An immersion heater timer switches your hot water cylinder on and off automatically, so a 3kW element only runs when you actually need hot water — not all evening because somebody pressed the switch at teatime.
That is the whole job. It is not clever, and that is exactly why it works.
The confusion starts because three different products get called the same thing: a full 7-day time switch, a simple boost controller, and the plain rocker switch on the airing cupboard wall. They do different jobs, they cost different money, and most people buy the wrong one.
Here is how to pick, from the counter that sells all three.
1. What an immersion heater timer actually does
An immersion heater is a big electric element sitting inside your hot water cylinder. Typical domestic elements are around 3kW, which is roughly the same draw as a kettle — except a kettle runs for two minutes and an immersion left switched on runs for hours.
The timer sits in the supply to that element and decides when it is allowed to run. Set it for an hour before you get up and an hour before the evening, and the cylinder is hot when you want it and coasting the rest of the day.
If you are on an Economy 7 or similar off-peak tariff, the argument gets stronger: the timer moves the heating into the cheap window instead of leaving it to chance.
Trade Tip: before you spend anything on a timer, put your hand on the cylinder jacket. If it is warm to the touch, you are paying to heat an airing cupboard. A £15 insulating jacket saves more than any timer will, and the two together save more again. We have watched people spend £60 on a programmable controller strapped to a bare 1980s cylinder.
2. The three types, and which one you need
A boost controller is a wall plate with one or two buttons. Press it, the immersion runs for a fixed period — typically an hour — and switches itself off. No programming, no clock to reset. For most households this is the right answer and they never realise it exists.
A programmable time switch gives you set on-and-off periods across the day, and a 7-day version lets weekends differ from weekdays. Right for a fixed routine, or where you are chasing an off-peak window precisely.
An analogue 24-hour timer is the old segmented dial. Cheap, robust, and repeats the same pattern every day with no weekday/weekend split.
The question that settles it is not "how much control do I want" — everyone answers that the same way. It is "will anyone in this house ever reprogram this?" If the honest answer is no, buy the boost controller. This is where the money gets wasted.
3. Boost controls — the one most people should buy
Sangamo PSB2 Powersaver Boost. The simplest thing that solves the problem: a boost control with no clock to set and nothing to go wrong when the power blips. If the household complaint is "somebody keeps leaving the immersion on", this ends it without a conversation.
Sangamo PSPS Electronic 24/7 Boost Controller with Programmable Operations. A step up: boost when you want it, plus programmable operations across the full 24/7 cycle. This is the one to fit where the routine is genuinely fixed and somebody will actually use the programming.
Sangamo PSPSS Powersaver Plus Select Controller, Silver. Same family, silver finish — worth knowing about when the control is going somewhere visible rather than inside a cupboard. It also comes in black (PSPSB) and white, so the plate can match the rest of the room's wiring accessories instead of shouting.
4. Wiring it in, and where DIY stops
An immersion heater is not a socket outlet. It is a fixed appliance on its own circuit, and the standard arrangement is a dedicated circuit feeding a double-pole switch, wired in heat-resisting flex for the final connection because the top of a cylinder gets hot.
Swapping a like-for-like control on an existing circuit is one thing. Running a new circuit, altering the consumer unit, or working in a bathroom is another — that is notifiable work and belongs to a competent person. If you are not certain which of those you are doing, you are doing the second one.
Trade Tip: heat-resisting flex is not ordinary flex with a different price tag. Standard PVC flex hardens, cracks and eventually fails where it meets a hot cylinder head — we see the evidence come across the counter as blackened, brittle tails. If the existing tails look tired, replace them while the cover is off. You will not want to do it twice.
While the isolator is off, it is worth reading what a fused spur is and when you need one, because the two get confused on exactly this kind of job.
5. Siting, and the mistakes we see every week
Put the control where the person who uses it can reach it. A beautifully programmed timer at the back of a loft is a timer nobody adjusts.
Three that come up repeatedly:
- Fitting a timer and leaving the old rocker switch on. The timer can only control what the switch lets through. Somebody flicks the switch, the timer is bypassed, and you conclude the timer is broken.
- Setting a one-hour boost on a large cylinder. A bigger tank from cold needs longer. Start generous, then trim it back over a week until the water is hot enough and no longer.
- Forgetting the clock after a power cut. Anything with a programmable clock drifts or resets. It is the single most common reason a working timer gets replaced.
Summary: which immersion heater timer to fit
| Type | Best for | Programming | Watch out for |
|---|---|---|---|
| Boost control | Most households | None — press and go | Fixed boost length may not suit a very large cylinder |
| Programmable 24/7 controller | Fixed routines, off-peak tariffs | Full on/off periods | Needs someone willing to set it |
| Analogue 24-hour timer | Simple repeating daily pattern | Segmented dial | No weekday/weekend split |
| Plain double-pole switch | Manual control only | None | Relies entirely on somebody remembering |
Expert Help in the West Midlands
We have been on Villa Road in Birmingham since 1975, and immersion controls are a conversation we have most weeks — usually with someone holding a browned-off old timer and a puzzled expression. Bring it in. We will tell you whether you need a like-for-like swap or whether the job has grown, and we will be honest when the answer is "get a spark to look at that".
The full range is on time switches and immersion timers, and the wider switches and controls range covers the isolators and plates that go with them.
Immersion heater timer FAQs
Will an immersion heater timer save me money?
Only if the immersion is currently running longer than you need hot water. If yours is already switched on and off by hand at sensible times, a timer buys you convenience rather than savings. On an off-peak tariff, or in a house where the switch gets left on, the saving is real.
Can I fit an immersion heater timer myself?
Replacing an existing control like for like on an existing circuit is within reach of a confident DIYer, with the circuit isolated and proved dead. New circuits, consumer unit work and anything in a bathroom are notifiable and need a competent person. If in doubt, don't.
What is the difference between an immersion heater timer and a boost switch?
A timer runs to a clock and repeats a programme. A boost switch runs the immersion for a fixed period from the moment you press it, then stops on its own. Most households get more out of the boost switch because it needs no maintenance and cannot be left on.