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Navien NCB-180 boiler not reaching set point temperature

mccaslin15
mccaslin15 Member Posts: 17
in Gas Heating
Hi,

I have a Navien NCB-180 boiler and it is not reaching the set point temperature. I am currently getting a supply temperature of 169F and a return of 154F while the set point is at 180F. This is the third heating season we are using it, and I hadn't noticed if this was happening before, but we are having a cold snap now and it is having problems keeping the house warm. I checked to see if it was being limited by the exhaust temperature limit dip switch but the dip switch is set to ignore the exhaust temperature. I have a 1600 sq ft. house in CT, where the temperature has been getting down to 0F at night. Is the boiler simply undersized for this kind of demand or should I look for other problems?

Thanks

Luke

Comments

  • Ironman
    Ironman Member Posts: 7,550
    What type of radiation do you have?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mccaslin15
    mccaslin15 Member Posts: 17
    Baseboard fin tube radiators.
  • Ironman
    Ironman Member Posts: 7,550
    Do you know how many total feet of element are in them?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mccaslin15
    mccaslin15 Member Posts: 17
    Not offhand, I can measure tonight. Do you suspect I don’t have enough radiators?
  • Ironman
    Ironman Member Posts: 7,550
    Yes, particularly if you had an oil fired boiler previously.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Ironman
    Ironman Member Posts: 7,550
    Is the outdoor reset curve setup? If so, what factor is it set at, 1.4?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mccaslin15
    mccaslin15 Member Posts: 17
    I did have an oil boiler previously. Out of curiosity, what makes you say the radiators may be undersized? Do oil boilers run hotter than gas boilers? I would believe that this is a problem, as the system responds slowly even at moderate outdoor temperatures.

    I have outdoor reset set up but it took a long time for the house to heat up in the morning if I turned the thermostat down at night, even when it’s not that cold outside, so I made the set point go to 180F at a fairly high outdoor temperature, I think 50F. I’m not sure what factor the curve uses, I left that at the default value.
  • Ironman
    Ironman Member Posts: 7,550
    A lot of times, old oil boilers were set to 190*+ to compensate for marginal baseboard. Install a mod/con, and you automatically will have less heat output no matter what it's btu rating is because it's usually limited to a lower max SWT.

    You shouldn't set your stat back with a mod/con: it's counter-productive to its design and logic and you'll actually use more fuel.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mccaslin15
    mccaslin15 Member Posts: 17
    I no longer set my thermostat lower at night as it was more trouble than it was worth.

    Should I be concerned that the boiler is not reaching the set supply temperature of 180F?
  • Ironman
    Ironman Member Posts: 7,550
    The flue gas temps may not actually allow it to do so.

    I don't have solid proof of this, but I suspect the dip switch only allows the flue gas temp to go higher but still limits it. Probably 20* higher.

    I can't imagine that a boiler manufacturer would allow someone in the field to totally disarm a safety feature like that given the catastrophe that could result.

    What type of venting material do you have? If it's PVC, you should set the switch back as you're far exceeding the temperature limitations of the pipe.

    More baseboard is probably your solution.

    Can you post some pics of the boiler and its near piping?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mccaslin15
    mccaslin15 Member Posts: 17
    I tried running with the dip switch in each position. The only difference I noticed was that when the outside temperature increased the boiler was able to reach the set point of 180F when the exhaust temperature limit is removed. When it is very cold outside the supply temperature hangs out around 170 regardless of the switch position. I expected a difference even at the cold temperatures so this was surprising to me. It makes me wonder if some other factor limits the supply temperature.

    One other thing I wanted to bring up, in going through the boiler settings I saw there is a limit on the return water temperature set in the mid 150s. I thought this was a limit on the return water temperature set point, if you chose to control based on return water temp, and not an actual limit that would have any meaning if you are controlling the supply temp. Is it possible that is causing a problem?

    I can post some pictures tonight. It sounds like the solution will be more baseboard though. Would it make sense to put some fans on my current radiators to try to pull more heat out as a short term solution/test of this theory?
  • Ironman
    Ironman Member Posts: 7,550
    That could indeed be the issue if it's set to the return water temp: the boiler is modulating based upon that.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Gordy
    Gordy Member Posts: 9,546
    raise the return water temp setting
  • mccaslin15
    mccaslin15 Member Posts: 17
    The dip switch is set to control based on supply temp so I don’t think the return set point should be doing anything (emphasis on the word think). I have it set to the maximum value, which is somewhere from 155-158, I’m having trouble recalling the exact number. Is it normal for there to be a limit on the return water temp even if the set point is based on the supply temp?
  • Paul48
    Paul48 Member Posts: 4,469
    What kind of documentation do you have from when the boiler was commissioned?
  • mccaslin15
    mccaslin15 Member Posts: 17
    I have the installation and owner’s manuals. I just looked up the “max return temperature” setting and it is pretty clear that it is only used when “return control” is on. I double checked and I’m in supply control mode so I don’t think this should be a problem.

    Is it common for mod-con boilers to be limited at ~170F as the highest temperature they can supply to the radiators?
  • Paul48
    Paul48 Member Posts: 4,469
    edited January 2018
    Parameter "F".....Max heat capacity for space heating, should be set to 100%. If you've selected "finned tube baseboard" as your type of heat load, the supply set point range is 120-180*. This is a combi unit?
  • Gordy
    Gordy Member Posts: 9,546
    Gross input is 80k on that boiler. Output is 74k, Doe with pickup factor is 64 k.

    If IIRC that on board pump can only move about 50k.


    Ironman
  • Ironman
    Ironman Member Posts: 7,550
    edited January 2018
    Gordy said:

    Gross input is 80k on that boiler. Output is 74k, Doe with pickup factor is 64 k.

    If IIRC that on board pump can only move about 50k.


    Agreed, but a 1600 sq. ft. house with a 30 btu per sq. ft. loss should be ok on it (48k btus).

    He's seeing only a 15* delta T, so that would indicate the boiler's not producing enough.

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mccaslin15
    mccaslin15 Member Posts: 17
    Good point, Bob. I think I should get at least a 20 degree delta T, right (assuming 5 Gpm)? So this would look like a performance issue?

    Should I be running any kind of cleaning solution through the boiler annually? The owners manual said to only do it when there is a decrease in performance, so I have not done it yet (after two seasons).
  • Ironman
    Ironman Member Posts: 7,550
    No, that's referring to descaling the domestic HX.

    The 15* delta T indicates that the onboard circulator hasn't reach its limit quite yet.

    It's definitely something not allowing the boiler to supply hotter water.

    What type of gas?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • mccaslin15
    mccaslin15 Member Posts: 17
    Natural gas
  • Ironman
    Ironman Member Posts: 7,550
    You can clock the meter with nothing else running and tell how much gas it's using and convert that into btus to see what rate the boiler's firing.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Brewbeer
    Brewbeer Member Posts: 616
    Baseboards clogged with dust and debris will also limit heat output. Check them and clean if necessary.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • mccaslin15
    mccaslin15 Member Posts: 17
    In trying to figure out what was going on I made a number of trips to the basement where the boiler lives. I realized it was really drafty and cold down there because I had forgotten to close a storm door at the bottom of the stairs that lead down from the cellar doors. I normally leave the door open all summer because it’s a bit of a pain. Closing that door made the basement maybe 15 degrees warmer and consequently the boiler is now keeps the house at the set point temperature. A little embarrassing how long it took to realize that but at least the house is warm now. I also sealed off a couple of drafty windows in the living room and that probably helped also.

    It’s still a bit of a mystery why the supply water temp hangs at 170 and never hits 180. I plan on doing the test Bob suggested of measuring the gas used. Every time I have tried to so far someone has done laundry or taken a shower though.
  • Paul48
    Paul48 Member Posts: 4,469
    I had considered having them go the route of clocking, but then realized this was a combi. The output for the domestic is much higher than space heating. If they're not having problems with hot water, it would suggest they can get the 80 for the space heating input. Clogged baseboards would not account for the boiler not reaching temperature.
  • Gordy
    Gordy Member Posts: 9,546
    Maybe however If not using full potential of domestic side so no indication if full output is being established
  • Paul48
    Paul48 Member Posts: 4,469
    Sure.....turn on the hot water in a bunch of fixtures and clock the meter.
  • Ironman
    Ironman Member Posts: 7,550
    If The DHW firing rate is higher than for space heating (which I believe it is), then you're not gonna get an accurate assessment.

    Remember, the purpose of clocking it is to see how much it's ACTUALLY firing at in space heating.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Paul48
    Paul48 Member Posts: 4,469
    Ahhh..........I was just ensuring that the unit was not starved for gas. Can you tell them how to lock it into hi-fire for space heating?

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