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Immersion heater timer switch for a hot-water cylinder

Updated: October 2026

The cylinder in the airing cupboard is expensive to leave on all day, and the timer you already understand is the plug-in beside the kettle. An immersion heater timer switch for that tank is a different product. A 3 kW element at 240 V draws about 13 A, and the Institution of Engineering and Technology says a BS 1363 plug, socket, or fused connection unit is generally unsuitable for that long full-power load. The [Timer switch comparison](/hub/electrical-equipment/timer-switch) is where the plug-in models in this set are compared. This page stays with the cylinder: why that shelf fails, what a wired 16 A timer is, and when a competent person has to connect it.

The cylinder in the airing cupboard is expensive to leave on all day, and the timer you already understand is the plug-in beside the kettle. An immersion heater timer switch for that tank is a different product.

Our top picks

Defender Defender 24 Hour Segment Timer Plug Socket

Best overall

Defender Defender 24 Hour Segment Timer Plug Socket
8.8/10about £7.95
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HBN HBN 24 Hour Mechanical Timer Plug Socket, 2 pack

Rank 2

HBN HBN 24 Hour Mechanical Timer Plug Socket, 2 pack
8.6/10about £12.99
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HBN HBN 24 Hour Mechanical Immersion Heater Timer Switch, 16 A

Rank 3

HBN HBN 24 Hour Mechanical Immersion Heater Timer Switch, 16 A
8.4/10about £15.99
View offer
On this page
  1. 1.Why the plug-in aisle is the wrong shelf
  2. 2.What a cylinder timer is
  3. 3.The wired timer in this comparison
  4. 4.Who connects it

Why the plug-in aisle is the wrong shelf

A 3 kW immersion stays at full power while it heats the cylinder, and shop pages still sell a 13 A plug-in as an immersion timer. The IET Wiring Matters note of September 2025 says a BS 1363 plug, socket, or fused connection unit is generally unsuitable for that long load. The same note puts an ordinary long full-power load on a BS 1363 plug at about 2.7 kW, unless the accessory maker says otherwise. A 3 kW element sits above that line. A listing rated up to 3120 W is a portable heater you unplug, not this cylinder. The timer switch buying guide is where a lamp or a fan belongs.

What a cylinder timer is

A cylinder timer is wired into the supply. It should state a resistive rating in the 16 A class, and it only switches that supply on and off. It does not replace the thermostat, and it does not replace the thermal cut-out. Bridging either control, so a one-hour setting runs the tank alone, leaves you without the safety the cylinder was built with. Timeguard’s NTT01 is the pattern: wired, 16 A resistive, 6 A inductive, 15-minute steps, and a qualified electrician to fit it. Match the element to the resistive figure. A lower inductive number is for a motor, not a reason to reject the timer. A segment dial for a plug-in lamp is a different fork, on mechanical vs digital timer plug. A screen does not make a 13 A plug suitable for a 3 kW element.
Defender Defender 24 Hour Segment Timer Plug Socket

Testsieger

Defender Defender 24 Hour Segment Timer Plug Socket
8.8/10about £7.95
View offer

The wired timer in this comparison

The only wired 16 A segment timer in this set is the HBN, B074FQ37KX, at £15.99, with a score of 4.2 from 988 ratings. The listing claims a cover, 15-minute steps, and 48 on/off actions, and it states no battery. Do not treat that listing as the same certificate as the NTT01. Reviews mention drift of about an hour a day and a clock that stops in a power cut. One review says there is no earth terminal. That is not a diagram to copy. The listed minimum depth is 15 mm, not Timeguard’s 32 mm, so the back box may be too shallow. A waterproof cover does not make it outdoor equipment.

Who connects it

Connecting this timer is fixed wiring. Timeguard and TFC both say a qualified electrician should install their immersion timers, and you stop if you cannot isolate the circuit and prove it is dead. You also stop if the next step is to omit or bridge the thermostat or the thermal cut-out. The timer only switches the supply. In England, Approved Document P makes a new circuit notifiable, while a like-for-like replacement is not on that list. Adding a timer is not automatically the non-notifiable case. Scotland and Northern Ireland have their own rules. Once a competent person has connected the timer, you choose the hours and set the time again after a power cut. Do not practise pin-setting by plugging the cylinder in.

Common questions

Can I put the immersion plug into a cheap timer?
No. A 3 kW immersion is a long full-power load, and the IET says a BS 1363 plug, socket, or fused connection unit is generally unsuitable for it in a warm airing cupboard. The note puts ordinary long full-power use at about 2.7 kW unless the accessory maker says otherwise. A cheap plug-in timer is for a lamp, not this cylinder, even if the heater lead already has a 13 A plug.
What rating should a wired immersion timer state?
Look for a wired timer stated at 16 A resistive. Timeguard’s NTT01 is 16 A resistive and 6 A inductive, TFC’s OP-IHTGPT is 16 A resistive and 8 A inductive, and Greenbrook’s T102 is 16 A resistive and 2 A inductive. Match the element to the resistive figure. The smaller inductive number is for a motor.
Will the HBN 16 A unit keep time after a power cut?
No. The listing states no battery, and reviews report a clock that stops in a power cut, plus drift of about an hour a day on some units. Timeguard’s NTT01 instructions say to turn the dial clockwise back to the right time after a cut. Reset this mechanical dial the same way, then check that the next on period still matches the hour you wanted.
Does the waterproof cover make it outdoor equipment?
No. The cover on the HBN listing is not an outdoor rating, and it does not clear a bathroom zone. IP20 is printed on the TFC sheet for that TFC model and means indoor equipment. Do not copy that mark onto the HBN, which has no matching IP claim in this comparison. Have a qualified electrician fit it as an indoor cylinder timer.
There is no earth terminal. What then?
Stop, and hand the job to a competent person. One review of this HBN says the clock has no earth terminal and that the earth was joined elsewhere. The protective conductor still has to remain continuous. This page will not give you a terminal or a join to copy. If you cannot see how the earth is maintained, you should not connect the timer.

Related pages

How to set a 24 hour plug timer

Timer switch

Best timer plug UK: which listing in this set fits

Plug in timer socket UK: choose the type before brand

Mechanical vs digital timer plug: daily dial or week

Sources

Public UK sources are listed below. Shelf prices and star lines are the figures on the saved amazon.co.uk listings.

  1. Electrical Equipment (Safety) Regulations 2016, regulation 35 (retrieved October 2026)
  2. GOV.UK consumer product safety (retrieved October 2026)
  3. Electrical Safety First, plugs and fuses (retrieved October 2026)
  4. Electrical Safety First, buying electrical goods online (retrieved October 2026)
  5. Approved Document P (England, 2013 PDF) (retrieved October 2026)
  6. BSI, BS EN IEC 60730-2-7:2020 (retrieved October 2026)
  7. Timeguard NTT01 instructions (retrieved October 2026)
  8. HSE electrical equipment user checks (retrieved October 2026)