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Old 10-09-2008, 07:09 AM   #1
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2 water heaters - saving energy


I have 2 water heaters on 2 completely separate systems. Used to have a family of 7 but now down to 3. Except when family is visiting, there seems to be little need to have 2 water heaters consuming energy constantly. The only fly in the ointment is that the showers being used now are on one system and the kitchen and laundry are on the other. What does anyone think of the idea of installing a crossover line (with a valve) to allow running everything off one of the heaters while shutting the other one down. The systems could be returned to normal operation when there is company (not that often) by closing the valve in the crossover line. Does this seem reasonable?
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Old 10-09-2008, 07:56 AM   #2
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I don't see why that couldn't be done. You'd of course have to shut the one heater off every time.

If the tanks were slaved together I could offer a solution. Many people in this circumstance use the first tank as a warming tank on a low setting and set the second tank as the one to get the warmed water nice and hot. That way they're both not running full bore trying to keep two full tanks of water hot at the same time, but recovery is a little faster than having just one tank.
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Old 10-09-2008, 02:08 PM   #3
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2 water heaters - saving energy


Agreed with KC in that a crossover wouldn't be a problem, but wanted to clarify that..

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Originally Posted by thekctermite
You'd of course have to shut the one heater off every time.
.. I think he meant shut the power off, in addition to the water. Just wanted to make sure that was clear, otherwise you'd still be paying to heat the tank that was being bypassed. And even if you bypass the tank on the hot side of the piping, you should also shut off the cold water supply to the tank. 2 HW heaters means twice the chance one will leak at any given time.

KC,

I'm with you on the benefits of slaving the tanks, but wouldn't the standby energy consumption still be higher? What I mean is that when in standby you're effectively doubling the heat loss (twice the surface area), and using twice as much power to bring the tanks back to set temp. Even if one is set at, say, 90deg and the other is 130deg, they might still both drop 5deg in standby and need to be reheated. So you need to heat twice as much water by 5deg vs. 1 tank. Am I on base here?
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Old 10-09-2008, 03:35 PM   #4
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That theory could have some merit, but I'm not sure. But, I'd rather pay to keep a tank full of water at 80 or 90 degrees as a preheat tank setup than to keep two tanks at a hot temperature in my cool basement. There's no way that the preheat tank will use as much gas as it would at a higher setting.
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Old 10-09-2008, 03:39 PM   #5
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Agreed it would be cheaper to have 1 low-temp and 1 high-temp tank rather than 2 high-temps. No question.

I was just trying to make the point that for the OP, with only 3 people in his house, 1 high-temp and 1 turned-off tank would be the cheapest way (energy wise) to go. Doesn't sound like he'd have demand issues.
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Old 10-09-2008, 04:03 PM   #6
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2 water heaters - saving energy


Quote:
Originally Posted by thekctermite View Post
That theory could have some merit, but I'm not sure. But, I'd rather pay to keep a tank full of water at 80 or 90 degrees as a preheat tank setup than to keep two tanks at a hot temperature in my cool basement. There's no way that the preheat tank will use as much gas as it would at a higher setting.

I think it's all a matter of physics.

It takes 1 btu to raise 1 pound of water 1 degree F.

No matter where that is done it will take the same energy.

If you throw in the heat loss from two tanks versus one tank then I would be on the boat that says one tank is better than two.

It's an interesting discussion but I think the overall the differences in energy savings would be small.

Most water heaters according to the stickers on the tank use about $10 per month to heat the water for the average family (whatever the average family is). Considering efficiency of 85% this means that about $1.50 a month is lost. The difference over a year would be $18.

I think the key to energy saving is to look at use of existing energy that is happening around us like sun and thermal ground cooling.

Anyhow, I love discussions about energy use and ways to "stick it to the man" when it comes to free energy.
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Old 10-09-2008, 05:22 PM   #7
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Quote:
Most water heaters according to the stickers on the tank use about $10 per month to heat the water for the average family (whatever the average family is). Considering efficiency of 85% this means that about $1.50 a month is lost. The difference over a year would be $18.
Hmmm.. that makes sense to me. So maybe the net result of all this is that rjamesh would probably spend more in time, pipe, and fittings than he would if he left both tanks the way they are for a couple of years.

Quote:
Anyhow, I love discussions about energy use and ways to "stick it to the man" when it comes to free energy.
Me too.. Until recently, my power company was doing a program with HUGE rebates for customers installing a grid-tie solar system. I did some calcs and found out that for my energy consumption, the install would pay for itself in about 2.5 years. Without the rebates I was looking at ~15 years, which is probably about the time I'd have to start putting money into it for repairs. They stopped doing the rebates a little while ago, and so I'll be SOL when I move into my house.

I think the point I'm trying to make is that it's hard to "stick it to the man sometimes".. tho I'd love to. (And DIY is out of the question for me b/c local code requires the work be done by a pro, and it'll be plenty obvious if I stick all that stuff up on my roof w/o permits ).
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Old 10-09-2008, 05:37 PM   #8
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Originally Posted by ScottR View Post
Hmmm.. that makes sense to me. So maybe the net result of all this is that rjamesh would probably spend more in time, pipe, and fittings than he would if he left both tanks the way they are for a couple of years.



Me too.. Until recently, my power company was doing a program with HUGE rebates for customers installing a grid-tie solar system. I did some calcs and found out that for my energy consumption, the install would pay for itself in about 2.5 years. Without the rebates I was looking at ~15 years, which is probably about the time I'd have to start putting money into it for repairs. They stopped doing the rebates a little while ago, and so I'll be SOL when I move into my house.

I think the point I'm trying to make is that it's hard to "stick it to the man sometimes".. tho I'd love to. (And DIY is out of the question for me b/c local code requires the work be done by a pro, and it'll be plenty obvious if I stick all that stuff up on my roof w/o permits ).
Yes, that is the problem. No DIY on alternative energy if you want the rebates.

I just buy as much as I can real cheap on cragislist and do my own install. At my vacation home I have wind, solar electric. solar hot water and hydronic cooling. I just got all the parts and did it myself. Even considering the rebates I still saved money.

Recently I just put in a high efficiency furnace and had to go through the company to get the rebates. Joe Diy'er can't do that.

To me this is a big deterrent to people going to alternative energy.
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Old 10-10-2008, 04:35 PM   #9
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Original poster here..........The replies are appreciated. It seems that I was a bit unclear in my question. YES - I would plan to shut off the power to one heater most of the time. Thus, when there is no demand and the temperature is at target, I'm maintaining temperature in just one and not two heaters. When there were more than 3 people overnight (not very often), I would position the valves accordingly and turn the power back on the normally unused heater.

Also, I earlier referred to a crossover line with a valve in it. I think I would also want another valve in the line coming from the unused heater just before the crossover line ties in. Otherwise, water from the online heater will be tending to go back toward the unused heater.
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Old 10-10-2008, 04:57 PM   #10
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Quote:
Originally Posted by rjamesh View Post
Also, I earlier referred to a crossover line with a valve in it. I think I would also want another valve in the line coming from the unused heater just before the crossover line ties in. Otherwise, water from the online heater will be tending to go back toward the unused heater.
I was going to mention this. Water flow has to be through the heated tank and not going into the unheated tank. You would wait a long time to get hot water if you went about this the wrong way.
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Old 10-10-2008, 05:30 PM   #11
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Diagram below is what I was thinking you had in mind (I left off some shutoff valves that I assume you have already.. water main & valves for HW1, etc..).

- HW 1 would always be used.

- HW 2 would only be used when you have people staying with you.

- Valve 1 & 2 would be closed to isolate HW 2, and HW 2 would be shut off. Valve 3 would open for the crossover.

- When you wanted both HW heaters working as you have them now, Valve 1 & 2 are opened, and valve 3 is closed.

If we're on the same page, then I agree, it's completely workable.

Sorry, we got off on a tangent before debating energy efficiency..
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Last edited by ScottR; 10-10-2008 at 05:42 PM.
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Old 10-10-2008, 05:39 PM   #12
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BTW - I clicked on my diagram to view it as full-size, but it actually came out much smaller. Don't know why. Let me know if it's not legible to you.
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Old 10-10-2008, 06:47 PM   #13
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Scott, you got it just right. Thanks. I'm not sure how long it will take to pay for the investment of 2 valves, 2 tees and about 15 feet of pipe but I can figure that out.

Everyone's input is appreciated. I think that's it for me unless someone else has a better idea for my situation. I'm always open for new ideas.
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Old 10-10-2008, 07:18 PM   #14
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I'm late getting in on this thread but I wanted to add. It would probably be a good idea to drain that by-passed tank when not in use. I don't have any data to relate but the idea of stagnate water sitting for months on end before use bothers me. I know it's a closed system but I believe there will be some residual effects from it unless the water was filtered before it entered the tank.

I get this from years ago when I had a boat with a hot water tank. Everyone advised that when I was away for a couple months I should always drain it.
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Old 10-10-2008, 09:54 PM   #15
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Quote:
I don't have any data to relate but the idea of stagnate water sitting for months on end before use bothers me.
Didn't think of that, but it would bother me too.

And also if you drain it, there's no chance it can leak.
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