Better to leave the hot water heater running 24/7, or switch on/off on demand?

[bobdole369]

Supposing you don't care about the time factor, say you have a timer that turns on the water heater an hour before you wake up, and turns it off an hour after you go to work, then brings it back on an hour before you get home, and then off about the time you go to sleep. So your heater would be on say 5-8a, then again 5-10p. So it would have its power cut for 16 hours a day.

Has anybody done the power calculations to see if this saves an appreciable amount on the electric bill? You would use a significant amount more power for a short time when the water heated up, and I suppose it all depends on the insulation factor of your water heater.

I know that we save about 30% over last year with a new programmable t-stat that lets me set the temp to 88 for a number of hours during the day, and to make it a couple degrees warmer at night.

 

[DesiPower]

Capacity? weather? country?

 

[bobdole369]

Generally speaking. Temperate climate, average insulation - controlled internal air temperature, nothing unusual happening.

I suppose capacity of the tank plays a huge part and the insulation amount is a big part too and those are 2 unknowns.

Just looking to see if anyone has done it and had noticed a big drop on the power bill.

 

[dainthomas]

I don't know, it would probably have to run constantly for an hour just to heat the water back up. Would probably negate any savings.

 

[Duder1no]

just get one of those new small water heaters that turn on automatically only when you open the hot water and turns off once you close it

i have one of these and they work great and are very economic gas-wise

 

[Capt Caveman]

I don't know about electric but my monthly gas bill for my hot water heater tank probably runs about $15 a month. If I were to turn it down or off, I might save $50 a year if I'm lucky? Not worth it imho. If I were going on vacation or away for multiple days, then maybe.

 

[Gooberlx2]

http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12980
http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=13110

It depends largely on the heater/tank combination to determine how well a timer would work. If your tank is well insulated and the heater is super efficient, then it could be a great strategy.

If you have crappy tank that leaks heat like crazy and a POS heater, you might spend more $$$ and energy heaving to re-heat cold water twice a day, and only ever get lukewarm showers.

 

[AMDZen]

24/7 is better.

If you have it off while you're at work, it gets cold. And thus it requires a lot more energy to heat back up again.

 

[BudAshes]

just get one of those new small water heaters that turn on automatically only when you open the hot water and turns off once you close it

i have one of these and they work great and are very economic gas-wise

They are rather unpractical for home use i've found. Or maybe the people i know just have shitty ones.

 

[DesiPower]

Generally speaking. Temperate climate, average insulation - controlled internal air temperature, nothing unusual happening.

I suppose capacity of the tank plays a huge part and the insulation amount is a big part too and those are 2 unknowns.

Just looking to see if anyone has done it and had noticed a big drop on the power bill.

I asked about country coz of the electricity bill vs average income. If u r in US then the difference will be negligible.

 

[dullard]

We are in such as sad state of our knowledge about heat transfer. Case in point:

If you have it off while you're at work, it gets cold. And thus it requires a lot more energy to heat back up again.

and

I don't know, it would probably have to run constantly for an hour just to heat the water back up. Would probably negate any savings.

The amount of energy you have to put in is proportional to the temperature difference between the room and the water setpoint. As the water temperature drops, you have to put in less and less energy. That is it; basically all other energy is energy you'd use anyways. So, you don't have to consider it. That energy heating it back up is less than the energy you would have used to maintain it at a hot temperature.

The real answer though, is unless your water heater is really crappy, the savings are almost nothing. In the summer I only use the water heater for gas. My summer monthly bills are $15 or less for gas. So, that means my water heater uses $15 or less a month and it is 13 years old without any maintanence ever done and little insulation. Even if your scheme saves me 50% (which it won't), that is less than $100 a year. Hardly worth the effort let alone the costs of installing the timer.

Instead, if you want savings, turn the temperature down a couple of degrees and put on more insulation if you want. Notice those copper pipes without any insulation at all on the hot water side? Copper is a great way to lose heat. A little insulation there will save money AND let you turn on your faucet to find hot water was still in the line (reducing that long wait period to get hot water). That will be 24/7 savings for life.

 

[KGB]

All it will take is one cold shower and you'll know what to do.

 

[kami333]

They are rather unpractical for home use i've found. Or maybe the people i know just have shitty ones.

The later.

Were the houses retrofitted to tankless?

 

[OutHouse]

Hot water heater?? why are you heating hot water?

 

[BurnItDwn]

Hot water heater?? why are you heating hot water?

To make it hotter

 

[LTC8K6]

The water heater doesn't run 24/7. It's controlled by thermostats.

 

[mshan]

I would be concerned that for standard tank water heater, constant on off cycles with large changes in temperature, might damage the glass lining and lead to earlier failure than designed.

I don't know if the concern is grounded in fact, but I would assume tank water heater was designed to be left on 24/7 at same temperature setting constantly.

 

[ElFenix]

gas heaters aren't constantly on, are electric?

 

[sao123]

why not just get tankless hot water? mine is great.

 

[DrPizza]

I would be concerned that for standard tank water heater, constant on off cycles with large changes in temperature, might damage the glass lining and lead to earlier failure than designed.

I don't know if the concern is grounded in fact, but I would assume tank water heater was designed to be left on 24/7 at same temperature setting constantly.

The change in temperature is relatively trivial compared to what those materials can withstand.

 

[bobdole369]

Great thread folks. Good info had. Silly me, I also eat tuna fish, typing in my PIN number at the ATM machine. Of course its a "water heater".

The real answer though, is unless your water heater is really crappy, the savings are almost nothing.

Exactly what I was looking for.

And yes I know the heater isn't constantly on. I don't get a tankless because I don't own this place (apt dweller).

The savings are dramatic with turning off the air during the day though. Nearly 30% last month over the month before, and its been a miserable late spring in sofla.

This says it all though:

All it will take is one cold shower and you'll know what to do.

Guaranteed the first time my wife can't get a hot shower and out the window this crazy scheme will go.

 

[Jeff7]

Adding to what dullard said about heat transfer:
High differences in temperature provide "motivation" for energy transfer, in the form of heat. So if you've got an object that's 1° hotter than its surroundings, it will lose energy much more slowly than an object that's 100° hotter than its surroundings.

Translate that to the water heater: If you're maintaining the water at a high temperature all the time, that means you've got a high temperature differential all the time.
That's the key part. This means that every second that the water spends lower than that high temperature means less energy is being lost to the cooler surroundings. That's why it's advised to lower your thermostat setting in the winter. If your house's internal temperature is closer to the outside temperature, less energy will be lost, and it therefore requires less energy to maintain that lower temperature. (Extend this example then to assume that the inside of your house is the same temperature as it is outside. No temperature differential, no energy transfer, no heating bills.)

Yes, you'll have to run the heater longer in order to get the water back up to that higher temperature. But you've also spent the whole period of time previous to that gaining "lost" energy by having the lower temperature differential - a period of time during which the heat source would have been going to make up for the energy losses incurred due to the high temperature differential.

That all said, repeating what dullard and others have said, water heaters are generally made to be well-insulated, so the loss of energy is low to begin with. You can go to a lot of trouble to micromanage your water heater, and you can save money. Then you can spend that money you saved on some statistical analysis software, so that you can verify that you actually did save any money at all.



For the electrical view of things, you've got Ohm's Law in heat transfer.
V = I R
Voltage differential = current * electrical resistance

ΔT = Q R
Temperature differential = heat * thermal resistance

They can even teach heat transfer as a resistance network, right down to series circuits (convection of water in a tank heats the wall of the tank, which then conducts the energy to the outer surface), and parallel circuits (a piece of metal that can both conduct and convect heat away).

It's just some difference in potential, which is equivalent to the flow of energy multiplied by the resistance through which it must pass.