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Navien "Mod-Con" Turn down

SpeyFitterSpeyFitter Member Posts: 420
First off, I am NOT a fan of Navien, or any of the wanna be tankless models trying to become condensing boilers. But that's just me, don't let my bias interfere with your opinion. However Navien apparently has their new CH series with is a combi series aimed at doing both hydronic heating and domestic hot water all in one. According to Navien they are listed with an ASME H-stamp, AND they have 3 models with turn downs from 10 or 9 to 1 or in that range (17,000 to 150,000, 20,000 to 175,000, and 20,000 to 199,000).

So they are essentially certified with an H-stamp with these turn downs, yet no one else can get these turn down ratios? Is there another standard more common mod-cons (e.g. Lochinvar, Viessmann) must certify to that we don't know about, or another set of rules to get higher turn downs?
Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber


  • HDEHDE Member Posts: 220

    I know of no ASME rules that control burners however, I know they accomplish the task by using a variable input gas valve with a 3-stage pre-mix burner allowing the wider range of firing rates. Controlling the variable fan/gas input across the different percentage of burner capabilities.
  • meplumbermeplumber Member Posts: 678
    I have been reading about this.


    There is a boiler coming out of western Canada, the IBC, which has a 10:1 turndown. However, they will not be offering that boiler in the US after March 1, 2012 (I think). I am still researching this to verify the information. There is a code or standard somewhere that is dictating the 5:1 turndown maximum. I just can't find it.

    I know that in Europe higher turndown ratios are more common. But as you pointed out, all the reputable boiler manufacturers in the US are selling with 5:1.

    Great question that has been bothering me for a while. I hope someone can answer this.
  • HDEHDE Member Posts: 220

    I know that on the boiler side, but the ever elusive combi boiler still has no definition and DOE, Energy Star, or institutions like AHRI have no idea or plans yet how to classify them thus they still fall under boilers. Combined Appliance -AFUE is a reality but doesn't matter since they only look at AFUE, which would be the boiler side is still the only one that counts.
  • SpeyFitterSpeyFitter Member Posts: 420

    meplumber - I'm quite familiar with the IBC unit in question - I've installed a fair amount of 15-150's seeing as how I live near Vancouver where they are manufacturered.  I know the story of how they attained 10 to 1 long ago, and IBC recently just had their new SL20-115 certifed at 5.75 to 1.  I have yet to hear of anything about them not being able to sell the 15-150 south of the border after March 2012, however, that's news to me. The 15-150 was certifed around 10 years ago and I've serviced a few of their early 15-150's installed around 2003-ish in the past few years.

    It just makes me wonder because the Navien was apparently certified recenty (2010?) mind you again perhaps the certification for firing rate is a different standard or certification as you eluded to.

    And as I've mentioned before, I believe the NYThermals Trinity has a higher than 5 to 1.
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
  • croydoncorgicroydoncorgi Member Posts: 83
    edited November 2011
    Horses for Courses

    From what I know about combustion technologies, I believe Viessmann, etc. are at the limit for turn-down ration at about 4:1 using the 'industry standard'  zero-pressure governor (don't know if this is the terminology used in N America).

    Other manufacturers such as Rinnai, that need to achieve a higher ratio, have to do it with different technology.  Rinnai water heaters achieve (I believe) 16:1 ratio using a multi-stage valve with a stepper motor in it (I know it's a stepper - I can sometimes hear mine gowling and chattering on the outside wall of my house as it adjusts the burner rate to the water flow.  Brilliant system for its purpose.

    But I question why you might need a bigger turn-down ratio for a combi boiler.  Millions of these are sold in Europe, especially UK.  Unless they're over-sized for the job, 4:1 seems fine to me - not much problem with the boiler cycling on an off at very low heat demands, in practice.

    (Being an ignorant ex-colonialist / imperialist here in UK, I know nothing about the content of ASME H standard / certification in N America!  What areas of 'heating equipment' does it cover?  To avoid contaminating this thread, maybe someone prepared to provide a brief overview will start a new thread.   Thanks.)
  • GordyGordy Member Posts: 7,122
    Large Turn down ratios

      I think one benefit is to the manufacturer. It would cut down on their product line, by only needing a few models to cover a wide range of heat losses for the installer. Take the IBC 15-150 it covers a wide range. Scenarios vary, maybe low load structure with a high load domestic demand.  Maybe climate that dictates large temp swings through out the heating season that puts some boilers on the edge at its coldest.

     Some manufacturers offer 6-8 different models where IBC can cover the same range with 4 models.

  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Turn-down ratios.

    I have the smallest W-M Ultra 3 (80,000 BTU/hr input) with a turn-down ratio of 5:1.

    In my opinion (I am a homeowner, not a contractor), I could use a much greater turn-down ratio.

    One could argue that my boiler is oversized (my heat loss calculation reveals I need between 30,000 BTU/hr and 35,000 BTU/hr when it is 0F outside, and design day temperature is only 14F), but that boiler is the smallest in the product line.

    The reason I need a much greater turndown ratio is because I have two heating zones. One takes about 24,000 BTU/hr when it is 0F outside, and the other takes around 6500 BTU/hr when it is 0F outside. Normally it is over 14F or warmer outside (it is 59F outside as I type this), so the heat losses are often much less than this.

    If only the small heating zone is running, I need 8:1 turn-down when it is 0F, and considerably more when it is warmer outside. Since I have only 5:1 turn down, the boiler cycles on and off quite a lot. I have diddled the controller to reduce the cycling rate (increased the dead-zone from 10F wide to 15F wide, reduced the maximum firing rate to 55% instead of the default 94%, changed the reset curve a little), so except for fairly warm days, it does not cycle more than 6 time an hour. I am sure a turn-down of 10:1 or more would be a lot better for me.

    Of course, a 40,000 BTU/hour boiler with 5:1 would be a step in the right direction, but they already have a lot of sizes in that product line. I wonder if it would be worth their while to make a smaller model? On my street, most of the houses are 1200 square feet cape-cod design. By now I assume most of them have had insulation installed, and perhaps better windows, leaks fixed, etc. So they need less heat than 60 years ago when they were built. I do not know what the other homeowners have done. They all had radiant slab heat downstairs and little boxes that looked like radiators with a 3-foot strip of baseboard in them for upstairs.
  • Tim McElwainTim McElwain Member Posts: 3,904
    I do not have answers to all the questions

    presented here concerning turn-down ratios.

    My knowledge of gas valve and burner design along with combustion limitations may be part of the answer. The standard you may be looking for is probably an ANSI Standard for boilers and or water heaters I do not have those handy as they cost a lot of money. I do know under conventional gas valve/equipment design they allow a minimum firing rate of 1/3 of input, by the way not BTU input but outlet gas pressure which is typically 3.5" W.C., what that gives as a minimum firing rate is around 1" W.C. (Note: some step opening gas valves open at around .7 to .9 " W.C) Then the concern is the port loading of the burner keeping in mind pressure is determined by the setting on the regulator and flow is determined by orifice sizes.

    When we get into Zero Governor technology an old term for a double diaphragm regulator which was the grandfather of our modern negative pressure gas valves. The negative pressure gas valves are able to work obviously at much lower pressures using the combustion blower as a balance for premix and firing ratio. There is really no limit to that as shown by some commercial boilers with very high turn down ratios. The burners have to be designed for that type of application such as the matrix from Viessmann and others.

    So what am I saying, I think the combi units are being looked at under a different standard than what are considered straight boilers. The Rinnai accomplishes multistaging on demand in a different way but it certainly could be used on a boiler just as easily as on a on demand water heater.

    It simply comes down to whether the manufacturers think there is a market for such an application and then getting the standard changed to meet that requirement. Many times a definite demand from the market place can stimulate change to meet the demand. This discussion keeps coming up here from time to time and I for one have passed on information to various boiler manufacturers
  • croydoncorgicroydoncorgi Member Posts: 83
    It's simplicity versus complexity = cost

    The reason (AFAIK) that the turndown ratio on zero-pressure gas trains maxes-out at about 4 or maybe 5 is to do with venturi design.  It's relatively easy to make a venturi where the pressure drop is roughly linear over the sort of flow rate range that 4:1 turndown implies.  But if you go further than that, the gas/air ratio will start to drift away from the desired point at one or both ends of the scale.

    Most boilers I work with have 2 adjusters:

    - throttle, which adjusts the 'aperture' of the pipe feeding gas to the venturi,

    - and an 'offset' screw on the body of the gas valve.

    Most boilers have a setup procedure which starts with getting the boiler at full power and then adjusting the throttle to give the correct CO2 or O2 percent as measured by a gas analyzer probe in the flue.  Once that's set right, then you reduce the output to minimum and adjust the ofset screw to the same or very similar value.  Then you put it back of full power and recheck the value and if necessary, readjust the throttle.  And so on ad nauseam until both the full and minimum power numbers are correct.  In practice, I've only ever had to go through the process at most twice.  Normally, unless the adjustment is completely screwed up, it's only one adjustment at each power setting.

    The worst case scenario is when the settings have been so badly messed up that the thing won't light, or lights with such a rich mix that the CO level kills the gas analyzer.  Then you have to whip the probe smartly out of the flue before the sensors are permanently damaged and adjust the valve by 'counting turns' of the throttle and / or looking at the flame picture and twiddling the throttle until it stops being bright yellow!  :=)

    Fortunately, most burners fire up at a reasonable level and stay lit at full power even if well off the correct setting.  It's at the low power setting that the burner goes out randomly!
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