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Lochinvar Combi 3-way valve sticks frequently

dac77 Member Posts: 1
Hi All,
I've just joined the forum, so please let me know if this post should be elsewhere.

I've had a Lochinvar NKC199N installed since November 2018. This is a result of the Columbia Gas Disaster in Massachusetts. The problem is: The 3-way DHW valve gets stuck on, indicating there is a DHW call, when there is NOT one. The heat is prevented from turning on while the DHW call is in effect, so no heat! I've been fighting this problem (and another) since a couple of months after it was installed! I won't bother you with the gory details. This fall (November) I flushed the DHW heat circuit with vinegar for an hour. Took the valve out, inspected it, it seemed to work beautifully smooth. Here it is 4 months later and AGAIN the valve is stuck in DHW mode, therefore no heat! I'm not a licensed plumber, but I am an electrician, and not incompetent with plumbing. A plumber I spoke with about this recommended a flush called Max Flow, or Max Flo, instead of vinegar. I have not been able to find it.

I have 2 questions:
1) Short term - Is there a better flush I should use?
2) Long term - Why is this happening even though I flushed it in November?

Any help is greatly appreciated.



  • hot_rod
    hot_rod Member Posts: 22,396
    Hows the water that you are refilling with? Hardness, TDS?

    If you have a lot of minerals in the fill water, they precipitate out when you heat the boiler up. And the hotter the water the more minerals come out of solution. So when the boiler goes into DHW it may run up to 180F or more.

    I think Lochinvar shows a hardness below 12 gpg, TDS below 350ppm

    The very best water to fill with is deionized water, water that has been stripped of all ions. I like to add a conditioner also, like Rhomar. That will buffer the Ph in the DI water, give you some O2 scavengers and help keep all the components working freely.

    If you have filled with hard water a few times, you might run a hydronic cleaner to clean all the internal surfaces of the scaling.

    I saw a plate HX out of a 1 year old combi in Bozeman that had the boiler side of the HX limed up so badly it would not keep up with a small faucet draw. We always look at the potable side, but either can scale from bad fill/potable water.

    Assuming it is not a control glitch keeping it in DHW mode? All sensors and connectors been checked?
    Power is on the valve and it fails to move?

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