Pablo - nice subject... I've been wondering this every day when I make my tea. The thing is, it seems there's a lot of subjectivity in the matter based on the nature of your tap.

My tap is blazing hot within seconds so I always fill the pot with tap water, then put it on the stove for the final heating. I'm fairly sure this saves gas.

Of course, I've got nothing to do with paying for the original heating, but as it's a large building (30+ units) I suspect that it's fairly efficient.

or even better,

one of these.

A "Hot Shot" from Sunbeam. Heats water to hot within 2 minutes. Perfect for a cup of tea, bowl of oatmeal, or to make french-press coffee.

Much more efficient than the stove or running the tap, as more of the energy goes directly towards heating the water than in the open systems of the hot water heater/pipes, or stove.

p.s. I don't work for Sunbeam, but do use this product several times a day.

Good point Steve, I am actually considering doing an comparison between a tea kettle, a microwave, and an electric kettle in the next few weeks. Stay tuned...

My hot water pipes have insulation around them from part of the trip. Not sure if helps.

Now here's one. What takes more energy to heat up(boil?), cold water or hot water? I've heard cold water, but I have no idea if that's true. Suppose I could google...

Ah It's false

Cold water boils faster than hot water. If hot water freezes faster, maybe cold water boils faster! Again, this defies common sense—and again, say scientists, it’s simply wrong. Hot water from the tap should in fact boil much faster than cold water. However, using hot water for boiling does not actually save any energy. You may use less gas (or electricity) on the stovetop, but your water heater will have used the same amount of energy to heat the water in the first place. (If you use solar energy to heat your water, of course, that’s a different story.) Some water heaters may introduce additional sediment into the water, giving you another reason to consider starting with cold—at least, if time is not of the essence.

Since this came up in the context of making tea, it should be noted that it is not advisable to consume water from a standard hot water heater. Anyone interested in doing so should remove a heating element and take a tour of the inside of their tank if they have one. Legionnaires disease is another risk to this practice. I have yet to take a look into tankless water heaters though so if anyone has info there please weigh in.

LOL what? Sounds like rubbish to me. Are you talking about build up?

Yes. Toxic metals, bacteria, and other contaminants are concentrated in a hot water heater tank. The hot water also more readily leeches the lead from the pipe solder if you have copper pipes between your heater and your faucet. Google it ;)

I would like to know if it's more efficient to boil water in my electric kettle (I am on a 100% green electricity tariff) or boil water in a kettle on a LPG stove...

Pipe solder does not contain lead. And hot water heaters draw water from the top, not the bottom.

I once saw the sludge in the bottom of a hot water tank that was being changed for a new one. Gross! Best to get your water as fresh as possible from the cold tap.

Lead-free soldier is a more recent standard. Older pipes would have solder joints containing lead.

Hot water heaters have water forced out of them, not really drawn per se. This is due to the pressure differential being fed on the supply side that points pressurized water straight at the bottom of the tank forcing water out the other pipe due to an opening down stream. The water swirls in the tank quite readily due to the turbulence of water coming in and going out at significant pressure stirring up those nasty contaminants.

On the topic of heating water, I wonder how a microwave factors in on the efficiency side.

The sludge in the bottom of a water heater is pretty disgusting. It's recommended that you drain some of it off, using the tap at the bottom, to improve the efficiency of the water heater. But maybe it's a good way to improve the safety of the water as well.

Where do you think the the "sludge " in the bottom of the hot water tank came from.... the water of course. So would you rather drink water that had the sludge still in it or the water that has (at least some) of the sludge removed via the hot water heater.

As for cold water boiling faster than hot...I don't think so. Conversely, there is speculation that "hot tap" water may freeze faster than "cold tap" water. The reasoning is that "cold tap" water has entrained air and/or dissolved air. Whereas water that has gone through the hot water heater has most of the air knocked out. The air acts as insulation to slow the cooling process. I do not ascribe to this theory, but I do not know if it has been empirically tested. However, if you use hot water in your ice tray, you will have very clear cubes (no air bubbles). Use cold water and have cloudy cubes (air bubbles).

Hot Water Heater? Why would you want to heat your hot water?

I have a friend who turns on the heater of the HW cylinder (electric) for hot water use once a week. He claims that heating from scratch once a week is more efficient than holding the water at hw temperature for a week. This is counter to my common belief that, for such a short time, turning off and reheating does not break even with holding the heat in the modern well insulated cylinder.
Any general opinion on this please. Also calculation formulas for proof!

Is it cheaper to heat a kettle of cold water on my kitchenaid range large burner or use an electric kettle. Which unit uses less energy? I do not have an electric kettle and am thinking about purchasing one.

Is it cheaper to heat a kettle of cold water on my kitchenaid range large burner or use an electric kettle. Which unit uses less energy? I do not have an electric kettle and am thinking about purchasing one.