Hot Water Cylinder Overflow Running or Water Boiling? What It Means and What It's Costing You
If your hot water overflow pipe is running — the one that discharges outside, often onto the roof or down an external wall — you're watching heated water go down the drain, and you're paying for every litre of it. An occasional drip after a heavy hot water draw can be normal expansion. Continuous running, and especially steam or visibly boiling water, is not: that usually points to a failed thermostat letting the cylinder overheat, and it should be dealt with the same day.
Steam from the overflow, or water that's scalding hot at the tap? Call 021 889 770 today — switch the cylinder off at the switchboard in the meantime if you can identify the circuit.
Read what the overflow is doing
Occasional drip shortly after heavy hot water use — usually normal thermal expansion as the cylinder reheats. Worth keeping an eye on, not an emergency.
Constant drip or trickle, day and night — commonly a failing pressure relief valve, which is plumbing work rather than electrical, but it's still costing you money and should be looked at.
Running steadily, water noticeably hot — the cylinder is likely overheating. A failed thermostat that no longer switches the element off is the usual electrical cause.
Steam, boiling, banging or rumbling from the cylinder — treat as urgent. Isolate the cylinder circuit at the switchboard if you can identify it, and call today.
No hot water at all, or lukewarm only — the opposite symptom, usually a failed element or thermostat, and also ours to fix.
Why it costs so much so quickly
Hot water is typically the largest single electricity load in a New Zealand home. A cylinder with a failed thermostat doesn't just heat to temperature and stop — it keeps heating, pushes water out the overflow, draws in cold water to replace it, and heats that too. The element effectively runs continuously. That's why a failed thermostat shows up as a sudden, dramatic jump on the next bill rather than a gradual creep, and it's the first thing we check when someone calls about a bill that's doubled with no lifestyle change. See our high power bill checklist for the rest of the diagnostic.
Electrician or plumber?
An honest split, because calling the wrong trade wastes a callout. The element, the thermostat, the wiring and the circuit are electrical — that's us. The cylinder itself, the pressure relief valve, the pipework and the overflow line are plumbing. Overheating caused by a failed thermostat is electrical even though the symptom you see is water. If you're not sure which you've got, describe it to us or send a photo and we'll tell you honestly — including when you need a plumber rather than an electrician.
Frequently asked questions
Can I just turn the cylinder off until someone comes?
Yes, and for a cylinder that's steaming or boiling it's the right call. Switch off the breaker labelled for hot water at your switchboard. You'll have no hot water until it's fixed, but you stop both the cost and the risk.
My overflow discharges onto the roof — is that normal?
It's a common arrangement in older Auckland homes with the cylinder in a roof space or upstairs cupboard, and it's why the problem often goes unnoticed for months — nobody sees the discharge. If you've had an unexplained bill increase and your cylinder is in the roof, this is worth checking specifically.
How much does a thermostat or element replacement cost?
It's typically a straightforward same-visit repair. Call us with the make and the symptom and we'll give you a number before we come out — and where the cylinder itself is at end of life, we'll tell you that rather than replacing parts in something about to fail.
Stop paying to heat the drain
📞 Call 021 889 770 — same-day for a boiling cylinder
📷 Text a photo of your cylinder and overflow to 021 889 770
Registered electricians on current EWRB practising licences · Certificate of Compliance and Electrical Safety Certificate issued within the statutory 20 days · Based in Remuera, working Auckland-wide