Summary

The number that decides whether a customer is happy with their new combi is not the heating output on the badge, it is the hot water flow rate in litres per minute, and most customers have never heard of it. They know the shower was "better before" or "rubbish since". The installer who explains ΔT 35, measures the mains and writes "12.2 l/min at a 35 °C rise, mains measured at 14 l/min" on the quote does not get that call. The one who sells a 24 kW combi to a house with a bath, two showers and a 9 l/min lead main gets it every February.

The physics is simple. A combi heats water as it passes through: the more kilowatts in the DHW heat exchanger, the more litres per minute can be lifted by a given number of degrees. Manufacturers quote the output at a standard temperature rise so boilers can be compared, and 35 K has become the UK convention (some data sheets add 30 K or 25 K figures that look flattering). What the customer feels is tap temperature and flow together, and both depend on the incoming mains temperature, the mains flow and pressure, the pipework, the outlet, and whether someone else has opened a tap. This guide gives the figures by boiler size, the mains measurements, the sizing rules for showers and baths, the regulations that touch the job, and the quote wording that keeps you out of trouble.

For whether a combi is the right choice at all, see combi vs system vs conventional; for mains pressure and flow in detail, see water pressure flow rate.

Key Facts

Quick Reference Table

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Combi DHW output Flow at ΔT 35 K (winter rating) Flow at ΔT 30 K Flow at ΔT 25 K What it runs comfortably
24 kW 9.8 l/min 11.5 l/min 13.8 l/min One shower or one bath; small flat or 1–2 bed house with one bathroom
28 kW 11.5 l/min 13.4 l/min 16.1 l/min One good shower plus a basin; 2–3 bed, one bathroom
30 kW 12.3 l/min 14.4 l/min 17.2 l/min One shower and a tap, or a bath in 7–8 min; 3 bed, one bathroom plus en-suite used in turn
32 kW 13.1 l/min 15.3 l/min 18.4 l/min Two modest showers at a push; 3–4 bed
35 kW 14.3 l/min 16.7 l/min 20.1 l/min Bath and shower household; two showers if the mains allows
40 kW 16.4 l/min 19.1 l/min 23.0 l/min Two bathrooms used together; needs 22 mm mains and 18+ l/min supply
42 kW 17.2 l/min 20.1 l/min 24.1 l/min Largest domestic combis; often a storage combi is the better answer

Figures are from the formula; manufacturers' quoted figures are usually within ±0.3 l/min because of heat-exchanger efficiency.

Detailed Guidance

The arithmetic, and why the data sheets look different

The heat exchanger in a combi transfers its DHW output into the water passing through. Water needs 4.18 kJ to raise 1 kg by 1 K, so a 30 kW boiler (30 kJ per second, 1,800 kJ per minute) can raise 1,800 ÷ (4.18 × 35) = 12.3 kg (litres) per minute by 35 K. The same boiler can raise 14.4 l/min by 30 K or 17.2 l/min by 25 K. Three figures, one boiler. The 35 K figure is the one that matters for a UK winter because the incoming water is cold enough that a 35 K rise is what a shower needs; the 25 K figure describes a summer afternoon.

When comparing boilers, compare at the same ΔT. A data sheet that says "14 l/min" with no ΔT is either at 30 K or is the flow restrictor's rating rather than the thermal capacity; the fine print or the ErP data table will say.

Measuring the mains before you quote

The combi is only half the system. Before you promise a flow rate:

  1. Dynamic flow. Outside tap or kitchen cold (confirm it is mains, not cistern-fed), fully open, 10 litre bucket, stopwatch. 10 litres in 40 seconds is 15 l/min; in 60 seconds, 10 l/min. Nobody else running water.
  2. Static and dynamic pressure. Gauge on the outside tap: static (no flow) and then with the kitchen tap open. A static of 3 bar that collapses to 0.8 bar under flow is a long or narrow service pipe — the boiler will fire but the shower will disappoint.
  3. Stop tap. Check the internal stop tap is fully open and not a half-closed, half-seized gate valve. A new quarter-turn stop tap is a cheap improvement.
  4. Service pipe. Lead or 15 mm galvanised iron to a 1930s semi is a 9 l/min main; a 25 mm MDPE service upgrade is a separate quote and often the real fix.

Write the measured figures on the survey and on the quote. "Mains flow measured at 11 l/min on the day of survey" is the sentence that ends the argument in February.

Sizing a combi to the household

The rule is: heating output sized to the heat loss, hot water output sized to the outlets, and because the hot water figure is always bigger, it decides the model.

Household Hot water pattern Suggested DHW output Mains needed
1-bed flat, shower only One outlet at a time 24–26 kW 10 l/min
2–3 bed, one bathroom with shower over bath One outlet; occasional bath 28–30 kW 12 l/min
3–4 bed, bathroom plus en-suite Two showers sometimes together 32–35 kW 15 l/min, 22 mm supply
4–5 bed, two or more bathrooms Simultaneous use expected 38–42 kW, or storage combi, or system boiler and unvented cylinder 18+ l/min, 22 mm or 25 mm MDPE

Where the mains cannot support the required flow, a combi is the wrong answer regardless of its output: an unvented cylinder with a mains booster or an accumulator (see mains water boosting) or a storage combi with an internal cylinder stores the hot water the mains cannot deliver on demand. Say so on the quote and offer the alternative.

Showers, baths and the "it goes cold" call

A thermostatic mixer shower holds temperature by throttling the hot or cold side as the mix changes. If the combi's output drops (someone opens a basin tap), the mixer closes the cold to compensate until it runs out of range, then the shower cools. The customer experiences a shower that goes cold when the dishwasher fills. The fixes, in order of cost: tell the household (free); fit a pressure-balanced or flow-prioritising arrangement where the shower is the first outlet on the DHW run; size the combi larger at the next replacement; or go to stored hot water. A thermostatic shower to BS EN 1111 (high-pressure) is the right valve on a combi, not a low-pressure BS EN 1287 valve designed for gravity — see thermostatic vs manual mixer shower comparison.

An electric shower is often the better companion to a small combi in a busy house: it takes its own cold feed and heats independently, so two showers can run together. Its own limits are its kW rating (an 8.5 kW electric shower gives about 3.5 l/min at ΔT 35 in winter) and the circuit needed — see shower types.

For bath fills, the complaint is the time: a 100 litre bath on a 24 kW combi is a 12-minute fill. Quote the fill time if the household has a big bath; a 30 kW unit is a small premium for a 4-minute saving.

Pipework and outlets that waste the flow

Worked example: quoting the flow rate honestly

Three-bed semi, Watford (hard water), one bathroom with a thermostatic mixer over the bath, kitchen tap. Measured mains: 13 l/min dynamic, 2.8 bar static, 1.6 bar dynamic. Existing 24 kW combi, 12 years old, customer complains the shower is weak and the bath is slow.

Options presented:
  28 kW combi:  11.5 l/min at ΔT 35 — mains supports it (13 l/min)
                bath fill ~8 min; shower 9–10 l/min when alone
  30 kW combi:  12.3 l/min at ΔT 35 — mains supports it
                bath fill ~7 min; shower 10+ l/min when alone
  35 kW combi:  14.3 l/min rated, but mains limits to 13 l/min
                — no benefit over 30 kW on this supply; not recommended

Recommended: 30 kW, flow restrictor 12 l/min, scale reducer on mains,
             full-bore flexis to bath, new quarter-turn stop tap.

Quote wording: "Hot water flow rate 12.3 l/min at a 35 °C temperature
rise (manufacturer's data). Mains flow measured 13 l/min on survey.
Flow is shared when two outlets run together."

The 35 kW unit would have cost £150 more and delivered nothing the customer could feel. The sentence on the quote is the warranty against the February call.

Regulations that touch a combi hot water job

A combi replacement is a Gas Safe job (Gas Safety (Installation and Use) Regulations 1998), notifiable to Building Control through the Gas Safe register for Part L and Part J. Approved Document L 2021 (England) requires the controls listed above; the Domestic Building Services Compliance Guide sets the minimum efficiency (92% ErP) and the hard-water scale reducer. The DHW pipework falls under the Water Supply (Water Fittings) Regulations 1999: WRAS-approved fittings, no backflow from the boiler to the mains (the combi has a double check valve on the filling loop, and the loop must be disconnected after filling). Bath outlets in new dwellings are capped at 48 °C under G3(4); on a replacement in an existing house it is good practice and a sentence on the quote. See heating controls for the Boiler Plus options.

Frequently Asked Questions

What is a good flow rate for a combi boiler?

For a one-bathroom house, 11–12 l/min at a 35 °C rise (a 28–30 kW combi) runs a good shower or fills a bath in 7–8 minutes. For two bathrooms used together, 14 l/min or more (35 kW upwards), provided the mains can supply it. A 24 kW combi at 9.8 l/min suits a flat or a small house where only one outlet runs at a time.

Why does my combi flow rate drop in winter?

Because the mains water arrives colder — 5–8 °C in January against 15–20 °C in July — and the boiler has to lift it further to reach the same tap temperature. Its output is fixed, so it lifts fewer litres per minute. The data sheet's 35 K figure is the winter number; summer flow at the same tap temperature is higher.

Can I increase the flow rate of my combi boiler?

Only up to what the boiler's output and the mains supply allow. Check the stop tap is fully open, replace restrictive flexible connectors and part-closed isolators, descale the plate heat exchanger, and confirm the flow restrictor is the correct size. Removing the restrictor gives more water but cooler. Beyond that, the options are a larger combi, a mains upgrade, or stored hot water.

Does a bigger combi mean more hot water pressure?

No. Pressure comes from the mains; the combi only heats what flows through. A larger combi gives more litres per minute at the set temperature if — and only if — the mains can supply them. On a weak main, a 40 kW combi runs at the same flow as a 30 kW one.

How many litres per minute does a shower need?

A thermostatic mixer shower feels good at 8–10 l/min and acceptable at 6–7 l/min. Large rain heads and multi-jet columns need 12–20 l/min and are usually a poor match for a combi unless it is 35 kW or more with a strong main. An electric shower runs independently of the combi but gives only 3–5 l/min in winter depending on its kW rating.

Regulations & Standards