Series or Parallel

RangerMike

I am considering installing two gas water heaters side-by-side.  Plumber says wrong to hook up in series, that correct way is in parallel.  Why?  Parallel requires tees, and check valves, etc, Series eliminates pressure differentials.  I assume plumber is correct but Why?

Steve Whitbeck

paralell

If you hook them up in paralell just make sure to have the hot water line come off of a T that is centered between both heaters. ( same length and fittings on each side.

Feed the cold water into the bottom drains. ( remove the drains and put a T then tie the bottoms of both water heaters togeather) T them the same as the hot on top. You don't need check valves. The water heaters will have the same level of hot water in each.

If you put them is series the first one does all of the work and will wear out first.

icesailor

Connections:

It is my experience over time that you do not need to connect the cold to the bottom of the heaters. The dip tubes on the cold water inlet will fulfill that function. I have always connected two water heaters as balanced with equal distances between connections. If I/you do three, you must do it as a parallel reverse return.

It was pointed out here on Heatinghelp.com that Parallel reverse return works just as well on two water heaters. You do not need check valves. I have never used them and found no need to do so. From this point on, when I need to connect two water heaters, it will be a parallel reverse return connection.

jumper

why not go commercial ?

I haven't priced them lately, but.....

saves room & piping complexity to buy a bigger water heater.

icesailor

Commercial:

It sounds to me like this is a residential application. If you go commercial, you are paying for an ASME rating that you don't need.

If you compare two 40gallon water heaters, and you go to one bigger one, say a 70 gallon, you get twice the recovery with two heaters than you do with the one.

Two always work better than one bigger one in my experience.

hot_rod

depends on water usage

on large DHW jobs for vacation or ski homes we would pipe in series. The first tank could be set low, off or at 140 when the house filled up, normally run on one tank when low occupancy.



That reduces standby loss and allows the first tank to collect most deposits.



Solar thermal tanks are usually piped like this in series, the solar pre-heats and the second fired tank takes it to setpoint temperature.



122F seems to be what most codes suggest, setting above that temperature would require a listed ASME mixing valve to protect from potential scald hazard.



I see Rheem has a small capacity high recovery tank on the market, something like 29 gallons with a 60,000 burner. XR90 is the model. Supposed to reduce operating cost up to 17% compared to 50 gallon. That may be enough to keep up with a constant 2 gpm load, reducing standby and footprint. They should market it as a mate to a solar pre-heat tank.



hr



hr