# 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. [View the web page](https://heatuneed.com/heat-pump-radiator-readiness-calculator/) ## 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. ## 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. ## What the calculator asks for ### The heat each room needs A design heat loss is the power a room loses on the local design-cold day. The best input is a room-by-room survey in watts. An early estimate can use floor area times a specific heat loss in watts per square metre, but that gives every room the same exposure and should not select final equipment. - I know the design heat loss in watts - Estimate it from floor area — Estimated design heat loss per square metre ### The output already installed Use each manufacturer's output at Delta T50 and add the ratings where a room has more than one radiator. If no catalogue value is available, the size mode estimates common steel panel types from their panel/fin arrangement, total width and height. It is deliberately labelled as an estimate. - Enter the manufacturer's Delta T50 output - Estimate output from panel type and dimensions - Type 10 - single panel, no fins - Type 11 - single panel, one fin - Type 21 - two panels, one fin - Type 22 - two panels, two fins - Type 33 - three panels, three fins ### Flow, return and room temperature Radiator output follows the difference between its mean water temperature and the room. The model uses the logarithmic mean of the entered flow and return temperatures, then applies Q = Q50 x (Delta T / 50)^1.3. The ladder evaluates the same rooms at each design flow below. - 35 °C - 40 °C - 45 °C - 50 °C - 55 °C ## Your radiator audit ### How readiness is decided A room passes only when its radiator output at the target temperature meets 100 percent of that room's design loss. Ninety to 100 percent is shown as nearly ready so uncertain inputs can be checked, but it is not silently counted as a pass. A whole-house surplus never cancels a deficit in another room. - **Ready** — Every room meets 100 percent of its design heat loss at the target flow temperature. - **Nearly ready** — Every room reaches at least 90 percent, but one or more still miss the full design-day load and need verifying. - **Targeted upgrades** — Some rooms pass and their radiators can stay; only the failing rooms need work. - **Major upgrades** — Most rooms are materially short, so fabric work, different emitters or a higher flow temperature must be considered. 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. ## Common questions ### Where do I find a radiator's Delta T50 output? 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. ### Why does a radiator lose so much output at 45 °C? 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. ### Should I aim for 35, 45 or 55 °C? 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. ### Can I test this by turning down my boiler? 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. ### Why can the total show enough heat while rooms still fail? 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. ### Does a failed room always need a new radiator? 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. - [Check app availability](https://heatuneed.com/app-store/)