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Solar DHW with Navien tankless (combi) issues - new possible solution

BP3 Member Posts: 7
Recently had a Navien NCB210E combi boiler installed with my existing solar DHW system and it doesn't seem to play well with my existing solar system.

With incoming water at anything above (or over) a delta 20 deg F of the set point, the Navien heats the water well over comfort range and short cycles, e.g., 120 F incoming, 1.2 gal/min, output 138-145 with set point at 125 F.
From what I have learned from Navien tech support is that the DHW will kick on to the min turn down when it senses flow, even if the incoming water is above the DHW supply set point, up until it gets too hot, then it shuts off and restarts.

I have read all of the other solutions posted suggesting TMVs, and/or a motorized 3-way valve, but I have another possible simpler solution that I am looking for feedback on. How about tricking the flow sensor to not sense flow until the Solar gets to below a point, then un-trick it. I am thinking an aquastat on the Solar storage outlet to interrupt the pulse signal to the flow sensor. Another option would be a controller of some sort with a temp sensor.

A couple of problems that might be an issue:
1. the unit will throw an error code if it senses an open in the signal circuit. I asked Navien if it runs a self-test on the signal circuit and the guy I talked to said it doesn't, but my contractor thinks it does. Should be easy enough to test I guess, I just don't want to fry my PC board. Even if it does self-test, a resistor connecting the normally open contacts on the aquastat to the input wire on the flow sensor to complete the circuit might keep it from throwing an error.
2. At the point when the aquastat completes the circuit after the Solar gets too cool, there would likely be a slug of cold water to the end user.
3. Running all the water through the tankless needlessly will wear it out. Although all that water would be flowing through it anyway if I didn't have solar DHW.

Any input is appreciated.


  • hot_rod
    hot_rod Member Posts: 18,927
    It can be done with motorized or even thermostatic valves. We build assemblies for that purpose, for the Euro market.

    Or assemble a couple off the shelf thermostatic valves, and build your own.

    Basically if the solar is hot enough it bypasses the heater. It can be done with two 3 way valves, adding a 3rd gives you additional mix temperature protection should the solar or tankless overshoot a safe temperature.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Steamback
    Steamback Member Posts: 36
    edited October 2019
    I hear your pain. I'm a solar hot water contractor and I recently went through this. The contractor in charge of the propane backup swore the Navien could do it, but he was wrong. I'm kicking myself for not confirming. Hot Rod is right...3-way valves are the answer on how to play nicely with a Navien boosting solar heated water. In our case we ended up putting in a 3-way valve controlled by a Johnson Controls A421. The 421 senses solar tank temperature and decides if solar heated water should enter the Navien or if unheated cold supply water should enter the Navien. So far so good. Not ideal for total system efficiency, but the cheapest and easiest fix we could think of...