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Benefits of tankless coil AND indirect water heater tank?

IM305 Member Posts: 1
Just joined/1st post so please be patient:

Recently moved my family of 5 into 4000sqft house to a Boston suburb. Built in 1920, relevant systems replaced ~2010. The old owners didn't leave any instructions on how they normally used the systems, so I'm figuring it out myself.

For heating a Viessman Vitola 200 gas fired boiler feeds cast-iron baseboards throughout. For DHW there is a Rinnai tankless water heater, and there is also a (large) Superstor Ultra indirect water heater. The Viessman has a switch for heating and DHW only (labeled by someone "summer") Whenever we turn on hot water, I hear the tankless turn on (and continues to run until hot water is off). I can't really tell the flow of the connections from the tankless and the indirect.

A few questions: 1) Is there a particular benefit to having both the tankless and the indirect water heater? 2)With the indirect tank, is it normal for the tankless to turn on any time we call for hot water? 3) Is there a benefit to flipping the Viessman DHW switch in the summer, over leaving it in heating/winter position year round?

Thanks. And please save your critiques, I merely inherited this setup!


  • Hot_water_fan
    Hot_water_fan Member Posts: 1,854
    There’s nothing “wrong” with having a tankless and an indirect if they’re set up correctly. It’s just extremely overbuilt - it’s like having a minivan that can go 200 mph. It’s got capacity you’ll likely never come close to using unless you have a huge tub. 
  • hot_rod
    hot_rod Member Posts: 22,157
    It could be that the indirect pre heats the water to the tankless. That would make some sense, keeping the indirect low, say 80- 90 degrees would keep the Viessmann running at a high efficiency.
    Pre heating the supply to the tankless would increase its capacity.
    Although tankless do occasionally need to be delimed, so there is some maintenance.

    Perhaps the previous owner had a large DHW demand. A large tub to fill, body spray showers requiring high gpm?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Alan (California Radiant) Forbes
    It could be that the tankless and indirect are separate, providing DHW to different plumbing groups.

    Easy to figure out; just turn off the cold water feed to one of them and see which fixtures don't have hot water.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab