I want to heat the contents (water) of a two liter thermos flask using a low power (50-100 watt) 12 volt DC heater.

Rather than buying a tiny little immersion heater, I've got some nichrome wire knocking about my workshop, would it be electrically safe to use a length of bare nichrome wire directly in the water to heat it?

My natural reaction is not mix electricity with water, but when I think about it I can't really envisage any real problems.

My intention is to use two length of wire, one about 100watts to heat the water in the first place, and a 2nd, about 3 watts to keep it warm once it's boiled.

Update for the curious... The flask is a traditional glass thermos, my intention is to to use this in a caravan with solar cells charging a 12v battery. The hope is that I can provide myself with a low power solution that will give me a permanent supply of hot water for occasional tea/coffee. Experimentation has indicated that I can keep a 2 liter thermos at about 95 degrees with only a couple of watts, and it only takes a few minutes of 100watts to heat the water again after drawing off a cup.

I think the electrical safety questions have been answered, but I'll try to do a bit more research about how healthy nichrome wire in drinking water may be.


  • 2
    \$\begingroup\$ How are you planning on having the nichrome not short out against itself in the bath? \$\endgroup\$
    – DKNguyen
    Mar 15, 2019 at 23:37
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    \$\begingroup\$ Another concern (not sure how well-founded since my chemistry skills are weak): A chance of metal ions, esp. chromium, leaching into the thermos? \$\endgroup\$
    – nanofarad
    Mar 15, 2019 at 23:39
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    \$\begingroup\$ @AndreyAkhmetov With the potential for electrolysis here, yes, I expect hexavalent chromium might have a chance of appearing. I don't have enough chemistry background to say for sure, but I do have enough chemistry background to know that this is probably a bad idea. \$\endgroup\$
    – Hearth
    Mar 16, 2019 at 0:28
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    \$\begingroup\$ If your flask is made of glass, no problem. Many decades ago we used a bare nichrom spiral with a 220V AC mains in a chemical-grade 3L flask, 2-3kW, to make a tea for the entire lab personnel, two times a day, for years. No problem, quick and most efficient. \$\endgroup\$ Mar 16, 2019 at 1:21
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    \$\begingroup\$ FOOD SAFETY, followed by the taste of your water are the two major considerations, followed by the element electrolysing away so it will fail after a time. Bare metal in water with a potential difference will dissolve metal ions in it. What the effect of those from commercial (not necessarily well specified to the analar level) nichrome will do are anybody's guess. \$\endgroup\$
    – Neil_UK
    Mar 16, 2019 at 6:26

3 Answers 3


You may or may not have this wire 'knocking about', and if you don't, it might be as expensive to buy as a proper 12v immersion heater.

Thin PTFE insulated copper wire has been used successfully as an in-thermos heater. You can recognise PTFE insulation, as it will withstand the heat of a soldering iron. My colleague built one using 7/0.1 wire, quite a long length bundled up, driven from 5v from a PC power supply, to get round the 'no private kettles' order in our workplace.


No, it doesn't seem to be safe. You have full potential gradient accross the wire immersed into water. Therefore, you have to consider electrolysis reaction where some interesting compounds may emerge depending on impurity of water. Even if you make it AC, I don't think that all of the reactions will be reversible with each alternation because of possible convection currents of liquid. So, isolation is required.


i say no. electrocution,poison metal element,molecular action, you might make drinking water deadly.dont be a miser,buy a comercially available heater that has safety markings on it,look for a ce mark,or equivalent,you risk death or worse. i would fail your idea if asked to test it as unsafe.any reasonable person would.have you thought of heating water by directly focusing sunlight on a water container ..difficult to do,but safer,resarch fully for problems if you do.


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