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Navien 200 Boiler, Supply Limit Control, High Delta T

On a new Navien NFB 200H boiler system, that runs three heating zones and an inidrect tank. Each zone and the system circulator pump has pretty beefy pumps, Grundfos UPS26, all brand new, no issues. Yet I am getting an alert on my screen that reads "Supply Limit Control! Check the boiler circulation Flow Rate" and the main symptom is a high Delta T of about 25. Supply 165, return 140. At first, I thought it was air in the system, but after air purging, the problem returns/persists. I've called Navien and the installer (both told me to purge air, and indeed I heard the air escape through the hose when I purged it the first time) but I hoped that you guys could give me some other ideas.

The first time it did this, the system went offline and we had no hot water--I would not notice an issue if the hot water had not gone offline. I unplugged, reset, and everything behaved normally again... until this alert and the wrench icon appeared.

My installer has come out twice, purged air, and the system seemed okay for several hours, then the alert recurred. No notice in poor performance, just the high delta T and the alert that says "Supply Limit Control! Check the boiler circulation Flow Rate".

The system is heating. We have hot water. Any ideas what this could be? The installer is slow to come out due to Covid issues.

The only other clue I have is that the system started doing this, only when we had a cold snap outside so all the heating zones were working at full. Still are. And by full, I mean the system is only at 40% capacity because there will be two more zones added in the future. It's a big system.

Two zones currently connected to old radiator piping, both heating well. One zone is baseboard, heating well. That's the three currently working. And we have good piping, with a large Caleffi hydro seperator with air and dirt release. That works well.

Any ideas as to what a chronic Supply Limit Control error could be?


  • Ironman
    Ironman Member Posts: 7,088
    Have you tested the value of the sensors?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Dave Carpentier
    Dave Carpentier Member Posts: 414
    Can you soft-cap the max burn % on that boiler ?
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • RobinInCali
    RobinInCali Member Posts: 36
    I have not tested the value of the sensors. I will ask about soft-capping the max burn.

    I will share some other data. Deep into the computer, I looked up and saw the Delta T Limit was 136 times, the Supply limit was 212, and the Delta T High was 487 times. That ain't so normal...

    I am trying to get another Navien tech in the Bay Area to come have a look. I've never had much trouble installing these, so troubleshooting is not my forte. It's plumbed right, was working fine, till the outdoor temps got into the 40s. Wow, so cold. Ha ha.
  • Ironman
    Ironman Member Posts: 7,088
    As far as system design and function, a 25* delta T should be fine and the boiler should have no problem operating within that range. Most I/O manuals allow it and even give pump flow rate charts showing 20*, 25*, 30* delta T.

    Again, you need to measure the value of the supply and return sensors to confirm their accuracy. You can also measure the inlet and outlet temps at the boiler and compare that to what the boiler’s seeing.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • RobinInCali
    RobinInCali Member Posts: 36
    Ding ding ding! And we have an answer!

    So after months of these error codes and visits by different Navien repair experts it turns out to be a malfunctioning check valve inside one of the brand new Grundfos pumps.

    This took three different Navien authorized repair people to solve. 

    And to the person here on the board who suggested a bad sensor you get credit also. One of the flow sensors wasn’t 100% so Navien is replacing that also. 

    This was about ten trips to my house, multiple flushes of the system, lots and lots of labor. 

    Grundfos says they won’t honor anything big because the check valve is a $6 part and ….

    Would anybody have known that or thought to check? We were flushing and we have a caleffi magnetic hydronic separator that I flushed regularly. I would have not though a new check valve could go bad. 

    Whew! Let’s hope this makes it go. It’s overwhelming to spend $25k on a new
    boiler system for a three unit apartment building with radiators, and to have it not work. And you lose heat and hot water regularly.  I hope those days are over.