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Non-oxygen barrier PEX in existing slab infloor heat

Krohner | Posted in General Questions on

Ok so I am still getting my head around my hydronic heating system. We have 5 zones  and one of the original zones from when the house was built has Non-oxygen barrier pex in the infloor heat. It is currently using a 3-mixing valve to maintain temperature, so the increased O2 can get back to the ferric components in the rest of the system. So my questions are:

1) How big of deal is it? I know O2 = Rust, but I have had different contractors  tell everything from it is no big deal since it is a low temperature loop. To it needs to be fixed and here is our solution for $7K.

2) If I were to isolate it from the rest of the system via plate heat exchanger with circulator, expansion tank,and makeup valve is there any best practices for the setup and control?

3) I notice most of the schematics I have seen have a temperature control ECM to “inject” heat into the heat exchanger  from the heating loop. I was wondering why that pump was needed could a ECM pressure control pump for all the loops  with another zone valve work instead or is there a reason for the required higher dedicated flow?

4) Any good resources for teaching myself hydronic controls?


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  1. Expert Member
    Akos | | #1


    Depending on the amount of steel (lot of steel is better, the lack of barrier on a short loop won't matter much) in your system and your boiler type this can be a big problem (more precisely, will be an expensive problem eventually). Non barrier pex lets through more than enough enough oxygen to cause sludge, low temp helps but doesn't solve the issue. This reacts with any steel in the system and creates sludge and eventual leaks. The number I've seen is something like 1 oz of corrosion product per 100' of pipe per year.

    Depending on the amount of pipe and the diameter in the zone, you probably don't need much pumping, something sized for hot-water recirculation would work (I like the B&G ecocirc e3 for small loops, ECM motor and and speed adjustment, uses almost no power) triggered from the existing zone valve. Adjust the flow from the zone valve to the plate heat exchanger until the loop is at the temperature you want, no need for any fancy injection or controls, set it once and forget it. Plate heat exchangers are typically low pressure drop, I doubt you will need a dedicated pump on the boiler side.

    If you don't need to integrate with any fancy controls for the problematic zone, another option would be to move the problem loop over to your domestic hotwater. You would still need a bronze/stainless pump, controls could be as simple as a timer, no need for heat exchanger. You would have to deal with the stagnant water issue outside of the heating season, which can be a daily timer or plumbimng the cold water feed through the zone.

    1. Krohner | | #2

      Thanks, I don't think codes let me do the hot water cross over in our area. I like the adjustable flow valve. It sounds like the simple straight forward answer I was looking for. It shouldn't be any different operationally from the 3-way mixing valve there today.

  2. Jon_R | | #3

    It's not clear to me why you don't convert the entire system to a low legionella risk, corrosion inhibitor filled, isolated system. Open systems (lots of oxygen) prove that corrosion inhibitors can effectively protect steel and iron.

    1. Krohner | | #4

      Thanks I will look into that, none of the contractors I have talked to mentioned that as an option.

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