GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Drainwater heat recovery on cast iron ok?

GBA Editor | Posted in Mechanicals on

I am installing a GFX (copper) drainwater heat recovery unit on my shower. about 10″ of horizontal pipe befero you get to it. Unbidden by me the plumber installed cast iron rather than the ABS called for. WOndering if the 10′ of cast iron pipe will rob the heat from the unit — more than the ABS would have?

Second issue — Can I assume the copper and cast iron do not corrode each other?

Thanks for any help.

ps: a blog of my project is at, in case you’re curious.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Interesting questions. Concerning the 10 feet of horizontal cast-iron pipe: my reaction is that yes, the cast-iron pipe will probably pull more heat out of the drainwater than would ABS. Ten feet isn't much; what I would do is simply remove the 10 feet of cast-iron pipe and replace it with ABS. Obviously, if the plumber didn't follow the job specs, the work shouldn't cost you anything.

    Cast iron and copper are usually joined with a rubber no-hub coupling that prevents any dielectric corrosion, so you should be fine on that account.

  2. Beatrice Dohrn | | #2

    Thanks - I decided to have it changed as well.

  3. david | | #3

    Have to wonder about the upfront cost of such a system, and the payback period.
    My low tech solution? Leave the warm water in the tub till it cools (heating the house)...then use the water to flush as needed. 2 for 1! No upfront cost!

  4. Riversong | | #4


    You wrote both 10" and 10' of horizontal cast iron drain pipe. Which is it?

    In either case, I would simply insulate the cast iron pipe with 1" closed-cell foam split insulation. The cast iron drain will be much quieter than plastic, particulary with insulation.

    Iron pipe will become the sacrificial anode to the more noble copper unless they are connected with a rubber hub-less connector.

  5. Riversong | | #5


    Your "low tech" approach is fine for a bath, as long as you're willing to scrub the bathtub ring. But for a shower, the drain heat-recovery system is about as low tech as you can get.

  6. Beatrice Dohrn | | #6

    It was 10 feet of pipe. I learned that the loss is fairly minimal and left it. Not sure that insulation does much since the heat is recovered from the inside temp of the pipe... In any case, it is all installed and now hidden from view behind a drywall chase. Question now is, is there any way to measure what it's actually doing.... It's plumbed into the COLD of my thermostatically controlled shower valve...

  7. Stephane Boisjoli | | #7

    Are you sure the shower valve is thermostatic and not pressure sensitive (most are pressure)? If you are not noticing a change in the temperature shortly after the shower runs, then it may very well be thermostatic. If it is pressure, you need only plug the drain after the shower is running for a bit to feel the difference.

    The easiest way is to take a temperature probe, stick it on the cold water pipe (near the water heater or wherever) and put some pipe insulation around the whole thing. Then take a temperature reading while someone is taking a shower. As well, you can put a temperature probe on the cold water as it comes into the house, to see how much the difference is. During the winter, with my Powerpipe, the water coming from the street is about 5C and the water from the Powerpipe is 18C.
    Alternatively, you could let a tap drip slowly while someone is taking a shower, you will get a tangible feel for the temperature difference.
    I also noticed the hot water seemed to run a little less long after (power vented tanks are so noisy).

  8. Riversong | | #8

    I suspect the heat loss is more than minimal (who told you that?). Cast iron is seven times as thermally conductive as ABS and has 2.5 times the specific heat (heat storage capacity) for the same thickness.

    The heat is not recovered "from the inside temperature of the pipe" but from the coil-to-pipe contact area on the outside of the drainpipe. The hottest water from the drain is going to transfer a good deal of heat to the cast iron pipe and reduce the recovered heat at the GFX unit. If you want to maximize heat recovery efficiency, then I would suggest opening up the chase and insulating the cast iron with the thickest closed-cell foam pipe insulation you can find for that size.

  9. Beatrice Dohrn | | #9

    Thanks to both of you for the responses. I will do the thermometer tests... I do have access just below the shower valve, so I think I can run some comparisons.

    1. I have a hansgrohe thermobalance 111 shower valve. Says it's Thermostatic.
    2. Unfortunately I can't retrieve the message, but I was in touch with the gfx'sinventor when I had the problem about the cast iron and he gave me some #s in terms of how many BTU's 10' of cast iron would absorb vs how many are in the water vs how many the GFX should recover and the proportion seemed minimal. Also, it seemed like since the insulation would be on the outside of the pipe, the majority of the damage would be done. In any case, the pipe is in the ceiling of a heated space, and it isn't getting insulation added now. PS: In case you're interested, I blog about my owner builder project at

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |