An immersion heater timer switches the element on and off automatically, so you are not paying to heat a full tank of water around the clock. Most homes still have the older mechanical kind: a clock face ringed with small tappets that you push in or pull out to mark the periods you want it running. Getting those tappets wrong is the single most common reason people think their timer has failed when it has simply been set for the wrong hours.
Reading the tappets
Around the edge of the dial sit a ring of small plastic segments, usually one for every fifteen or thirty minutes. Pushed in towards the centre, a segment tells the timer to switch the heater on for that period. Left out, it stays off. The pointer or hand on the clock moves round in real time, and the heater follows whatever the tappet underneath it says at that moment. It is a simple mechanism, but it is easy to misread which direction means on.
Why people get it wrong
- Confusing a 24-hour dial with a 12-hour one — pushing in the segments for 7am to 9am on a 24-hour face actually sets 7pm to 9pm.
- The clock losing time after a power cut. A mechanical timer has no memory of the real time once the power drops, so it needs resetting by hand or it will run its schedule at the wrong hours.
- Forgetting it after the clocks change in spring and autumn — the timer keeps its own time regardless of what the rest of the house does.
- Assuming a heater that never seems to come on has failed, when the tappets are set for hours nobody is checking, such as the middle of the night.
Setting it to your off-peak hours
If you are on an off-peak tariff such as Economy 7, the point of the timer is to run the heater inside the cheap-rate window rather than around it. Your supplier can confirm exactly when your off-peak hours fall, since it varies by region and by tariff. Once you have that window, set the tappets to cover it with a little margin either side rather than cutting it exactly to the minute — a tank that has not quite finished heating when the cheap rate ends is a common complaint, and a slightly wider window usually fixes it.
Digital and smart timers
Digital replacements do the same job with a keypad or app instead of tappets, and they hold the correct time through a power cut, which is the main advantage over the mechanical kind. Some smart versions can also be boosted for a single extra heating period without disturbing the rest of the weekly schedule, useful if the household needs an unplanned hot water top-up outside the normal hours.
If the tappets are set correctly and the heater still is not switching on or off when it should, the fault is more likely in the timer itself or the circuit behind it than in how it was programmed — worth reading alongside our guide to immersion heaters that have stopped working. We fit and replace immersion heater timers as part of our heater repair work.
If you are unsure, it is always cheaper to ask than to guess. Give us a call and we will tell you straight.