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Immersion heater timer switch

Updated: October 2026

You want the cylinder hot before a shower, and you are looking at a plug timer because it is the thing in the search results. An immersion heater timer switch for a fixed element is a fused spur on the wall, not an adaptor in a socket. This page uses the two spur rows in the [digital timer switch comparison](/hub/electrical-equipment/digital-timer-switch): the Timeguard FST24, captured at about £33 with 4.5 from 905 reviews, and the Candeo C-TSFS at about £32.99 with 4.4 from 127. This is not a lab test, and it is not a wiring diagram. A qualifying purchase can earn a commission at no extra cost to you. The general choice between a portable plug and a spur is on [plug in timer or fused spur](/hub/electrical-equipment/digital-timer-switch/sub/digital-timer-switch-plug-in-or-fused-spur).

You want the cylinder hot before a shower, and you are looking at a plug timer because it is the thing in the search results.

Our top picks

Timeguard Timeguard FST24 fused spur timer

Best overall

Timeguard Timeguard FST24 fused spur timer
9.2/10about £33
View offer
Candeo Candeo C-TSFS fused spur timer

Rank 2

Candeo Candeo C-TSFS fused spur timer
9.0/10about £32.99
View offer
On this page
  1. 1.Why a plug-in timer is the wrong object
  2. 2.What the FST24 listing states
  3. 3.The 3 kW figure and the IET caution
  4. 4.When you stop and book an electrician

Why a plug-in timer is the wrong object

A plug-in timer expects an appliance you can unplug. A fixed immersion is wired to a spur or a switch beside the cylinder. Cutting the plug off an adaptor and joining it to the cylinder cable is not a conversion. The NOVKIT's 2,900 W line and the ORIDGET's 13 A title are plug ratings for portable kit. They do not become an immersion controller because the number looks similar to 3 kW. None of the plug-in ASINs here is the object for this job. People reach for a plug because the search result is about £6 and the spur is about £33. The price gap is real in the captured files. It does not change the way the cylinder is connected. A plug also blocks a socket and is easy to knock out, which is a poor way to leave a 3 kW element.

What the FST24 listing states

The Timeguard FST24 repeats up to four on and off periods every day, so a cheap-rate window is the same on Saturday as on Tuesday. The bullets state 13 A and 3 kW resistive, a BS 1362 13 A fuse, double-pole switching, and a one- or two-hour boost. They ask for a back box at least 25 mm deep and recommend professional installation. Reviews say the supplied screws are too short for a recessed metal back box. The Candeo C-TSFS is the other spur if you need 7-day, 5-day or 2-day patterns. It does not state a wattage beside the fuse. Reviews mention Wi-Fi, Tuya and Alexa. The bullets do not. The FST77 is the weekly Timeguard in the trade, and it is not an ASIN here.
Timeguard Timeguard FST24 fused spur timer

Testsieger

Timeguard Timeguard FST24 fused spur timer
9.2/10about £33
View offer

The 3 kW figure and the IET caution

Timeguard says the FST24 suits immersion heaters at 3 kW resistive. That is the maker's claim. The IET, in Wiring Matters in September 2025, says a 3 kW immersion at a nominal 230 V is generally unsuitable for a BS 1363 plug or a BS 1363-4 fused connection unit unless the accessory manufacturer confirms otherwise. The same article suggests about 2.7 kW as a general ceiling for long loads on that gear. Hold both sentences. Cable size, hours, and what else shares the way are a circuit question an electrician can confirm. The Candeo's 13 A fuse without a wattage is not a 3 kW claim. Ordinary plug loads are on the digital timer plug buying guide.

When you stop and book an electrician

Book an electrician when the work is more than a like-for-like swap of a spur you already understand. Approved Document P, the 2013 edition, makes a new circuit, work at the consumer unit, and work in a special location notifiable. A bathroom zone is that kind of location. A back box shallower than the 25 mm the FST24 bullets ask for is a reason to stop. If the box is metal and recessed, the supplied screws may not reach. Isolate before any cover comes off. If you are not sure the circuit is dead, leave the cover on. A plug-in programming sequence will not set a spur.

Common questions

Can I plug the immersion into a digital timer adaptor?
No, not when the immersion is a fixed element wired at the cylinder. A plug-in timer is for an appliance that already has a BS 1363 plug. Cutting that plug off to join the timer to the immersion cable is not a method this listing describes.
What is the boost button for?
The FST24 boost runs the output for one or two hours and then turns it off. Reviewers use it when they want extra hot water outside the programmed window. It is not a second weekly programme, and it does not change the 3 kW question.
Do I need an electrician?
You need one when the circuit is new, when the work is at the consumer unit, when the spur is in a bathroom zone, or when the cable and the box do not match a simple swap. The FST24 listing itself recommends professional installation.
Is a 13 A fused spur enough for a 3 kW element that runs for hours?
Timeguard says yes for this model, at 13 A and 3 kW resistive, with a BS 1362 fuse. The IET says a 3 kW immersion at 230 V is generally unsuitable for a BS 1363-4 fused connection unit unless the accessory maker confirms otherwise, and it floats about 2.7 kW as a general ceiling for long loads on that gear.
How deep is the back box?
The FST24 bullets state a minimum of 25 mm. Reviews say the supplied screws are too short for a recessed metal box, and one buyer used 50 mm screws. A Candeo review describes the switch fouling plasterboard back-box tabs, with screws of about 60 mm mentioned for a deep box.

Related pages

How to programme a digital timer plug

Digital timer switch

Best digital timer switch UK: lamp, countdown, spur

Digital timer plug buying guide: form, load, schedule

Plug in timer or fused spur timer: which object fits

Sources

Public UK sources from the research digest. Shelf prices and star lines are the amazon.co.uk figures in the display file, captured on 20 October 2026.

  1. Electrical Equipment (Safety) Regulations 2016, regulation 35 (retrieved 20 October 2026)
  2. GOV.UK: Electrical Equipment Safety Regulations 2016, Great Britain (retrieved 20 October 2026)
  3. BSI: BS EN IEC 60730-2-7:2020, timers and time switches (retrieved 20 October 2026)
  4. Approved Document P, 2013 (retrieved 20 October 2026)
  5. IET Wiring Matters, September 2025: 3 kW immersion and a 13 A fused connection unit (retrieved 20 October 2026)
  6. Electrical Safety First: plugs and fuses (retrieved 20 October 2026)
  7. Electrical Safety First: portable heaters (retrieved 20 October 2026)
  8. Timeguard FST24 fused spur timeswitch (retrieved 20 October 2026)