A “35 kW combi” badge tells you little about whether the shower will perform. Correct specification uses the technical datasheet: separate heating and DHW outputs, flow rates at defined temperature rises, minimum and maximum modulation, and mains pressure requirements.
Heating kW vs hot water kW
On most combis the marketing kW figure refers to domestic hot water output. Space heating output is often lower — sometimes substantially. Example: a unit marketed as 35 kW might deliver 35 kW for DHW but only 24–30 kW to radiators. Always read both figures against your heat-loss calculation.
For heating, compare maximum and minimum modulation range. A 24 kW boiler that can turn down to 5 kW suits a well-insulated home better than one that only modulates to 12 kW.
DHW flow rate and ΔT
Manufacturers quote hot-water flow in litres per minute (L/min) at a specific temperature rise (ΔT) — typically 30 K or 35 K. That means:
- Flow quoted at 35 K ΔT: if incoming mains is 10 °C, the boiler must raise it to 45 °C at that flow rate.
- Winter mains at 5 °C needs more kW for the same outlet temperature — performance drops unless flow is reduced.
Example: A datasheet states 12.4 L/min at ΔT = 35 K. That is a reasonable single-shower figure. A rain shower needing 15 L/min at 42 °C may exceed what the boiler can do in midwinter — even if the headline kW looks adequate.
Relating kW to flow and ΔT
A useful approximation for specification:
DHW kW ≈ 0.07 × flow (L/min) × ΔT (K)
So 12 L/min with a 35 K rise needs roughly 0.07 × 12 × 35 ≈ 29 kW of hot-water output. Cross-check this against the datasheet DHW kW column.
What else to read on the datasheet
- Minimum DHW flow to fire — Boilers need a minimum flow to detect hot-water demand; low basin flows can cause cycling.
- Maximum heating flow temperature and recommended operating range.
- Mains pressure range — Minimum dynamic pressure at peak flow is critical; insufficient pressure causes poor DHW and fault codes.
- Gas input kW — For gas pipe and meter sizing.
- Electrical consumption — For fused spur rating.
- Flue and clearance dimensions — For siting surveys.
Mains dynamic pressure and flow testing
Static pressure at a tap (no flow) is not enough. Before promising combi performance, measure under realistic load:
- Record static mains pressure at the incoming stop tap or cold kitchen tap.
- Open the cold tap fully and measure dynamic pressure (pressure while flowing) — or use a flow cup to measure L/min at that outlet.
- Repeat with other outlets that might run together (e.g. kitchen + bathroom).
- Compare results to the boiler manufacturer’s minimum inlet pressure at maximum DHW flow.
If dynamic pressure collapses when flow increases, the mains supply or internal pipework may be restrictive. Fitting a larger combi will not fix it — stored hot water or mains upgrade may be needed.
Common specification mistakes
- Selecting boiler kW from old boiler badge without heat-loss calculation.
- Ignoring winter mains temperature in DHW sizing.
- Assuming marketing flow figures apply at shower temperature (40 °C+) rather than datasheet ΔT.
- Oversizing heating kW “to be safe” — causes cycling and reduced condensing time.
- No flow test in properties with 15 mm lead, shared supplies, or long meter runs.
Putting it together
- Complete room-by-room heat loss → heating kW required.
- List hot outlets and realistic simultaneous use → required L/min and ΔT.
- Shortlist boilers whose datasheets meet both, with modulation headroom.
- Confirm mains dynamic pressure and flow on site.
- Document choices on Benchmark and customer quote.
See also sizing & efficiency, combi vs system vs regular, and commissioning checklist. Manuals and datasheets: BoilerManuals.com.
Technical specification must follow the manufacturer’s current documentation for the exact model and fuel type. This guide is for educational purposes for competent heating engineers.