I am replacing one of two 40-gallon gas hot water tanks. They both have cold water inlets. Why did they install them in parallel-series. I live in a two-story home with 3,500 square feet.

Would it be to my advantage to install a 50-gallon tank in place of the old 40-gallon? Is this connection described above a feature for additional hot water or is one tank serving as a reserve (pre-heat tank) for the home. And does it save any money on gas?

Is it parallel or is it series, it can't be both. Once the hot water pipe leaves the heater do the two come together making one supply to the house or do they each serve different parts of the home? The square footage of the house does not demand multiple heaters but length of run and numbers of persons does have an effect. Tell us about those two toipics.

Both of the two 40 gallon tanks supply hot water to the same line. Would it be to my advantage to change out one tank to a 50 gallon?

Both of these 40 gallon hot water tanks supply a single line that splits to service the east and west sides of the home.

In my opinion, serial installation is better system. Cold water should enter your heater #1 on cold water side. Then, it should exit hot water side - and connect to the cold water side on heater #2. Hot water then leaves heater #2 and enters your house. In other words, your 2nd heater is being fed by preheated water from your first heater.

Thermostat on Heater #1 should be set to about 100F. Thermostat on Heater #2 should be set to about 125 F. First heater preheats water and second heater brings temperature up to desired temperature.

This works well with hot water circulation system.

We usually do this type of installation when client has - let's say 100 gln residential w/h and looks for cheaper way to replace it. He will get same amount of hot water for lot less money.

Yes, you can install 50 gln heater in place of your aging 40 gallon heater. If you do so, you will increase your hot water capacity by about 12.5 %.

Both of the two 40 gallon tanks supply hot water to the same line.

This is a parallel hook up giving you a 80 gallon volume. The only fault I can find with using two water heaters is, 1) the initial cost of two heaters, 2) the extra installation cost **and** 3) the cost of maintaining two heaters instead of just one. Oh yeah! 4) the space taken up by the second heater.

Would it be to my advantage to change out one tank to a 50 gallon? No! Milo covered that. Good luck in whatever you decide. Tom

I am replacing one of two 40-gallon gas hot water tanks. They both have cold water inlets. Why did they install them in parallel-series. I live in a two-story home with 3,500 square feet.

Would it be to my advantage to install a 50-gallon tank in place of the old 40-gallon? Is this connection described above a feature for additional hot water or is one tank serving as a reserve (pre-heat tank) for the home. And does it save any money on gas?

If it is a parallel installation, then both tanks need to be of the same capacity.

Differentials in capacity and even length of inlet/outlet piping can cause one tank (the one with the smallest capacity and shortest inlet/outlet piping) to be overworked.

If you are going to replace one of the tanks with a larger tank, then the most efficient installation method would be to put them in series.

Hate to seem dumb but based on the description, exiting hot feed 2nd tank cold inlet, this sounds like series to me not parallel. Maybe my head is blocked but I think parallel would be they each have their own dedicated cold feed and hot exits. If the two hots come together later this makes them parallel, if the hot feed totally separate parts of the home they are neither parallel or series. I think I should drink my coffee and reread this.

Both of the two 40 gallon tanks supply hot water to the same line.
Smells like a parallel hookup to me Bob.

I think parallel would be they each have their own dedicated cold feed and hot exits. Then they would just be two heaters supplying separate areas.
And Growler,
If it is a parallel installation, then both tanks need to be of the same capacity.
GLC already stated that the two tanks were 40 gallon. So it's, indeed, a parallel installation.
Cheers, Tom

I just spotted my error. I read Milo's suggestion , post #5,and mistook it for the home owners description.