Navien NCB240e Questions

Discussion in 'Boiler Forum' started by jeff711981, Dec 28, 2020.

  1. jeff711981

    jeff711981 New Member

    Dec 28, 2020
    Fowlerville, MI
    Well, I've got roughly 30 feet of fin-tube in the problem zone. At 120 degree water temp and 1 GPM flow it's at 190 BTU/ft according to the chart you shared. And let's assume they're not working great since they're old and they definitely have some deposits inside reducing the efficiency. So let's call it 150 BTU/ft. At 30 feet, that's 4500 BTU.

    So let's do some rough back of the napkin math... (referencing:

    The room is 20x20 = 400 sq/ft
    It has standard 8 foot walls, but the ceiling is vaulted, so let's assume an average of 12 foot ceiling height (since some is 16 and some is 8)

    400x12 = 4800 cubic feet

    Currently with temps around 30 I'm trying to maintain a 40 degree temperature differential.


    192000 * 0.135 = ~26000 BTU

    The site I reference admits this is a conservative number and actual numbers are 65-80% of this calculation. So...

    26000*.65= about 17000
    26000*.8 = about 21000

    Taking into consideration that this is a 40 year old addition built onto a home with which the original structure dates back to the late 1800s, I'd probably err on the high side of that range as many years have gone by for insulation to be damaged and also taking into consideration 40 years ago homes weren't sealed up as well as they are today, plus there's a wood burning fireplace and pellet stove, both of which penetrate the wall and allow more cold air in than if those didn't exist.

    This increases to 45000 if I'm trying to maintain a 70 degree differential if it were 0 degrees outside.

    So unless I've made an error in my math, this one 400 sq/ft room exceeds the minimum capacity of my boiler, and my fin-tube elements are probably only capable of putting out 25% of the required BTUs currently, and only 10% of the required BTUs when the temps drop to 0, as they often do in Michigan during winter.

    It is for this reason that I believe installing additional radiant elements in the room would help the situation. In doing some research, I've found that on the low end, radiant floors provide roughly 35 BTU per square foot, and I assume that's at a maximum water temp of 120 degrees since in my research it seems nobody recommends exceeding 120 degrees for radiant floor heat. So 400 square feet in the room * 35 = an additional 14,000 BTU/hr.

    This would put me at a total of 18,500 BTU/hr in the room, which is getting into the neighborhood of what the room requires based on the formula above for days like today with temps around 30F.

    If I replace my existing fin-tube with new ones that are larger and produce 25% more BTU/hr, that puts me just under 20,000 total BTU/hr.

    These numbers still don't make a lot of sense since by this math, my fin-tubes are only capable of about 4500 BTU/hr, but they're maintaining a 30 degree temperature delta in that room (it's never dropped below 60, even with temps dipping into the 20s overnight), which should require as much as 20,000.

    Again, lots of assumptions... but given all the variables in regard to holes in the walls for fireplaces and pellet stoves, the age of the construction... I don't know how I'm going to get a more accurate calculation. All I can say with certainty now is that the way it's currently running it's only capable of maintaining a 30 degree temperature differential in that room and if I want to be able to maintain a 70 degree differential for the coldest days of the year, I need to at least double the heating capacity. The boiler is perfectly capable of doubling its output, but I don't have the radiant element to actually put it in the room.
  2. jeff711981

    jeff711981 New Member

    Dec 28, 2020
    Fowlerville, MI
    In addition, I located an old IP camera and positioned it in front of the boiler so I can stop going down into the basement to look at it... and as I mentioned, it's always lit. No short cycling.

    Attached Files:

  3. fitter30

    fitter30 Well-Known Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
  4. jeff711981

    jeff711981 New Member

    Dec 28, 2020
    Fowlerville, MI
    Don't have one, but I did set a thermometer oh the highest ledge I could set something and it's only a 1 degree difference from the thermostat which is about 5 feet off the floor
  5. jeff711981

    jeff711981 New Member

    Dec 28, 2020
    Fowlerville, MI
    Just keeping this up to date so others can learn from my experiences, too.

    With the camera in place, I did catch some short cycling of the boiler. The temps in MI have been in the 40's during the day with sun and still above freezing overnight, so the boiler doesn't have to work too hard. Most of the house had no problem staying around 70 with the zones only calling for heat a few times a day - the sun helped a lot.

    However, the problem zone doesn't get as much sun - fewer windows. So with it being the only zone calling for heat for hours on end, the return water temp crept up and the boiler was short cycling. To combat that, I changed the Burner Off Temperature (setting L) from the default of 4 degrees to 30 degrees. My theory being that if it overshoots the target temp, it will take me out of condensing temps on the return side and be a bit less efficient than I want, but the heat won't be wasted - in fact, the supply temp got up to 150 with return temps in the low 140s before another zone called for heat and began circulating 70 degree water from the rest of the plumbing and brought the temp back down and now I've had 2 zones calling for heat for a couple hours now and the boiler is sitting in the mid 120s which is where it should be based on the outside temp and the ODR curve I've configured.

    So this boiler, as others have said, is definitely oversized when just one zone is calling for heat... I've considered the possibility of having the company who installed it swap it out for a smaller unit... maybe the 180 or even 150... but instead, I think I'll keep playing with this - fine tune the settings, add some additional radiation in the form of retrofit radiant floor once the weather turns around and I'm also toying with the idea of renovating the garage and attaching it and running some radiant floor heat into there as a 4th zone since the boiler seems to have the excess capacity.
  6. Dana

    Dana In the trades

    Jan 14, 2009
    OK, so where the hell is the other (~18000-4500 = ) 13, 500BTU/hr ending up to allow continuous burns? That's a LOT of heat- there's no way it's not cycling when only the problem zone is calling for heat. 13,500 BTU/hr is 225 BTU/minute. Even assuming 22.5lbs of water + water-equivalent thermal mass in that zone loop (2.5-3 gallons of water), the water temp would be rising 225/22.5= 10F per minute, which would be all but guaranteed to be delivering burns of less than two minutes when serving only that zone, maybe even less than 1 minute.

    If there are no check valves on the zones it's possible that there is back flow on the other zones when only the problem zone is running, which would be one possible explanation of where that heat might be going.

    That tool is utter crap, not even worth the time to enter the data.

    Heat loss isn't a function of volume- it's function of exterior area broken down by surface type (window, wall ceiling, etc) and the U-factors of those surface/construction types, the amount of air leakage + ventilation into that space, and the indoors to outdoors temperature difference.

    Here is a primer on the classic I=B=R method of load calculation, which is very similar to (actually a subset of) ACCA Manual-J, which is the current gold standard for the HVAC trades. High performance building designers prefer to use more sophisticated modeling tools, but unless yours is a WAY better than code house, IBR or Manual-J isn't going to overestimate your load by much if you are appropriately aggressive on input assumptions. Freebie online CoolCalc or LoadCalc use Manual-J(ish) methods and will typically overshoot reality by 15-35% ,

    That's more than the entire heat load of my antique sub-code 2400' fully conditioned above-grade floors + 1600' of insulated basement if I fully heated the basement. To get 45,000 BTU/hr out of 30' of fin-tube would be 1500 BTU/foot- absolutely impossible out of a hydronic boiler, but if you're up for using molten sodium as the working fluid you can probably get there (although it's likely to ignite any wood that's in contact with the plumbing.)

    To get that much heat out of 400 square feet of radiant floor would require a floor surface temperature north of 130F in a 70F room.

    Suffice to say that 45K number is simply RONG, and not by just a little bit.

    Run a more realistic and intelligent load calculation, and refrain from being conservative about the anticipated degraded condition of the insulation or air leakage. Even when pretendjng it's all perfectly installed R13 or R19 or whatever as well as perfectly air tight, the Manual-J tools can sometimes overshoot by more that 10%. If the place leaks air like a tennis racquet, assuming 5cfm per leaky window or door might be a reasonable max. Otherwise ASHRAE's 62.2's 3cfm per 100 square foot of floor area (12 cfm for a 400 square foot room) would be reasonable, and usually an overstatement of reality.
    Getting 35 BTU per square foot (see the left side scale) out of an under-the-subfloor solution is going to require 180F water, even with aluminum heat spreaders. Even with above-the-subfloor solutions such as WarmBoard or Roth Panels will require temps above the condensing zone. The subfloor + 3/4" wood flooring is going to have total R-value between R1.5-R2 (R values are indicated on the right side scale of this nomograph.) You're looking at ~15-20 BTU per square foot out of it at condensing temps, best case:


    Run a Manual-J type load calc on the room, being aggressive rather than conservative about air tightness, R-values, and window U-factors. Don't count any wall/floor/ceiling area that has conditioned space (even an unheated basement) on the other side, only the exterior surfaces in contact with outdoor air.

    It's likely that 30' of flat panel radiators replacing the fin-tube would work, for a lot less money and effort than an under the floor The Manual-J heat load numbers would be necessary to spec how tall & deep the rads would have to be.
    Last edited: Jan 15, 2021
    jeff711981 likes this.
  7. Dana

    Dana In the trades

    Jan 14, 2009
    I wrote:

    Hammering home the point, on their page they write:

    "In order to illustrate the point further, here is a sample calculation: if you face 30-degree temperatures in your region and you want it to be 70 degrees in a 3,000 sq foot home with 8-foot ceilings, your calculation would look like this: 3000 x 8 x 40 x .135 = 129,600 BTUs Keep in mind that this is a very conservative estimate, meaning you probably will not need an HVAC system that puts out 129K BTUs. "

    Even a 3000' tent or green house that size would barely have a heat load that high at a 30F delta-T:

    The U-factor of a simple air barrier (such as single pane glass) is about 1 BTU/hr per square foot per degree-F difference.

    Assuming a 60' x 50' footprint that's 3000' of U-1 ceiling, and 220' of perimeter, for 220' x 8' = 1760' of wall area, total area for a total area of 4760'.

    U1 x 4760' x 30F= 142,800 BTU/hr plus infiltration.

    Even at ASHRAE's ventilation rate of 30 cfm/1000' or (x 3000' =) 90 cfm , or 5400' /hr, it would only deliver an additional ...

    ... 0.018 BTU/cubic foot per degree-F x 5400 x 30= 2916 BTU/hr

    Call it 145K, for a TENT that size!

    OK, lets look at 2x4 framed house with empty wall cavities and no attic insulation:

    The U-factor of an UN-insulated 2x4 16" o.c. stud wall is about 0.25 BTU/hr per square foot per degree-F, a clear glass (no low-E) double pane (or a clear storm over a single pane ) is about U0.5. Assuming a window/floor ratio of 15% there would be 450' of U0.5 window for window losses of:

    U0.5 x 450' x 30F= 6750 BTU/hr.

    Subtracting out the 450' of window area from the 4760' of wall + ceiling area leaves 4310' of U0.25 wall & ceiling assembly for a loss of:

    U0.25 x 4310' x 30F= 32,325 BTU/hr

    Add in 2916 BTU/hr of ventilation/infiltration losses and you're still at only 6750 + 32,325 + 2916= 41,991 BTU/hr, call it 42K.

    That would make the recommended 129K heating system only about 129K/42K = 3x oversized for the load. That heating plant wouldn't start to lose ground until the indoor to outdoor difference was (30F x 3= 90F, say, -20F outdoors, 70F indoors. And that's for an UNINSULATED house. (Imagine what it's like in a code-min house!)

    That's not "...a very conservative estimate...", rather, that's a very LUDICROUS estimate.

    An oversize factor of 3x or more is really common with hot-air furnaces, which is the opposite of comfort. The duty cycle is so low even during cold weather that it delivers a short & ferocious hot blast followed by an extended chill. The average temp might be just fine, but it's about as comfortable as standing next to a roaring fire outdoors in a T-shirt & shorts on a 0F outdoors night- you will either be freezing your face while roasting your butt, or conversely. But the average temp is just fine.

    So if you want to be comfortable in the addition, run the heat load numbers aggressively but carefully, and size the radiation appropriately for the desired water temps. That way when it's cold outside the radiation will be emitting constantly rather than duty-cycling, keeping the room temperatures comfortable and stable.

    But true comfort starts with addressing any inadequacies of the building first. If the walls are air-leaky and full of mouse-eaten R19s or R13s it's worth dense-packing cellulose over the cavity insulation to 3-lbs per cubic foot or better, which will dramatically reduce air leakage that might be happening through the sheathing or drywall.

    Windows often/usually leak more around the window trim than even un-weatherstripped window sashes, so popping off the casing and insulating/sealing between the window unit & rough framing with purpose-made low-expansion latex foam sealants (eg DAPtex Plus) is worth it. If the gaps between window & rough framing are large, a low-expansion polyurethane against the exterior side of the cavity to seal it, then stuffing the rest of the space full of shredded batt insulation (pack it in there to where it's fairly firm to a finger poke) or cellulose to reduce convection in the cavity works. If the gaps are really tiny its sometimes just fine to seal/glue the trim in place with polyurethane caulk.

    Band joists & foundation sills are another potentially large leak to air seal & insulate, mitigating against cold spots in the floor near the walls.
    jeff711981 likes this.
  8. jeff711981

    jeff711981 New Member

    Dec 28, 2020
    Fowlerville, MI
    For anyone following this and in a similar situations, one of the best configuration changes I've made to the boiler so far has been the "burner off temperature" setting. I set that to 50... which means the burner doesn't turn off until the supply temp reaches 50 degrees over the set temp.

    I've found this eliminates the short-cycling of the boiler. In summary, my boiler is oversized for my home - I wish I had the opportunity to do all this research before I had to have it replaced, it but it was basically an emergency situation heading into winter and I accepted a recommendation from a friend of the family and had someone come out and quote the replacement of our failed boiler.

    Anyway, I ended up with the 240e, and as you may have seen by reading the previous posts here, I have a problem zone that doesn't have enough radiation installed to heat the room without supply temps above 160 when it's cold outside. In fact, right now it's 37 degrees outside and with a water temp of 147, it's losing ground in this zone. The set temp is actually about 120 based on my custom ODR curve that I've configured, but because I've changed the burner off temp, the burner steps down as far as it can, but stays on and continues heating the water - up to 170 if the set temp is 120, before it turns off and cycles. So the boiler is running at about 10% capacity, the zone can't radiate that much heat, so the supply temp rises.

    Eventually, one of my other two zones calls for heat and 70-80ish degree water ends up getting mixed in from those zones, which brings the temp back down, and sometimes even causes the boiler to step up to maintain the supply temp based on the ODR. More importantly, the energy that went into heating the water to the 140-150 range isn't wasted - as the temp creeps up and the supply temp rises because of the lack of radiation, that is eventually used when a supply temp of 120 or so is all that's needed for the other zones... so they get quick blast of heat as the 80 degree water mixes with the 150 degree water and the burner usually doesn't even step up.

    This is working perfectly for me now... so well in fact that I've abandoned the idea of installing radiant floor heating in all but the kitchen, which I bypassed due to leak and really poor looking connections, and I'm just going to replace the fin-tube elements that have very corroded connections with higher BTU units in the 3rd, problem zone and possibly add some retrofit radiant floor heat for the kitchen.

    This is basically what my ODR curve looks like, and it's working great!

    Last edited: Mar 20, 2021
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