Water heater on but the water’s cold? Sometimes it’s a small valve, not the heater

How a tank water heater works, and why it may be fine

An electric tank water heater takes cold water in at the bottom, the heating element warms it, and the thermostat switches the element off when the water reaches the set temperature and back on when it cools. A second safety cut-out switches the element off if the temperature goes too high. On the cold supply into the heater there is a non-return valve, known in Saudi homes as «الرداد», that lets water flow one way only. If it sticks open, or a tap mixer is worn inside, the hot and cold lines meet in the pipes: hot water pushes back into the cold line, or cold water gets into the hot line and cools it before it reaches you. It feels as if the heater isn’t heating while it is working perfectly well.

Five simple things to check first

1. The breaker and the light

   Check the heater’s breaker in the panel and the heater switch if it has one. If the breaker is down, reset it once only. If it trips again, don’t keep trying: that’s an electrical fault and needs a technician.

   You can check it
2. The temperature control

   Make sure the heater’s temperature dial isn’t at its lowest. If the heater was off, or you’ve just drained and refilled it, give it time to heat the whole tank before you judge — a big tank takes a while.

   You can check it
3. One tap or the whole house?

   Try the hot water at more than one tap. If only one shower or basin is affected and the rest run hot, the heater is probably fine and the problem is that tap’s mixer.

   You can check it
4. Feel the cold tap once the heater has been running

   Open the cold tap after the heater has been on for a while. If the first water out is warm or hot and then turns cold, hot water is pushing back into the cold line — a sign of a check valve that doesn’t close or a mixer letting water through.

   You can check it
5. The safety valve (the one that drips)

   On the heater inlet there’s a safety valve that lets out a little water when pressure rises during heating. Electrolux’s water-heater manual, for example, describes a few drips while heating as normal. Water running out constantly, or water from under the heater body itself, is not.

   You can check it

6 signs that tell you where the problem is

If the simple checks didn’t make the cause clear, compare what you notice with these signs:

1. Hot or warm water comes out of the cold tap, especially when you first open it

   Likely cause: Hot water is flowing back into the cold line: the check valve on the heater inlet isn’t closing, or a mixer in the house is passing hot water into the cold side

   Needs a technician

2. Only one tap or shower runs lukewarm and never gets fully hot or fully cold

   Likely cause: The mixer’s cartridge is worn inside and blending the water. The cartridge or mixer needs replacing; the heater has nothing to do with it

   You first, then a technician

3. The water comes out hot for a moment, then goes cold much sooner than usual

   Likely cause: Either cold water is getting into the hot line through a worn mixer, or an element or thermostat is weak. The mixing test below tells them apart

   You first, then a technician

4. The breaker is on but the heater light is off, and the water is completely cold

   Likely cause: Power isn’t reaching the element: the safety cut-out inside the heater has tripped, or a thermostat or connection has failed

   Needs a technician

5. The heating light stays on for a long time and the water never warms

   Likely cause: The heating element has most likely burned out, or the thermostat isn’t passing power to it. A technician confirms it with a meter

   Needs a technician

6. The breaker trips every time you reset it, or water drips from under the heater body

   Likely cause: A short in the element or wiring, or a leak from the tank or its connections. Switch the breaker off and leave it off

   Needs a technician

If the tap is far from the heater, the first water out is what was sitting in the pipe and has gone cold. Let it run a little before deciding the heater isn’t heating, and compare with a tap close to the heater.

A simple test: is cold water getting into the hot line?

• Switch the heater’s breaker off at the panel as a precaution.
• Close the cold-water valve feeding the heater — usually on the pipe going into the heater from below.
• Open a hot tap only. The flow should weaken and stop after a short while, because no water is going into the heater.
• If water keeps running strongly and turns cold, cold water is reaching the hot line from somewhere else: usually a tap or shower mixer letting water through. Test each mixer tap on its own to find which one.
• Reopen the valve, run a hot tap until the water flows with no air so you know the tank is full, and only then switch the breaker back on.

If the water stopped and every tap behaves normally, mixing probably isn’t the cause, and the problem is in the heater itself: the element, the thermostat or the safety cut-out. If one tap turns out to be letting water through, the fix is that tap’s cartridge or mixer, not the heater. And if hot water comes out of cold taps all over the house, a technician starts with the check valve on the heater inlet.

What a technician checks