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Direct to Indirect Switch?


In December, had a new Viessmann Vitocrossal CU3A-57 installed to replace a 1950's-era oil boiler. We had to run using propane for the winter but the nat gas line was just installed. We are going through the quote now to convert the boiler back to nat gas, run nat gas lines for future conversions of existing appliances, and conversion now of the existing Rinnai direct HW heater to nat gas.

Due to the Rinnai being 15 years old, they included a more expensive option to replace with Mega-Stor MS-53 (50 Gallon) indirect stainless steel water heater with circulator, valves, and controls for a new zone off of the Viessmann.

Not an expert at all, but had always heard that direct was the better option as tanks deteriorate and leak and you're paying to heat water constantly when it may not be needed, so I had a few questions for the remarkably knowledgeable people on this board.

1. Does the very high eff of the new boiler and leveraging of an already running boiler now negate the efficiency issue I thought indirect had?
2. The current system can take a while for HW to come out of the tap, sometimes as long as 30 seconds. Not sure if this is complicated by old piping added over time across a rambling house, but would this be better with an indirect?
3. Thoughts on the Mega-Stor? I also read somewhere that the lifetime warranty may be voided by chlorine treatment of the water. Is that a standard thing for town water, which I have?

Thanks for your time and thoughts.


  • hot_rod
    hot_rod Member Posts: 20,836
    pros and cons as you detailed. Definitely check your water, all stainless tanks have a very clear caution pertaining to chloride levels.

    Tanks give you high dhw dump load coverage, quick delivery depending on piping length.

    Many codes require recirculation loops if the distance is over 50' to most remote fixture, either a tank or tankless can have recirculation added..

    Tankless heaters or combination boiler do not store dhw, so less standby loss and less legionella concern. Flow rate is based on incoming water temperature and BTU output. A 120K combi should supply around 3 gpm constantly. So if you have long dhw demands, or want endless hot water you might prefer the combi.

    Most combis have two settings for hw, either maintain the boiler at temperature, or fire up on every dhw demand, so a bit of a lag on the cold boiler start option. A bit more energy consumption if you keep the boiler hot.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,497
    Who sized your new boiler? Unless your house is 8,000 sq ft, it is way oversized.

    I think generally if it is setup and installed correctly, the indirect option will run more efficiently than your present setup. Right now your water heater cycles ever time you use hot water. This cycling wastes energy.

    It is hard to say whether the delay you experience is because of piping distance or a performance issue with the Rinnai. The indirect will give you immediate hot water at the tank, without recirc you may still have to wait at the faucet.

    Chloride levels high enough to eat stainless steel are pretty unusual. You might check with a local supply house to see if they see tanks being returned.

    15 years is a long life for a Rinnai, if it were mine I would be installing the indirect.

    You might also consider your gas line sizing in the equation. When switching from propane to NG, you lose significant capacity. Your piping may not be adequate to carry 400,000 btu+.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 20,836
    I'd certainly check your chloride level occasionally. Public water supply can be well beyond what the tank manufacturer suggests. Warranty is void of course if levels exceed their limits. It's usually the welds that suffer the leaks due to crevice corrosion, the aggressive water in contact with the weakened metal caused by the weld process.

    This is an older study, I suspect levels have increased.

    Part of the Flint, Michigan water issue was a result of switching to the salty river water, road salt and deicer run off. Combined with the chemicals they used to treat the water it turned into a highly aggressive water.

    Colorado and Minnesota are about the two top users of chlorides for de and anti icers. The I-35 bridge collapse was also traced to corroded bolts and rivets from high chloride levels of the de icers.

    Chlorides stay wetter longer, really tough on steel bridges, vehicles and electrical components. A huge expense for the trucking industry that travel mag chloride highways.

    The public demands safe snow and ice free highways, that comes at a cost for water users and wildlife. Can you imagine telling skiers to travel I-70 to the resorts on snowpack roads. If CODOT decides to just plow and not de-ice :)

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream