Delta T 25K Decoded: What a 10 kW Heater Really Delivers, and the Three Questions to Ask Before You Compare Quotes

You usually meet this number for the first time in three quotes side by side. All three say 6 L/min, or 5, or 5.5, and all three look equivalent, so the decision drifts toward price. Then the container lands, a customer fills the unit from a 4°C stream in November, and the shower runs lukewarm at half the promised flow.

The flow figure is real. It is simply quoted at one temperature rise, and if that rise is not the one you operate at, the number does not describe your job. This article gives you the formula, walks one 10 kW unit through it, and then turns the arithmetic into three questions you can send to any supplier before you compare their prices.

The Physics: Heat Load Is Flow Multiplied by Rise

Heating water takes energy in direct proportion to two things: how much water, and how far you have to raise its temperature. Written out:

P = ṁ × c × ΔT

where P is heat power in watts, ṁ is mass flow in kg per second, c is the specific heat of water (4,186 J/kg·K), and ΔT is the temperature rise in kelvin.

Because 1 liter of water weighs 1 kg, liters per minute converts to mass flow by dividing by 60. That single step is where most buyers lose the thread, so it is worth doing explicitly.

Take the rated point: 6 L/min at a 25K rise.

• 6 L/min ÷ 60 = 0.1 kg/s
• 0.1 × 4,186 × 25 = 10,465 W ≈ 10.46 kW

The rated input of the Thermostatic model is 10 kW. A theoretical requirement of 10.46 kW against a 10 kW nameplate is a spec sheet doing its job — the small gap is the rounding between a clean 25K rating and real heat-exchanger efficiency. If a manufacturer's numbers come out far below the nameplate, the flow figure is optimistic. If they come out far above, the nameplate is.

The same formula, rearranged, answers the question buyers actually have:

Flow = P ÷ (c × ΔT) × 60

Engineering: Two Bottlenecks, Not One

A 10 kW heater has a fixed heat load and a fixed maximum flow. Whichever limit you hit first is what you feel at the shower head.

The heat-load limit only binds when the incoming water is cold. At 4°C inlet and a 40°C target you need a 36K rise, which caps the unit at roughly 4.0 L/min no matter how far you open the tap.

The rated-flow limit binds when the incoming water is warm. At 20°C inlet you only need a 20K rise, and the arithmetic says the unit could theoretically pass 7.2 L/min — but the rating stops at 6 L/min. Above that ceiling, extra flow buys you nothing; the outlet temperature simply drops.

This is why "10 kW" is not a promise of "more hot water". It is a promise of a specific amount of recoverable flow in cold water. A 6 kW unit in the same portable family carries 60 percent of the heat load: at a 36K rise the arithmetic would put it near 2.4 L/min, but that model is rated to a 25K maximum rise, which is the honest way of saying it is built for milder inlet water rather than near-freezing streams.

There is a third number on our sheet that clarifies all of this: maximum temperature rise, 55K. That figure is not available at full flow. A 10 kW heat load can only lift water by 55K when it is passing about 2.6 L/min, because 0.043 kg/s × 4,186 × 55 is 10 kW. Read "maximum temperature rise" as the limit at minimum flow, never as a figure that coexists with the rated flow. In practice the settable outlet range of 35–55°C runs out first: from 4°C inlet, the 55°C set-point is reached at a 51K rise, before the machine ever touches its 55K ceiling.

The Thermostatic model also carries a high-quality copper heat exchanger and a settable outlet range of 35–55°C, so the target temperature in the tables below is a setting, not a fixed output.

The number on the screen is the set point — what the shower actually gets depends on how cold the water going in is, and how much of it is flowing.

Comparison: What 10 kW Delivers Across Inlet Temperatures

Theoretical maxima, assuming no heat loss to the surroundings. Real output runs a little under these.

Two readings matter here. First, the unit's supplied shower head runs at roughly 3 L/min, so a 4 L/min winter output still fills a comfortable shower — the machine is matched to its own head. Second, if you swap in a North American 2.5 GPM head (about 9.5 L/min), you are asking for more than twice the water at the same heat load, and the outlet temperature will sag accordingly. Flow and temperature trade against each other; they never both go up.

The 25K reference point is simply a convention. North American sheets commonly rate at a 45°F rise, which is the same 25K, and the trade rule of thumb — 500 BTU/h per GPM per °F — folds to the identical physics.

What to Ask Before You Compare Quotes

Three questions, sent in writing, that make a three-way comparison fair:

1. "At what temperature rise is your flow figure rated, and what inlet temperature does that assume?"

A supplier who answers "25K, which is 20°C in and 45°C out" has quoted honestly and given you something usable. A supplier who answers "6 liters per minute" without a rise has given you a number that cannot be checked.

2. "What is the maximum temperature rise, and at what flow does it apply?"

Anyone who states a maximum rise as though it were available at rated flow has not read their own sheet. The two figures live at opposite ends of the flow range.

3. "What outlet temperature do you guarantee at 5°C inlet, at 3 L/min?"

This is the question that matters for a Nordic, Alpine or Canadian program, and it is the one most often deflected. A supplier who cannot answer it either has not measured or does not want to publish the answer — both are information.

If the answers come back with measured values rather than theory, the supplier has a test bench. If they come back as a copy of the marketing sheet, you have learned something cheaper than a returned container.

Practical Pitfalls

• Do not read the flow figure as a year-round number. It is quoted at one rise. Winter changes the rise, not the machine.
• Check your inlet temperature before sizing. Groundwater and unheated tank water in late autumn sit far below the 20°C a spec sheet quietly assumes.
• Do not buy a big shower head for a small heater. Head flow and heater ceiling must be matched, or you get a weak warm stream instead of a strong hot one.
• The 55K maximum rise is a low-flow figure. If you need genuinely hot water from near-freezing inlet, you are accepting reduced flow to get it.
• This heater draws from a container using its own 25 W self-priming pump, and runs on its internal lithium battery, so it needs no mains wiring and no water pressure. It still needs a charged battery and a connected gas canister before it will start.
• Portable outdoor water heaters must never be used in tents, vehicles, or any enclosed space. Combustion produces carbon monoxide, which is colorless, odorless, and toxic.
• SEFFY portable heaters are designed to international gas-safety standards; we are actively preparing for EU CE certification, expected soon.