passive greywater heat exchanger

it might not surprise you that i spend a decent amount of time researching ways to reduce the household energy use and increase efficiency.  while doing research on grey water collection i was reading about a device that collects the heat from greywater and radiates it to the house as it drains into the collection tank via a heat exchanger.

this seemed like a pretty complex device to collect what didn’t seem like that much heat.  i don’t use much hot water for washing dishes or taking a bath, and most of our clothes washing is done in cold water.

it did get my gears turning.

what if you could do something that required no work and could save this heat?  that would be worth it right?  it occurred to me that if you just left your hot bath water to sit until the water in tub was cold, it would have already achieved the task of extracting all the heat out of it before it went down the drain, making the need for a greywater heat extractor unnecessary.

so thats what i do now, leave the water in the tub until it has become cold.  how much this saves in energy savings, i have no idea, but if any one technically minded wants to calculate this, i would be very interested to find out.



5 responses to “passive greywater heat exchanger

  1. To figure out the btu’s or kilojoules you can go here:
    You just need to identify a few things (Imperial method):
    how much water
    specific heat capacity of water in btu’s = 8.3
    temperature when initially filled
    temperature when drained (which should equal the surrounding air temp) to get the temperature difference to get the difference or delta T.
    So if you fill the tub with 25 gallons(?) to bathe and the incoming temperature of the water is 100F and the temp when drained is 66F
    Btu= (100-66)*25*8.3
    7,055 btu’s

  2. That calculation is good enough to see how waste energy can be utilized. 7.055 btu is about 2 kilo watts That is enough energy to heat a 140 sq.ft room for two or three hours.
    You can imagine how much energy could be saved if millions of people did it. If you shower let the water collect in the tub and then dump it when cold.
    The down side is cold scummy water eventually fouls the drainage pipes so every couple of weeks you should run hot water for a 2 or 3 minutes to clear away the goop. Same applies in the kitchen sink but the fouling is more severe due to grease, so a daily hot soapy flush is necessary.

  3. just wanted you to know that i visited your blog today and thanks for the many lessons.. love ya! Ginny

  4. Nice post…the information here is something everyone should consider..thanks

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