Tale of Two Water Heaters.

Can you hook up two gas water heaters together? Looking for documentation from manufacturers one way or another and can’t find Installation instructions from any manufacturer to say either way?


I am looking for this info only and am aware of other issues you will see in the pic…but please limit help here with just the requested installation help.

As a side note, these are two 40 gal heaters in a 2600 square foot home. They are over 12 years old and have safety issues that we are just going to recommend replacement and to one larger and properly sized heater. This would make the point mute on this type of installation, but I would like to find documentation somewhere how you can install or why you can’t install this way.

Thanks so much in advance.

quick glance of the photo indicates that the 2 are plumbed in series.

What were your noted concerns?

Hooking up two together is fine but it should be done in parallel instead of series like your picture.


Thank you Larry.

…glad it was helpful.

Series installation is a very good way to do it.
Parallel allows both tanks to become warm as the cold enters both.

I have mine in series and it works very well.
I keep the first one turned off unless we have company.

I concur with Larry because in a Series type installation, water flows into and out of the first unit and then flows into and out of the second unit. The second unit in this type of setup will receive preheated water. The second unit will not function properly if it is supplied with preheated water.

In a Parallel type installation, both units receive cold water at the same time and water leaves both units relatively at the same time. The plumbing pipe is separated in two (Y configuration) before entering both units and is joined back again after leaving both units. In a parallel configuration, the units will function properly since there is no preheated water entering either unit.

So, Parallel piping is much better in my mind.

This is what it should look like…

“Click to Enlarge”

David, I agree if you are trying to even out the lifespan of the two units.

However, Bruces experience is also valid IMO. With the HW tanks in series the first tank is able to act as a tempering tank and bring the water closer to room temperature before it is additionally heated in the second unit. When the need arises for increased capacity the first heater can be activated to provide more output during the time of need. The only drawback I see is the need to turn up the T-stat on the first tank from time to time.

I highly suggest Twin Parallel Water Heaters with complete bypass systems for each, over a single large water heater!

Normally, both water heaters do not die at the same time. And, when do they usually go? In the evening on the weekend or on a Holiday.


  1. Faster recovery time. (2@37+ gal/hour or 1@50+ gal/hour)
  2. If you do not need large amounts of hot water you can completely shut down one water heater. Energy Saving (Teenagers have gone off to collage)
  3. If one water heater fails on a Friday night you can completely shut that one down and still have Hot water until the weekday when a service call will be less expensive.
  4. Initial install cost is about the same for 2@40 gal w/bypass or 1@80 gal, including parts & supply’s.
  5. Try to find an 80 gal propane water heater in stock anywhere when you need one. (Most stores & plumbers do not have 80gal water heaters in stock.)
  6. Less expensive to replace 1@40 gal water heater then 1@ 80 gal.
  7. You will have a warning to start putting money aside to replace the second water heater, it will most likely need replacement in the next 6 months.


  1. 2@40gal uses 1.5 to 2 times the floor space of 1@80 gal water heater.
  2. Second 40 gal water heater will be in need of replacement most likely within the next 6 months. (Not a real CON in my book, see PRO #7.)
    I can not think of any other CON’s.

I think the PRO’s out weigh the CON’s.

Even if they are connected in series I would personally like to have bypasses for both tanks. This way if one fails I can shut down the failed unit and still have Hot water while waiting for the other to be replaced.

Anyone else have any thoughts about this list?

The AO Smith Tech Bulletin referenced in the document above:


Gentleman, thanks! Those are excellent posts and reasons for one way or the other. That is exactly the info I was looking for. Again thanks a million!

Just for future knowledge, so you don’t say it to a client. It is a moot point, not a mute point.

Now that would be a mute point?:mrgreen:

… or you could just buy a larger water heater, Ive seen them up to 119 Gallons. I wonder which would be more cost effective?? Anyone? Larger water heater or connecting smaller ones?

See my post… Post #10, Pro’s & Con’s.


Sorry Jason, breeeeeeezed right over this. thanks

That’s a great reference - I must be having a good day - I read that and said, “Duh!” it made absolute sense!:shock:

I have seen two hooked up in series and it worked great.
40 gal to do main part of the home .
A small kitchen 35 feet away took forever to get warm water .
12 gal in crawl space Hot water fed from the 40 gal almost instantly and they never ran out of hot water in the kitchen .
Worked just the way they wanted it too.

I understand, Some of my posts have been becoming a bit long lately. :smiley: