Why Does a DAFI Heat Differently in Summer and Winter? Seasonal Differences Without Panic  - Where those temperature swings come from, how to set the flow, and how to keep showers comfy all year

Why does a DAFI heat differently in summer and winter? The simple truth about mains water

Seasonal differences can feel like “device magic,” but it’s just physics and… the cold tap. When the temperature of the mains water changes, the starting point your DAFI has to “chase” up to comfort changes too. In winter, icy water arrives from the pipe (often single-digit °C); in summer—pleasantly mild. Same heater, same wattage, same outlet, totally different shower feeling. A DAFI instantaneous heater works the moment you open the hot tap: flow triggers the switch, the element heats only while you’re drawing water, so warmth is “on demand,” but the final temperature depends on a combo: inlet temperature + pressure/flow + power. Lower flow = hotter water; higher flow = cooler water. That’s not a quirk—every tankless heater behaves like this.

To get a feel for scale: “FT – flow/temperature” tables give indicative outputs for 15 °C inlet and 200 kPa (2 bar). For example, with 5.5 kW, getting ~40 °C means ~2.9 L/min. In winter, when mains water is ~5–8 °C, the same 40 °C requires a lower flow (because the temperature lift is larger). In summer, you can open the flow more and still land in the comfort zone. The numbers vary by model (3.7–11 kW), but the principle never changes.

What really makes the difference: power, flow, pressure and… voltage

Heater power sets the ceiling: how much heat can be pushed into the water passing through. But the temperature you feel right now is decided by… your hand on the tap. Ease the flow down—you raise the temp; open wider—you lower it. That’s the official, basic rule for using DAFI and the key to winter tuning. Then there’s system pressure: the unit is for low-pressure household installs and works within typical domestic ranges; if your building has bigger swings or exceeds the top limit, a pressure reducer helps; if pressure is too low, performance and comfort drop.

One more thing: supply voltage. Every percent drop below nominal means less power delivered to the water—roughly a 2% drop in heating effect for each 1% voltage drop. When winter evenings hit and everyone in the block fires up kettles and heaters, your line voltage can sag, and water may feel a touch cooler at the exact same tap position. Normal, not a fault. If you want it hotter, trim the flow.

A small but mighty detail: the spout aerator and the inlet filter screen. A clogged aerator can reduce flow in a “random” way (sometimes so much the heater won’t trigger), and a dirty screen can choke the inlet. Cleaning the aerator’s rubber nozzles and checking the filter quickly “gives back” perceived power—especially after months of hard water.

Winter settings without the stress: how to set the flow for real warmth

The instinct in cold months is to “open more.” Paradoxically—that hurts comfort. With a tankless unit, the trick is to give the element enough time to lift the temperature of the water rushing by. First, reduce the hot flow a bit; only then—if you really need—add a tiny touch of cold (or none at all). That one move delivers the immediate “ahh, that’s warmer,” and the heater still draws power only while water actually runs.

A rough example for a 5.5 kW model (FT data for 15 °C inlet show ~2.9 L/min for ~40 °C). In winter, when inlet is ~5 °C, you need a lift of ~35 °C instead of ~25 °C—so bring flow down to roughly 2.0–2.3 L/min. In summer, with ~20 °C inlet, you can go up to ~3.4–3.6 L/min and stay comfy. No need to obsess over decimals—just build the reflex: colder in → slower out.

More comfort hacks for cold-inlet days: shorten the pipe run from heater to tap (fewer losses), use the supplied shower heads/low-flow aerators (they spread the stream so it feels warmer at a lower L/min), and—at installation—place the unit as close as practical to the point of use. These aren’t “nice-to-haves,” they genuinely influence comfort and consumption.

Summer is your ally: use the “free” degrees and save

When mains water arrives pleasantly warm, a DAFI really spreads its wings. At the same power, you can enjoy a clearly stronger stream and still hit a comfortable shower or quick dish rinse. It’s a perfect time to “relax” the element: open the hot a bit more, let the aerator do its job and… forget about mixing cold. The payoff? Less fiddling with handles, steadier temperature, and lower bills—because the heater warms only while water flows.

Practically, it’s the same rule as winter, just flipped. The higher the inlet temperature, the more flow you can pass. If you’d like a gentler, spa-like feel in summer—no problem: nudge the hot slightly down, the stream softens, and thanks to the element’s low thermal inertia, the response is instant. That’s a hallmark of DAFI—no waiting for a tank and no delayed “heat wave.”

Just keep up simple hygiene: “flick” the aerator’s rubber tips and flush, and pop out the inlet screen to rinse it. Limescale or renovation grit can mess with flow any time of year—either overheating a too-tiny stream or preventing the trigger altogether. Five minutes of care buys you months of smooth operation.

Basin, kitchen, shower: seasonal scenarios for 3.7–11 kW

Basin / small sink (3.7–4.5 kW). In this territory, summer feels “easy”: handwashing, quick face rinse, cup rinsing. Winter stays comfortable too, but the keys are a moderate stream and a short run to the tap. For 4.5 kW, FT tables at 15 °C inlet suggest ~2.4 L/min for ~40 °C; in winter (~5–8 °C inlet) a sweet spot is ~1.7–2.0 L/min. In summer you can nudge it up to ~2.8–3.0 L/min without chill on your hands.

Kitchen (5.5–7.3 kW). This is stop-start territory: rinsing, washing, quick pot clean. With 5.5 kW, 40 °C ≈ ~2.9 L/min (at 15 °C inlet). In winter, pick one comfortable mixer position and stick with it; if your kitchen aerator is clogged—clean it, because a fake-low flow ruins the whole “math.” At 7.3 kW, the power headroom is bigger, so winter compromise is less noticeable; in summer—you’ll enjoy surplus flow.

Shower (7.5–11 kW). This is where comfort “rides” most on the mains temperature. In apartments with decent pressure, 9–11 kW models handle a typical hand shower even in winter, but it still pays to use low-flow showerheads (they spread the spray and your body perceives it as “warmer”). If you feel winter “waves” in temperature, check whether the riser has automatic flush valves—those can steal pressure briefly. DAFI supports shower sets and connections, but remember: don’t mount the heater inside the shower enclosure.

When to look beyond the season: quick checklists

Seasons aside, there are other “culprits.” If the water suddenly feels too cool for your usual handle position, check in order: is your voltage sagging (evening drops of several percent are noticeable at the tap), is the pressure unusually low (maintenance flushes, well pumps, neighborhood works), are the aerator and filter screen clean. Pressure and voltage swings affect heating effect, and too-low pressure can even interrupt operation. If local pressure tends to run high, a reducer stabilizes comfort and protects the system.

If your building has lots of auto-flushing fixtures, fluctuations can be short and annoying. Remedies exist: lower-flow showerheads, shorter hoses from heater to outlet, and in tough cases—an installer’s look at pipe diameters and a pressure reducer. As a low-pressure device, DAFI likes stable conditions within the manufacturer’s working range; you’ll feel the difference in day-to-day comfort.

Calm takeaway: seasonal differences are normal—set the flow and enjoy

In winter the water starts colder, in summer warmer—period. A tankless DAFI responds exactly to how you “play” the tap: slower = hotter, faster = cooler. Add a clean aerator, a clear inlet screen, sensible pressure and no big voltage drops—and the “why does it heat differently?” question stops being a mystery. In the cold season, run a slightly lower flow; when it’s warm outside, treat yourself to a fuller stream. Simple, logical, effective.