# Series or parallel?



## Qplumb (Dec 19, 2015)

I always plumb 2 water heaters in series and recently another plumber I know said he always plumbs them parallel because it's better. I asked him why it's better, he didn't really have an answer. 
What do you guys prefer & why? 

My reasoning. ....
only 70% of the hot water in a water heater is usable because the other 30% gets diluted by the cold coming in. A 50 gallon will give about 37 gallons of usable hot water. In parallel two 50 gallon water heater will only give 74 gallons of hot water. In series the second water heater is being filled with hot water from the first giving you a full 50 gallons of usable hot water. Combined with the first water heater that's 87 gallons of usable hot water. 
Second reason is the efficiency, in parallel when you use a few gallons of water both water heater kick on using about 80,000 BTU. In series only one would kick on. The other plumber said it shortens the life of the first water heater but in 18 years of doing this that doesn't hold true.


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## plumberkc (Jun 23, 2011)

The problem with running in series with the first at a lower temp is the potential for bacteria growth.


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## Qplumb (Dec 19, 2015)

I always set them to the same temp. Usually 140° unless customer wants lower temp.


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## TerryO (Oct 12, 2012)

According to AO Smith you should always pipe in parallel. Their reasoning is that if piped in series the first heater does the bulk of the work and will wear out earlier. When piping parallel though it's important to run equal length and fittings to each to insure each gets equal flow. 

This is from AO Smith


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## Redwood (Sep 8, 2008)

Series or Parallel?

Neither!

And I don't care what AO Smith says...
They make crappy water heaters...


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## Mykeeb33 (Mar 6, 2015)

What's wrong with them?


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## plumberkc (Jun 23, 2011)

Redwood said:


> Series or Parallel? Neither! And I don't care what AO Smith says... They make crappy water heaters...



Opinions are just that. My site http://waterheaterdb.com/brands provides actual instances proving that AO Smith makes the longest lasting heaters.


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## Redwood (Sep 8, 2008)

plumberkc said:


> Opinions are just that. My site http://waterheaterdb.com/brands provides actual instances proving that AO Smith makes the longest lasting heaters.


But then again that would be your opinion...:laughing:


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## Blackhawk (Jul 23, 2014)

I will plumb it how ever it makes it easiest for me on replacement or service work...

If I am starting a new install I pipe units in parallel.


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## plumberkc (Jun 23, 2011)

Most of the heaters here are in series, I offer to upgrade to parallel for an extra $100 on every job. Biggest advantage is if one leaks you can shut it down and still have hot water.


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## plumberkc (Jun 23, 2011)

Redwood said:


> But then again that would be your opinion...:laughing:


I really don't have a preference as long as it's not Rheem. I get BW for the same price as AO and install them whenever somebody chooses to go with American made. Well, the gas valve is still made in Mexico.


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## tdp1 (Sep 21, 2010)

there is also a way to pipe them with a few extra valves to where you can use series or parallel.


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## GAN (Jul 10, 2012)

Series uneven use potential mineral deposits worse in first one.

Parallel, even use across the board, more volume to draw off of. Of course if the piping is not equal you will draw off of one more than the other.


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## Redwood (Sep 8, 2008)

I said neither...


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