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Poor Man's Solar Water Heater

Discussion in 'Home Power and Microgeneration' started by David Williams, Jun 21, 2007.

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  1. -> First, by way of background... I consume a little over 800 litres
    -> (210 U.S. gallons) of heating oil per year for space heating and
    -> domestic hot water purposes. By my estimates, water heating accounts
    -> for over half this amount and a large percentage of this can be
    -> attributed to stack and tank related losses. My hot water consumption
    -> is rather modest and there's probably not a whole lot more I can do to
    -> reduce it any further (well, not without causing marital disharmony,
    -> that is).

    -> And therein lies the rub. I can't justify the cost of installing a
    -> solar how water system when my DHW demand doesn't warrant it.

    Have you thought of recycling the heat that goes down the drain when
    you use hot water? When I re-did my bathroom, some years ago, I
    installed a heat exchanger that allows heat to pass from the warm water
    in the shower drain to the incoming cold water that is going to the
    "cold" supply to the shower, and also to the water-heater. (Doing it
    for the shower is especially easy, since warm water is going down the
    drain at the same time as cold water is being drawn in.) I rigged up
    the heat exchanger myself, using a 2-inch copper pipe, about ten feet
    long, to carry the drain water, and a 2.5-inch pipe arranged
    concentrically with it, so the narrower pipe runs inside the wider one,
    with the cold water flowing in this "jacket". The flow directions are
    opposite, which maximizes heat exchange. A bit of scrounging around in
    plumbing stores let me find some fittings that I could use to make this
    arrangement work. The pipes are installed horizontally, so the drain
    water flows at a leisurely pace, giving up its heat. I did some
    measurements which showed that it recycles about 30% of the available
    heat, which is a heck of a sight better than none!

    dow
     
  2. Hi David,

    That's a clever idea. The first fill on my front loader is set to
    warm, but all subsequent fills (rinses) are cold, so there's probably
    not a whole lot to be gained by recapturing this waste heat.

    I did look at installing a GFX on my shower drain, but even at today's
    relatively high fuel costs, the economic payback is not all that
    compelling. In rough terms, two five-minute showers at 2 gallons per
    minute and an average temperature rise of 60F represents about 10,000
    BTUs of heat demand. That translates to be something in the order of
    0.35 litres of fuel oil per day or approximately 128 litres (34
    gallons) per year. If the GFX could recapture half that (and that
    might be somewhat optimistic), I would expect to save about 65
    litres/year or about $55.00 at current fuel oil prices (i.e., $3.20
    per US gallon). If there were teenagers in the household, it would be
    a done deal, but since there are only the two of us and we're both
    pretty water and energy frugal, I really couldn't justify it.

    Cheers,
    Paul
     
  3. Eric

    Eric Guest

    I have serious doubts about the effectiveness of such a system. Drainwater
    is less than 100F (probably less than 85F by the time it gets very far down
    the drain) and is only in brief contact with the heat exchanger. Given the
    flow rate through the heat exchanger for the incoming cold water, I'd be
    surprised if it raised the temperature of the water by even 1 degree.
    Eric
     
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