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Turndown ratio?

HydroMan Member Posts: 14
OK, I'm confused by all these turndown ratio claims. Some boiler manufacturers claim 20:1, some now claim they can do 10:1, while the majority have stuck with 5:1 turndown. I've even heard the mfgrs that claim high turndown ratios actually ship them to the jobsite with 5:1 ratio, and then rely on the startup technician to change it upwards accordingly. So, what are the thoughts out there from you engineers on turndown? Is 20:1 or 10:1 really warranted and attainable IN THE FIELD?


  • Brad White
    Brad White Member Posts: 2,398
    As a practical matter

    no, not yet.

    The Aerco Benchmark started the 20:1 turn-down challenge maybe 8 to 10 years ago and I agree, 5:1 is as wide as reliably seen, mostly in Lochinvar boilers. I understand that Lochinvar is coming out with a boiler with a deeper "actual" turn-down. Most have, we all know, a comfortable 3.5:1 or 3:1 turn-down.

    What we have found with the Aerco Benchmark line is at least a 10:1 ratio but in order to prevent nuisance tripping, the low fire limit is set higher, 8:1, 6:1, just guessing, and left there. The rationale has been that these are typically installed in a battery of four to five units and so reduced turn-down means less in the company of other modulating boilers.

    By "nuisance tripping", or rather, what we have found that prevents the lowest, deepest turn-downs, has to do with flame failure. Not sure which code it was.  In our area, our gas pressure is not often the greatest and the ratio of gas to air becomes more difficult to maintain below certain levels. Given low gas pressures and precise but low differential pressures, a little difference means a lot. So technicians we have spoken too found that limiting the range made the difference and the plants still work. There is more to how and why, I do not know but am always open to learning more.

    The negative pressure gas valves, such as Dungs, from Germany, also have their range limitations. These are used on the HydroTherm KN line and their Ray boiler, I believe, as well as Munchkin, correct me if I am wrong.

    But in practice, so long as we are installing and controlling multiple boilers in sequence, we can amply vary the plant output to conditions, well.  So, warranted? Not that low. Attainable? Not often.

    Now, a single boiler with maybe 8:1 turn-down, reliably across low gas pressures, would suit me fine.

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
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