What Is a Flow Switch?

An instantaneous heater only fires when it knows water is moving, and the flow switch is how it knows. No flow signal, no ignition; a weak flow signal, and the burner cuts out mid-shower.

Why It Matters

• It explains the most common continuous-flow complaint: hot water that stops and starts. The heater isn’t dying: the flow through it isn’t enough to keep the switch triggered.
• Blocked inlet filters are the usual culprit. These small filters block up regularly, choking flow below the ignition threshold. Cleaning them is quick and often fixes the fault outright.
• Low-flow tapware can contribute: a heavily restricted outlet may sit right at the switch’s threshold, flickering the burner on and off.

The Usual Fix Sequence

1. Restart the unit (genuinely fixes some electronic sulks)
2. Find and clear small blockages: clean the inlet filters
3. Replace minor internal parts if worn (including the switch itself)
4. For old units, a straightforward swap-over, since they’re an easy replacement

Most of these are simple, inexpensive repairs: a long way from the new-system quote people fear.

Common Mistakes or Misunderstandings

• “The unit cuts out, so it’s finished.” Nine times out of ten it’s flow, not the heater. Diagnose before replacing.
• “More hot water = turn the unit’s temperature up.” If the problem is flow, temperature settings won’t stop the cut-outs.
• “It only happens in one shower, so the heater’s fine.” Partly true: that outlet’s flow is the marginal one. It still deserves a look; thresholds drift as filters clog.

When to Call a Plumber

Stop-start hot water, ignition failures, or a unit that works for some taps and not others: quick diagnostics pinpoint whether it’s filters, the switch, or the associated valves.