Radiator Output at Low Flow Temperature
Check radiator output at heat-pump temperatures, find each room’s minimum flow, size replacements and explore a heating curve and cost-per-degree scenario.
A radiator rated at 75/65/20 °C can deliver much less heat when a heat pump supplies cooler water. This radiator output at lower flow temperature calculator compares the available heat with each room’s design heat loss. It is for engineers, installers, students and homeowners assessing whether existing radiators could work with a lower-temperature heating system.
Enter one to four rooms. For each room, enter the installed catalogue output, the manufacturer’s radiator exponent, the desired room temperature and a separately calculated design heat loss. All entered ratings must use the same reference flow, return and room temperatures. The defaults are illustrative room data; they do not identify a particular radiator or house. For more than four rooms, repeat the comparison and retain the highest minimum flow temperature.
Choose the proposed flow temperature and the flow-to-return temperature drop. The room schedule shows the available output, heat shortfall and whether more output is needed. The selection schedule gives the catalogue rating required at those operating conditions, assuming the replacement has the entered exponent. Select the next suitable product and check its actual exponent and installation corrections. A room with spare output cannot compensate for a shortfall elsewhere.
The calculation follows the manufacturer-published EN 442 exponent method with a logarithmic mean water-to-air temperature difference. Both operating and reference conditions use the same logarithmic convention. At 75/65/20 °C, the reference mean excess is about 49.83 K. Do not mix these results with a correction table based on an arithmetic mean without checking the manufacturer’s convention.
The highest room minimum flow is the design target for the compared rooms. The animated output chart follows the limiting room as you change the flow temperature or press play. It sweeps steady operating points; it does not simulate radiator warm-up. The suggested heating curve reduces room loads linearly towards the outdoor balance temperature and assumes constant circulation, so the water temperature drop decreases with load. Heating is off at the balance point.
For heat pump radiator sizing, the annual electricity and cost-per-degree figures are separate scenarios. Enter useful space heat, seasonal COP, electricity price and a supported COP sensitivity to higher flow temperature. The example tariff and sensitivity are assumptions dated 27 October 2026, not a live price or a universal heat-pump efficiency claim.
This tool does not calculate building heat losses, pipe sizing, hydraulic balancing, heat-pump capacity, defrost performance or hot-water demand. Covers, niches, connection arrangements, very low water flows and mixed radiator types can change output. Confirm low-temperature performance with the manufacturer. Check against the standard in force; installation work belongs to a qualified person.
How it is calculated
The radiator conversion follows the manufacturer-published EN 442-2:2014 framework (German edition DIN EN 442-2:2015-03), using logarithmic means at both rating and operating conditions. Kermi 2025 p. 135 and Zehnder 2021 p. 115 provide independent examples.
For flow , return and room air :
The equal-water-temperature limit is . At 75/65/20 °C, the reference excess is 49.8329 K. The room shortfall is ; a negative result needs more emitter output. Required catalogue output is with the same entered exponent.
The following inverse is derived here. With a fixed sizing water drop and required mean excess :
The system minimum is the maximum of the room minima. The displayed residual checks output at each solved minimum against its room heat loss.
The heating-curve scenario uses . Each room load becomes and the water drop becomes , representing unchanged circulation. Apply the same inverse at each outdoor temperature and take the highest room flow. The zero-load endpoint is the highest room setpoint; the heating is off there.
The separate cost scenario assumes . Annual electricity is and the extra electricity for +1 K is . Multiply by the entered tariff for annual cost. This is a sensitivity scenario, not a heat-pump model.
Assumptions
- Water-filled passive radiators, steady state and adequate circulation. Rating conditions are shared; exponents are entered separately for each room.
- The room design heat losses and all example room values are illustrative inputs, not inferred from floor area or copied product data. Sum radiators within one room only when their reference conditions and exponents match.
- The sizing comparison keeps the entered flow-to-return drop fixed. The outdoor heating curve instead keeps circulation constant by scaling that drop with heat load. Valves must still be balanced and commissioned.
- Replacement catalogue ratings assume the same exponent; re-enter the new product's exponent before selecting it. Required-rating ratios describe thermal output, not a direct physical-size ratio.
- The tariff, seasonal COP and logarithmic COP sensitivity are editable scenario assumptions, dated 2026-10-27. Currency is whatever currency the tariff uses. COP sensitivity defaults to 0.02/K and is not a universal measured value. Annual heat demand remains unchanged in the +1 K comparison.
- Drawing labels explicitly use °C and W; numerical input, result and table fields support the viewer's unit switch.
Where it applies
Use flow ≥ return > room air at the reference rating, and operating return above every active room temperature. The tool's operating flow range is 30–85 °C. A solved minimum above 85 °C flags a selection requiring revision; extrapolated manufacturer output must be confirmed. Exponent input bounds are numerical guardrails, not manufacturer validity limits.
Use catalogue data for the exact radiator and the manufacturer's allowed temperature and flow range. Very low circulation, covers, recesses, nonstandard connections, fan assistance, mixed emitters and thermal transients need additional corrections or a different model. The method does not certify heat-pump readiness, determine heat losses or replace a hydraulic design. Check against the standard in force. Installation work belongs to a qualified person.
The inverse solve is blocked when an extremely small room heat loss and large fixed water drop would require a return temperature indistinguishable from room air at numerical precision. Reduce the water drop and confirm realistic circulation.
References
- Kermi, Konvektoren Preise + Technik I/2025, original p. 135; attached extract s1 p. 1. Logarithmic conversion and worked 55/45/20 °C example. https://www.kermi.com/media/documents/KITI_PL_Konvektoren_I-2025_DE.pdf
- Zehnder Group Sales International, bathroom radiator technical documentation, 2021, original pp. 114–115; attached extract s2 pp. 1–2. Conversion equation and Yucca YSD-130-080 worked 60/45/24 °C example. https://zehnder.hu/wp-content/uploads/2021/11/Z-SI-V1220-RAD-PRL-Bad2021-en_01.08.2021screen.pdf
- DIN EN 442-2:2015-03, German version of EN 442-2:2014, Radiators and convectors — Part 2: Test methods and rating. Standard metadata only; no standard tables reproduced. https://doi.org/10.31030/2157935
Inputs
| Input | Symbol | Unit | Default |
|---|---|---|---|
| Rooms in this comparison Compare up to four rooms. For larger homes, repeat for each group and use the highest minimum flow temperature. | N_rooms | 4 | |
| Flow temperature — ▶ sweeps radiator output Water entering the radiators. Play sweeps a steady-state sizing curve; it does not model warm-up time. Keep flow above return and all room temperatures. | T_flow | °C | 45 |
| Flow-to-return temperature drop Return = flow minus this drop. Held constant for sizing; the heating curve assumes constant circulation and scales the drop with load. | dT_water | K | 5 |
| Rating: flow temperature Use the actual catalogue rating conditions, shared by all radiators entered. | T_ref_flow | °C | 75 |
| Rating: return temperature | T_ref_return | °C | 65 |
| Rating: room air temperature | T_ref_room | °C | 20 |
| Installed catalogue output Sum ratings of radiators in this room only when they share the same exponent and rating conditions. Defaults are illustrative, not product catalogue data. | P1 | W | 2000 |
| Manufacturer exponent n Enter the model's exponent. The default 1.30 is an approximate example, not a universal radiator value. | n1 | 1.3 | |
| Room design heat loss Enter the room heat loss at the design outdoor temperature from a separate heat-loss calculation. | L1 | W | 800 |
| Room air setpoint | Tr1 | °C | 20 |
| Installed catalogue output Sum ratings of radiators in this room only when they share the same exponent and rating conditions. Defaults are illustrative, not product catalogue data. | P2 | W | 1200 |
| Manufacturer exponent n Enter the model's exponent. The default 1.30 is an approximate example, not a universal radiator value. | n2 | 1.3 | |
| Room design heat loss Enter the room heat loss at the design outdoor temperature from a separate heat-loss calculation. | L2 | W | 450 |
| Room air setpoint | Tr2 | °C | 20 |
| Installed catalogue output Sum ratings of radiators in this room only when they share the same exponent and rating conditions. Defaults are illustrative, not product catalogue data. | P3 | W | 1000 |
| Manufacturer exponent n Enter the model's exponent. The default 1.30 is an approximate example, not a universal radiator value. | n3 | 1.3 | |
| Room design heat loss Enter the room heat loss at the design outdoor temperature from a separate heat-loss calculation. | L3 | W | 400 |
| Room air setpoint | Tr3 | °C | 20 |
| Installed catalogue output Sum ratings of radiators in this room only when they share the same exponent and rating conditions. Defaults are illustrative, not product catalogue data. | P4 | W | 1000 |
| Manufacturer exponent n Enter the model's exponent. The default 1.30 is an approximate example, not a universal radiator value. | n4 | 1.3 | |
| Room design heat loss Enter the room heat loss at the design outdoor temperature from a separate heat-loss calculation. | L4 | W | 500 |
| Room air setpoint | Tr4 | °C | 22 |
| Design outdoor temperature The outdoor temperature for the room design heat losses. This calculator does not determine those losses. | T_out_design | °C | -10 |
| Balance outdoor temperature Space-heating demand falls linearly to zero here, representing internal and solar gains. | T_out_balance | °C | 16 |
| Annual useful space heat Illustrative annual delivered space heat for the compared rooms or whole system. Excludes hot water. | E_heat | kWh | 12000 |
| Seasonal COP at the selected flow Use measured or manufacturer-based performance for your system. This is a scenario input, not predicted heat-pump performance. | COP_base | 3.5 | |
| Logarithmic COP loss per degree of higher flow Assumption: 0.02/K means COP is multiplied by exp(−0.02) for each 1 K increase. Replace with a sensitivity supported by your heat-pump data. | s_COP | 1/K | 0.02 |
| Electricity price (currency/kWh) Enter your tariff in any one currency per kWh; results use that currency. Illustrative price, not a live German tariff. | price | 0.3 |
Results
With the default inputs above, the calc gives:
| Result | Value |
|---|---|
| Minimum design flow — all compared rooms | 53.8 °C |
| Limiting room number | 4 |
| Rooms short of heat at selected flow | 4 |
| Available heat at selected flow | 1,799 W |
| Scenario extra annual cost for +1 K (currency) | 20.78 |
| Scenario extra annual electricity for +1 K | 69.3 kWh |
Checked against sources
Each case below was calculated by this calc and agrees with the value from its source (within the tolerance where one is shown).
Kermi: available output, replacement rating and inverse flow
Source: Kermi Konvektoren Preise + Technik I/2025, original p. 135; attached extract p. 1. https://www.kermi.com/media/documents/KITI_PL_Konvektoren_I-2025_DE.pdf . Published 1960/1.96 = 1000 W uses a rounded factor. Unrounded logarithmic conversion gives about 1001 W; tolerances preserve the correct formula and printed rounding.
Inputs: Rooms in this comparison = 1; Installed catalogue output = 1,960 W; Room design heat loss = 1,000 W; Flow temperature — ▶ sweeps radiator output = 55 °C; Flow-to-return temperature drop = 10 K; the others as above.
| Result | Source | This calc |
|---|---|---|
| Available radiator outputs [1] | 1,000 ± 2 W | 1,001 W |
| Required catalogue rating at selected flow [1] | 1,960 ± 4 W | 1,958 W |
| Minimum design flow — all compared rooms | 55 ± 0.1 °C | 54.976 °C |
Zehnder Yucca YSD-130-080 at 60/45/24
Source: Zehnder Group Sales International 2021 bathroom radiator technical documentation, original p. 115; attached extract p. 2. https://zehnder.hu/wp-content/uploads/2021/11/Z-SI-V1220-RAD-PRL-Bad2021-en_01.08.2021screen.pdf . Rated 1024 W, n=1.30; published correction factor 0.469 and output 480 W. Tolerances cover printed rounding; the unrounded model gives about 480.16 W.
Inputs: Rooms in this comparison = 1; Installed catalogue output = 1,024 W; Room design heat loss = 480 W; Room air setpoint = 24 °C; Flow temperature — ▶ sweeps radiator output = 60 °C; Flow-to-return temperature drop = 15 K; the others as above.
| Result | Source | This calc |
|---|---|---|
| Available radiator outputs [1] | 480 ± 1 W | 480.2 W |
| Minimum design flow — all compared rooms | 60 ± 0.1 °C | 59.993 °C |
Responses