Navien condensing boiler settings troubles
Hello to everyone! Last fall I had installed a Navien NHB-110H condensing boiler for my central heating in my 117 year old 2 story brick house. This replaced a Baxi Luna 310fi combination boiler that worked well for almost 20 years. I live in Canada, and the temperature ranges between as high as 40 degrees Celcius in summer to as low as -40 degrees Celsius in winter where I am. The house is just under 1100 square feet of living space, and there are 5 cast iron radiators, which is quite adequate. In fact, I have one rad's ball valve shut off in an upstairs room that I don't use. All of the hot water piping dates back to when the house originally had a gravity fed boiler. Apart from the boiler-controlled circulation pump and an added expansion tank, the pipework has remained essentially the same. The whole system is one loop, with each rad having it's own manual valve. There is no common pipe between the supply and the return lines. So, that's essentially the layout here. My trouble lies with controlling this new Navien boiler. A major feature of this boiler is to utilize an outdoor reset controller to achieve high efficiency operation. I can select from a list of 6 preset outdoor reset curves, and one that can be user-defined. The presets correspond to different heat loads, such as fan coil, finned tube, radiant and cast iron, etc. If I enable the outdoor reset option, it works well enough at temperatures below, say, -20C. Anything milder than that, and it just gets way too warm in here and the boiler keeps running regardless of whether of not there is a call for heat from the thermostat. So for any temperatures above around -20C, I have to disable the outdoor reset option and set the space heating supply temp manually in order that the boiler is controlled by the thermostat setting. This is really inconvenient, and my installer is unavailable for comment or service. Any help and advice from the community is greatly appreciated. Thanks!
Comments
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Is dip switch 7 off?
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Select the user defined curve and adjust it so till it doesn't overheat when warmer. This also gets you the most amount of condensation thus best efficiency out of the unit.
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The boiler shouldn't fire if your thermostat is off unless you have a constant circulation system.
Do you know what your thermostat is wired to? Directly to the boiler?
And if you want to get granular, go to "Main Menu", select "1. Status Information" and then go to item "10. Outdoor Temp" and see if the temperature shown is accurate. You may have a bad sensor or the wiring has shorted.
@Kaos suggestion to enter your user defined values is spot on.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab2 -
This sounds like the outdoor reset curve is too aggressive for your radiation and piping. When the curve is set too high at mild outdoor temps, the boiler keeps targeting a supply temperature that overheats the space, so the thermostat never really shuts it down.
What usually fixes this is flattening the curve. Lower the warm weather supply temp and raise the minimum outdoor cutoff so the boiler stops running when it does not need to. With old cast iron radiators and a single loop, you often need much lower water temps above minus 20 C than the factory presets assume. If the boiler runs without a call for heat, also check that the control is not set to constant circulation or warm weather shutdown is disabled.
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@Kaos As far as I can tell, the only parameters I can set for the user-defined curve are the highest and lowest boiler set temperatures. I thought this might be the correct curve to use, but I have not been successful with it as yet. The outdoor temperatures here can fluctuate from daytime highs of say, -10C to night time lows of -30C or lower, and that variance seems too great for any of these curves. If you have any setting suggestions for the user-defined curves, I'd be grateful. Thanks
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When you have tried this, what temps have you tried for those "highest" and "lowest" settings?
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@Alan (California Radiant) Forbes My outdoor sensor is pretty accurate, differing by maybe 1 degree C from my other outdoor thermometer (unrelated to heating system). My thermostat is wired directly to the boiler at the T/S_1 connector. The circulation pump is connected at the "Boiler Pump" connector, CNP2. It does seem to be running constantly, although I can't say for sure 100%. Thanks
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@Perr_y32 Thanks for your insights here. This does appear to be what's happening. I'm a little unclear about the right setting for minimum outdoor cutoff, and I haven't seen a setting for constant circulation. Also, I'm a bit confused about where the warm weather shutdown should be set to.
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My sincere thanks to everyone who took the time to read my post and offer suggestions to fix my issue. You guys are great!
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The manual has the info on how to set it.
Section 11.8
It also has the warm weather shutdown as well.
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@Kaos After looking at the online manual from the link you shared, I see that it's the same as the paper version I have here, and have been using as a guide to better understand how this system works. Unfortunately, I find that the descriptions are short on explanatory context.
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Some manuals are really good and some not so much. Even people in the business struggle with equipment they are not familiar with.
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@EBEBRATT-Ed Thanks! It's good to know that it's not just me!
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Okay, here's an update on what I've done so far… From the service menu, I lowered the absolute minumum and absolute maximum supply temperatures to 28C and 72C respectively. From the regular menu, I set the outdoor reset to the user-defined curve, changed the outdoor high temperature setting(for outdoor reset control) to 18C, and disabled the WWSD function. From last night's low of -23C to today's high of -11C, the boiler seems to be following the thermostat setting nicely. Things seem to be on the right track now, but I'll have to wait for some wider variations in the outside temps to know for sure. In the meantime, I'd like to thank all of you again for the guidance and great suggestions you've given me!
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How long were your thermostat cycles when it was close to -23C that night? In other words, was the thermostat calling for heat for for 2+ hrs straight or did it cycle off? I'm wondering if you can turn down your high setting even further, below 72C. You might not know for sure until you have a design temp of -40C outside?
Turning that lower setting to 28C should really help out with the overheating problem you described. Is 28C the lowest option?
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@bjohnhy I'm not sure how long the heating call cycles have been, but I'd say not longer than about an hour or so. I'll try and keep a closer eye on this. The boiler itself rarely stops to go into standby, and the same goes for the circulation pump. What I do see is that the boiler's heating capacity is modulating between around 25% to 35% capacity. At these outdoor temperatures, I'm sure I could set the high setting lower than 72C, but I fear the boiler won't be able to keep up when the deep freezes hit, say -35C and lower. The house's original boiler and the Baxi boiler that followed always made the rads piping hot on the coldest days, so maybe that's what I'm expecting this one should do too. I guess the only way to find out is to keep searching for the sweet spot. Currently, the temperature dfferential between supply and return is typically 3 degrees C. About the absolute low setting, I think you're right that it's a big part of the issue. The factory default is 40C, but I can lower it to 25C. I should probably try it at 25C, just to see. Do these numbers sound like they should? One other detail about my house is that its construction (1909) is 2 courses of brick with an inch of airspace between, strapping, lath and plaster for the exterior walls. No insulation in the walls, but around R30 in the attic, and roughly R12 for most of the full basement. The house breathes without being drafty.
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Hearing those details of the house, I suspect you may have to go higher than 28C on the low end of that curve. Maybe not. You'll just have to see. Go ahead and try 25C and see if it keeps up on those not so cold days. Paying attention to the length of thermostat call can help decide if you can move further down on curve. After you make additional air sealing and insulation improvements you can further lower both points on the custom curve.
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@bjohnhy Thanks for the helpful hints here, as they help confirm that I'm on the right track dealing with this issue. Seems to be just a matter of dialing things in from here, which will take some time, naturally. I am unlikely to upgrade the values of the current insulation, unless I go with rigid foam exterior insulation and vinyl siding over the brick. Time will tell. For now, I have further lowered the operating temps to a minimum of 25 degrees C and a maximum of 68 degrees C. So far, the heating is very even and smooth, completely unlike the characteristics of my previous boilers. Woohoo!
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