rail the limiting emitter. Possible solutions may include a larger towel rail, additional panel radiator, fan-assisted emitter, underfloor heating or a higher-output combined solution.
KEY TAKEAWAY
A towel rail can look large and still provide very little useful output at low water temperatures. The aim is to heat the room properly, not simply retain a towel rail because one is already there.
Mixed Emitter Systems
Not every heat-pump system uses radiators alone. The property may include radiators, underfloor heating, fan convectors, towel rails and other low-temperature emitters.
Each room still needs to be assessed against its own design load. The wider system then needs to work at operating conditions that suit the chosen emitters.
For mixed systems, the radiator schedule should identify any rooms that require a different approach rather than forcing one assumption across the whole property.
What You Need to Send Us
If you already have the room heat losses, the main information required is typically:
- Room-by-room heat-loss figures
- Room design temperatures
- Existing radiator dimensions
- Existing radiator type
- Photos where identification is unclear
- Intended flow temperature if already selected
- Return temperature or design temperature basis where available
- Available wall space
- Customer restrictions
- Existing UFH or other emitter information
If the room-by-room heat loss has not yet been completed, Heat Loss Hub can also provide that stage.
Explore: Room-by-Room Heat Loss Calculations for MCS Installers
What You Receive
The aim is to provide a schedule the installer can actually use. For each room, this may include:
- Room name
- Room heat loss
- Existing radiator details
- Existing emitter output at the design condition
- Required output
- Retain / replace / supplement decision
- Proposed replacement where required
- Design emitter temperature
- Notes on wall-space or customer constraints
The wider schedule can also identify limiting room, key assumptions, problem emitters, revision number and current issue date.
The output should tell the installer what goes in each room, not just whether the existing radiator passes or fails.
Example Radiator Schedule
A finished schedule may look something like this:
| Room | Heat Loss | Existing Emitter | Output at Design Condition | Status | Proposed Action |
|---|---|---|---|---|---|
| Lounge | 1,450 W | T22 600 × 1200 | 1,520 W | Retain | None |
| Bedroom 1 | 850 W | T11 600 × 800 | 480 W | Replace | Larger T22 |
| Bathroom | 700 W | Towel rail | 310 W | Supplement | Additional emitter |
| Hall | 620 W | T22 600 × 900 | 680 W | Retain | None |
The purpose is to turn calculations into clear site decisions.
What if the Design Flow Temperature Is Not Yet Fixed?
The emitter schedule can help establish it. You do not always need to decide the final flow temperature before looking at the radiators.
Different design temperatures can be assessed to see which rooms pass, which rooms fail, which room becomes limiting and what changes would allow a lower operating temperature.
For example: a system may initially appear to require a higher flow temperature. But if one or two weak radiators are upgraded, the whole property may be able to operate at a lower design temperature.
KEY TAKEAWAY
Flow temperature and emitter sizing should be developed together, not treated as separate guesses.
What Is Not Included in Radiator-Sizing-Only?
Radiator sizing is one stage of the wider design process. Unless separately agreed, it does not automatically include:
- Room-by-room heat-loss calculation
- Final heat-pump model selection
- Cylinder sizing
- Pipe sizing
- Hydraulic design
- System schematic
- Controls design
- Full MCS design pack
You can retain those stages in-house. Or Heat Loss Hub can continue with them if required.
Explore: Outsourced MCS Heat Pump Design
Revisions and Design Changes
The radiator schedule depends on the design information it was based on. If that information changes, the emitter decisions may also change. Examples include room heat loss revised, flow temperature changed, different radiator selected, customer rejects proposed radiator, available wall space changes, fabric improvements added or room use changes.
Where the change is material, the emitter schedule should be reviewed.
KEY TAKEAWAY
If the room load or design temperature changes, the emitter schedule may need to change with it. Each revised schedule should be clearly identifiable so the installer knows which issue is current.
One Project, Batch Work or Ongoing Support
You can use the service for:
One Project
You have a single job where emitter sizing is taking too much time.
Occasional Support
You normally size radiators internally but need extra capacity.
Batch Work
Several heat-loss calculations are complete and you want the emitter schedules produced together.
Ongoing Support
Radiator sizing becomes a regular outsourced stage of your design workflow.
You stay in control of how much work is handed over.
White-Label Radiator Schedules
Emitter sizing can also sit inside a white-label workflow. Depending on the agreed arrangement, Heat Loss Hub completes the technical work, your business remains customer-facing, your customer remains your customer, reports can follow the agreed branding approach, and the emitter schedule can sit inside your wider design pack.
This can add design capacity without changing the customer journey.
Radiator Schedule Quality Checklist
A useful emitter schedule should let you answer:
- What does each room need?
- What does the existing radiator deliver?
- Which radiators can stay?
- Which need replacing?
- What replacement is proposed?
- Which rooms need supplementary emitters?
- Which room limits the design temperature?
- What operating condition was used?
- What assumptions were made?
- Which revision is current?
If those questions cannot be answered, the schedule may not be detailed enough to install from.
Frequently Asked Questions
Yes. If you already have room-by-room heat-loss figures, Heat Loss Hub can complete the emitter-sizing stage without taking over the rest of the design.
Yes, radiator sizing needs a reliable room design load. If you do not already have one, Heat Loss Hub can also complete the room-by-room heat-loss calculation.
Yes. Existing radiators can be checked against the room requirement at the proposed operating condition.
No. Some existing radiators may already be large enough. The correct approach is to assess each room individually.
They should be assessed at the design condition intended for the heat-pump system. That means considering flow temperature, return temperature and room temperature rather than relying on the standard catalogue output alone.
Yes. Emitter sizing can be assessed against different operating temperatures where required. The room heat loss and complete design condition still need to be considered.
That room becomes a potential limiting room. It may be more efficient to upgrade the emitter in that room rather than raise the operating temperature of the entire system.
Potentially. They need to be assessed against the bathroom heat loss at the intended operating temperature. In some cases an additional emitter may be required.
Yes, where the required information is available. Each room and emitter type should still be assessed against the relevant design load.
Yes. If your survey records the radiator dimensions, type and other relevant information clearly enough, it can be used as the basis of the emitter assessment.
Alternative emitter options can be considered, such as higher-output radiator types, taller emitters, additional emitters or fan-assisted units. The design still needs to satisfy the room requirement.
This can depend on the agreed scope. The schedule can identify required output and suitable size/type, with specific manufacturer selections included where agreed.
Yes, a white-label workflow can be agreed where required.
Need the Emitter Schedule Without Outsourcing the Whole Design?
Send us the room-by-room heat-loss figures and existing emitter information. We assess what can stay, what needs changing and what each room requires at the intended heat-pump design temperature.
SUBMIT A PROJECT
Send us your room loads and existing radiator details and we’ll return a clear, room-by-room action list.
Explore: Heat Pump Design for Installers