If you have a new account but are having problems posting or verifying your account, please email us on for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact

Boiling the kettle with hot tap water

  • 09-06-2022 11:23am
    Registered Users Posts: 59 ✭✭

    Is it cheaper to boil the kettle starting with water from the hot tap which has already been heated by air to water?

    I know there are many variables but my mind boggles thinking about them all.

    There is also the cost of running the tap until it warms up, wasting the length of pipe to the hot tank pipe full of hot water to cool down, this would depend on the length of pipe too. Also, if the hot tap was already running hot vs waiting for it to heat up.

    There would be the economy of scale to think about, maybe because of pipe heat wastage it's more expensive for one cup, but cheaper for a full kettle

    Also I know that people are often reluctant to drink from the hot tap, as they visualise the dead mouse floating in the tank in the attic, but our system doesn't use the tank.

    Your thoughts?


  • Registered Users Posts: 793 ✭✭✭reklamos

    The cheapest is to boil as much as you will use at that point in time.

  • Registered Users Posts: 11,004 ✭✭✭✭the_amazing_raisin

    Probably cheaper but water from the hot tap is generally not drinkable since it could have been sitting stagnant for some time

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 2,471 ✭✭✭bullit_dodger

    Generally I'd advise against doing that (drinking from the hot tap). If you have a new'ish house your probably ok in terms of lead piping leeching into the water, but if you were in an old building, it's possible that that there might still be some lead piping knocking about. Hot water is also hanging about your system for longer and heat (not boiling of course) is known to increase bacterial production.

    Sure, your boiling the water which should kill 99.99% of bacteria, but in terms of energy saving it's just not worth the risk.

    (Energy consumption of kettle) x (time) x (price per unit) = 2Kw x (6 mins) x (€0.30) = (2) x (0.1) x (0.30) = 6 cents.... from cold.

    Considering you'd probably have to boil the same water from (say) 60c -> 100C which is half way, you'd save maybe 2-3cents per kettle. Not counting the energy you used to get the hot water up to 60C.

    Nahh, math isn't there for all the downsides, not work the single cents to risk a "stomach cleanse"

  • Registered Users Posts: 26,958 ✭✭✭✭Dempo1

    Wouldn't be boiling a kettle using already heated water personally. I boil what I need, one cup at a time but also regularly descale kettle base with vinegar. I've a lot of limescale build up due to hard water. Descaling allows water boil quicker in a kettle and limescale build up slows boiling making it even more expensive.

    Is maith an scáthán súil charad.

  • Advertisement
  • Moderators, Sports Moderators Posts: 18,801 Mod ✭✭✭✭slave1

    I'd never use hot water that has been sitting in the pipes/tank for some time, always kitchen tap which comes straight in fresh off the mains feed.

    If you are looking at the cheapest then just stick it in the microwave for two minutes

    My stuff for sale on Adverts inc. outdoor furniture, roof box and EDDI

    My Active Ads (

  • Moderators, Home & Garden Moderators Posts: 5,902 Mod ✭✭✭✭graememk

    We use a Britta filter to stop the limescale (and also removes/,greatly reduces the scum in the tea!)

    We change the filter whenever the kettle starts making more noise than usual.

    But I'd not be boiling from the hot tap for the above reasons. I do get the OP's thinking though.

  • Registered Users Posts: 5,112 ✭✭✭John mac

    should always use water direct from the main for potable water . mised the bit about the tank in the attic , how is the water heated so. ?

  • Registered Users Posts: 14,828 ✭✭✭✭loyatemu

    if you've no tank then the water is being heated by a combi boiler? In which case you'll save yourself some time but whether it saves you money depends on the relative of cost of gas vs electricity (gas is generally cheaper but if you've solar panels then obviously that is cheaper).

    ignore - I missed the part about it being heated by A2W, I've no idea so.

  • Registered Users Posts: 59 ✭✭Fern Bench

    The water is heated in a tank which comes from the mains, doesn't use an attic tank which would often be open to bugs and stuff, hence the usual advice never to drink it.

    Re: the point about water coming fresh off the mains feed, ours goes through a lot more pipes and does sit it a tank as it has to go through a pressure unit in the shed. Doesn't all drinking water spend time sitting in pipes and tanks?

    I do get the point about people's reluctance to drink warm water that has been sitting in the immersion tank.

  • Advertisement
  • Moderators, Sports Moderators Posts: 18,801 Mod ✭✭✭✭slave1

    Doesn't all drinking water spend time sitting in pipes and tanks?

    Yes, but mains feed cold water is potable/consumption friendly. Water when heated will release any chemical additives not to mention micro contaminants from the tank/pipes.

    My stuff for sale on Adverts inc. outdoor furniture, roof box and EDDI

    My Active Ads (

  • Registered Users Posts: 78,312 ✭✭✭✭Victor

    "Sure, your boiling the water which should kill 99.99% of bacteria" - while the bacteria might be dead, the poisons they produce might not have been destroyed.

    Using previously heated water means that much of the (small but necessary amount) of chlorine will have been released, in water that might be sitting there for days.

    "(Energy consumption of kettle) x (time) x (price per unit) = 2Kw x (6 mins) x (€0.30) = (2) x (0.1) x (0.30) = 6 cents.... from cold."

    This might be a better calculation:

    85 degrees (from 15 degrees to 100 degrees) x 4182 J/kg°C = 355470 J

    => 0.1 kWh for 1 litre of water, which costs about 3 cents. This doesn't allow for heat loss or other inefficiencies.

    Post edited by Victor on

  • Registered Users Posts: 984 ✭✭✭Still stihl waters 3

    Can afford air to water system but trying to save cents to boil a kettle

  • Registered Users Posts: 59 ✭✭Fern Bench

    No I'm talking about water intended for drinking, as I said earlier I take the point about heated water being potentially harmful. My point is that even the cold water I drink spends time sitting in a tank and pipes.

    I've always used warm water to fill the kettle, partly for speed, partly to to make myself feel like I was getting some value out of the a2w system, and partly to make myself feel as if I'm saving the planet. Don't think I'll do it anymore after the stuff I've read since about drinking from the hot water tap.

    @Victor am I reading you correctly that you're saying 6c from cold and .3c from hot (obviously not regarding all the other expenses)?

  • Registered Users Posts: 78,312 ✭✭✭✭Victor

    @Fern Bench I've revised that calculation. The value or "1 J = 2.77778e-7 kWh" that I got on the internet doesn't seem to be right. 1 kWh =3600000 J.

    So 3 cents from cold for a litre of water (many kettles hold 1.5-2.5 litres, although 1 litre is 2-4 cups). Less for pre-heated water. The 6 cents was an example I was responding to.

  • Registered Users Posts: 193 ✭✭harderthanf

    I have a Thermos pump jug. I tend to boil the kettle once a day, in the morning and fill the jug. It keeps the water at almost perfect boiling temp for 8ish hours. If we run out during the day we just boil a full kettle again (trying to wait for some surplus!). The still hot water left the following morning goes back into the kettle.

    Apart from the small savings the benefit of having tea/coffee without waiting for a kettle to boil is great.

    Jug cost about 35 euro in Argos

  • Registered Users Posts: 6,194 ✭✭✭Ubbquittious

    Cost wise it wouldnt really be worth it. You might save a couple of quid a year if you are lucky but the kettle does boil faster if you fill it with hot water

  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 48,654 CMod ✭✭✭✭magicbastarder

    I'd suspect boiling the water kills a hell of a lot more than 99.99% of bacteria. 65C is the temp you're advised to bring water to, for 5 minutes, to kill bacteria.

  • Moderators, Science, Health & Environment Moderators Posts: 4,689 Mod ✭✭✭✭Tree

    Just to reiterate, killing bacteria does not equal "make safe for consumption". Safe for washing your hair, sure, but consumption is different. Certain types of bacteria have a toxin in their cell membranes that comes loose when they die, and can still cause illness. Others may release their toxins into the environment while alive, and again, may not necessarily break down by simply boiling in water.

  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 48,654 CMod ✭✭✭✭magicbastarder

    Not disagreeing with you there, but is that a worry in this context? I did some googling and the gist of it seemed to suggest that can be the case but generally it didn't seem too big a concern - but most of the info really talked about boiling completely untreated water, not boiling slightly stale water that had reasonably recently been treated.

  • Advertisement
  • Registered Users Posts: 13,283 ✭✭✭✭fits

    Genius idea. The sound of the kettle drives me mad in the evening when I’m watching to. This would get around that.

  • Registered Users Posts: 18,940 ✭✭✭✭Del2005

    A rolling boil is what they advise to purify water, a kettle won't do a rolling boil unless you are using a stove top kettle as electric kettles are designed to stop once they reach boiling temp. So the OP has the worse of all worlds tepid water in a kettle which they don't properly sterilise to save <10c for a full kettle of water, they'd be better off to boil a cup full of potable water than a kettle full of unpotable water.

  • Moderators, Category Moderators, Arts Moderators, Sports Moderators Posts: 48,654 CMod ✭✭✭✭magicbastarder

    oh, i agree it's barely worth the hassle, a 2kW kettle running for 5 mins probably costs about 5c in electricity.

    but i'd hesitate to call it 'the worst of all worlds'. reboiling what had been treated water is not exactly going to be dangerous in the grand scheme of things.