Mind Teaser – Water Heater Wars

At the urging of several readers I have decided to, from time to time, present a little puzzle or “thinking exercise” here on the blog.  Maybe it will help those of us who are advancing in age to stave off the effects of Alzheimer’s disease or, better yet, just offer something to think about on long commutes or airplane trips.  I thought I’d start off with one that has been bugging me for many years now.  Truthfully, I do not know the answer and would really appreciate input from readers who are either opinionated or who actually know how to solve the quandary.  So here goes – –

My daughter, who lives in Texas, and her husband own a rather large house located outside of Dallas.  It isn’t like they own the largest house on the block because all they have in Texas are large houses.  Anyway, in the garage, there are two typical domestic water heaters connected in series.  The heated water from the first one flows into the second one and from there to the hot water plumbing for the house.  Both water heaters are set to just above medium on the temperature scale.  The engineer in me wants to question this arrangement – is this the best way to do things or a plumber’s “off the cuff” solution to providing more hot water?  The alternative, of course, is a parallel arrangement.  The two options are shown below.

Assumptions –

• All water heaters are identical and have the same capacity
• The water supply pressure is the same in both cases
• The water supply temperature is the same in both cases
• The aim is to supply water at a given temperature (140ºF, for example) at a specific flow rate for the maximum time.

Although this problem may look simple in the beginning, it gets more complex the more you think about it.  For example –

• Does the rate of temperature increase slow as the temperature of the water in the tanks increases due to a lower ΔT?
• What is the effect on heat loss through the tank  wall as temperature is increased?
• What effect does the introduction of cold water have on the temperature in the tank?  Does lower flow (as in the parallel arrangement) help prevent mixing of the incoming cold water with the hot water already in the tank thereby helping extend the time that the water exiting the tank remains at a higher temperature?

And a few other things –

• What effect will each arrangement have on energy efficiency?
• What should be the settings on the two hot water tanks for maximum performance and efficiency with each arrangement?
• What would be the effect of a spike in consumption (2 showers, the dish washer and the washing machine operating at the same time) in each case?

As I said at the beginning, I don’t know the answers to these questions and haven’t been able to find them using reasonable effort in internet searches.  Maybe they’re the same but I doubt it.  Readers, please help me out on this one.  Please flood the comment area below with responses and opinions.  If you can’t figure out the comment section, email me at jfuchs@ctgclean.com and I’ll take care of the rest.  If this was your house and you had the option, how would you connect the water heaters?

–  FJF  –