My tap is faster, but is my stove more efficient?
February 28, 2011 2:25 PM   Subscribe

Which wastes more energy, heating water at the tap, or on the stove?

My sink puts out really hot water, so when I want to boil water, I like to speed up the operation by running the water really hot, then putting the pot on the stove. Someone said it was a massive waste of energy that way, because the water was heated, then ran about 10 ft through a cold pipe (thus losing some of its original heat), then had to be heated again. So does it waste more energy heating the water to temperature X using the tap, or when I take unheated water from the tap, put it in a pot, and heat it using the electric coil on the stove? It always seems to take longer on the stove, but it does make sense to me that direct application of heat would be more efficient...and yet... those coils seem to waste a lot of heat into the room...to say nothing of all that water wasted running it hot at the tap... Can you help? I'd like to know.
posted by Ys to Science & Nature (21 answers total) 1 user marked this as a favorite
 
I don't think it matters either way: you should not be drinking hot water from the tap. Hot tap water dissolves contaminants in your pipes (read: lead, if you have lead piping) and they end up coming out of the tap into whatever you are drinking.
posted by Cat Pie Hurts at 2:32 PM on February 28, 2011 [9 favorites]


It's a moot point, because you shouldn't cook or drink hot tap water. The water sitting in your hot water tank is, in layman's terms, gross. The New York Times agrees.
posted by sdrawkcab at 2:33 PM on February 28, 2011 [3 favorites]


This question is complicated by the fact that your hot water heater is keeping a large volume of water hot 24/7, and if you don't use it, then that energy is essentially shot.

If you ignore that though, your friend is right. Heating it on the stove (in a covered pot) is almost certainly more efficient.
posted by chrisamiller at 2:34 PM on February 28, 2011


For even more efficiency, consider using an electric kettle. Lickety-split!
posted by bq at 2:37 PM on February 28, 2011 [2 favorites]


I will assume that you're boiling this water for non-consumption purposes so that the lead and other metallic content of hot tap water is not a relevant health concern.

I would disagree with chrisamiller. The advantage of the hot water heater is that the heating element is in very good contact with the water. (similar to the way plug-in electric kettles are fast and efficient ways to boil water) A water heater is, as might be expected, very well optimized for heating water.

(okay, though, I agree with chrisamiller's point about the fact that 30 gallons of water is being kept hot whether you use 1 gallon of it or not, so heating that one gallon is not as much change in energy output as the change between the stove being on or off.)
posted by aimedwander at 2:38 PM on February 28, 2011


I doubt it's much of a difference, assuming you heat no more than a few pots of water per day. If your sink produces unnecessarily hot water, though, you could probably save energy by turning down your boiler temperature.
posted by domnit at 2:39 PM on February 28, 2011


There is a book called How Bad Are Bananas? that goes over questions like this. It doesn't talk about hot water from the tap, probably because it's not very common:
"To summarise, kettles are better than saucepans, and gas [kettles] beats electric – but only if you are not in a hurry or you want to heat your room anyway. Just as important, of course, is not to boil more water than you actually need."
posted by acheekymonkey at 3:04 PM on February 28, 2011


The wasted energy in your case isn't in the the small volume of water that you are actually boiling, but in the water that goes down the drain while you wait for the water to run really hot. All the energy that went into heating up that water goes two places: 1) into the air in your house/basement from the pipes that it was sitting in and 2) down the drain. Any tap water that goes down the drain at any temperature above cold tap temperature is taking wasted energy with it into the sewer. So, yes, you are wasting a lot of energy.

The solution for fast boiling water is, as mentioned, an electric kettle.
posted by ssg at 3:06 PM on February 28, 2011


I'm constantly amazed that people boil kettle any other way than using a kettle (filled from the cold tap).
posted by turkeyphant at 3:11 PM on February 28, 2011


The stove top is a very wasteful way to warm water. It's estimated that you waste about 2/3rds of the heat.

Microwave heating wastes about 1/2 the energy.

Heating directly, electrically, wastes about 1/10th of the heat (or less).
posted by bonehead at 3:15 PM on February 28, 2011 [3 favorites]


The advantage of the hot water heater is that the heating element is in very good contact with the water. (similar to the way plug-in electric kettles are fast and efficient ways to boil water) A water heater is, as might be expected, very well optimized for heating water.


My thinking is probably being swayed right now by the fact that my bathroom is on the third floor and our water heater is in the basement. That's a long way for hot water to travel through some pretty darn cold pipes in this old building.
posted by chrisamiller at 3:28 PM on February 28, 2011


NEVER drink the hot water from the pipes. Your water heater disolves a lot of nasty things into it.
posted by majortom1981 at 4:05 PM on February 28, 2011 [1 favorite]



The stove top is a very wasteful way to warm water. It's estimated that you waste about 2/3rds of the heat.


That depends. The "wasted" heat here is wasted in the sense that it doesn't heat the water, but the reason it doesn't heat the water is that it's escaping into the surrounding air. In the wintertime, when you're heating your house anyhow, you're really not wasting that heat at all since the furnace won't have to run quite so long. In the dog days of summer, however, there's a lot of waste because heat escaping into the air (1) fails to heat the water, and (2) increases the amount of heat that your air conditioner has to remove to keep the room comfortable.
posted by jon1270 at 5:21 PM on February 28, 2011


The stove has to be better. The excess heat from the stove heats your house -- if you have a gas stove, it may be more efficient than your furnace. The water heater sits in a cold basement and is connected through very, very cold pipes (at least in my house).

I'm constantly amazed that people boil kettle any other way than using a kettle (filled from the cold tap).
Turkeyphant, in the US we only get 110V through wall outlets so it takes roughly twice as long for water to boil in an electric kettle as it does in the UK. Between that and our proliferation of drip coffee makers, most Americans have probably never seen an electric kettle.

posted by miyabo at 5:34 PM on February 28, 2011


Miyabo, those tests I linked above were conducted with 120/110v kettles. The majority of Canadian homes have electric kettles. They start at $10. The lack of kettles in the US market is mostly cultural, and has little to do with efficiency or utility.
posted by bonehead at 7:53 PM on February 28, 2011


Thanks all! The debate may not be settled, but I think I may look into an electric kettle --y'all have made some pretty convincing arguments for not drinking hot tap water.
posted by Ys at 8:18 PM on February 28, 2011 [1 favorite]


I'll nth the "don't drink your hot tap water" thing. I did this until I helped my dad pull out our old hot water service in the 90's. We cut it open and had a look in the bottom... Wow! Don't drink that shit. (Lots of blacky/browny - you don't want this on your hands, let alone in your body - gunk)
posted by deadwax at 9:07 PM on February 28, 2011


Late to the party here - but one other thing re hot water heaters - remember to drain a few gallons of water out of the bottom tap on the thing every six months or so.

It'll help remove the sediment that accumulates at the bottom. Too much sediment, and it cuts down severely on your efficiency if you're using a gas water heater, and can really shorten the life of the thing. Localized heat buildup can cause problems - crack the glass liner and water gets to metal, which leads to corrosion, which leads to leaks, which leads to "Oh, shit, turn off the water!" and a replacement water heater and associated frivolity, usually at the least convenient time.

However, if you HAVEN'T flushed your tank in, oh, a decade or so... it might be best to leave it as it is. Disturb the rust, and you'll likely get a leak. (He says, from sad experience...)

If you're using an electric it's not so critical, but you'll want to get the glop from the sacrificial anode (which combats corrosion inside the tank) off the bottom of the tank.
posted by JB71 at 6:57 AM on March 1, 2011


Forgot to add - run the water until it's clear out of the tap at the bottom of the tank. This might take 10 gallons, might take 2, depends on how long it's been since the last flush.

If you try to open the tap and get a trickle of rusty water that then stops... close the tap and think about getting a new water heater real soon. That one's a goner, it just hasn't started leaking YET, and you'll want to swap it out before it does.
posted by JB71 at 7:07 AM on March 1, 2011 [1 favorite]


These answers have assumed that it's not a tankless water heater, which might change things a bit.
posted by Chrysostom at 8:11 AM on March 1, 2011


I didn't see bonehead's link before, but I actually have the exact electric kettle in the picture in the link. It works great. I'm sure it's more efficient than stovetop heating, but it does take a little longer -- there's just no way a 1200 watt electric heater can compete with a 6000 watt gas flame.
posted by miyabo at 6:15 AM on March 2, 2011


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