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Radiant Heating System Corrosion

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Looking for help with my radiant heating system. I built a new house in 2016 with radiant heat and indirect DHW. Not long after moving into the house I started having to replace the Taco (cast iron) circulators as they were corroding/scaling and having bearing issues. The circulators sometimes only lasted a year. After about 6 years being in the house the expansion tank developed a pin hole leak and a few months later (early 2022) the heat exchanger in the Weil Mcclain boiler developed a leak. It was decided at that time to replace the boiler with a Navien boiler, add an additional micro bubble resorber and to replace all the zone pumps. There had been numerous issues with the WM boiler from day one. In the process of replacing the expansion tank a hand tight pipe fitting was found and was thought to be a possible source of the Oxygen entry into the system.

The water in the system from day one always had an orange rust color to it and still does. I remember growing up helping my dad install some cast iron radiators in our family house and the water was always gray black which I took as a sign of an anoxic system. As an aside his American Standard hot water boiler has been running since the early 1950s and is still going strong.

I was working in my basement tonight and I notice a dry water spot on my floor near the boiler. Upon investigation I found staining and what appears to be a leak in the bubble resorber that was installed when the new boiler was installed. The feed water for the boiler comes from my city domestic water that is softened with a Culligan unit because the city water is very high in calcium hardness.

Any help would greatly appreciated.



Comments

  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    edited September 2023
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    Does your tubing have an Oxygen barrier?

    A chemical analysis needs to be done on your system water and your fill water. 

    With 8 zones, how big is this system? 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • NewHouse2016
    NewHouse2016 Member Posts: 5
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    The system is Onix staple up tubing with Pex in the basement slab. Both have an O2 barrier.

    I can have the water tested, are there specific things that I should test for?

    The basement is 3500 Sqft - three zones. Requires very little heat.

    First floor is 3500 Sqft - three zones.

    Second floor is 3,000 Sqft - four zones (One for each bedroom).

    Boiler output 293,000 BTU
  • Big Ed_4
    Big Ed_4 Member Posts: 2,831
    edited September 2023
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    Check the printing on the tubing to make sure they installed heat Pex and also test for a slab leak . To test for a leak , shut off the boiler feed valve , then watch for the system pressure drop on the pressure gauge ... Incorrect Pex and or a leak will introduce oxygen into the system which will rot out the iron .....

    There was an error rendering this rich post.

  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    Onix.....

    That's your problem. Sludged up and rusty symptoms and Onix, they seem to go together. 

    I'd install a flat plate heat exchanger and isolate the Onix from the rest of the system. 

    Or replace all the Onix with hePEX
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    kcopp
  • kcopp
    kcopp Member Posts: 4,448
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    I will second Onix being the primary culprit.
    They claim an O2 barrier but its a poor one at best.
    I have been able to work on these systems by replacing as many ferrous components as possible and doing a complete system flush.
    Adding a system inhibitor is a must....Rohmar, Adey, Fernox...
    Find a contractor who is familiar w/ working on these type of set ups.
    Where area are you located?


  • NewHouse2016
    NewHouse2016 Member Posts: 5
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    Being that I have the Onix buried in drywall ceilings replacing it is not really an option.
    All the Pex in the system appears to be correct with O2 barrier and proper temp rating.
    The only ferrous components in the system are the circulators and two pipe nipples on the indirect water tank. All other ferrous fitting that were installed I had them remove them when the system was being installed, I missed the two on the DHW tank.

    Are there concerns about adding chemicals to the system when it is being used to heat an indirect hot water tank? I would be concerned about the coil leaking and allowing treated water to enter the potable water?

    I questioned the Onix before it was installed because I had read mixed reviews online. I was assured by the installer and the rep selling it that it was the best product to install. But they also talked me out of heat transfer plates with pex and also warmboard. I am an engineer but not an expert in heating and cooling, so I listened to the recommendations from the installer and the rep.

    I am located in rural northwestern PA and good contractors for this type of work are basically non-existent in my area.