Given the option of tank or tankless, which do you choose

Discussion in 'Tankless Water Heater Forum' started by chefwong, Mar 20, 2014.

  1. chefwong

    chefwong Member

    Messages:
    710
    Location:
    District of Columbia
    I'm mulling over switching over to tankless in the new crib.
    My hestitation is just the ducts involved which is a major b*t#h.

    Tankless is more money, and I get the capacity of hot water I want.
    Not sure if it's improved over the years, but I had tankless in another place and it saved my a whole $5 per month.
    There was surges of colder water - when someone use hot from another faucet.

    Current boiler room will require me chopping the wall/redoing the pipe to flue if I want something larger than a 50 gal.

    Got a nice big gas feed coming into the room, so gas volume should be okay.
    The only drawback is just the mess and work involved in properly running the ducts if I go tankless.
  2. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    If you have a hydronic heating boiler, use an indirect-fired hot water tank and NOT a tankless. The additional load of for the boiler improves it's as-used efficiency, and you won't have the flow limitations or cold-water sandwich quirks of a tankless, and an indirect tank doesn't need another flue- it's heated by the boiler. Cost-wise it'll probably be cheaper than a tankless install.

    If the boiler is some decrepit oversized piece of junk from the 1930s, replacing it with a better more efficient and right-sized boiler and an indirect-fired tank is appropriate, but even a 3x oversized mid-efficiency 85% AFUE cast-iron beastie controlled with a heat purging controller (eg Intellicon 3250HW+, but there are several others.) would come in about the same net hot-water efficiency of a condensing tankless. Standard D.O.E. EF testing over-rate the true efficiency of tankless units, since it's 10 equal-sized draws per day rather than 25-30 draws, most of them short-cycles. An indirect married to a cast-iron boiler has a much faster recovery rate than most standalones- how fast the recovery is, and how much continuous flow you can keep up with depends on the boiler's output ratings and the size of the heat exchanger in the indirect, but most homes with a big boiler can do just fine with a 30-40gallon indirect, whereas they might need a 50 if it were a standalone tank.

    Putting some numbers behind it, the typical burner output of a 50 gallon standalone is 28-35,000 BTU/hr, the output of a 50KBTU/hr modulating-condensing boiler is over 45,000 BTU/hr. Oversized cast iron is often over 80K, sometimes over 100. A normal-flow shower takes less than 75 KBTU/hr, and with even 30 gallons of buffering storage to manage other intermittent simultaneous draws the hot water performance can be pretty good, but a 50K mod-con won't support a 24/7 endless shower condition (unless you also installed drainwater heat recovery, which is a whole 'nuther subject.)

    The simplest approach is to just size the indirect tank for the biggest tub you'll ever need to fill and be done with it, which works pretty well even when the boiler is a smaller low-mass modulating/condensing boiler. If you're anticipating installing a six side spray monster-gusher luxury shower you'll have to do some math.

    So, how much boiler do you have? (Type, approximate age, rated input-BTU/hr, D.O.E. output BTU/hr on the name-plate, if you can find it.)
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