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Descaling tankless vs indirect experience? What have you seen?

knkreb Member Posts: 13
I have a nursing home with a domestic water heating system that has always had issues. Just due to the shear amount of volume of water going through the system for the kitchen and the laundry. In it's history, it started with a Raypak boiler, that had all sorts of issues with flow etc. It finally started leaking at the 20 year mark. It was replaced with a Lochinvar, similar low mass, but DHW setup. It went through two heat exchangers. The latest iteration, is a Rinnai hybrid with two tankless 199's and a tank. They are fighting scale build up and plugging screens.

They've been very disappointed with the Rinnai's, but really, you can't fault the unit if it's a water quality issue. They are on city water. Everything looks to be in tolerance for Rinnai's specs as far as pH, iron, etc.

My question is about changing up the way the system works and the benefits.

Many years ago I was at a Crown boiler class. The guy teaching was talking about a hotel that went through water heaters every couple of years. They changed over to an indirect system and the scaling issues that were eating up the traditional style water heaters went away. This was due to lower TD heating the water at the point of heat exchange. Instead of a hot flame on the other side of the heating vessel, a closed loop water circuit at 180 was heating the water. Less TD meant less build up, but still had the heating performance.

So, my question for the fine folks here on the wall who have dealt in hydronics for centuries of experience here, do you find that a conventional boiler DHW coils and indirect water heaters scale less than today's tankless water heaters?

I was thinking of installation of flat plate heat exchangers to heat heat the domestic water off of a heating closed loop. This would help to eliminate the beating of tankless units with essentially a sacrificial unit in between.

Crazy or not?


  • Robert_25
    Robert_25 Member Posts: 533
    Generally speaking indirect water heaters will tolerate minerals better than tankless units, but if you have excess hardness in the water that will cause problems for appliances regardless of your choice of water heater.

    Get the water tested.  
  • clammy
    clammy Member Posts: 3,113
    Look into turbo max units . The coils are copper on the portable side and are pretty easy to flush if piped to do so and de scale if necessary and even easier to flush residue out of then tanks . I would think that you need some type of water treatment or possible increase storage and or lower the tank temps . I ve noticed the higher any tank indirect or storage is held at the more amount of lime and scale build up . I know that the tanks have to be set Mim to prevent and inhibit bacteria growth . I have a couple of smaller turbomax out there and haven’t had any issues and I ve installed a few w other professionals and there’s been no issue everybody's happy zero complaints and no lime ,calcium issues and the water is our area is terrible . If you don’t have a softener you have rings and stains . Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • hot_rod
    hot_rod Member Posts: 22,472
    In a facility like that you will want to maintain the tank at at least 140f for legionella potential mitigation. That is the most at risk population. So they is no way to stop the mineral precipitation by running lower temperatures.

    How hard is the water? The mineral precipation starts at day one, never stops, and efficiency drops from day one.

    Is softening an option? You don’t need to go down to 0 gpm, just lower it to a more acceptable range. I would soften just cold supply to the water heater. A wide varsity of opinions on magnetic or TAC treatment out there. Zilmet was showing a unique treatment at AHR.

    Tankless water heaters in Europe often add a phosphate doser on the cold feed to mitigate hard water scaling.

    Smooth coil I ndirects handle mineral build up the best. Reverse Indirects run much hotter boiler water side for their performance numbers in many cases, so the mineral precipitation is even higher. Turbulence through the coils helps keep them performing, but they also scale.

    Of all the HX methods, flat plates do pretty well in combis, small size and very high channel velocity may be part of it? Certainly easier to clean compared to an entire tank that is scaled and the outside surfaces of the coils in the tank. The Energy Kinetic HX system makes sense.

    Fix the water😏

    Perdue did a good study on DHW scaling years ago, I’ll try and find that
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream