Preheat Tank

Discussion in 'General Discussions' started by Marc, Feb 3, 2009.

  1. Marc

    Marc New Member

    I believe this may have been covered before, but the search on this site is not very good.

    I've read that indirect hot water tanks (such as solar) are a good choice for preheat tanks but everyone I see is more expensive than an electric tank.

    Is there a certain brand out there that has a reasonably priced 50 gallon preheat tank or could I just use an electric tank and not hook it up to any power?
     
  2. Waukman

    Waukman Member


    I have the electric tank but not connected. Seems to me to be the best solution....
     
  3. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    ditto for me
     
  4. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    This is what most people do.
     
  5. Johndoe

    Johndoe New Member

    Added Question

    If you have a 85 gal & a 50 gal tank which tank should be used for the preheat tank? The 50 gal tank should be enough to take care of our needs. Are you wasting elect by heating the 85 gal?

    Thanks
    Johndoe
     
  6. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    An educated guess

    Johndoe,

    I'll take a stab at this purely on an energy savings level. I would suggest using the 85 gal as the preheat tank and the 50 gal as the day-day assuming that is enough for your requirements. My thoughts are that it is cheaper to heat a smaller tank than it is a larger tank.
     
  7. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    I agree with urthboy
     
  8. Looby

    Looby Member Forum Leader

    My geo designer/contractor recommended a 50 gal preheat tank
    feeding an 80 gal water heater. It seemed kinda strange at first,
    but he really seems to know what he's doing, and I've come to
    trust his judgement & experience.

    Like you, 50 gal water heater would usually be fine for us, but
    80 gal is really nice with an occasional house-full of guests. If
    the tanks are well-insulated, the additional heat loss from the
    larger "hot" tank is very minimal.

    Try this logic(?): The DSH can heat the smaller preheat tank
    more rapidly -- so, in everyday use, hotter water is available
    to replace what we draw from the big tank. There's no need
    to preheat more than we can possibly use (99% of the time).

    ...works for me,

    Looby
     
  9. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Asymetric tank sizing - interesting question

    Some random thoughts (select the ones you like best)

    1) Preheat works well at one day's typical use because preheat generation is slow but long (make warm water throughout cold night (or hot afternoon in summer)) and have it ready for heavy short hot water loads at ends of day (morning showers, evening laundry / dish washing)

    2) Maximum total heat recovery occurs with lowest EWTs into desuperheater exchanger. The larger the tank, the more total recovery.

    3) Weather variation favors a preheat tank sized for more than one day - extra preheated water from a high load day carries forward into a milder day.

    4) Standby losses will be slightly higher with a larger main tank.

    5) One option to temporarily increase total hot water (guests in town) would be to wire preheat tank but leave it off most of the time. Do this if electrical service is cheaply available for this addition.

    6) Looby's point is valid - fewer hotter gallons of preheated water may result in lowest run times of main heater - essentially trading quantity for quality.

    I lean toward the bigger preheat tank but would want to consider the following?

    A) Does user want / need fairly hot domestic water, 130+ vs 115? Need for very hot water favors small preheat tank.

    B) Is climate extreme? Long run times, summer and winter, favor larger preheat tank.

    C) Geo unit tonnage / operating efficiency / staging considerations: A small unit operating highly efficiently (open loop or very well designed closed loop), mostly in stage one will have much less superheat available for recovery, so the smaller preheat might work better.

    A large unit, and / or one roaring along in stage 2 ten+ hours a day will have lots of heat for recovery, so a larger preheat tank would best absorb it.
     
  10. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Great points

    Engineer,

    I may be printing this one off...
     
  11. Looby

    Looby Member Forum Leader

    All good points, engineer. BTW, my engineering intuition makes me
    suspect that the "optimum" solution would be equal-sized tanks
    (sort of analogous to impedance matching). OTOH, I also suspect
    that the difference will be minimal in either case -- as long as the
    electrically heated tank is large enough to satisfy normal usage.

    Optimist: "The glass is half full."

    Pessimist: "The glass is half empty."

    Engineer: "The glass is larger, heavier, and more expensive than necessary."

    Looby
     
  12. Johndoe

    Johndoe New Member

    Thanks Guys
    Johndoe
     

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