# Tank Water Heaters in Parallel?



## Johnnyvee (Apr 22, 2012)

A client asked me to make a recommendation on replacing their water heaters. The 6 unit, 3 story condo has two 75 gallon, gas burning, piloted, low efficiency, tank type, A.O. Smith water heaters installed in parallel. Neither heater is leaking and both are working, but one of the owners is complaining of not having hot enough water for showers.

I did a sort of a draw down and make-up test on each of the units, and one tank showed a much lower makeup capacity, it took much longer to heat back up. Both tanks are identical and of the same age, about 10 years, but the lower performing tank has the recirculation system piped through it.

The tanks were installed in parallel. My first thought was that dual tank systems were usually installed in series, but I'm not a professional plumber, and whoever did this job was. I wonder, what would be the reason that dual tank type heaters would be installed in parallel vs series? Should I recommend re-plumbing these in series to fix the lukewarm water issue? Perhaps I should recommend replacing just the deficient heater? Any advice?


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## KillerToiletSpider (May 27, 2007)

You're in Illinois, so I would suggest you call a plumber, you are not licensed or insured to do this work, and I would never advise someone to break the law.


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## Johnnyvee (Apr 22, 2012)

My clients already had two licensed, insured plumbers come out, and spent 5 minutes each looking at the installation before giving them quotes. Neither of the professionals did any kind of test, nor explained what they would do to give my clients an installation that would be any better than what they already had.

I am only seeking to make a educated recommendation. I certainly would never attempt to do anything as obviously complicated and difficult as installing a water heater.


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## KillerToiletSpider (May 27, 2007)

Johnnyvee said:


> My clients already had two licensed, insured plumbers come out, and spent 5 minutes each looking at the installation before giving them quotes. Neither of the professionals did any kind of test, nor explained what they would do to give my clients an installation that would be any better than what they already had.
> 
> I am only seeking to make a educated recommendation. I certainly would never attempt to do anything as obviously complicated and difficult as installing a water heater.


I haven't even seen the job, and I don't need to do any tests at all to tell you that what is currently installed is not fit for the job it is expected to do. You have six condos, so six separate single family homes, and two water heaters that are designed to heat water for one single family home, do the math, you're four water heaters short. Residential water heaters are not designed to deliver the volume that you need for this application, you need a commercial water heater.


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## matttheplumber1 (Apr 26, 2012)

good luck


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## Johnnyvee (Apr 22, 2012)

*Water heaters Piped Series vs. Parallel - online research*

Although my question wasn't answered here, I was encouraged to research more on my own. Google searches of "Water Heater Series" and "Water Heater Parallel or Series" revealed a lot of discussion of this topic on various sites. Opinions of which is best seemed quite polar, with staunch supporters of both strategies. Many persons suggested that series installations had worked for them in many cases, while others said that they only installed in parallel, and had good results. I encourage anyone reading this message to search for themselves to find the various opinions and testimonials.

I would like to post here links to various piping diagrams and discussions of series vs. parallel water heater piping that I found on manufacturers sites.

Bradford White suggest parallel installations in most cases:
http://www.bradfordwhite.com/images/shared/pdfs/Piping/2_RESI_ATMOSPHERIC_VENT_MODELS_WITH_TOP.pdf
Bradford White shows series connection when a passive storage tank is used in conjunction with a tank type water heater:
http://www.bradfordwhite.com/images/shared/pdfs/Piping/1_ RESI_ATMOSPHERIC_VENT_MODEL_WITH_TOP.pdf
Bradford White technical staff are on record as recognizing the usefulness of both connection methods. These comments were attributed to Mike Gordon of Bradford White Corp. In Nov 1, 2003 of "Contractor" he reportedly said:

“Parallel and series installation of water heaters both have advantages and disadvantages. So the answer would be ‘it depends.’

“Parallel equalizes the run time of the heaters, distributes any mineral accumulation evenly and has lower water-side pressure drop. Series installation requires less piping and may be less expensive to install. In the BWC commercial installation manuals, we recommend parallel reverse return.

AO Smith suggests parallel connection in most cases. Document 315268-000 at this location is typical:
http://www.hotwater.com/resources/product-literature/piping-diagrams/residential-piping-diagrams/
For other applications, AO Smith recognizes the value of a series piped strategy. See document AOSCG61180 for "Single Flue (2 Unit) Two Temperatures" at this location for an example of series piping:
http://www.hotwater.com/resources/product-literature/piping-diagrams/commercial-tank-type-gas/

I found an interesting document from Rheem that pictures both connection methods and analyzes them in a general way. Rheem also seems to favor parallel connection:
http://www.texasinspector.com/Water Heater Plumbing.pdf

The engineers at Bock Water heaters up in Madison, Wisconsin, seem a bit more creative and prolific than the technical fellows at the big three. Their hookup diagrams show many interesting piping diagrams including a triple series hookup and dual temperature systems with and without mixing valves. They also discuss the relative merits of each strategy:
http://www.bockwaterheaters.com/technical/engineering_PDFS/5-Installation_08_06.pdf

My personal analysis is that both parallel and series installations have their benefits and faults, that I will attempt to summarize here:

Parallel piping
1) Requires identical heating units, in storage capacity, recovery capacity, and flow characteristics for full benefit.
2) Requires hookup piping to be identical in flow resistance.
3) Offers higher flow rate because heating units are in parallel although flow rate will likely be influenced much more by downstream piping than by flow resistance through the heaters themselves.
4) Because of Item 1, heaters should be replaced simultaneously, because age will affect both a heating unit's flow and heat transfer capacities.
5) With valves at both inlets and outlets of both heaters, a system can run from any heating unit by itself or with others, that is, any heater can be individually replaced, although this might affect item #1.

Series piping
1) Any capacity heating units, both in volume and heating rate, can be hooked up in series.
2) Flow rate of the system will be slower than flow rate through a parallel system using the same heating units, because the flow resistance of the heaters themselves is in series rather than parallel. Like the parallel system, the overall flow rate will likely be limited more by downstream piping than by its path through the heaters. 
3) System is not sensitive to flow balancing so heaters can be replaced or retained one at a time.
4) If plumbed with a bypass for each heater, in addition to valves at both inlet and outlet, system can be run from any single heating unit in the system.
5) If heater thermostats are set to split the overall heat gain, heating work will be distributed over the series. (If heater thermostats of both are set to the desired outlet temperature, the 1rst unit in the series will do a greater share of heating)

My personal opinion is that it is generally helpful to consider the principle of cui bono, or "who benefits" to understand why the parallel piping schematic is preferred by manufacturers of water heaters. A parallel system will require both water heaters to be replaced at the same time in order to function properly.

I hope this discussion is helpful to others. I am NOT a plumbing professional. I am a handyman, and also a degreed mechanical engineer.


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## KillerToiletSpider (May 27, 2007)

See post four, I answered your question.

It's not my fault you can't grasp the answer.


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## Inner10 (Mar 12, 2009)

KillerToiletSpider said:


> See post four, I answered your question.
> 
> It's not my fault you can't grasp the answer.


So parallel or series? :laughing::laughing::laughing:


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## Johnnyvee (Apr 22, 2012)

Sorry Toilet Spider,

I assumed that you were joking. It's pretty hard to take an answer seriously that starts with the suggestion, "I don't need to see anything, or test anything" to know the answer. assuming of course, first one got past your qualifications, i.e. specializing in "blowing things up", and your jokey avatar and goofy account name.

I just assumed that you were some kinda troll or clown, Sorry if you were seriously trying to help. If you read the original post again, you will see that I was primarily looking for general ideas of why a parallel vs. a series installation would be preferred.

Sorry I didn't include more info about the specific application, that wasn't the point of my post. But to satisfy your curiosity, these are small condos, converted from apartments, 1 bath, 1 kitchen, 1 stacked washer dryer, 1 dishwasher, installed in each of the 6 - 2 Bedroom condos. 1 unit is currently vacant, 1 unit has two adults, one working one not, and a toddler. 4 units have 1 working adult in each.

Personally I think it's important to note, that they were perfectly happy with the system for 10 years, as I suggested, and only recently a problem has been reported.

The other two pros, the ones who actually "Saw" but did not "Test" the installation, both suggested simply replacing the existing installation, 2 new 75 gallon, high recovery heaters, hooked up in parallel. Probably if one Pro hadn't quoted twice what the other one did, they would never have called on an amateur like myself to render an opinion.

But I'm not a pro. I would never have the confidence to suggest that I knew the answer without knowing anything, or to insist that I'm right when I haven't even considered what question is being asked. 

So, assuming that I was asking about sizing the installation, which I wasn't, I was asking about parallel vs. series, what sizing procedure should be applied?

Very simplistically, assuming that everyone takes a 3.0 gpm , 5 minute shower at the exact same moment, somebody runs a load in their dishwasher, and somebody else a load in their washing machine, it's pretty hard to come up with a peak usage rate much higher than 125 gallons in an hour, which it seems to me, the two 75K BTU each, 75 gallon each, residential heaters, should be able to handle.

What sized commercial heater(s) would YOU be recommending?


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## donerightwyo (Oct 10, 2011)

I think you might be smarter than most of the plumbers I know:clap:


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## KillerToiletSpider (May 27, 2007)

First of all, never size a system based on expected usage, you size it to allow for every fixture running at the same time. Now lets move along, a typical 75 gal. residential water heater is going to recover about 50 gals in an hour, this number decreases as the heater ages, so now you have six single family homes trying to share a hundred gals of hot water, two units doing laundry at the same time can run you out of hot water in no time fast, and it is going to take a full hour to get it back. On the other hand, a 120 gal commercial water heater can recover it's full working capacity in half that time.

I won't even attempt to size a water heater for a job I have no fixture unit sizing on, but I do know that what you have there was inadequate from day one, and has gotten worse as it aged.

But then again what do I and my joke of a username know? I'm not a mechanical engineer.


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## Tinstaafl (Jan 6, 2008)

KillerToiletSpider said:


> But then again what do I and my joke of a username know?


Quite obviously from historical evidence, a lot. But it does sadden me that it takes provocation to bring forth that knowledge without withering sarcasm. :sad:


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## Tom Struble (Mar 2, 2007)

besides..i'll do the clowning around here


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## KillerToiletSpider (May 27, 2007)

Tinstaafl said:


> Quite obviously from historical evidence, a lot. But it does sadden me that it takes provocation to bring forth that knowledge without withering sarcasm. :sad:


Sarcasm?

Me?


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## Tinstaafl (Jan 6, 2008)

KillerToiletSpider said:


> Sarcasm?
> 
> Me?


Tu, Brute.


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## Johnnyvee (Apr 22, 2012)

Wow Toilet Spider,

Now I am SURE that you are just having a joke on me.

It's hard to know what to say to your suggestion about sizing an installation not based on expected maximum usage, but on assuming that every fixture and appliance in the place is running full volume. With that approach, I imagine I would also need to try to sell this client a 1.5" gas line, and a larger sewer main to handle all that hot water running down the drain.

Since this thread is about parallel vs. series installation of water heaters, and I don't want to waste any more of anyone's time eliciting your "unusual" views, I won't say much, except that you seem to be "Unique" in your approach to sizing an installation. Every other source I've been able to find, suggests that sizing a water heater based on peak usage is the best strategy, and that the more apartments or condos or households that share a single system, the more "diversity factor" allows reduction of the total expected demand. I'm not looking to argue this point, but in the interest of helping other people who might read this in the future, I feel it's right to put the majority viewpoint forward.

I have heard that in ancient Sparta, issues were decided in favor of whoever could shout the loudest, or perhaps the longest. I know that you have much more time to post and pontificate than I do, so I'll leave you to it. Have fun!

Bradford White's tech. resources seem to be suggesting that a single 75 gallon, 75,000 BTU heater will be capable of supplying water to 6 apts with 6 bathrooms and 6 laundries:
http://rightspec.bradfordwhite.com/Sizing/Apt.aspx

AO Smith's tech folks think that 100,000 to 120,000 BTU/hr are needed, coupled with as little as 60 gallons of storage:
http://www.hotwatersizing.com/ApplicationData/Apartment.aspx

Rheem's sizing suggestions can be found here:
http://www.rheem.com/products/tank_water_heaters/how_to_size_a_water_heater


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## Kent Whitten (Mar 24, 2006)

I think that you may have ADD or something. What you think is going to work is irrelevant. The owner hired the wrong person to give him a recommendation. Exactly what is he supposed to do with your information? Tell a professional plumber your calculations? It's a joke, a farce. 

You're barking up the wrong tree.


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## Splinter (Apr 5, 2005)

Wow, someone wants to have a real discussion in the dead plumbing forum, and even the moderator gives him sh!t... 

Thanks for the interesting read Johnny.. Nice to see someone do some research first instead of the "lets try this and see if it works" approach.


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## txgencon (Jan 4, 2011)

Check the dip tube in the underperforming unit.


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## KillerToiletSpider (May 27, 2007)

Johnnyvee said:


> Wow Toilet Spider,
> 
> Now I am SURE that you are just having a joke on me.
> 
> ...


Wouldn't every fixture in use at the same time = whatever "peak usage" is?

Now I'm not an engineer, nor did I stay at a Holiday Inn Express last night, but when I went to school to earn my plumbers license we learned to correctly size systems, we didn't use the opinions published by manufacturers that are intended to sell product be it correct or not.

We also learned to understand the difference between storage capacity, and working capacity, and how British Thermal Units affect these capacities and why.

We also learned how to correct the misconceptions of the architects and engineers that are consulted on the prints we actually use to build buildings.


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