Rocket hot water for hot tub / spa

Discussion in 'Designing, building, making and powering your life' started by John Morrison, Mar 25, 2015.

  1. John Morrison

    John Morrison Junior Member

    Joined:
    Oct 2, 2014
    Messages:
    18
    Likes Received:
    5
    Trophy Points:
    3
    Im interested in designing a rockety fuel stove to a hot water heat exchanger suitable for a spa or hot tub. The tub holds approx 5000 litres and its enjoy it at about 36c. We expect to have a small pump to cycle the water. Ive looked at and visited Geoff Lawtons system and seen various systems on line but most are set to heat water for a few showers.

    I imagine it should take about 8-10 hours to heat this volume of water from 22-36c and as the spa is well insulated it could be maintained at this temp easy enough.

    Does anyone out there have experience in such a thing.
     
  2. Topher

    Topher Junior Member

    Joined:
    Feb 19, 2015
    Messages:
    25
    Likes Received:
    2
    Trophy Points:
    3
    Occupation:
    Energy & Permaculture Consultant
    Location:
    Midcoast Maine
    Home Page:
    Climate:
    Cold Temperate
    Greetings,

    No experience, but, 5000 liters going from 22°C to 36°C with a 90% efficient stove requires about 17 kilograms of wood.

    Spas tend to have about 3 inches of foam insulation, so expect to lose about 1.5°C per day.

    Thank You Kindly,
    Topher
     
  3. John Morrison

    John Morrison Junior Member

    Joined:
    Oct 2, 2014
    Messages:
    18
    Likes Received:
    5
    Trophy Points:
    3
    Thanks for that fantastic info where do you get such info.
    This spa is mounted in the ground and made from concrete so more loss would be expected initially the top is covered with 75mm 3 inch of foam.
     
  4. Topher

    Topher Junior Member

    Joined:
    Feb 19, 2015
    Messages:
    25
    Likes Received:
    2
    Trophy Points:
    3
    Occupation:
    Energy & Permaculture Consultant
    Location:
    Midcoast Maine
    Home Page:
    Climate:
    Cold Temperate
    Greetings,

    I am an energy auditor, so I know some numbers, but for your situation, it was just a matter of running the numbers.
    Wood is 8000 BTU/pound, Foam is R-5 per inch (US units), Water changes 1 degree per pound for each BTU added or subtracted.
    Heat loss (or gain) is Area * (1 / R-value) * temperature difference. That plus some conversion factors is it.

    Concrete insulation value is miniscule, less than 1/100th of foam. You are going to be losing a couple of degrees an hour through that.

    Thank You Kindly,
    Topher
     
  5. Pakanohida

    Pakanohida Junior Member

    Joined:
    Feb 27, 2011
    Messages:
    2,984
    Likes Received:
    20
    Trophy Points:
    38
  6. John Morrison

    John Morrison Junior Member

    Joined:
    Oct 2, 2014
    Messages:
    18
    Likes Received:
    5
    Trophy Points:
    3
    This books seems like a good read but I can't seem where I can get one
     

Share This Page

-->