two water heaters

We recently moved into a new 3200 sf home, and there are only two of us. We have two 40 gal hot water heaters in the attic; one serving downstairs, and one sserving upstairs. The only time the upstairs is used is when we have guests, which in not often. We have a large whirlpool tub that my wife likes to use, but the 40 gal heater is barely enough to get water above the jets. How difficult would it be to connect the two heaters together to be able to take advantage of the extra capacity just for use of the tub? Would like self-perform the work, and I am pretty handy. Just don't want to screw something up, since I am not really that familiar with plumbing. All plumbing piping in the house is poly.
Thanks for any help.
R
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you haven't said where the heaters are. if they're next to each other, it's pretty easy to make some sort of manifold to connect them. if not, you may have to repipe and tear into quite a few walls to do this.

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They are next to each other in the attic, so no problem getting to them.
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Can be done, but may not be the best idea. Why keep 40 gallons of water hot for days on end if it is not being used? Consider one of the point of use heaters that works only on demand.
At least talk to someone that knows about them. If you use the tub daily, that is one scenario, but if it is a once a week treat, it may pay to go the instant route.
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Actually, I was thinking that what he wants to do is the best choice. Plumb the heaters in series, then add a way to turn the first one off/on. You can use the other one normally, and power up the first one when you will be using the tub a few hours later, or when you are expecting a bunch of guests. Then, you can add (since they're in the attic) some solar collectors to the first one at a later date to pre-heat the water.
Bob
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Bob wrote: ...

Sam thought I had.
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Joseph Meehan

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Get one Rinnai or Takagi gas tankless you will never run out of HW and thay are more efficient
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Do not want to go to the expense of a point of use heater at the moment, but a good wuggestion for the long term.
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Crazy non standard solution would be to plumb a separate hot pipe from the upstairs tank to the tub downstairs with its own separate faucet (i.e. no cross connection) You will have the benefit of filling the tub twice as fast. Pipe the cold for that faucet from anywhere convenient. Nothing in code I am aware of prohibiting multiple taps in a tub.
Try diagramming the plumbing in your house, this will help you communicate options to others for advice. Scan and post the diagram on a website and post a link here. Cross connecting the two hot systems could cause problems if there is a pressure differential anywhere, you could have water flowing in an unintended direction. Depends on where/how you do it.
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This is not a bad idea at all. Filling large tubs take quite a while, so you get the added advantage of halving the time.
Another approach, assuming they're relatively adjacent, is to "plumb them in series", and move all your hot plumbing to the "downstream" tank, so all hot water has to go through both.
Instead you could plumb a line from the output of the tank _not_ feeding the tub into the input to the other tank. Both are in series for the tub, and only the first tank is used for other things.
The latter two don't fill the tub any faster, but will provide enough hot water. It's perfectly okay to feed hot water _into_ the input of a HWT.
From the perspective of cost of operation, an "idle" electric HWT consumes about $3 worth of power a month just to maintain temperature. Short of bypassing/decommissioning one of the HWTs, that will be your approximate incremental cost of operating both.

Cross-connecting - meaning, hooking them in parallel. Depending on how your plumbing works, it would likely mean that one of the HWTs is effectively useless.
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Chris Lewis, Una confibula non set est
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Maybe. In my house, the limiting factor is how fast I can get water out of the pressure tank, so having pipes from two separate water heaters would just get me anemic flow at both taps.
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I like the idea of plumbing them in series, if I understand it correctly. The cold water input would go to one tank, and the output of that tank would go to the second tank. Hot water to the fixtures would come from the second tank. Is this correct? So you are dumping cold water into one tank, and pre-heated water into the second. I will make a diagram and study it. Thanks to all for your input.
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Yes.
The second tank is pretty much only maintaining the temperature.
Some homes do this if there's a _long_ hot run - install a small 2-5 gallon heater at the far end in series with the long run. It's almost "instant hot water", and doesn't suffer from low flow rates from all but the more high capacity tankless heaters..

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Chris Lewis, Una confibula non set est
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Your spa tub must be huge. I have one that once I made a slight adjustment with the temp setting my 30 gallon water heater does the trick.
I would try raising the set point before I played plumber, but that just me.
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I agree. I have a 52 gallon tank, and a smaller than what I grew up with bath tub, and It takes me almost the full water heater to fill the tub. I know, because if I fill it to the top and it's either too hot or too cold, and I drain some out, there's often not enough hot water to fill it up again.
I used a measured bucket and ladled the water out of the tub and it was about 100 gallons, so I'm using about 50% hot water and 50% cold.
I'm a little slow witted, but someone else pointed out to me the solution, to make the hot water hotter. I haven't done this, but I'm the only one hear and I can plan my water use without surprises.
How did the previous owners handle this?
Remove NOPSAM to email me. Please let me know if you have posted also.
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I can squeeze enough extra from my 40 gal tank by running the hot water long enough to get it to the tap then waiting for the tank to finish reheating before actually filling the tub (10-20 minutes). I get a warm volume of water right away from the pipe (it is insulated most of the way) and warmer than normal water (since it just finished burning rather than sitting for a while) from the tank. This gets me just enough to fill the tub (~70 gal).
I do have two fill taps (one shower head and one spigot) but they are fed from the same tank. It does speed up filling somewhat but since they share the same 3/4" pipe, turning one on does reduce the other but not by all that much.
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Why do you want to heat hot water?
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wrote:

It costs less than heating cold water.
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FWIW, the Journal of Light Construction had an article about the proper way to connect two water heaters. It was in the october 2004 issue and was titled: "Kitchen & Bath: Adding a Second Water Heater "
View it at www.jlconline.com
I think it's free, but there may be a small charge to view it.
This is a great magazine, by the way.
HTH,
Paul Franklin
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