Dual water heater installation

Should two gas water heaters be installed in series or parallel?

By in series, I mean cold water flows into the first water heater, then the hot water flows into the second water heater, then distributed.

Series installation is worthless. . .

Why would you do this?

Thanks guys. Great help!

I didn’t set this up. I’m doing an HI and was being challenged. Just wanted some confirmation.

Electric water heaters in series works very well, I have mine setup this way.
I just turn the power on the 2nd tank when we have company.

I don’t see why gas would be any different.

Why would you want cold water entering both tanks and preventing the use of a whole tank of hot water as would be the case with a parallel setup?

With series plumbing, you get to use the whole tank which refills with all hot for awhile and then warm as the other tank cools down due to the cold entering.

A single large 80 gallon would be another good option.

Some interesting reading and opinions.

Congratulations, Your Water Heater Had Twins!

Inspect NY Article

Great Reading! Thanks Michael.

I also have my electric water heaters in a series. Gas WH’s should not be any different if done the right way (which it sounds to me these are not).

The ideal thing is to zone the house, but since that is so often not feasible then I prefer parallel. Equal usage and the ability to shut one of the units down.

Thanks Michael.

Nice Link

I have the ability to shut down one of my series tanks, it works very well.

The first tank that the cold well water reaches has a 240V switch on it.
When it is off, it serves as a storage tank that gets warmer from the garage in the summer (well water is very cold).

In the winter, it does not get any colder since the garage stays around the same temperature as the well water due to the slab thermal mass absorbing the ground heat.

This really is a great system, extra tank of hot water when you need it and a circulation pump on a timer and a few small “point of use” tanks for distant sinks when the loop is not running.

Here is an excellent site with diagrams of both types.
In series the larger tank should be plumbed first.

In parallel you will have a better output, for large amounts of use in a short time.

Think about this reasonable scenario,

How much good hot water can you get out of a single 50 gallon water heater with 55-60 deg water entering as water is used?

And remember, that heater (gas or electric both) will not heat quick enough to use all 50 gallons.

Lets guess the average useable hot water available is 30 gallons from that 50 gallon unit before it cools down below a desired shower temperature.

So two of these in parallel will give you about 60 gallons plus the added effect of two heaters running while trying to keep up so figure 65 gallons.

With two tanks setup in series, you will get almost 50 gallons out of the “output” tank and 30 gallons useable from the “input” tank for a total of about 80 gallons of good useable hot water.

Other variables such as heat loss, heat gain, flow, pressure etc. are not a big factor in this discussion since they exist in both setups.

All high end homes I inspect either have two tanks in series or one 80 gallon tank.
Then some have one or two tankless units.

When I designed my plumbing system, at first I thought parallel was the only way to go until I thought about it for five minutes. We have enough hot water for 5-6 showers even with two running at the same time with a 50 and 38 gallon electric in series.

With parallel, to turn off one unit, you have to turn off the gas valve or power and a water valve. With series you just turn off the power to the input tank.

Also with series, there is absolutely no real need to have one tank larger than the other as that web site states.