Drain Heat Recovery Unit?

Discussion in 'Remodel Forum & Blog' started by michaelene, Feb 21, 2009.

  1. michaelene

    michaelene New Member

    Messages:
    5
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,615
    Location:
    Cave Creek, Arizona
    drain heat

    Yes, SAVE YOUR MONEY. The temperature of the drain water will be warm at best, and the water flowing through the coil will be in contact with it for such a short time that ANY heat transfer will be difficult to measure. Wrapping it around the heater's flue, on a gas heater, would make more sense, with only slightly better results.
  3. chris8796

    chris8796 New Member

    Messages:
    100
    Location:
    Illinois
    I have a different maker and it works well in our situation. My general guidelines have been, if your average cold water temp is below 55 and you take at least 30 minutes in showers a day you should give them a hard look. We have the ideal situation in that we take 3 or 4 showers back to back in the mornings. We were limited to a smaller one 40" because of space issues. We average about what the manufacturer reported (40% recovery at 2.5gpm), that is 40% of the remaining heat, you lose 15% in the showering process. For us, this results in a 20 degree increase in cold water temperature. When I measured mine, the cold water was 45 F before the exchanger and 65 F after. For showering, the average temp is 105 F, that is a 60 degree rise from 45, the exchanger provides 33% of the heat needed for my shower. You maybe able to squeeze 50% of the heat needed if you use a larger one 70-80". While I enjoy the 33% savings in showering heat costs the biggest benefit has been greatly extending the capacity of our HW tank. We can get 3-4 showers back to back out of a 40 g NG tank when before we were limited to 2 max. I can easily the tell the difference the 20 degree temperature rise has on the cold to hot water ratio. I also like the fact it is essentially idiot proof with no moving parts. The only limitation I'm aware of is some pressure loss at very high flow rates 7+ gallons /min.

    Here is the brand I have, I wish I lived in MN so I could get the $350 credit. I DIYed mine for $500 total and save probably $100 a year. I think it is was worth the investment, not many things give you 20% ROI a year.

    HJ, what is your experience to make you doubt them?
  4. michaelene

    michaelene New Member

    Messages:
    5
    Thanks for sharing your experience!

    Thanks for sharing your experience!
  5. dx

    dx General Contractor

    Messages:
    156
    Location:
    Michigan
    It is only practical in a single-story house. In a 2-story, most often all bathtubs/showers are on the upper floor. Also best with copper drain piping, which is not common.

    The "results" can be false/misleading. If you run a long copper line to the "recovery coil" at the shower drain and back, you pick up a lot of heat from the ambient of the house. And you pick it up all the time, not just when taking a shower. This doesn't require the coil, just some long lines.

    I'd be curious how Chris measured his 20 degree increase in cold water temp. Was it at the input and output of the coil itself or did it include the rise in the lines due to ambient?

    As already stated, it makes more sense to wrap it around the flue. Then it captures heat every time the heater runs, not just when taking showers. And more heat. Also would require only one coil per house regardless of how many bathrooms/showers you have.
  6. chris8796

    chris8796 New Member

    Messages:
    100
    Location:
    Illinois
    There is no limitation of being a single story house. The 2 showers we use are on the second floor and the exchanger is in the basement. I think it would be worse with a copper drain. You waste energy heating up the drain pipe, instead of making it to the heat exchanger.

    I haven't measured it explicitily, but once the piping and water are the same temperature there is very little heat transfer from the house to the water in the 30 feet from the basement to the upstairs. Heat transfer from still air (inside the wall cavity) to copper is very slow on the order of 2.3 btu/ft^2 hr. I don't see this being a factor, although it is easy enough to measure.

    I measured immediately before and after the exchanger, the easiest way to measure it.

    I wouldn't do this for several reasons. Combining different metals will result in a galvanic reaction and corrode the metal. How are you suppose to get good contact between the flue and copper pipe? You would also condense the flue gases making a big mess. You then need to drain this acidic condensate as it ate away your flue, just like you have to do with a condensing water heating.

    An average 40 g NG water heater is about 35000 btu/hr, assuming forty percent goes up the flue that is 14000 btu/hr you can capture (assuming you can capture 100%, probably be lucky to get 30%). In an hour long shower, I'm getting (2.5 g/min x 60 min x 8.8 lb/gal x 20 F) 26400 btu/hr. As a funny comparison, my well insulated 2600 sq/ft home had a heat load of 15000 btu/hr during the month of Jan when the average temperature was 17 F (70 F inside). Ain't math fun?
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