# Whats more economical, filling pot with hot water, or cold, to boil water?

1. May 19, 2012

### hxtasy

I've always wished i could figure out how to calculate this. But don't know where to start.

scenario: you have to boil a liter of water. if you fill it with cold water from your faucet, it will bill in X amount of time.

if you fill the water with hot water from the faucet, it will boil quicker.

but to fill the pot with hot water, you will be using heated water from your water heater (pretend water heater and hob are both fueled by natural gas). So I am wondering, if the hot water boils say 4 minutes faster, thats four minutes of time that your gas burner on the hob isn't running, but how much energy is going to be used to replace that hot water you used from the water heater?

which method would be cheaper? Im not sure this is even worth calculating, of course its not, but it's always bothered me!

what would be some other pros/cons from using hot water or cold water, for cooking purposes?

2. May 19, 2012

### Staff: Mentor

Hot water from the water heater will almost certainly be cheaper, since a lot less of the combustion heat will be absorbed by the pot than by the water heater.

3. May 19, 2012

### Andre

But then again, if the distance between faucet and water cooker is substantial, then a lot of water is wasted before hot water comes out and the pipes are left with hot water to cool without a purpose.

4. May 19, 2012

### hxtasy

Well those are two good points i haven't thought of. a Pot is designed for cooking, not for heating water so the water heater is definitely more efficient at heating water.

Also, in this scenario, or at least mine, i would let the tap water run until it became hot.

so far it seems like, if you are already using hot water, that it would be more efficient to fill the pot with hot tap water.

5. May 19, 2012

### KiwiKid

Theoretical considerations are fine, but this seems easy enough to test in a specific setup.
Boil a hundred pots of water with both methods, checking your electricty (if you use an electric water heater) and gas meter after using each method. The downside to this is having other appliances using gas or electricity at the same time complicates thing. Oh, and boiling so much water takes quite a bit of time.

6. May 20, 2012

### Ivan Seeking

Staff Emeritus
The correct answer is... that's the wrong question.

Anecdotally at least, I have heard quite a bit about the potential hazards of using water from the water heater for drinking or food preparation. I grabbed the first related link that I saw.

7. May 20, 2012

### Jack21222

I don't think the opinion of one engineer meets the peer-review standards most of us would be interested in.

8. May 20, 2012

### Ivan Seeking

Staff Emeritus
I didn't think giving one easy reference suggested that as an information limit. Are you suggesting it isn't logical and therefore not worth further investigation?

9. May 20, 2012

### D H

Staff Emeritus
Only Use Cold Water for Consumption
Use only water from the cold-water tap for drinking, cooking, and especially for making baby formula. Hot water is likely to contain higher levels of lead.The two actions recommended above are very important to the health of your family. They will probably be effective in reducing lead levels because most of the lead in household water usually comes from the plumbing in your house, not from the local water supply.

10. May 20, 2012

### ViewsofMars

My house has copper piping so I'm safe to use the hot water coming out of the kitchen sink when I cook.

11. May 20, 2012

### D H

Staff Emeritus
How old is your house? Lead-based solder is perfect for joining two pieces of copper pipe.

12. May 20, 2012

### ViewsofMars

Aitimony solder was used on the copper pipes. It doesn't contain lead. I don't live in a home with lead joints. Yes, it is my house that I own. I also have a hot water dispenser attached to my kitchen sink. It's especially nice when I want a cup of hot tea.

Thanks. I must admit that I'm exhausted today from a late night adventure. lol I think I'll have a cup of coffee fresh from the pot.

Last edited: May 20, 2012
13. May 20, 2012

### edward

When I was making large batches of humming bird nectar (1gallon) at a time I discovered a way to speed up the old "watched pot never boils" scenario.

I would preheat filtered tap water one quart at a time in the microwave then pour it into the pot on the stove. As soon as I poured the first quart into the pot I would set the burner on high.

I was going for doing it in the least possible time and it really worked out. While water was heating I would be measuring the sugar. I could boil mix and be done in less that 15 minutes.

Edit I didn't do any energy calculations as I was going for time. A microwave oven will definitely heat water faster than an electric stove. I also had the advantage of heating water in two places at the same time.

Last edited: May 20, 2012
14. May 21, 2012

### Monique

Staff Emeritus
The OP forgot an option, I always boil water in my electric water boiler before putting it into a pan. Definitely a lot quicker than boiling it on a stove and a lot hotter/cleaner than warm water from the faucet.

15. May 21, 2012

### Andre

And as a bonus, Monique, your veggies like beans and broccolli keep their fresh green color. When those are heated slowly they turn into some dull olive green.

16. May 21, 2012

### Monique

Staff Emeritus
Yes. The exception being root vegetables such as potatoes, which need to be placed into cold water and brought up to a boil slowly.. but that's more relevant for the food thread

17. May 21, 2012

### alt

Only Use Cold Water for Consumption

I strongly support this. Two different plumbers have told me the same. Not because of contamination from the pipes, because you'd get that in hot or cold tap water, but because of contamination from the heater itself - particularly when water is very hot in it.

I can actually taste the difference, ie, tea made from water from cold tap then boiled, as opposed to tea made from hot tap then boiled.

18. May 21, 2012

### Andre

Oh dear, I have this built in water cooker used for instant soup, green broccoli and tea. Am I doomed now?

Last edited: May 21, 2012
19. May 21, 2012

### Pkruse

Several months ago I read a related article in a medical publication in my doctor's waiting room. Infection rates had gone up in a hospital. They traced it to bacteria in their wash water. More tests followed. The root cause were the new water saver devises they had installed, to automatically turn the water off when your hands were not under the tap. So they went back to the old valves that let the water flow the whole time the staff took to wash their hands, which is several minutes if done properly. That solved the problem by keeping the hot water system flushed out.

I've got 3 adults using one 30 gallon heater, and nobody thinks much about saving water. That keeps it flushed out, clean, and safe.

None of this matters in the current case because he is boiling the water anyway. That will kill anything growing in it. Metals in the water are only a concern with very old plumbing systems. Even those are of limited concern in the system sees enough flow to keep it flushed out.

As for the original question as to which is quicker, it could work out either way for the reasons already mentioned. But the difference will be too small to worry about.

20. May 21, 2012

### alt

Probably not - not because of that, anyway.

I was more referring to large water heaters that heat, then keep stored a high volume - 275 litre to 415 litre ones are not uncommon round my way.