Setback for hot water tank?

[ano]

SteveQ, how many degrees does the hot water drop when you typically turn it off the hot water?  If it is a rather short period of time, and I don't know where you live or where your hot water heater is located, you could be fine, but your savings seem to be totally associated to your rate schedule.  Three water heater replacements tells me that dropping the temps more than 5 or 10 degrees DOES cause water heater leaks from the constant temp changes.  Ideally you would want to preheat your water a bit hotter before your shutdown period, but that also means you can't use much water during that shutdown period either. 
 
You say you save $5-$10 a month?  I was having to replace my water heater every 1 to 2 years, and even with the warranty paying for the new water heaters, the labor charges would eat all the savings.

 

 

[purple_motion]

SteveQ, how many degrees does the hot water drop when you typically turn it off the hot water?  If it is a rather short period of time, and I don't know where you live or where your hot water heater is located, you could be fine, but your savings seem to be totally associated to your rate schedule.  Three water heater replacements tells me that dropping the temps more than 5 or 10 degrees DOES cause water heater leaks from the constant temp changes.  Ideally you would want to preheat your water a bit hotter before your shutdown period, but that also means you can't use much water during that shutdown period either. 

 
I don't quite understand this... isn't the temperature in the tank in flux quite a bit?  Here in Canada in the winter there is some very cold water (5-10 degC). If I draw out 20 gal of hot water, it is being replaced with this very cold water. The tank temperature will have dropped considerably (at least at the bottom of the tank). This is normal usage. I suppose it is uncommon that the top of the tank will have cold water (unless you use all the hot water).  Are you sure there is not some other factor at play causing the tank faliures?  BTW: I have an electric tank and it can take 40min to reheat the water after a shower... so the water is cooler than the setpoint for quite a while. 
 
I have considered turing off the tank during peak electrical hours but the times were adjusted to make it a non issue. i.e off-peak starts at 7pm. Kids have their baths around that time so it is pretty easy not to draw hot water during peak hours (no dishwasher, no washing machine).

 

[fcwilt]

Well if nothing changes other then the time the water heater is on (from a expensive time to a cheaper time) there would be a savings for sure. But to get a completely accurate picture would take some detailed information regards water usage, temp of water used, etc. It's not an simple problem. For example, it may be that extra electricity is required to restore the heat to the water after being off but the cheaper electricity still allows for a savings. A full analysis would take some work but the results could be quite interesting.
 
I avoided the whole issue by installing tankless water heater. <g>

 

[apostolakisl]

I don't quite understand this... isn't the temperature in the tank in flux quite a bit?  Here in Canada in the winter there is some very cold water (5-10 degC). If I draw out 20 gal of hot water, it is being replaced with this very cold water. The tank temperature will have dropped considerably (at least at the bottom of the tank). This is normal usage. I suppose it is uncommon that the top of the tank will have cold water (unless you use all the hot water).  Are you sure there is not some other factor at play causing the tank faliures?  BTW: I have an electric tank and it can take 40min to reheat the water after a shower... so the water is cooler than the setpoint for quite a while. 
 
I have considered turing off the tank during peak electrical hours but the times were adjusted to make it a non issue. i.e off-peak starts at 7pm. Kids have their baths around that time so it is pretty easy not to draw hot water during peak hours (no dishwasher, no washing machine).

 
 
I also think this is the case.  If the tank cools off during a shut down, it is very slowly and evenly cooling down.  When it cools off from usage, it can be quite harsh with water as cold as 40 degrees suddenly coming in contact with parts of the tank that had a second earlier been 120 to 140 degrees.  It seems to me that this sort of hit would make the 2 or 3 degree drop from a passive shut down of the tank be irrelevant.
 
Regarding tankless.  In hind site, I wish I had used the heat pump units in my house, instead of the tankless ones.  I have heat pump in my office and find it to be better for several reasons.  1) It costs about the same to operate as compared to propane.  2) It cost wayyyyyyyyyyy less money to install (when you figure that I had to bury a propane tank in my yard, and do all that propane plumbing work), and lastly, hot water recirc pumps are very tricky with on-demand heaters, and I really like the instant hot water I have at the faucets in my office.  The only thing I really like about the tankless, is that they take up basically no space (mounted in the stud space).  If I had NG available, I might say different.  Also, if I lived in a cold climate where I wouldn't want the cold/dehumidified air blowing into my house and had to deal with 40 degree incoming water, I might also think different.

 

[oberkc]

For example, it may be that extra electricity is required to restore the heat to the water after being off but the cheaper electricity still allows for a savings.

Count me among the group that believe this to be false.  I don't think it takes any EXTRA energy to retore heat than to maintain heat.  If anything, I believe it to be less, due to reduced energy losses from the cooler tank.

 

[apostolakisl]

Count me among the group that believe this to be false.  I don't think it takes any EXTRA energy to retore heat than to maintain heat.  If anything, I believe it to be less, due to reduced energy losses from the cooler tank.

 
Those who have actually measured these things state it is almost exactly the same when you shut down the hot water heater for short periods of time when no hot water is being used, like 12 hours or less.
 
This makes perfect sense.  The rate of energy loss is proportional to the delta between the water temp and the ambient air temp around it.  A cooler tank looses less heat, but hot water tanks that aren't being used take a long time to cool off, so shutting a tank off for 8 hours hardly changes the delta since the temp hardly changes.  Water holds a lot of energy (high specific heat), so a good amount of energy has to escape before the temp changes very much.  Between that and the pretty descent insulation they have, it is a slow process for a tank to cool just sitting there.
 
The heating element will be slightly more efficient at delivering energy to the water when it is cold, again, a delta between the temp of the element and temp of the water.  The energy transfer is greater when the water is colder.
 
But in the end, it takes so long for a tank to cool, that none of this is worth didley unless you are shutting the tank off for days at a time.

 

[oberkc]

Those who have actually measured these things state it is almost exactly the same when you shut down the hot water heater for short periods of time when no hot water is being used, like 12 hours or less.

Agreed.  This is, as you point out, consistent with the physics lessons we were all taught. 
 
The point is that some believe it takes MORE energy to reheat the water after a shutdown than it takes to maintain the energy level over that same period.  I find this to be inconcsistent with my understanding of thermodynamics.  I suppose one could argue that it is less efficient to run the heating elements for long periods of time rather than short, but I doubt it.

 

[fcwilt]

In a perfect system it would NOT take more energy to reheat the water and I am not saying it does.
 
I am saying that it is possible that a given water heater design (not that any such exist or ever will exist) might make maintaining temp more economical then raising temp. Imagine a system that had one mode of heating that was great at raising water temp quickly and another that excelled at maintaining temp and the later was much more economical then the former. So the laws of physics always apply but that actual system has characteristics that determine its real world behavior.
 
Also, if a tank is so well insulated that the temp drop over, let's say, 8 hours is negligble, so little that the heating element never comes on, then shutting off the tank during this interval is not going to save much money.
 
So it may well be that with proper design and the right circumstances turning the heater off may save money.

 

[fcwilt]

Regarding tankless.  In hind site, I wish I had used the heat pump units in my house, instead of the tankless ones.

 
I had not heard of these. Do they supply hot water or demand like a tankless?

 

[oberkc]

Frederick C. Wilt, on 04 Apr 2013 - 15:05, said:
In a perfect system it would NOT take more energy to reheat the water and I am not saying it does.

I am saying that it is possible that a given water heater design (not that any such exist or ever will exist) might make maintaining temp more economical then raising temp. Imagine a system that had one mode of heating that was great at raising water temp quickly and another that excelled at maintaining temp and the later was much more economical then the former. So the laws of physics always apply but that actual system has characteristics that determine its real world behavior.

Also, if a tank is so well insulated that the temp drop over, let's say, 8 hours is negligble, so little that the heating element never comes on, then shutting off the tank during this interval is not going to save much money.

So it may well be that with proper design and the right circumstances turning the heater off may save money.

I can see where a "dual fuel" system (similar to my house heatpump/gas) might be more economical to maintain rather than restore. (And, yes, they do exist now that you bring it up. I saw one at Sears not long ago. It appeared to be an interesting design, but I could not help but wonder...given that the heat pump part of the water heater used conditioned air as an energy source, is this not simply a case of transfering part of the water-heating burden to the house HVAC system?)

Unfortunately, I have heard so many suggest that (all other things being equal) it takes more energy to restore rather than maintain, I thought I would chime in.

 

[fcwilt]

Unfortunately, I have heard so many suggest that (all other things being equal) it takes more energy to restore rather than maintain, I thought I would chime in.

 
Well at first glance that makes perfect "sense" doesn't it. How much energy does it take to raise 100 degree water to 100 degrees? Zero right? How much energy does it take to raise 50 degree water to 100 degrees? More then zero.  <g>

 

[apostolakisl]

In a perfect system it would NOT take more energy to reheat the water and I am not saying it does.
 
I am saying that it is possible that a given water heater design (not that any such exist or ever will exist) might make maintaining temp more economical then raising temp. Imagine a system that had one mode of heating that was great at raising water temp quickly and another that excelled at maintaining temp and the later was much more economical then the former. So the laws of physics always apply but that actual system has characteristics that determine its real world behavior.
 
Also, if a tank is so well insulated that the temp drop over, let's say, 8 hours is negligble, so little that the heating element never comes on, then shutting off the tank during this interval is not going to save much money.
 
So it may well be that with proper design and the right circumstances turning the heater off may save money.

 
Excellent point Fred (can I call you Fred?)
 
In fact, this very thing does exist, I own one, it is a heat pump hot water heater.  If they get behind, a regular heating element kicks in which is way less efficient than the heat pump (1/2 to 1/3).  The heat pump is slower at delivering heat to the water, so they are programmed to kick on the regular element when they start falling behind.  Technically not dual "fuel" but dual systems of using the same fuel with different efficiency.  You do also have to realize that these units are getting the heat from the air in your house, so it is particularly good during the AC season, but is working against you in the heating season.  
 
Just saw your next question:
 
Heat pump units look pretty much like regular hot water heaters.  They come in 50 gallon, 80 gallon, and other sizes.  They have a compressor on top that works like a fridge in reverse.  They sell them at Lowe's for about $1000.  I think you get a $300 tax credit now, but I'm not sure on that, maybe that was my local thing.

 

[fcwilt]

Hello Lou,
 
I go by Frederick so if you don't mind that's what I would prefer.
 
We had a situation where tankless was ideal, a 8 bedroom home that during the summer is typically full (extended family) but during the winter is usually just my wife and me. The tankless insured that we had hot water for all during the summer.

 

[apostolakisl]

I came across some hot water storage tanks on the web.  These are not heaters, just storage tanks.  They advertise the heat loss of less than 1/2 degree per hour.  So if you shut off your tank for 8 hours, that is only 4 degree drop at most.  Probably not enough to notice.  I think this is quite consistent with people who report monitoring their hot water heater over 8 hour spells of non-use and find that it doesn't come on.

 

[SteveQ]

Ano posted, "SteveQ, how many degrees does the hot water drop when you typically turn it off the hot water? If it is a rather short period of time, and I don't know where you live or where your hot water heater is located, you could be fine, but your savings seem to be totally associated to your rate schedule. Three water heater replacements tells me that dropping the temps more than 5 or 10 degrees DOES cause water heater leaks from the constant temp changes. Ideally you would want to preheat your water a bit hotter before your shutdown period, but that also means you can't use much water during that shutdown period either.

You say you save $5-$10 a month? I was having to replace my water heater every 1 to 2 years, and even with the warranty paying for the new water heaters, the labor charges would eat all the savings."
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I do not have any temperature sensors in my water heater. I researched this several years ago and I did not find any practical way of getting a temperature sensor inside a water heater. If you have done this, I would be interested in knowing how.

I can tell you that after being off all night, my water heater runs for about 20 minutes when it comes on at 7:00 AM. My water heater is in the garage. This time of year the average temperature in the garage is around 45 degrees.

My current water heater is 10 years old. I replaced one of the heating elements 3 years ago. I do not believe that thermal shock is causing your water heater to fail. I believe water heaters are designed to handle the thermal expansion associated with heating water from room temperature to their maximum set point. If you have teenagers in your house, you know that a 20 minute shower will create more thermal shock to the water heater ( and to you if you want to shower after them) than the gradual cooling that occurs when you turn off the water heater!

Steve Q