# dual water heaters: parallel or series??



## broox

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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## the_man

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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## GottaFixIt

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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## Work4living

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.


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## GottaFixIt

Work4living said:


> 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:


Nick DIY said:


> 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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## The Engineer

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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## broox

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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## Work4living

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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## Docwhitley

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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## Work4living

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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## nap

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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## Plumber26

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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## LateralConcepts

> 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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## LateralConcepts

> 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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## Docwhitley

Plumber26 said:


> 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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## the_man

just wondering... are these gas or electric tanks. I'm a big fan of the Bradford White GX high performance tanks. if i remember right the 55 gallon tank has like a 200 gallon first hour rating. if ya haven't seen it and it'll work, i'd consider it


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## TheEplumber

I don't put valves on hot lines for residentual but it seems like almost all commercial tanks have h&c shut off valves. Can anyone tell me where in the UPC it says not to? I'm to lazy to get my code book out of the truck :whistling2:


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## Docwhitley

TheEplumber said:


> I don't put valves on hot lines for residentual but it seems like almost all commercial tanks have h&c shut off valves. Can anyone tell me where in the UPC it says not to? I'm to lazy to get my code book out of the truck :whistling2:



I think it is a local code around here. Some of those are just ridiculous


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## the_man

TheEplumber said:


> I don't put valves on hot lines for residentual but it seems like almost all commercial tanks have h&c shut off valves. Can anyone tell me where in the UPC it says not to? I'm to lazy to get my code book out of the truck :whistling2:


lazy [email protected]$terd  i'll go grab mine and look it up


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## TheEplumber

the_man said:


> lazy [email protected]$terd  i'll go grab mine and look it up


I'll be waiting :drink:


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## the_man

from what i've seen, its not prohibited. 605.2 specifies cold water inlet valve, but also says that one shall be installed on the discharge piping from water supply tanks. Does a water heater count?:whistling2: 608.3 says that anytime restriction of thermal expansion is restricted, an expansion tank is required. Does that (required) valve count? :thumbup: 608.6 doesn't say anything about it, but does talk about valves piped in after the water heater but before a water storage tank. From all that, all my customers now need a valve on the outlet and an expansion tank :laughing:


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## Docwhitley

Good luck with that... We were losing a bunch of jobs when we were following the new Local code that said All houses needed a PRV, Dual Check and exspansion tank. Now we tell them the new code requires them but id you do not want them we understand the difference in the price is _____ , It is bad when some follow the code and others do not know it. I have had to e-mail the code to show we were telling the truth


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## Work4living

There is nothing in the code against valves on the hot side. The only stipulation is there has to be one on the cold supply before the thermal expansion tank. Doc must be dealing with a local addendum.


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## TheEplumber

Docwhitley said:


> Good luck with that... We were losing a bunch of jobs when we were following the new Local code that said All houses needed a PRV, Dual Check and exspansion tank. Now we tell them the new code requires them but id you do not want them we understand the difference in the price is _____ , It is bad when some follow the code and others do not know it. I have had to e-mail the code to show we were telling the truth


My city is asking for double checks on commercial now. We remodeled a shower rm, it had an existing WH that served this bathrm/shower group. The inspector wanted an expansion tank on it because of the new double check. He was so happy about the correction notice that missed the other 2 WH in the bldg :whistling2:


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## Work4living

LOL, I agree this day and age the codes have moved way overboard. Problem is that overtime when we (The Plumbing Profession) improve things the result is always opposite and equal. Frigging Newton pisses me off. Not to mention the blatant violators that cause the increased strigency of the code.

I used to be under UPC and at that time they required rpz check valves on all domestic supplies . Now I am under IPC and they don't but are moving that way. They just started requiring thermal expansion on water heaters, and that is even at the inspector's discretion.


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## Ltnicks

the_man said:


> from what i've seen, its not prohibited. 605.2 specifies cold water inlet valve, but also says that one shall be installed on the discharge piping from water supply tanks. Does a water heater count?:whistling2: 608.3 says that anytime restriction of thermal expansion is restricted, an expansion tank is required. Does that (required) valve count? :thumbup: 608.6 doesn't say anything about it, but does talk about valves piped in after the water heater but before a water storage tank. From all that, all my customers now need a valve on the outlet and an expansion tank :laughing:


dont forget to put a vacuum breaker on the faucet on the bottom of the water heater :laughing:


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## Michael Thomas

AO Smith's opinion: Parallel vs. Series Piping - Technical Bulletin Number 64


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## Work4living

Like I said parallel!!!! I'm right...... I win!!!!!!


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## broox

Work4living:congratulations!! I will mail you your reward.

The reward is the old leaky 50 gallon water heater I removed. 

All you have to do is pay shipping and handling. :euro:


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## Work4living

Woooooooooo Hoooooooooooo.....


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## Plumber26

Had the same arguement w/ a co-worker. His take on it was, "There's no way to ensure that they will be used equally." My response, "Rephrase, 'There's no way YOU can ensure they will be used equally... I know how to use my tape measure."

Thanks Micheal Thomas... I'm gonna print out that bulletin and slam it on the windsheild of his van in the morning.


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## Docwhitley

Plumber26 said:


> Had the same arguement w/ a co-worker. His take on it was, "There's no way to ensure that they will be used equally." My response, "Rephrase, 'There's no way YOU can ensure they will be used equally... I know how to use my tape measure."
> 
> Thanks Micheal Thomas... I'm gonna print out that bulletin and slam it on the windsheild of his van in the morning.



I would wonder if using it as a warmer as I do would still cause this Problem... Basically I put the first water heater on Med and second on hot so they are both still being used all the time


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## Work4living

Doc,

Personally I don't think it is a concern if you turn the first heater back. The question is what is the Delta T.... normally this is approx 20 degrees difference. Not really sure there.

plumber26, tell your coworkers that you can't fight physics. Many have tried and lost. 

It's always hard to teach old dogs and young pups!


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## cudaman

*Very active thread!!!*

Ok boys.... What would you do?

Just failed an 8 year old AO smith 66gallon electric

Wanted the tax credit and save on money, so installed the GE Geospring 50 gallon unit.

Running out of hot water and water pressure is much less on the hot side, I'm assuming the GREEN GEOSPRING has a flow restriction.

I like the geosprings features, warrenty, low cost... I'm thinking about getting another Geospring and plumb it parallel. This should double out amount of storage, increase hot flow rate, and still be cheaper to operate per year than a single 100 gallon. Agree or disagree with my gameplan?


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