I’m installing an electric tank water heater (Rheem Marathon) in a conditioned space. The hot pipe will be insulated all the way.

But how many feet of the cold (supply) line should I insulate? The pipe for it goes up 6in, sideways 8in, then down 6 ft through a floor.

CA requires insulating the first 5’ (p. 25 here), but couldn’t find anything in IPC. I’m looking for best practices more than a code citation though. I’m guessing the need for this has to do with insulating the part of the pipe that may become heated by thermosiphoning… and with my pipe route, that’d probably only be a couple feet, right? Let me know what you think, and thanks!


1 Answer 1


Heat moves from hot to cold objects. So insulating the hot line is good. ( barrier from heated water to colder air.) Insulating the incoming water may be counter productive, since the tank is in a conditioned space. The water coming into the tank may be colder than the air around the piping. Thus you can be preventing some of the cold from the water to be moved to the air. It all depends on the temperature of the water and the air around the piping. Perhaps only insulate that part of the pipe that is subject to colder air.

  • If you touch the incoming cold pipe on any hot water heater, you'll find that it's warm. This is due to thermosiphoning... the hot water rises and forces its way into the cold pipe. This introduces some inefficiency and makes your water heater run more than it should. "The water coming into the tank may be colder than the air around the piping." Yes -- but that delta-T will be maybe 15-25 degrees, assuming ~40-50F groundwater and ~65-75F air temp. Compare that to the delta-T of the water in the tank vs. the water in the cold pipe. Some inefficiency either way, but the latter's greater.
    – borisj
    Nov 7, 2022 at 14:01
  • 3
    Warm incoming pipes are due to simple conduction as much as anything. Copper is an excellent conductor.
    – isherwood
    Nov 14, 2022 at 21:45

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