There is a line T-ing off of the DRAIN line at the bottom of my water heater and I don't know what it is for. It is UPSTREAM of the valve that I need to open annually to remove sediment from the water heater. Does that mean that it is full of (hot) water under pressure? The water heater is located in the ground floor garage,. It feeds only my home, which occupies the top 2 stories of a 4 story townhouse/condominium. This line T's off from the drain line and is insulated up to the ceiling of the water heater closet (where it disappears from view...) Any ideas what it might be for would be appreciated. Thanks!

  • Do you have two pipes coming out the top of the water heater? Where do they go?
    – longneck
    May 28, 2015 at 22:34
  • On the top of the water heater there is the inlet pipe (water supply), the hot water outlet, and the pressure relief valve... The relief valve pipe goes off to the side of the water heater. The water supply and hot water outlet pipe penetrate the ceiling of that closet (right next to the 'unknown destination' pipe i'm asking about.
    – lady_diy
    May 28, 2015 at 23:08
  • Do you have an upper level laundry? If so, do you have a washing machine drain pan? This would explain it since those are often installed as emergency drains with no trap and don't use the main sewage piping. If the pipe in question really is just a tap onto the pressure relief drain pipe then it can't be a hot water recirc, because those dump back into the cold inlet side of the piping.
    – Jeff Meden
    May 29, 2015 at 12:18
  • @JeffMeden Only crap-tastic retrofit hot water recirc uses the cold water as a return pipe. A properly designed system uses a dedicated return pipe and does not make the cold water side of things hot. Given that the Tee is "UPSTREAM of the valve that I need to open annually to remove sediment" it would be spewing water into the drain pain if it was what you think it is...
    – Ecnerwal
    May 29, 2015 at 18:20

1 Answer 1


Most Likely - the return line of a hot water recirculation loop. ie, "instant hot water" by means of a circulating pump which pulls hot water from the hot line and returns it to the hot water heater through this tee on the drain line, after running it through the supply lines to your faucets. Given your location 2 (3?) stories above the heater this seems very likely (no guarantee that it's working, but the system was installed at some point, anyway.)

So yes, it's full of hot water under pressure. Sounds like decent workmanship if it's fully insulated.

If you do not have a long wait for hot water, it's probably functioning.

  • Thank you so much for the answer. This is very possible, although the construction of my home in 2000 was pretty "cheap" in many other regards so it would surprise me. It takes about 15-20 seconds to get hot water up 3 stories to the 4th floor tub... is that a "long wait"?
    – lady_diy
    May 29, 2015 at 10:28
  • Based on crude testing, yes. May not be the most effective possible setup (design details affect actual "time to hot water" even with a recirculation setup, depending what recirculates and what doesn't) but I get 45 seconds from "tap on" to "warm" (not fully hot) for hot water in the basement and sink on the second floor (no recirculation here) so 20 seconds for the 4th floor sounds like recirculation is working.
    – Ecnerwal
    May 29, 2015 at 18:29

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