I see this a lot but not sure what it's called or what it does?

Silver thingy on boiler pipes

  • Are you talking about a hydronic heating water heater, a potable storage water heater, or an actual steam boiler? And is this in the supply/feed, the return, or the hot water outlet? Aug 16, 2016 at 5:34
  • Are there any markings on the device? Maybe at the side or back. Could you take a close-up photo using some much stronger lighting? Aug 16, 2016 at 8:58
  • Ordinary budget/domestic combi boiler. Couldn't see markings. From below its a cold water inlet, with a cross hose to top up the CH
    – Stilez
    Aug 16, 2016 at 11:45
  • It's holding the house-genie. Don't let it out! Aug 16, 2016 at 18:36

2 Answers 2


The above mentioned devices is a Magnetic Limescale Inhibitors which prevents limescale build-up by influencing the type (crystal structure) of the limescale precipitated. Laboratory research has shown that the growth of limescale crystals is affected by the application of the magnetic field causing more ‘needle-like’ aragonite crystals to be formed (which find it harder to stick to heat-exchange surfaces) than the normal calcite (which forms harder surface deposits).

Magnetic Limescale Inhibitors are best for individual appliances such as boilers

  • When you say "the above mentioned device", its ambiguous whether that's referring to the original question, or the picture by @RedGrittyBrick. Can you clarify your answer a bit, thanks
    – Stilez
    Aug 17, 2016 at 14:26
  • Sorry for mistake but both the devices the one in question and other in the 1st answer are same both are M.L.I
    – user182647
    Aug 17, 2016 at 19:12
  • Thanks. And also bonus thanks for providing a science-based reason why it might have an effect on limescale (as opposed to being a product of handwaving woo)
    – Stilez
    Aug 20, 2016 at 10:28

It might be some sort of woo/voodoo device sold to separate the gullible from their hard earned cash.

Possible example

enter image description here

I've no idea if this sort of device has any basis in real science but I am sceptical. As are others. Your mileage may vary.

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