Assume that my water is used only for showering (i.e disregard dishwashers and washing machines).
Assume that my gas water heater is very very well insulated (it is!).
Question: is it more efficient for me to heat the water to a high temperature like 150 degrees F and, in the shower, use a lot of cold water to bring it down to showering temp (115?) or should I heat to 115 and use no cold water?
With the 150 degree water I would use less heated water but I would have used more gas to heat it. At 115 I would use a lot more heated water but it would have been heated to a lower temperature.
MrFloppy, consider that the heat energy content of the water going down the drain is going to be the same whichever way you do it. However, if you set your water heater to 105 °F and don’t run any cold water in the shower, your water heater and hot water pipes will waste only the amount of heat they can throw out at 105 °F. If you set your water heater to 211 °F and only use a little of it, your hot water heater and hot water pipes will heat part of your house along the way.
You could probably get a mathematical proof to back up kanicbird’s kind and correct offering if you paid a mathematician to do it.
I cranked up my heater temperature a few years back so that I could obtain a longer shower before it goes cold (by mixing with alot of cold thereby conserving more hot). But I am fairly certain that this is more expensive due to the fact that 23 3/4 hours out of 24, your hot water heater is loosing heat no matter how well insulated it is. I have no proof other than what I hear from plumbing experts on shows such as This Old House or HomeTime.
As an aside, I had a friend over one day who decided to wash his dish, which was nice of him. He turned on the hot water started to do the dish until he started burning his hand. He said “Wow, you guys have really hot water in this city.”
Heat transfer is directly related to the temperature difference (Delta T).
So the heat loss (Q) per unit of time for a system is Q=C*Delta T, with C being some constant. C is independent of Delta T, so the difference in heat loss for a system at two different values of Delta T is proportional:
Q1/Q2 = Delta T1/Delta T2.
If the ambient temperature is 70 degrees, then Delta T1 = 115F-70F = 45F. Delta T2 is 150F-70F = 80F. The water heater holding 150 degree water is losing almost twice as much heat as the same water heater holding 115 degree water.
However, for standard gas fired water heaters, there should be a minimum temperature that you shouldn’t go below, in order to keep the flue gases hot enough to vent without condensing. 115 degrees sounds like it might be a little low. A high efficiency heater can probably handle the condensation.
If you have an automatic dishwasher it may require water hotter than 115 degrees. Most of the ones I have owned require water at least 140 degrees F. Your dishwasher may have an integral heater, but it may not.
Andy Padian, who is the leading authority on how to reduce energy use in old urban apartment buildings, recommends 120. He also emphasizes that 137 will burn your skin.
Let’s pretend that my water heater is *extremely * well insulated (it’s a super-duper model that incorporates vacuum insulation). With such a water heater, maintaining a static temperature of 200 °F won’t require much more energy than maintaining a static temperature of 140 °F, right?
Assuming this is the case, setting the water at 200 °F will require less hot water to be drained from the tank for given desired temperature at the faucet. Less water drained from the tank = less cold water to heat = less energy being used.
However… there’s the issue of *dynamic * temperature changes in the water heater, i.e. it takes more energy to heat cold water to 200 °F vs. 140 °F.
So… if I had a water heater that had perfect insulation, and if the pipes were perfectly insulated, would I use the same amount of energy regardless of what temperature I set it to? In other words, would the amount of hot water to heat be a perfect inverse relationship w/ set point temperature, such that the effects cancel each other?
Does it? Do you have any before and after numbers for how much you saved? I am curious not dising you I would like to know what kind of savings can be had.
In a perfect world with no losses at all, even in the pipes, I’m not sure, but I think it would still be better to leave it at the lowest temperature as a gas flame can only get so hot, and besides higher temperatures wear out the water heater sooner.
At the very least you would have a transient loss, for when you dispose of this tank you will be throwing away very hot water instead of sort of hot water.
>So… if I had a water heater that had perfect insulation, and if the pipes were perfectly insulated, would I use the same amount of energy regardless of what temperature I set it to?
As usual, Crafter_Man, you’re on to something. We also have to add that the flue gasses leaving the water heater would be at the same temperature, implying that they lose heat equally well to water at all temperatures, but then the energy would be the same. All the relationships would be a little nonlinear because the specific heat of water isn’t quite constant at all temperatures, but it would work out as you suspect.