Indirect vs Storage HW Tanks

Discussion in 'Water Heater Forum, Tanks' started by Greatwhitewing, Apr 25, 2011.

  1. Greatwhitewing

    Greatwhitewing New Member

    Apr 19, 2011
    MA, United Staes
    I have talked to 4-5 professionals in the heating business. Three oil companies that install Boilers and tanks and two so-called energy companies who don't sell oil but design and install systems.

    I have quite a mixed bag of opinions although the majority favor indirect, two of the guys said the storage tank is the way to go. One each from oil company and one energy company

    By storage I mean a tank getting hot water from tankless coil and storing it with a re-circulation pump if temps drop.

    I understand the operational theory of both but why the large difference of opinion? Interested in how two professionals from the same type of business model can come to such widely differing conclusions.

    I currently have a tankless ONLY system and it will soon be gone. Boiler being replaced.
  2. johnjh2o1

    johnjh2o1 Plumbing Contractor for 49 years

    Aug 9, 2009
    I would go with the indirect. Using a tankless coil and a storage tank will give you some of the same problems that you have with a tankless coil. (coil getting plugged, low volume of hot water and having to maintain a boiler temperature in the summer) All these problems will be none existent with a indirect system, plus the saving in fuel.

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  4. Dana

    Dana In the trades

    Jan 14, 2009
    The efficiency of indirects vs. tankless coils depends a lot on the oversizing factor of the boiler to the heat load and the insulation level of the boiler with the coil, but the standby losses of having to keep the boiler hotter than necessary for space heating usually tips strongly in favor of indirects, espeicially with cold-start boilers. See:

    The best net performers tend to be with cold-start boilers using heat-purge controls.

    If your heating system is broken up in to several zones it's often more efficient to use a "reverse indirect" as the hyraulic seperator between boiler-loop and heating loops, and slave the boiler to the buffering reverse-indirect as it's only "zone". The temp of the reverse indirect should be 140-145F or whatever the min return water spec on the boiler is (+5F), or to whatever the radiation temp requirements are, whichever is higher. That way when the boiler loop starts the mass of the indirect pre-heats the boiler, and even boilers not designed for cold start can be set up to cold-start. Combined with a heat-purge control, the boiler is always cooled down to the indirect's temp at the end of a burn, minimizing boiler standby loss.


    The difference between one of these and a standard indirect is that the bulk water stored is heating system water, and the potable is heated as it passes through the internal heat exchanger. The advantage is that it can doulble or triple the thermal mass of the a low heating system resulting in fewer, longer, more effcient burn cycles independent of t-stat calls from zones.
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