I know very little about water heaters and plumbing. Done some minimal work, but never water heater hookup.

I need to replace my 20 year old AO Smith 50 gal gas water heater in a house I've owned for nine years. Water heater is in the basement, all pipes are copper. The hot water outlet pipe at the top of the water goes up a few feet and branches to several pipes supplying water to various areas of the house.

But there is something curious about the way the drain on the water heater is setup.

A plumber giving me a bid noted that the drain outlet on the water heater has a valve, which opens to a two way junction. One side of the junction goes immediately to a standard outdoor hose faucet (seems normal so far). The other side of the junction is a pipe that leads back above the water heater, through the basement wall, and eventually rejoins one branch of the hot water outlet pipe (probably 10-12 feet away), specifically the one going to the kitchen sink.

The plumber, who seems very experienced, was perplexed by this configuration, said he'd never seen it and didn't know why it would be this way, and told me he'd have to undo this when installing the replacement.

There is a 2nd water heater in another part of the basement that isn't quite as old (also gas, 40 gal), and that I eventually would like to decommission. This 2nd water heater appears to have been installed about seven years after the first, but appears to have the same drain configuration, same two-way junction with one side leading to a pipe that travels some distance away and rejoins another hot water outlet pipe.

Somebody obviously went to a lot of work to setup the drains on both water heaters this way.

I'm wondering what function this configuration would serve, whether it is important, and should I ask that it be retained. Thanks!

  • This is the regular maintenance drain, not the T&P relief valve, right?
    – mmathis
    Commented Jan 23, 2018 at 19:12
  • Correct, this is the drain at the bottom of the water heater, not the temperature & pressure relief valve (which is near the top).
    – DS_
    Commented Jan 23, 2018 at 19:38
  • Why do you need two heaters , with about 90 gallons that is huge amount. If not needed one would be enough maybe even 30 gallon tank ..The money put in to doing this would be paid back in time off your bill Also have your electric hook up looked at seems 20 years or older
    – user101687
    Commented May 26, 2019 at 2:12

2 Answers 2


What you describe sounds like a Gravity Fed Hot water re-circulation system.

It's designed to keep hot water available at the kitchen sink without having to flush cold water out of the pipe first. The trade off is waste heat because the line out to the kitchen sink in your case is always kept hot.

Sometimes these systems have a pump, but they don't need to especially if the piping is above. As the water cools it becomes more dense and thus heavier, which creates the circulation.

The following diagram is shamelessly stolen form the internet.

enter image description here

Strictly opinion follows: it seems like an experienced plumber would recognize this method, it's been around a very long while.

  • I wonder if this can help minimize temperature stratification inside the water heater?
    – BillDOe
    Commented Jan 23, 2018 at 21:10
  • I've never seen a thermal loop hooked onto the drain. Seems kinda hacky to me. What that diagram shows isn't really going to do anything. The loop needs to go as high and as close as possible to that fixture above (like where the arrow is for #1). As shown, it'd just be like moving your tank to the other side of the basement. I've never seen a loop in a single story house... But +1, 'cause that's what it is, hacky as it may be.
    – Mazura
    Commented Jan 24, 2018 at 1:57
  • Also, thermal siphons don't need to go into the bottom of the tank. The tank anywhere is warmer than your pipes, guaranteed. So is the pipe coming just out the top; where they usually tap in. That way, you can just pull the tank w/o messing with the pipes ever again - assuming you use flexible connectors - request them if they haven't been mentioned.
    – Mazura
    Commented Jan 24, 2018 at 2:01
  • 1
    As @Tyson illustrated, I had this setup at my last house and it worked perfectly. I always had 40 gallons of hot water in the 40 gallon tank and almost instant hot water at all faucets.
    – d.george
    Commented Jan 24, 2018 at 11:25
  • 1
    @Mazura Thermal siphons do need to re-enter the bottom of the tank, it's part of what makes them work. The fact that the tanks heats internally and creates upward thermal convection is part of the magic that help "pull" cold water from the return end of the loop back in. Maximum "fall" outside the tank is also part of the magic that makes it work.
    – Tyson
    Commented Jan 24, 2018 at 13:28

Just a note supplementing the prior answer. The gravity feed need not be a large diameter pipe. Often 1/4" is used if the run is not long.

One advantage is that dishwashers will work better because each draw does not take in cold water for half the fill.

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