# dual water heaters



## kellybhutchings

Is it okay to hook up two water heaters of the same size and make in parallel if they are on opposite sides of the house?


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## Artisan

My vote is no..
Do it in series and insulate all piping well.
The different lenghts of piping, both hot and cold will give one heater an advantage if you will, via less friction loss thus it will work more and when it runs out of hot water it will deliver cold water and your other water heater could sit there and stay hot and not be drawn from.


(Plagerized below)
Another important point of parallel installation is the length of the supply piping and delivery piping – they must all be the same length. As shown in the drawing below, section *A-E *must be the same length as section *A-*B; *C-D *must be the same length as *F-*G; *E-F *must be the same length as *C-*B. The same holds true for the hot water outlet side. Because the water pressure is constant along the cold inlet piping and hot water supply piping, the heater with the closest 'run' will do the majority of the work. To prevent this we ‘balance’ the unit with equal pipe lengths. When installing heaters (and storage tanks) in parallel, it is important to accurately plan and measure the distances from the cold water supply pipe to the heaters and from the hot water outlet on the heater to the hot water supply line. This will equalize the work between the two water heaters.


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## smellslike$tome

Tanks in series, tankless in parallel.


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## user4

I would never pipe heaters in series unless I was doing more than two, and if that was the case I would consider a boiler and storage take above multiple heaters.


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## Helgore

Pipe one as a storage tank, that is the way to do it.


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## Artisan

He said there on differnt sides of the house, an existing condition I do believe and not a perfect world, yes?


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## TotalPlumber

I always reccomend parallel. I don't see the advantage of series piping , at all.

Total


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## para1

I'm sure helgore will explain.:jester:


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## kellybhutchings

A friend piped the water this way in a new house, I believe he already split the system back up. When he first piped it that way it was on accedent and he asked me if it was okayto leave it. I told him it was okay piping the heaters in parrellel but I didn't know about having them on opposite ends of the house.


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## Redwood

Parallel piping of equal length...

That's easy just leave the extra pex spooled up in the attic...:laughing:

Helgore will explain it all!:laughing:


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## SewerRatz

You all ever plumb units in parallel using first in last out method?

Here is a couple diagrams by Bradford White. It works real well with more than 2 heats.

2 water heaters. http://www.bradfordwhite.com/images/shared/pdfs/Piping/2H_TCNR.pdf

3 water heaters http://www.bradfordwhite.com/images/shared/pdfs/Piping/3H_TCRC.pdf


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## trout lake

SewerRats
I don't particularily agree with BW's piping schematic. What is it about the design that you like? I have to wonder about the draw on a last out method.
tl


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## nhmaster3015

Avoid the confusion, sell them a bigger tank :laughing:


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## SewerRatz

trout lake said:


> SewerRats
> I don't particularily agree with BW's piping schematic. What is it about the design that you like? I have to wonder about the draw on a last out method.
> tl


 Well first you can install an odd number of heaters easily. Plus it is easer to keep this type of manifold evenly sized on both sides. I see the first in last out install mostly in commercial installs and see all heaters working evenly. Where as the standard parallel install I see in residential homes, I have seen more failed water heaters due to, where one pipe leading into the tee is a hair longer than the pipe from the other heater, which causes the heater with the shorter piping to work harder. 

The first in last out method is also taught by the Local Union shops out here.


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## trout lake

SewerRatz
I'm just not quite convinced. Your points are good but I'd really be interested in BW engineered studys to have them promote this type of installation. These guys are in the business of selling product. It would suprise me if they gave us a model whereby it promoted a longer tank life. From their drawing, the pressure release point is after the second tank. The draw, in my opinion, is heavier on that tank. A good old flow meter would either prove their design either right or wrong.
Don't get me wrong. I'm not being critical of their opinion. I only hope that this kind of dialogue makes all the more informed at the end of the day. This is a great form for all of us to exchange ideas and be masters of our trades.
cheers
tl


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## Protech

BW's diagram will work perfectly. Show me why it wouldn't.


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## bob young

*twin heaters*



smellslike$tome said:


> Tanks in series, tankless in parallel.


ao smith also has some excellent piping diagrams


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## trout lake

Protec
Do you think the tanks draw evenly? If you do, what is it based on?
Here's where I'm comming from.
Anytime piping is uneven or there are restrictions of any kind, you will encounter hydraulic resistance. That resistance will have a direct effect on flow and in closed piping systems, head.
The system BW put out for us to follow is fine. It will work. But nobody should be under the illusion that it is the end all be all of a multi tank system.
The 2 problems I have are with the longer, and uneven pipe length from the second tank. Even though the difference is minor, there is a difference. That difference will have an effect on flow, albeit small. But I wouoldn't want to be on record saying the first tank works harder because of the unbalanced piping. Why? I think a much bigger problem in a system piped like this is the fact that the first tank comes out of a 90 and the second out of a "T". What do you want to bet that the first tank bully's it's way past the second. If the "T" was directional, different story. But how many designed systems have you run into with directional "T's". Not many I bet. It`s like the old argument about air flow in a ducted system going into a straight or directional defuser. Where do you get the best flow in this case. I think the BW system will definately work in a pinch.
I like an equal draw system that is balanced. But that`s only my personal preference. The science of hyraulics is a bit fun isn`t it though.
tl


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## Protech

See pic


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## trout lake

So Bw has èxactly the same lengths of pipe in the system. Since the pipe lenghts are identical you say the draw is equal. Are you saying the system draw evenly because BW says it is


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## Protech

Pipe friction head is equal along both flow paths no matter which heater you choose to look at. It's that simple. This of course only applies if your heaters are piped exactly as shown down to the fitting.


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## trout lake

I`don`t understand the term `friction head`. Head in a system is comprised of pressure head, velocity head and elevation head.
You are correct when you say the hydraulic resistance is equal along both equal lengths of pipe for each tank. The only difference is, is that the resistance feeding the tank is independent from the one drawing. In other words, the only one that matters is the one on the draw side. And it matters because it effects the first tanks flow.
Let me ask you this. BW`s system is fine. It comes close to perfect but it is not because it`s not exactly balanced. If they maintain that the equal lengths of pipe alone give you balance, what if the tanks were 100 meters apart. Or 500 meters apart. Is the system them in balance.
tl:no:


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## trout lake

Bob
What does AO have to say on the subject.


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## Protech

Friction head is the amount of pressure lost to pipe friction when under flow.

The amount of friction head on the "in" lines is equal to the friction head on the "out" lines.

The green path is the same length and has the same number and type of fittings as the purple path. The distance between the heaters is irrelevant as far as pressure drop and consequently flow balance are concerned.



trout lake said:


> I`don`t understand the term `friction head`. Head in a system is comprised of pressure head, velocity head and elevation head.
> You are correct when you say the hydraulic resistance is equal along both equal lengths of pipe for each tank. The only difference is, is that the resistance feeding the tank is independent from the one drawing. In other words, the only one that matters is the one on the draw side. And it matters because it effects the first tanks flow.
> Let me ask you this. BW`s system is fine. It comes close to perfect but it is not because it`s not exactly balanced. If they maintain that the equal lengths of pipe alone give you balance, what if the tanks were 100 meters apart. Or 500 meters apart. Is the system them in balance.
> tl:no:


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## SewerRatz

Just because BW shows the piping as first in last out set up does not mean they invented the idea. This way has been taught in many plumbing classes. A.O Smith does a parallel set up as well but splitting the tanks between a pair of tees.

The first in last out is much simpler than the picture below for a parallel set up less fittings and easer to keep all pipes at equal length.


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## SewerRatz

Did some looking at AO Smith multiple tank set up on a 4 water heater to 1 storage tank they also recommend the first in last out. http://www.hotwater.com/lit/piping/A2270.pdf


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## smellslike$tome

If they need more than 1 water heater then they need a tankless (or 2, or 4, or 10!).


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## SewerRatz

smellslike$tome said:


> If they need more than 1 water heater then they need a tankless (or 2, or 4, or 10!).


 Not with the hard water conditions some of these people have. They do not want the added maintenance involved with Tankless, and do not want the added expense of having a water softener system installed.


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## trout lake

Protec
Thanks for the clarification. What you refer to as friction head I refer to a viscous friction. That being the energy developed in the actual fluid stream and along the pipe walls.
It's been a great debate and I thank you for that. But at the end of the day, we'll have to agree to disagree. I am a proponent of equal draw. I believe it is a better system than any first in last out system some manufacturer says is gold. These are the same guys that ship circ pumps out on boilers, on the wrong side, because it's convienient for shipping. Everyone that understands hydronics knows the pump should be on the return side.
Our chat has stirred the minds of others and thats always good.
tl


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## Protech

I'm not trying to put anyone down, I just want the readers out there to get good info trout. In this case I can't just agree to disagree because I know for a fact that the reverse return aka first in last out method IS an equal flow configuration.

It seems as though everyone out there agrees with me too. If you care to disagree, please do so with a formula and demonstrate your claims.

Respectfully yours,
Protech


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## Protech

and another


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## trout lake

Protec
Debating the pro’s and con’s of the two tank hook up, is simply that. I, like you, have no interest in blowing smoke (so to speak) just to hear myself. You have a point on how you like to do tanks and so do I. I, like you, am not trying to influence anybodys thinking on the subject. I think most of the guys hanging out on this site are quite capable of making up their own minds on this matter. It’s not a matter of who agrees with you or me. It’s simply having a discussion with you and exchanging ideas.
You say to me to put the science out there to back up my views. What do you do? Send me some pictures of somebody else’s views, with not one drop of back up, that you obviously agree with, and say “there, take that.” This is fact.
On the last drawing you sent from SRCC, I do agree with their views on the top drawing as being equal. I don’t with the second.
By the way…..photo one and two are very close in their deliveries. So close some may say we’re splitting hairs. You started by saying BW’s way was bang on and “show me why it won’t work.” I never said it wouldn’t work. I only said it was NOT the end all be all as they and you profess.
I simply do not agree that the viscous friction (friction head) is identical “no matter which tank you look at.”
The relationship between head loss , flow rate and hydraulic resistance is based on piping and it’s devises. It is represented in a formula: HL= r(f)1.75 ….where HL=head loss…..r=hydraulic resistance of the pipe…..f=flow rate gpm……1.75=power of f (an exponent).
The resistance in the formula can be broken down to R=(acl) …..where a=fluid density…c=pipe coefficient (inside dia)….l=length of pipe
The fluid properties a=(d/u) x (-.25) …….where d=density of the fluid lbs/ft3……u=dynamic viscosity of the fluid.
So put it to work:
What’s the pressure drop in 100 feet of ¾ inch copper piping with the water temp at 140f and at 5lbs pressure?
A=(d/u) x -.25.…equals 61.35(density of [email protected]) divided by u (.00032) = (191919) x -.25...= .04779
SO……….HL = (acl) (f) x 1.75.…=((.04779)(.061957)(100)(5)x1.75.….=4.95 feet of head.

Head loss to pressure = delta p=HD/144.…=(4.95)x(61.35)/144.….=2.11psi

I hope you can follow the math but there it is. It’s the science behind the view that came from someone a heck of a lot smarter than me.
Anytime piping is NOT of exact lengths and restrictions pressure changes will happen.
There will be a difference in draw as related to pressure drops between the first and second tank. Your assertion that the drop is equal across the system simply because the piping lengths’ on the in and out on the tanks is even, is simply not correct. Even though the pressure drop, once a demand for water is initiated, is similar, it is not exact at the point exit of the second tank and first.
The guys that put their views out that manufacture tanks are not god. If I don’t agree with them I have no problem saying so.
Best regards
tl


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## Protech

Look at the developed length for each branch of heater pipe. The blue and red is irrelevant because it's common to both.


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## Protech

Actually, you may be right to a degree. I forgot to factor in the reduced viscosity of the hot water lines. If it's a really large bank of heaters or a great distance between 2 heaters the difference in viscosity between the hot and cold could actually make a significant imbalance of flow between the heaters.

Rather than doing a page worth of D'Arcy-Weisbach equations, I just shortened the hot lines by 10% to make up for the difference in viscosity. After doing the math I suspect it would work out to be a hair less friction head than what I have shown.


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## bob young

*Hydronic Heating System*

[On a hydronic heating system , the best piping arrangement is to pump away . [ circulator on supply to system] once you try it you will be convinced forever. quote=trout lake;41631]Protec
Thanks for the clarification. What you refer to as friction head I refer to a viscous friction. That being the energy developed in the actual fluid stream and along the pipe walls.
It's been a great debate and I thank you for that. But at the end of the day, we'll have to agree to disagree. I am a proponent of equal draw. I believe it is a better system than any first in last out system some manufacturer says is gold. These are the same guys that ship circ pumps out on boilers, on the wrong side, because it's convienient for shipping. Everyone that understands hydronics knows the pump should be on the return side.
Our chat has stirred the minds of others and thats always good.
tl[/quote]


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## trout lake

Bob young
Just why is it so good? Give me an example on the pressure difference in the system with the pump on either side. Is it greater or less on the supply or return? What effect will it have on the system?
tl


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## trout lake

Protec
Touche'!!!!!
I had a student like you one time. I'll take your "partially right" as a small victory and turn the page on this debate. I like your cock sure attitude. In life this is good. Always stick by your truths and convictions. 
The fodder with you has been stimulating.
:thumbup:
tl


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## SewerRatz

trout lake said:


> Bob
> What does AO have to say on the subject.


 This is from AO Smith http://www.hotwater.com/bulletin/bulletin64.pdf


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## trout lake

I agree!


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## bob young

*Hydronics*

IF You have ever done any h.w. heating systems , particularly residential baseboard loops , this piping arrangement virtually elimininates any air problems. purging the system is a dream. give it a shot & you will thank me forever. american standard actually recommended this method back in the early seventies. i saw the diagrams but being primarily a plumber did not get it. i do now. if you want shoot me a private e-mail. & will give you more details. [email protected]


trout lake said:


> Bob young
> Just why is it so good? Give me an example on the pressure difference in the system with the pump on either side. Is it greater or less on the supply or return? What effect will it have on the system?
> tl


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## trout lake

Hey Bob
I've done my share of residential hydronic systems that's for sure. You mentioned that your way is the way to go and I'd thank you for it is I ever used it. But what way?
How do you lay it out? You gave no particulars so I'm bumfuzzled. Where do you put the pump in relation to the expansion tank?
tl


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## bob young

*pumping away*

always pump away from expansion tank. an effective piping arrangement is to install an air separator on supply & connect ex. tank or extrol type tank to tapping supplied w/ pres. red. supply valve , then circulator pump & finally the flow control valve. a bypass is recommended to temper return circuit or primary , secondary circuit can be alternative option depending on complete system engineering & piping design. a purging station is fabricated at return circuit before final connection back to the boiler with pertinent valves etc. this pumping away design virtually eliminates air problems . also extremely effective for a split circuit basboard heat of hot water loop connected to low water on steam boiler. hope i explained this clearly. i rest my case !! 


trout lake said:


> Hey Bob
> I've done my share of residential hydronic systems that's for sure. You mentioned that your way is the way to go and I'd thank you for it is I ever used it. But what way?
> How do you lay it out? You gave no particulars so I'm bumfuzzled. Where do you put the pump in relation to the expansion tank?
> tl


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## njoy plumbing

*Excellant potty time reading!*

Here is the best info I ever read on"pumping away subject. No kidding its called,.... wait " pumping away" by Dan Holohan. Trust me he's right.


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## trout lake

Bob
The idea of this form is to exchange ideas. If someone one makes a statement that you feel might be contrary to your thinking, it is appropriate to ask for an explanation. What one doesn't expect is a BOLD type response back with an "I rest my case following". Something like this Bob, simply pisses me off!
I asked you to explain yourself as to your views. Take a look at what you wrote. You've read something somewhere, tried to explain something you obviously are not expert in, and it came out "garbliegoop". I would only ask anyone that knows to read your explanation and draw their own conclusions.
Your right about pumping away from the tank. The only trouble is, is that I don't feel from your explanation that you really, really know. Am I wrong?
If I am, explain to me in language for all to see, why the tank should aft of the circulator? Explain to me it's effects on pressure in the system and why it's good or bad. What is the problem associated with a circulator before the expansion tank. With a 15 lb tank, how much drop in the system with a tank before system. Is there a risk of actually drawing air into the system instead of expelling it?
Keep it civil, and I will
Best regards 
tl


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## trout lake

Bob
My apologies for my rude comments. There is no place on a form like this for rudness. After reviewing what I said, I was rude.
Hope you can forgive this error. Even tho you and I have made contact through e-mail and sorted this issue, I wanted the good fellows on this site to know that I was absolutly off base with my comments to you.
best regards
trout lake


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## bob young

*Trout Lake*

you have certainly shown yourself a a sincere Gentleman by the way you have conducted yourself. I appreciate that, sir. you & I are good , that is a fact. It is up to us to lead by example. bob young


trout lake said:


> Bob
> My apologies for my rude comments. There is no place on a form like this for rudness. After reviewing what I said, I was rude.
> Hope you can forgive this error. Even tho you and I have made contact through e-mail and sorted this issue, I wanted the good fellows on this site to know that I was absolutly off base with my comments to you.
> best regards
> trout lake


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## jayson22

I agree, I would do it in series as the tank that fixtures of the house will continue to take water from the tank of least resistance.


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## Protech

come again?


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## trout lake

Jayson
We have had some good debate on this subject. Almost 900 hits and lots of feed back. I think that my astute colleague's "come again" comments were directed your way. You said you agreed but didn't say with what. You need to be more spacific when you throw your view out. I think where my colleague was confused was by your general comment on the subject. You need to read what you wrote and see if it makes sence to you. If it does, your on the wrong form and obviously lite years ahead of me.
With respect, 
trout lake


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