My wife and I recently purchased a new house. The house has several bedrooms, and therefore two water heaters. Because the crawl space is not real tall the builder went with electric 50 gallon water heaters. They are located side by side. Here is my question. They have these heaters plumbed parallel so when hot water is called for it comes from each of the heaters. That then calls for cold water to enter both tanks and turns on the element in each of the heaters. It seems to me that is they were plumbed in series, the first tank would heat the water and the second would keep it hot. When hot water was called for the water comes out of the second tank and is refilled by hot water. Therefore the first tank would be heating it and the second would most of the time just be maintaining. I can't find anyone who can give me an answer to this. Any ideas?

The thought is that you want both tanks to wear and age evenly.

Also, you'd be surprised at just how often the elements kick on in the other tank to maintain a static temperature in a series installation.

Most Plumbers aren't aware of this, but a proper parallel installation requires an equal length of piping to the inlet of the tanks from the central source, including the calculation of friction loss with the addition of each 90 degree change in direction.

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Originally Posted by iamgrowler

The thought is that you want both tanks to wear and age evenly.

Also, you'd be surprised at just how often the elements kick on in the other tank to maintain a static temperature in a series installation.

Most Plumbers aren't aware of this, but a proper parallel installation requires an equal length of piping to the inlet of the tanks from the central source, including the calculation of friction loss with the addition of each 90 degree change in direction.

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Thanks Growler
I doubt that the plumber gave any thought to the tanks wearing evenly. I just can't seem to ask this question correctly. Wouldn't the amount of electricity used be less to maintain the heat than to have both heater heating cold water? The issue is too complicated for me to get my mind around. If you start with two tanks or 100 gal of hot water, when you take out 10 gal of hot and put 5gal back in each tank haven't you cooled both tanks down?

I have never hooked up water heaters in parallel to get more volume. We just increase the capacity of the heater. While our homes are slab houses and the water heaters are located in the garage we don't have a problem fitting a heater in. It seems to me with the average home water heater size at 40 gallons that 100 gallons is excessive and is costing the home owner too much in electrical cost.
While I've never hooked water heaters up in parallel I have added a "kicker heater" in series to furnish hot water to master baths that were a long distance from the primary heater. Growler was correct when he stated, "Most Plumbers aren't aware of this, but a proper parallel installation requires an equal length of piping to the inlet of the tanks from the central source, including the calculation of friction loss with the addition of each 90 degree change in direction." Good point! In answer to Rockys question, common sense dictates that it costs more money to heat and maintain 100 gallons of water then to heat and maintain 50. Regards, Tom