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Old 12-12-2010, 10:09 PM   #1
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dual water heaters: parallel or series??


I have two questions, one theoretical and one practical:
1. Which is the better or more efficient method of hooking up two
identical water heaters? AND WHY???

2. If I have a parallel piping job on two water heaters and I have to
replace one of them and the new one is taller and has fresh new
heat traps, won't that effect the balance? Should I pipe those in
series?

Series is where the cold water goes into one water heater then through the second water heater.

Parallel is where the cold supply has a tee which feeds each water heater independently and joins them back together to one hot water supply.

THANKS FOR YOUR INPUT!!!!

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Old 12-12-2010, 10:15 PM   #2
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dual water heaters: parallel or series??


i've always liked series. in parallel it always seems you'll get more draw from one tank, and might not hardly use the second one at all. series you've got a lot of water available and it'll be heating water in the second tank just in case you need more

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Old 12-13-2010, 09:36 AM   #3
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dual water heaters: parallel or series??


Broox, What about installing a small tankless unit before the old tank? You'd see significant energy savings vs. heating 2 tanks all the time, and you'd never run out of hot water.
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Old 12-13-2010, 11:48 AM   #4
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dual water heaters: parallel or series??


Parallel is my preference. If both heaters are piped equal then water is drawn from them equally.

A small tankless is a bad idea in my opinion. If you go that route you may as well go with one sized for the application.

If piped parallel, then if one leaks you can valve it off at still have the other work. In series this is not so, unless you pipe in a bypass and again you should've piped parallel.

Last edited by Work4living; 12-13-2010 at 11:53 AM.
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Old 12-13-2010, 12:35 PM   #5
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dual water heaters: parallel or series??


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Originally Posted by Work4living View Post
A small tankless is a bad idea in my opinion. If you go that route you may as well go with one sized for the application.
Being an owner of a tankless wh (only) from a practical standpoint, I have to disagree. See my post from a previous thread:
Quote:
Originally Posted by Nick DIY View Post
Curls,
I own a Bosch 1600h tankless water heater. It does require significantly more gas flow than your normal tank water heater, therefor larger pipe, double wall flue... The unit itself is quite compact though, and that can be a big benefit.

My biggest complaint is the "warm up time". You'll run a significant amount of cold water through the heat exchanger before the fire kicks, and even then, you'll continue to wait while it heats up before you'll see hot water at the faucet. Additionally, unlike a tank wh, the water in the lines won't be kept warm by the hot tank that they're connected to. So, the water comes out quite cold for quite a while before it gets warm, then eventually hot.
Even if you turn off the hot water, then back on, you'll get hot, followed by cold, by warm and then eventually hot again.

The cold outside temps definitely has a significant impact on the performance. Takes longer to heat, and not as hot once heated.

I do love the energy savings, particularly because for the last 1 1/2 years, I spent the majority of my time away from my house and used very little hot water. And I do love that I can run back to back hot showers without worrying about ever running out of hot water.

I guess it's a personal decision, but I hope this helps!
I have a friend who was constantly running out of hot water, considering upgrading to a huge tank. He inquired about my tankless. He decided to leave his current tank & install a tankless as a pre-heater. He absolutely loves it.

It's more or less a series setup, except you're only keeping 1 tank hot. You enjoy all of the benefits of the 2nd tank, but without the added energy costs when you're not using hot water.

Just one more option to consider.
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Old 12-13-2010, 03:51 PM   #6
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dual water heaters: parallel or series??


Parrallel is the way to go for two water heaters

In series, one heater will be doing most of the work heating the water, while the 2nd one is acting like a storage tank only firing to maintain temperature under no flow. If you want to increase storage capacity, it would be better to just install a storage tank with a circulator tied back into the single water heater to maintain the hot water temperature in the storage tank. Adding a 2nd water heater to do what a storage tank could do, would be more work with gas and flues that you need to do. If you really want two water heaters, install them in parrallel with equal lengths of pipe between the cold tee and the hot tee, or in a "first in, last out" configuration and you will have a reliable system that uses both heaters equally.
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Old 12-13-2010, 04:13 PM   #7
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dual water heaters: parallel or series??


So more people say parallel, but what about question #2. Remember- one of these water heaters is seven years old and one of them is brand new-Does that change the answer??

Tha water heaters are electric.
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Old 12-13-2010, 04:30 PM   #8
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dual water heaters: parallel or series??


Broox,

It reinforces my answer. In parallel one can be replaced while the other still works.

Tankless is not the answer unless you take the full leap. Otherwise you're wasting your time and money. A small tankless will require 240v/50a circuit and will only heat approx 2gpm @ 80 deg rise. I appreciate Nick's thought about a tankless for preheat but is a waste of money and energy.

I do this stuff all the time it is my profession. Series is an inexpensive way to plumb 2 heaters.

Engineer is right with his post.
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Old 12-13-2010, 04:45 PM   #9
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dual water heaters: parallel or series??


I bounce back and forth between how I like to hook them up... I also like to do a little extra work if I do hook them in series

I feel that you are thinking of the wrong thing when talking about the age of the water heater.... I would guess that 9 out of 10 old water heaters we replace are from leaks not from the Heating elements failing... Of course this is only a guess I never really counted it and am not sure where I could find this statistic.... So here is what I would look at.

What do you need the Hot water for? is it a Large tub that you need all of the water at one time or is it because you have 15 kids and you need it for them to take showers 1 after the other?

In the First Scenario I would do it in Parallel... Water will be drawn off of both water heaters so you will get about 80 gallons of hot water out of 2 50 gallon water heaters the would still be Hot hopefully.

Now Scenario 2- I would do series but I would use the first tank as a warming tank in other words as the second water heater uses the Hot water it will pull in the water that is pre-heated from the first tank so it will take less time to heat up between showers.

In my House I have a Water alarm that also shuts off the water in the event of a leak... I highly recommend them. I have seen it save a Finished basement When the tank ruptured when the person was away. only had 50 gallons of water instead of 1000's

I also have a valve on bot the hot and cold side (not allowed by code) because When it goes I want to be able to remove it with out draining my house. To get it to pass inspection I removed the Handle from the hot side valve.
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Old 12-13-2010, 06:58 PM   #10
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dual water heaters: parallel or series??


Broox,

Even if you look at your 2nd scenario. If piped properly and you leave room in the vertical drops to the heaters, for varied heights of different heaters. You can still make parallel work just fine. Also as you can see everyone has their own opinions. There isn't a right or wrong. All I can tell you is what I have seen, and encountered. Both ways work, and both have their drawbacks. I prefer Parallel.

It's truly a matter of personal preference.
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Old 12-13-2010, 07:48 PM   #11
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dual water heaters: parallel or series??


I like the single tank with an added storage tank in series the best.

Parallel would be great if you could assure equal draw from the tanks but I do not believe you will get that in the real life situation. The tanks also need to be of equal capacity or you will start using tempered water as soon as the smaller tank starts running out of hot water. Just not a realistic solution IMO without a lot of electrical controls to assure equal flow from both and to shut down one or the other if it starts running out of hot water.

So, since I don't know where to buy a storage tank, I would simply use 2 heaters in series and since I don't want to hook up a circulating pump as you would need when using a storage tank and a heater in series, I would simply hook up both heaters and be done with it.
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Old 12-14-2010, 08:07 PM   #12
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dual water heaters: parallel or series??


I also have a valve on bot the hot and cold side (not allowed by code) because When it goes I want to be able to remove it with out draining my house. To get it to pass inspection I removed the Handle from the hot side valve.[/quote]

Doc, Just curious, I always put a valve on both sides and have yet to have it fail. I'm in NC, but I was just wondering what state you were in. I do it that way b/c when it's time to replace it, I shut both off, unscrew the unions, slide the old out and the new in. But if there is some reason not to do it I want to know. The only thing that comes to mind is thermal expansion.
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Old 12-14-2010, 08:17 PM   #13
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dual water heaters: parallel or series??


Quote:
I also have a valve on bot the hot and cold side (not allowed by code) because When it goes I want to be able to remove it with out draining my house. To get it to pass inspection I removed the Handle from the hot side valve
You shouldn't need valves on both sides to remove the heater unless you have a bad cartridge somewhere in the house. Once you shut off the cold side, just burp a faucet, and the tank will vapor lock. Just make sure no one tries to turn a faucet on before you hook up the new tank. Same with replacing a gas valve. Might get a 1/2 cup of water or so.
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Old 12-14-2010, 08:27 PM   #14
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dual water heaters: parallel or series??


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But if there is some reason not to do it I want to know. The only thing that comes to mind is thermal expansion.
That makes sense to me. It seems that if the hot valve was accidentally shut off for a period of time, and the tank remained operational, thermal expansion could put excess pressure on the tank itself.
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Old 12-14-2010, 09:40 PM   #15
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dual water heaters: parallel or series??


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Originally Posted by Plumber26 View Post
I also have a valve on bot the hot and cold side (not allowed by code) because When it goes I want to be able to remove it with out draining my house. To get it to pass inspection I removed the Handle from the hot side valve.
Doc, Just curious, I always put a valve on both sides and have yet to have it fail. I'm in NC, but I was just wondering what state you were in. I do it that way b/c when it's time to replace it, I shut both off, unscrew the unions, slide the old out and the new in. But if there is some reason not to do it I want to know. The only thing that comes to mind is thermal expansion.[/quote]


You got it,,, The inspector that failed me said it was not allowed because a homeowner could shut off the Hot side allowing the pressure to build in the tank. Then grabbed my crescent and told me to remove the handle and passed me. I am like you and like to be able to remove the water heater with out having to wait for the house to drain. I also do not have the problem of water in the pipe when I do it this way. If it is a customers house I usually only do the one because it only takes the 1 customer that ruins your life.

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