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Are your radiators ready for a heat pump?

Test every room at a 35 to 55 °C flow temperature. See what your existing radiators really deliver, the lowest temperature that works, and exactly which ones need upgrading.

Heatuneed

Are your radiators ready for a heat pump?

Prepared

A heat pump can keep the radiators - but not their old output

The output printed in a radiator catalogue assumes water far hotter than an efficient heat pump normally supplies. This tool derates each radiator to your proposed flow and return temperatures, compares it with that room's design heat loss, and keeps the room-by-room mismatch visible instead of hiding it inside a whole-house total.

Start from a typical system

System and temperatures

A room-by-room heat-loss calculation is the right input. Floor area is useful for an early check, not final radiator selection.
Use the catalogue's watt output at Delta T50 whenever you can. Dimensions are only a generic estimate because brands and models differ.
°C
Lower is more efficient for the heat pump. Click a row in the temperature table to compare 35, 40, 45, 50 and 55 °C.
K
5 K is a common heat-pump design value. A larger drop lowers the radiator's mean water temperature and its output.
°C
Use the indoor temperature the rooms must still reach on the local design-cold day.

Rooms and existing radiators

Add one line per heated room. If a room has several radiators, add their Delta T50 ratings together or enter their combined width when they are identical.

Your inputs

Your radiator audit

Updated live as you change a room or temperature.

Add a room to start the check

Enter a positive heat need for at least one room and the readiness result will appear here.

Rooms ready at target —
Lowest flow for every room —
Rooms to upgrade —
Total design heat loss —
Output at target —
Whole-system coverage —
Delta T50 rating to add —
Catalog output retained —
Design return temperature —

Whole-system capacity at the design condition

Heat the rooms need —
What the radiators deliver —

A passing total does not prove every room passes. The room table is decisive: spare capacity in the kitchen cannot heat an undersized bathroom.

What must change, room by room

The required flow is the temperature at which the current radiator just meets that room's load. The upgrade figure is a catalogue rating at Delta T50, so it can be used to select a replacement.

Room Heat loss Output at target Covered Flow needed Action

What each flow temperature changes

Same rooms and radiators; only the design water temperature changes. Click a row to use it as the target. The star marks the lowest listed temperature at which every room passes.

Flow Total output Rooms ready Rating to add

This is a design-screening calculation, not a room-by-room heat-loss survey or an installation design. It assumes clean, unobstructed radiators with adequate water flow and uses an exponent of 1.3. Actual output changes with the product, covers, curtains, recesses, hydraulic balancing and pipework. Use manufacturer data and a competent heat-pump designer before ordering equipment.

Calculated with the free tool at heatuneed.com/heat-pump-radiator-readiness-calculator/

How the calculation works

The tool puts the room load and the radiator rating on the same design condition, then solves the water temperature and replacement size room by room.

1

Start with the design heat loss

Each room needs a power figure in watts for the coldest design condition. Use a room-by-room survey when available, or floor area times an approximate W/m² rate for an early feasibility check.

2

Put the radiator on a Delta T50 basis

Modern radiator catalogues publish output at a 50 K mean water-to-room difference. Enter that figure directly; if it is unavailable, the dimension mode makes a generic estimate from panel type, width and height.

3

Derate it to heat-pump temperatures

The logarithmic mean water-to-room difference is calculated from flow, return and room temperatures. Output is then Q50 multiplied by that difference divided by 50, raised to the 1.3 radiator exponent.

4

Solve the weak rooms, not just the total

For every room the model solves the minimum flow temperature and the Delta T50 replacement rating needed at the target. That identifies exactly what can stay, what needs verifying and where a larger or fan-assisted emitter is required.

Common questions

Look for the model and dimensions in the manufacturer's catalogue or data sheet, then use the watt figure headed Delta T50, EN 442 or 75/65/20. Add the figures if the room has more than one radiator. Very old catalogues may quote Delta T60 instead; do not enter that number as Delta T50 without converting it. If the model cannot be identified, use the dimension estimate as a screen and verify the likely replacement with an installer.

Its catalogue rating assumes a mean water temperature around 70 °C in a 20 °C room: a 50 K difference. A heat pump flowing at 45 °C and returning at 40 °C has a mean difference of only about 22 K. Natural convection weakens non-linearly as that difference falls, so a standard panel radiator retains only about 35 percent of its Delta T50 rating in that case.

Use the lowest temperature that still heats every room on the design-cold day. 35 °C is excellent for heat-pump efficiency but asks roughly six times the Delta T50 radiator rating with a 5 K water drop and a 20 °C room. Around 45 °C is a common retrofit target; 55 °C can preserve more existing radiators but makes the heat pump work harder. The ladder shows the trade for your rooms rather than choosing it in advance.

It is a useful reality check. On a genuinely cold day, cap the boiler flow near the proposed heat-pump temperature, run the heating continuously and leave radiator valves open, then see which rooms fall behind. It is not a substitute for a design heat loss: a mild test day, intermittent controls or a boiler display that differs from actual flow temperature can all give a false pass.

Water and heat are distributed by circuits, valves and emitters; spare radiator capacity in one room cannot be reassigned to another. Whole-house output is useful for scale, but readiness is a room-by-room constraint. That is why the verdict follows the room count and the table, not the total capacity bar.

No. First verify the heat loss and catalogue rating, remove covers or blocked airflow, and balance the system. Fabric work may cut the room load enough to keep the radiator. If a genuine gap remains, the table gives the total Delta T50 rating the room needs: that can come from a larger panel, a thicker type, a second emitter or a fan-assisted low-temperature radiator.

Need the room heat losses that make this check reliable?

Heatuneed scans the building with LiDAR and calculates heat loss through walls, windows, doors, floors and roofs, room by room - the design input this radiator audit is built around.

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