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Old 07-04-2017, 12:21 AM   #1
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Install Timer on Hot Water switch (DC side)

Im about to install a timer on the hot water switch the DC/Gas side not mains. The timer can handle 12v @ 8 amps.

I suppose the 12v is only for starting the gas furnace. And the switch cant be more than 10amps.

Does anyone know what amount of current that flows through the switch?
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Old 07-04-2017, 12:59 AM   #2
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Some googling came up with 0.17A when idle and 0.70A when heating for the fairly common Suburban water heaters.
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Old 07-04-2017, 03:38 AM   #3
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Cheers.... thought it would have been minor but better to be safe than have a fire. Thank you
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Old 07-04-2017, 12:53 PM   #4
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Timer is a good idea. Ours gets left on all the time. Waste of propane.
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Old 07-05-2017, 12:32 PM   #5
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Im about to install a timer on the hot water switch the DC/Gas side not mains. The timer can handle 12v @ 8 amps.

I suppose the 12v is only for starting the gas furnace. And the switch cant be more than 10amps.

Does anyone know what amount of current that flows through the switch?
I have been thinking about this as well.

What switch are you using? Where to obtain, if possible.

Pictures please of install.

Thanks in advance for your response.

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Old 07-05-2017, 12:40 PM   #6
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I'm trying to understand the reasoning for this. Are you just wanting the water heater to only fire up at a certain time and thus not keep the water temp heated at other times?

You may need to do a test and see if it takes more propane to heat the water from a cooled down temp, than it does to just let the thermostat do it's function and keep the temp maintained in the tank as needed.
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Old 07-05-2017, 01:01 PM   #7
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...You may need to do a test and see if it takes more propane to heat the water from a cooled down temp, than it does to just let the thermostat do it's function and keep the temp maintained in the tank as needed.
I too wonder but I guess that not running it all night will save some propane. It will always save energy to let it cool down since the "cooling" is merely the head escaping to the environment through the insulation. The lower the water temperature, the less energy escapes to the environment as that is the only energy consumption possible...as long as you don't use hot water and replace it with cold.
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Old 07-05-2017, 01:21 PM   #8
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I'm trying to understand the reasoning for this. Are you just wanting the water heater to only fire up at a certain time and thus not keep the water temp heated at other times?

You may need to do a test and see if it takes more propane to heat the water from a cooled down temp, than it does to just let the thermostat do it's function and keep the temp maintained in the tank as needed.


For us, switch on when needed and the switch would turn itself off at a preset or set amount of time..............

When camping DW and I do not need hot water all that much. Only need it every few days or so. We turn it on when needed and forget to turn it off. It will fire up to maintain hot level when not needed, for an extended time, for us. It will always fire up in the middle of the night and wake me as it is right under our bed in the underbelly.

Would be nice to just push the button or flip the switch and not have to remember to turn it off, it would just turn off by itself.

Others might find this not necessary but I would prefer this convenience.

Don't know or really feel like doing the "tests" to verify/confirm the actual savings, or not, as we do not use that much hot water. If we did I might be more concerned about the potential "waste" or savings of turning off/on only when needed.

These RV water heaters are not very efficient, so they do use a lot of energy to maintain heat level.

Hope that helps with our reasoning.

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Old 07-05-2017, 02:54 PM   #9
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We turn it on when needed and forget to turn it off.

It will always fire up in the middle of the night and wake me as it is right under our bed in the underbelly.

Would be nice to just push the button or flip the switch and not have to remember to turn it off, it would just turn off by itself.
I do understand this aspect of it. Thanks for the clarification.
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Old 07-06-2017, 10:47 AM   #10
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It will always save energy to let it cool down since the "cooling" is merely the head escaping to the environment through the insulation. The lower the water temperature, the less energy escapes to the environment as that is the only energy consumption possible...as long as you don't use hot water and replace it with cold.
That is true. Allowing it to cool and then reheating it uses less energy than leaving it on.
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Old 07-06-2017, 11:00 AM   #11
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That is true. Allowing it to cool and then reheating it uses less energy than leaving it on.
And you verified this how?
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Old 07-07-2017, 10:05 AM   #12
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And you verified this how?
It is a direct corollary of Newton’s law of cooling, which states that the rate of heat loss from an object is proportional to the difference in temperature between it and its surroundings.

I have created two graphs to help explain how this applies to a water heater. Each shows how the difference in temperature between the water and the surroundings varies with time, starting with the water hot. In the upper graph, the water heater is left on, and the water temperature falls and rises slightly as the thermostat switches the heater off and on. In the lower graph, the heater is switched off, the water cools, the heater is switched on again later, and the water heats back up. (The water temperature will actually follow curves, but straight lines are easier to draw.)

Since Newton’s law of cooling says that the rate of heat loss from the water heater is proportional to the difference in temperature, the graphs also show how the rate of heat loss (BTU/minute) varies with time. Integral calculus then tells us that the shaded area within each graph represents the total amount of heat lost (BTU) during the time period. (Think of it as Rate of heat loss x Time = Total amount of heat lost.)

Since the temperature at the end of the period is the same as that at the beginning (hot tank at the beginning returning to hot tank at the end), the amount of heat (from propane or electricity) put into the tank during the period must equal the amount of heat lost to the surroundings.

You can see that the shaded area within the upper graph, with the heater left on, is greater than the area within the lower graph, with the heater switched off. More heat has therefore been lost with the heater left on, and more propane or electricity must be used to replace that heat.

Ask any high school AP Physics student, and you will get the same answer.
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Old 07-07-2017, 10:15 AM   #13
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That Newton guy was a visionary when it comes to water heaters!
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Old 07-07-2017, 10:54 AM   #14
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It is a direct corollary of Newton’s law of cooling, which states that the rate of heat loss from an object is proportional to the difference in temperature between it and its surroundings.

I have created two graphs to help explain how this applies to a water heater. Each shows how the difference in temperature between the water and the surroundings varies with time, starting with the water hot. In the upper graph, the water heater is left on, and the water temperature falls and rises slightly as the thermostat switches the heater off and on. In the lower graph, the heater is switched off, the water cools, the heater is switched on again later, and the water heats back up. (The water temperature will actually follow curves, but straight lines are easier to draw.)

Since Newton’s law of cooling says that the rate of heat loss from the water heater is proportional to the difference in temperature, the graphs also show how the rate of heat loss (BTU/minute) varies with time. Integral calculus then tells us that the shaded area within each graph represents the total amount of heat lost (BTU) during the time period. (Think of it as Rate of heat loss x Time = Total amount of heat lost.)

Since the temperature at the end of the period is the same as that at the beginning (hot tank at the beginning returning to hot tank at the end), the amount of heat (from propane or electricity) put into the tank during the period must equal the amount of heat lost to the surroundings.

You can see that the shaded area within the upper graph, with the heater left on, is greater than the area within the lower graph, with the heater switched off. More heat has therefore been lost with the heater left on, and more propane or electricity must be used to replace that heat.

Ask any high school AP Physics student, and you will get the same answer.

Bravo!

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Old 07-08-2017, 08:09 AM   #15
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Sub'd for installation details. Not a Newton's Law lesson.
Thanks​.
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Old 08-31-2017, 10:46 PM   #16
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I would think adding additional insulation would help.
Here is some info that could partially be used to study.
http://aceee.org/files/proceedings/1...apers/0114.PDF
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Old 08-31-2017, 11:12 PM   #17
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That is true. Allowing it to cool and then reheating it uses less energy than leaving it on.
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And you verified this how?
Quote:
Originally Posted by johntaylor View Post
It is a direct corollary of Newton’s law of cooling, which states that the rate of heat loss from an object is proportional to the difference in temperature between it and its surroundings.

I have created two graphs to help explain how this applies to a water heater. Each shows how the difference in temperature between the water and the surroundings varies with time, starting with the water hot. In the upper graph, the water heater is left on, and the water temperature falls and rises slightly as the thermostat switches the heater off and on. In the lower graph, the heater is switched off, the water cools, the heater is switched on again later, and the water heats back up. (The water temperature will actually follow curves, but straight lines are easier to draw.)

Since Newton’s law of cooling says that the rate of heat loss from the water heater is proportional to the difference in temperature, the graphs also show how the rate of heat loss (BTU/minute) varies with time. Integral calculus then tells us that the shaded area within each graph represents the total amount of heat lost (BTU) during the time period. (Think of it as Rate of heat loss x Time = Total amount of heat lost.)

Since the temperature at the end of the period is the same as that at the beginning (hot tank at the beginning returning to hot tank at the end), the amount of heat (from propane or electricity) put into the tank during the period must equal the amount of heat lost to the surroundings.

You can see that the shaded area within the upper graph, with the heater left on, is greater than the area within the lower graph, with the heater switched off. More heat has therefore been lost with the heater left on, and more propane or electricity must be used to replace that heat.

Ask any high school AP Physics student, and you will get the same answer.
The problem with your example is that we don't know what the actual time periods are. If you NEED hot water at any of the high points in your first graph, you will be out of luck if you are operating it in accordance with your 2nd graph. The OP stated that he only needed hot water every couple of days, so it's pretty obvious he'll save energy. If you needed hot water every couple of hours, you would not save energy, especially, as someone noted, you would be adding cold water every time you used it. Somewhere between "hours" and "days" is the cross-over point. Without some experimentation or very good instrumentation and calculation, we don't really know where that cross-over point is. We can only deal with the two extremes.

So your original statement, allowing it to cool and then reheating, is probably only true if you meant to allow it to "cool down to ambient" (i.e., "cold") which at least on my HWH takes many hours or a day.
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Old 09-01-2017, 02:01 PM   #18
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Why would you not just put in a tank less on demand water heater. That is how they are designed to work.
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Old 09-01-2017, 05:36 PM   #19
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With the TT, we just turn on the gas heater switch about 15 minutes before wanting hot water, then turn it off when we're done.

BUT, on the boat, we don't want the 120vac water heater coming on when the a/c is running or it will trip the generator. So we use one of these to limit the time the water heater can be on - and once hot it stays hot for a day or more.



https://www.amazon.com/Intermatic-FF...ng+wound+timer

Same switch could be used for 12vdc.
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Old 09-02-2017, 12:45 AM   #20
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I turn the water heater off most of the time. If the A/C is running I turn on the gas for only as long as I need hit water. During the summer it stays warm enough most of the day for occasional hand washing etc. For showers in the morning I will use gas a electric together to heat the water fast and turn off the gas first, using electric for dish washing etc. Then I turn off the electric, until I need more than what is in the tank, or the tank has cooled too much.
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