Water in a DAFI Cools After Washing a Few Dishes — Is the 2.5 mm² Cable to Blame?  - Step-by-step diagnosis: cable, voltage, pressure, flow, and accessories that decide your temperature

Water in a DAFI cools down after washing a few dishes — what’s really going on?

When, after two–three minutes of doing the dishes, the stream turns lukewarm, the first thought is often: “it must be that 2.5 mm² cable isn’t coping.” Tempting, because it’s simple. Reality is less binary: outlet temperature in an instant heater depends simultaneously on supply voltage, flow rate, pressure, and… the temperature of cold water at the inlet. In DAFI, the heater fires only when the flow sensor “sees” water moving, and the water heats up only as much as the unit’s power and the current conditions allow. Lower flow — higher temperature; higher flow — lower temperature. That’s how the device is built, not a quirk. After opening the hot-water valve the heater engages, and the temperature depends on inlet pressure and water temperature; you “regulate” it by slowly closing/opening the valve — in other words, by steering the flow.

Contrary to folklore, “it cools down” doesn’t necessarily mean the heater stops heating. Much more often it’s simply too much flow, a voltage drop in your electrical line, or a pressure blip (someone flushes a toilet or an automatic valve kicks in). As a low-pressure appliance, DAFI has a comfort zone: a minimum and a maximum operating range. Below the minimum the heater may not engage; above the maximum, a pressure reducer is a very good idea. Typical residential ranges are about 1–6 bar; when you see fluctuations and excursions toward the top of that range, expect temperature swings. If evenings feel fine but mornings during peak usage are cooler, hydraulics are the likely culprit — not the cable.

The 2.5 mm² cable: when it’s enough, and when it makes life harder

Many installers ask whether 2.5 mm² “will do.” For single-phase 3.7 kW and 4.5 kW units, 2.5 mm² is the minimum; for 5.5–7.3 kW, 4 mm² is recommended (or, in some cases, a ring of 2×2.5 mm²). In 400 V three-phase models (7.5–11 kW), expect 2.5–4 mm² between phases. Practical caveat that matters more than people think: verify conductor size against the installation method and voltage drop — long runs, high ambient temperature, being buried in plaster or squeezed in conduits all increase resistance and the voltage that “falls” before it reaches the heater’s terminals. That’s the exact mechanism that “eats” hot-water performance: every 1% of voltage drop below nominal reduces effective heating output by roughly 2%. With, say, a 5% drop, your “useful heating headroom” shrinks by ~10%. Result: the same stream that felt hot in autumn turns merely warm in winter.

Second issue: protection and connection method. Units above 3.7 kW must be hard-wired (no plug), with protective earth and the proper two-pole breaker/RCD on the panel. Nameplate currents tell the story: 4.5 kW draws about 19.6 A, 5.5 kW about 24 A, 7.3 kW about 31.7 A — and that’s what you size conductors and breakers for. If the heater “hangs” on an undersized branch, a too-long 2.5 mm² cable, or — worse — the same line as other heavy loads, the under-load voltage sag will be real and immediately visible as lower water temperature.

Pressure, flow, and fittings: why the stream steals your heat

A flow heater lives on balance: the more liters per minute you push through the heating block, the less time water spends in contact with the element, and the lower the outlet temperature. That’s why small hydraulic details have outsized impact. Start with the humble aerator: scaled-up, it can alternately choke the stream (heater won’t start) and then let it surge, which yields on-off heating and that “sometimes hot, sometimes not” effect. Regularly “pop” the limescale out of the aerator’s rubber nubs and rinse it. Add the inlet mesh filter to your routine (typically on the left tap stub) — when temperature yo-yos and the flow dwindles, that’s diagnostic step number one. A mechanical pre-filter in the line to the heater is strongly recommended too: debris destabilizes flow and wrecks temperature stability.

If you’re running 3.7–4.5 kW models under the sink, the included or recommended spray nozzle makes a night-and-day difference. It breaks the stream into fine jets, raises perceived temperature at the same water use, and calms the flow; for these power levels it’s a straight-up best practice. Don’t forget the basics: bleed the heater of air before applying power, and remember that pressure spikes (e.g., from automatic flush valves nearby) can cause momentary heating dropouts. If your system swings toward >6 bar, a pressure reducer isn’t optional — it’s what prevents roulette.

Heater power vs. expectations: how many liters per minute do you actually want?

“Cooling down” very often means expecting too much flow at a given power. As a ballpark with 15 °C inlet and ~2 bar pressure: a 4.5 kW unit delivers around 2.4 L/min at 40 °C or about 2.0 L/min at 45 °C. 5.5 kW manages roughly 2.9 L/min (40 °C) and 2.5 L/min (45 °C), while 7.3 kW can reach ~4.0 L/min (40 °C) and ~3.3 L/min (45 °C). These are indicative values: when winter drops inlet water to 5–10 °C, shave them down; when summer brings 18–20 °C at the inlet, you’ll get a boost. So for dishwashing you either limit the stream (aerator/spray), accept a slightly lower temperature, or consider a higher-power unit. No magic, just the arithmetic of heating water on the fly.

In practice, a 4.5–5.5 kW under-sink unit feels very comfortable if you keep the flow reasonable and keep pipe runs short (minimize losses on the way). The manufacturer’s guidance is to install the heater as close to the outlet as possible so you don’t waste heat into the pipe. If the device is tucked far back in the cabinet or behind a wall, after closing and reopening the tap you’ll get a brief cool spell — the “stale” water in the pipe hasn’t been reheated. A short composite line from the heater to the spout often bumps perceived warmth up a notch.

Diagnostic checklist: quick tests that make a real difference

1. Flush and bleed. Open hot water and run through the heater for at least a minute to purge air. Only then apply power. If, during use, it “spits” air, shut the tap and power off immediately, then bleed again. It’s foundational for safe operation and… stable temperature.
2. Aerator & mesh. Remove and de-scale the aerator; clean the inlet mesh filter. Clogged elements create the illusion of “cooling” because flow falls below the activation threshold and the heater cycles on and off. If you use a spray kit — de-scale it regularly.
3. Pressure. Check with a gauge before the heater. When it’s much below ~1 bar, the unit may not start; when it spikes toward 6 bar, fit a reducer and “calm” the system. Near public-style toilets or automatic valves, this is practically mandatory.
4. Electrical supply. Confirm that any model >3.7 kW is hard-wired, on a dedicated circuit, with proper breaker/RCD and protective earth. Match the nameplate current (e.g., ~19.6 A for 4.5 kW) and conductor size. A long plaster-buried 2.5 mm² run plus several connectors = larger voltage drop. If the branch is shared with a kettle/induction hob/boiler — split it out. Remember: every 1% of voltage drop costs roughly 2% of heating performance.
5. Accessories matched to power. For 3.7–4.5 kW, use a spray/flow-shaping nozzle — it materially improves dishwashing comfort without raising energy use. If you want more flow and high temperature, step up to 5.5 kW or 7.3 kW — after verifying your wiring and protection can safely carry the current.
6. Heater location & pipework. Shorten the distance from outlet to spout and avoid turning a long copper run into a heat sink. Mount as close as you reasonably can; using a low-loss composite tube to the faucet helps curb losses.

Action plan: stop fighting lukewarm water and get comfort back

Tidy up hydraulics first: aerator, mesh, pre-filter, pressure test, and — if needed — a reducer. Then match accessories to power: on 3.7–4.5 kW, a spray/stream-shaping tip is a small cost with a big effect. Next, fix the electrics: dedicated circuit, correct cross-section (2.5 mm² typically fine for 3.7–4.5 kW, but for long hot runs and harsh conditions it’s sensible to consider the next size up), solid terminations, correct protection. Finally, adjust usage habits: instead of “full blast then heat,” go “smart” — gently closing the hot-water valve raises temperature without changing your bill. Within those bounds, a DAFI flow heater does exactly what it’s designed to do: hot water on demand, no standby losses. Put these pieces in place and that “it cools after a few dishes” issue fades into the background.