Outdoor reset curve Navien NCB 240e

Discussion in 'Boiler Forum' started by Matt13, Dec 31, 2017.

  1. Matt13

    Matt13 New Member

    Joined:
    Dec 31, 2017
    Location:
    02176
    Another person seeking help trying to dial in their outdoor reset curve...

    I had a Navien NCB240e installed in November 2017 replacing a 40 year old oil fired system. House is a 1900 sq ft. 1948 Cape with cast iron radiators. One zone covers approx 90% of the house (3 BR, 2 baths, LR, kitchen, DR, finished basement) and another zone covers a former breezeway converted to a four season room. There is also electric heat baseboard in a 1 room addition off the living room that was added in the 80's or early 90's. We only turn the electric baseboard heat on when someone is using that room. The former homeowner had Mass Save insulate and install new windows around 2010.

    I did not know much about condensing combi boilers when I had the unit installed... just that it was the most efficient and recommended option from several installers. So I did not ask the right questions and trusted the contractor when he said everything was set up correctly and not to touch anything.

    Upon further investigation the outdoor reset curve was installed but not activated. The space heating supply was set to 140 which seemed to work ok even during the bitter cold in the Northeast the last 2 weeks.

    So I am now trying to get the ODR properly set up.

    The below are the ODR settings I've used for the last two days where the actual outdoor temp has been between 0 and 20 degrees. The system is cycling on about once per hour for about 5 minutes at a time. (House is set to 65f) From what I've read in other posts that cycle time is not nearly enough? So my question is what input should I be looking to adjust? I already have the lowest outdoor temp set at the system minimum. I also noted when the system cycles on, the supplied hot water temp is close to 160f. Given the house was doing ok at 140f, it seems like this is obviously inefficient.

    B: 3 (I know 6 is technically correct for my house but a previous post said to try 3 first)
    C: -4 (Went as low as I could go and unit is keeping up fine. Currently 2 degrees outside Boston)
    D: 60
    E: 30
    F: 100 (my guess is this is what I should be adjusting)
    G-W: All at system defaults

    Also, does this mean my system is oversized? I mentioned to the contractor that we would likely be adding a bedroom over the garage in the next two years and would like to eventually convert the electric baseboard onto the gas system, so I think he took that into consideration.

    Thanks for any help!
     
  2. Dana

    Dana In the trades

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    At it's maximum output NCB 240 is oversized for space heating loads for 99.5% of the houses in the MA but not necessarily for the domestic hot water loads. The critical aspect is the minimum firing rate output relative to your average load, and whether you have sufficient radiation to emit the minimum output at condensing temperatures (average water temps under 130F.)

    The heat load of a reasonably tight, 2x4 framed decently insulated 2000' cape plus full basement with U0.32 (code min in 2010) windows in MA is typically on the order of ~30,000 BTU/hr @ 0F outdoors, 70F indoors if the foundation walls are insulated, 35-40K @if the foundation isn't insulated. The 99% outside design temps of most of MA is in positive single-digits, low double digits on the coastal areas from Boston south.

    The minimum fire output of the NCB 240 in condensing mode is 17,000 BTU/hr, or roughly half the load or a bit higher, which while biger than ideal, isn't terrible from an oversizing point of view. Set up correctly and with sufficient radiation it can modulated in condensing mode at least most of the winter. It'll be cycling a bit during the shoulder seasons, but with the thermal mass if high volume radiators it probably won't short cycle.

    The F: part of the menu sets a max firing rate for space heating. If set to something like 30% and it's max output will likely cover the load at down to negative single digit out put temps but recovery from setback might feel a bit slow, if using a setback + boost. Try 40%, which would 48,000 BTU/hr-in,45K out, maximum, which should cover your load and allow something faster than glacially slow recovery from setback.

    The curves and max water temperatures are set by what you selected in the "B:" part of that menu. Since you state that it keeps up fine at 140F you can select a curve with lower defaults for water temperatures, either 4 (low mass radiant- max output temp 140F) or or 5 (high mass radiant, max output temp 120F). If it doesn't quite keep up when set to 5 but does when set to 4, you can try tweaking it more finely by implementing a custom curve (7). See the menu items N: through Q: for setting up a custom curve.

    At the low temperature output try timing the burn cycles after the outdoor temperatures rise into the 30F range (roughly the mid-winter average temp in the Boston area). If the burns are more than 3 minutes and fewer than 5 burns/hour even when cycling you'll be fine. Check the burn times again in the spring when temps are in the mid-40s just to be sure (but it'll probably be fine.)

    If you're not using overnight setbacks , in menu part "E:" disable the boost time outs. When boost is enabled it raises the water temps to finish out the call for heat, and to speed up recovery from setback, but with the reset curve set up reasonably it just adds to the numbers of burn cycles, and lowers the average combustion efficiency slightly.
     
    Last edited: Dec 31, 2017
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  4. Matt13

    Matt13 New Member

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    Thanks for all of the good info.

    I just changed F to 50% (that was as low as it allowed me to go) and put E to 0. I'll leave everything else as is for now it see what these changes do for now.

    What exactly is the change you would expect to see from adjusting the firing rate?

    PS- I grew up in the 01609 section of Worcester. I imagine the big leaky victorians in that area are the .5% that the NCB240 is not oversized for!
     
  5. Dana

    Dana In the trades

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    Leaving B: set up for 3 will still end up with the high/very-high peak output temperatures that you don't need or want.

    Set B: to 4, and see if it keeps up (probably will). If it still keeps up and cycles on/off when set to 4, then try it set to 5 to see if it loses ground. If it'll keep up even at 0F with it set to 5 just leave it there. If 4 doesn't quite keep up during the cold snaps, doing a custom curve with the max output temp set to 150F probably would.

    With the firing rate limited to 50% (limiting it to 60,000 BTU/hr max firing rate, half the 120,000 BTU/hr maximum heating side firing rate) it'll still be oversized for your real loads by about 2x, but won't overshoot the thermostat set point temperatures as quickly when you the outdoor reset curve is set way too high. Once you have the outdoor reset curve dialed in a bit closer it won't really matter, since overshoots would be rare.

    I'm sure at least a couple of the larger historical district houses in 01609 could use the full output of an NCB 240, but those that have been retrofit insulated and fitted with storm windows probably don't. I live in a 1920s antique 1.5 story bungalow, ~2400' fully conditioned with ~1600' of insulated basement, and it comes in just shy of 40,000 BTU/hr @ 0F at the full 70F indoor temp (including basement), about 35K @ +5F (the local 99% outside design temp). Larger houses tend to have a lower heat/floor-area ratio. A 5000' Victorian would have to be pretty air-leaky to be over 100,000 BTU/hr @ +5F (but to be sure, some of them really are that leaky.)
     
  6. Matt13

    Matt13 New Member

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    Last night the unit ran almost all through the night but maintained temperature. The output temp was usually about 130 degrees. This was with B set to 4. So far during the day, the unit has been running 90% of the time and briefly cycled off 1 or 2 times in 4 hours.

    Some threads have noted you want the unit to be running almost constantly. So is my current situation what I am looking for, or should I do a custom curve set to 150f so it cycles on/off a bit more frequently?

    Thanks again for the help!
     
  7. NY_Rob

    NY_Rob In the Trades

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    With a mod-con you are looking for long , uninterrupted run periods with the lowest supply water temp that will maintain the desired temp inside the house.

    130F supply water to the radiators is good (if it's enough to keep your house warm).

    Keep in mind that your boiler ODR curve is linear... plug your current values in to Excel for a visual representation.
    Unfortunately, in the real world your home's heatloss is anything but linear. Think of your heatloss on a sunny, calm, 32F day... now compare that to your heatloss at 32F at night with 35mph winds. Your boiler only gets input from a single outdoor temp sensor which doesn't account for solar gain and wind infiltration- so while your ODR curve may sometimes work at 32F, it might not keep up with heatloss under other conditions.
     
  8. Dana

    Dana In the trades

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    If you have a 90% duty cycle and it's operating at condensing temperatures even when it's ~0F outside there isn't much to be gained by tweaking the curve much finer. Only if it doesn't keep up under NY_Rob's high wind example would fine-tuning a custom curve make any sense.
     
  9. Matthew Bohas

    Matthew Bohas Matt_Toronto

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    I know it's an old thread but I thought I read somewhere that the outdoor reset with the Navien does not work with multiple zones, is this true? I want to start using this.

    Secondly, is it easy to install myself?

    Thanks!
    Matthew
     
  10. Dana

    Dana In the trades

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    You can't set separate curves for different zones (the way you can with some Lochinvars) but it's fine to run multi-zone systems with Navien boilers under outdoor reset control, as long as there is sufficient radiation (or thermal mass) on each zone to not short cycle. The nakpin-math analysis on how to determine the short cycling risk lives here.

    Installing a boiler (particularly a modulating condensing boiler) is more than just a plumbing exercise. Even if you had all the right parts there are dozens of ways to screw it up. Before attempting a DIY installation, read up on it (including what it might do the warranty support).

    Combi boilers like the NCB series are also a lousy fit for most homes. They are better suited for home with moderate hot water needs and LARGE heat loads. Most homes trend in the opposite direction, with design heat loads smaller than even the mid-range of the heating side output. Many small to mid sized homes don't have sufficient radiation to run the bigger NCBs in condensing mode without short cycling.
     
  11. Matthew Bohas

    Matthew Bohas Matt_Toronto

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    Sorry, the boiler is already installed, I meant just installing the outdoor reset senor. I'm fine having the outdoor reset sensor for both zones though, I just thought I read somewhere that this was not possible...
     
  12. Dana

    Dana In the trades

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    Even non-rocket-scientists and non-heating engineers like this guy can correctly install the sensor (on what appears to be a 2 zone system). Be prepared to spend some time and maybe even some crayon-on-napkin math setting up and optimizing the reset curve though.
     
  13. FDNY/RETIRED

    FDNY/RETIRED Member

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    Before I started my own thread asking about trying to figure this out. I read this over and over. Since it isn't winter in NY yet I am not sure what to do. The B to F settings guess I am going to have to deal with when winter really hits. In my post I said I have Convection radiators. So I am not sure if I use 1 or 6 for my A setting. It is only about a 10*F difference between the High setting. Return is about 6*F higher on 1. It also says in my manual that 1 is the default setting. The B,C,D,E,F are the ones I am not sure how to tweak. But I guess the F setting is the one that I am going to have to deal with more then the others. Today 10/16/2020 it is supposed to get down to 47*F at night. Right now I do not have ODS activated, and wonder if I should just wait till it stabilizes to colder weather in a month or so. I just want to get this unit to condense the proper way. I always thought the hotter the water the better it was. But with these units I see that's not true. This Navien unit replaced a Quietside DPW120A. I don't even want to go into it.
     
  14. Matthew Bohas

    Matthew Bohas Matt_Toronto

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    Question about some settings with the outdoor reset.

    I set it up to start as low mass radiant.

    My understanding based on the chart in the manual is when setting the lowest outdoor temperature is the scale linear?

    Example under low mass radiant the supply temperature is 140 at 14 degrees F. If I change the lowest temperature to say 20 degrees F does it change the whole scale? Meaning supply temperatures are higher at say 25 degrees F when the lowest temperature is set to 20 instead of 14?

    Lastly, setting L "setting the burner off temperature"

    Default is 4f, does this mean when the supply temperature is 4 degrees higher than the burner off temperature is when it turns off? If I made this higher what does that do?


    Thanks
    Matthew
     
    Last edited: Nov 1, 2020
  15. Dana

    Dana In the trades

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    When referencing a chart in a manual it's useful to supply a link to the manual (if it's online), as well as the chart and page number you are referring to. Trust me- nobody has a photographic memory of any version of the manual.

    Yes, moving the minimum & maximum outdoor temperatures affects the entire curve. If you have calculated the whole-house heat load at the local 99# outside design temperature it's fine to use the outside design temp as the minimum temp. If you can calculate or know how much heat the radiation emits across temperature it's a good starting place to set the low-outdoor temp water temp to something like the average water temp needed to match that temp, and fine tune it when colder weather arrives.

    Tip: Many or even most low-temp applications work more efficiently setting the return water temp curve rather than the supply temp. (Dip Switch #1, position #1, as described on p54 of the version of the manual linked to above.)




    It's poorly stated in the manual (on p.68), but that sets the max difference in temperature higher than the reset curve temp and control (whether under return temp or supply temp control). eg: If at at the current outdoor temp the curve calls for 117F water (in either supply or return control mode), with parameter L set to 4F the boiler will keep going as long as the supply temp is less than 117F + 4F = 121F.

    See parameter M, which allows you to program how cool the water is allowed to drop below the immediate reset curve target temp before refiring.

    Those parameters can be quite useful for suppressing short-cycling on marginal systems/zones when the radiation can't emit the lowest firing rate output at condensing temperatures. The wider the swing between re-fire and turn-off, the fewer burn cycles there will be during a call for heat on an under-radiated zone. On many micro-zoned systems there is simply never going to be enough thermal mass in the zone radiation to suppress shorty cycling but with a well tuned reset curve and a wide temperature window between N & L it can still avoid most short-cycling whenever there is enough total load on the house by increasing the likelihood of zone calls overlapping in time.
     
    Matthew Bohas likes this.
  16. Matthew Bohas

    Matthew Bohas Matt_Toronto

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    Thanks, amazing reply clarified alot.

    Yes, I should mention the manual :) I've been staring at it lately so its memorized it now ha
     
  17. FDNY/RETIRED

    FDNY/RETIRED Member

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    Excellent explanation it took me until last week to understand this. On my unit I tried the Low mass and High mass heat loads. The water was not hot enough and it was coming back below 120*F. I had a Navien tech over and he said I was pretty close when I set it to #1 for Heat load. Then he set my F: Max heating capacity down to 80% for now. My PCB dip switch #1 is set to 1. He then explained to leave L and M at default. But he said the same thing you just posted. So far all is operating fine. though my boiler has only turned on 2 times since the last time I posted. It has been only in the mid 50's to 60's at night around here. The last couple of days it has been in the mid 70's. to 75 in the day.
    I also added two Nest E thermostats, and my house is in the 70's all day. Through the night it does not fall below 67-69*F. The nest said only my zone 2 turned on for about an hour and half 2 days ago. That was at 3:30-or 4am. Ill take the weather like this till next April. So my next step will be when it does get colder seeing what I need to do with the F,L and M settings.
    The next part of my heating project. Is heating my Garage and Storage /shop Space. I was going to go with gas fired Modine units. Two 45,000 BTU's. My Garage is 4,488 cuft. which comes out to about 35,063 BTU's. My storge space is 4,446 cuft which is about 34,728 BTU's. Cant use one unit because I have a ICF wall separating both rooms. PITA if I had to run vents. Though I was going to use the Modine separate combustion units.
    So talking to the Plumber who checked my boiler reset curve. He said I should go with Hydronic units. After doing some research I want to use the Modine Lodronic low temp units. I could get away with a HCH 67 in the garage and a HCH-39 in the storage space. I will have 2 more zones put in. Then do the reset curve all over again. But I should have more then enough BTU's to deal with doing this. I would like to have a outside air supply to the fan, but I think the Hydronic is the way to go. My storage space is also a machine shop with a South Bend heavy 10 and a Delta Rockwell lathes, Bridgeport knee Milling machine, Surface grinder, Floor drill Press and some other stuff. Including a Ingersoll Rand Type 30 7.5-HP 80-Gallon Two-Stage Air Compressor. Some Tig, Mig and Arc and gas welding. Plus water soluble Rustic cooling oil. Which is why I favor an outside air supply. Still doing research on what to do.
     
  18. FDNY/RETIRED

    FDNY/RETIRED Member

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    Its been 32*F here the last 2 days and yesterday 11-17-20 with the wind blowing it showed 29 to 30*F. I had the upstairs zone 1 set at 72*F on the Nest E and the downstairs set at 69*F on the other Nest. Neither stat in Eco mode, and I had short cycle problems. With the Heat load set to the default #1.
    So today I did a custom setting.
    L: left at 4
    M: left at 5
    N: 138*F supply Max Temp,
    O: 102*F Supply Min Temp,
    P:120*F Return Max Temp,
    Q:102*F Return Min Temp,
    F:60% for the Max heating capacity. I started at the 80% but I was still going to high on my supply and return temps. At 60% I was sitting at 130*F to 133*F on the Supply. But it did go up to 138*F a couple of times but didn't stay there. On Return 120-126 and was pretty constant. I was able to get to 75*F while I was allowing it to run and see if it would short cycle. It didn't and for the 10 hrs I let it run it stayed on. Except for 2 times the burner went off. I had condensation fill up and pump out of my neutralizer. So I figured I am in a pretty good zone.

    I then lower my stats on both zones to 71*F and zone 1 cuts off after a while. Now here is my problem. On Zone 2 the stat stayed on at 71*F and all of a sudden I am cycling about 5-7 times in 45 min. With the Supply water temp staying within 136-138*F, But my Return water temp kept going above 127*F. It would come on when it went to the 120*F mark. Zone 2 has a 12ft slant/fin fine/line 30 baseboard against the exterior insulated wall. I figured I need 1,632BTUs for the 1,016 cuft room?

    With zone 1 on and zone 2 off, no problem Supply 130-132*F and Return is staying at about 122*F. No cycle problems. What can I do to have zone 2 not cycle. Also it says I should be able to get my Supply Max water temp between 36*F to 194*F. Why cant I get it below 138*F. Do you think if I went to 50% on my Max heat capacity setting this would help? Been messing with it almost all day and its 28*F outside now. But I am not going to play around till tomorrow.
     
  19. fitter30

    fitter30 Well-Known Member

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    Nest stats have a setting delivery system make sure it set on radiator.
     
  20. FDNY/RETIRED

    FDNY/RETIRED Member

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    Yes my Net E learning gen 1 stats are set to radiator. I only have heat and they asked me in the setup what I had.

    Now I have tried my reset curve on #6 Radiator, #4 Low Mass Radiant and #5 High Mass Radiant. The Tech set it to #1 and he just lowered my F setting to 80%. It was doing ok except for my return water was in the 130*F to 135*F. So I was not at the 95% AFUE and condensing the right way. When it went down to 28*F to 32*F on the setting I he set it up to. That's when my short cycling started. My supply temp went up into the 160-180*F which caused my return to increase and I thought that is what was causing my cycle problems.

    So I tried my own custom curve the one above trying to get my supply to at least 138*F and my return to 120*F-125*F. Which I did accomplish with 2 zones running. But I have to admit the house was hard to get to 71*F, with the stats set at 75*F. Yet the boiler was condensing and staying on. I even had the neutralizer pump condensing water out a couple of times. Until when one or the other zone was turned off them it was short cycling at the lower water temp. Today while it was still in the high 20's. I tried my F setting at only 50% and it still short cycled. Dip Switch #1 is set for return water reading. I double checked that today.

    I think I will never get this to optimum AFUE because my heating system was designed for a oil burner when I bought the house 20yrs ago. Then I had it converted to gas, because they were doing it for free and offered a rebate on Combi boilers. A Quietside DPW 120A. Which was the biggest mistake because I wish I knew back then what a POS it was. Then my house was raised and about another 1200sqft was added. To get my house heated comfortably, it seems I can't use low temp heating water. Or the reality is this unit is way over kill even at 50% Heat Capacity.

    For now I put it back to what the tech put it at. It was in the 40's* and when it ran I didn't have short cycle. But more then I want it cycling anyway. It is doing about 168 *F supply and about 148*F return. So I have a 20* deltaT if I am saying it right. My T-Stats are in Eco mode and the house is comfortable. Upstairs 69*F and Downstairs 66*F. Outside temp is 47*F. Last time the unit ran was 4 hrs ago for 20min. upstairs. Downstairs last time it ran was 11AM for about the same time.

    Maybe I should have gone with a smaller unit and I would not be having this problem. Maybe if I put the Modine Lodronic Hydronic heaters one in the Garage and one storage area which is also my shop. It will allow me to run at a water return of 120*F without short cycling and a lower water supply long enough to heat the house. Or I am setting myself up for different problems. I need to heat an extra 8,934 cuft since my house was elevated after Sandy.
     
    Last edited: Dec 1, 2020
  21. fitter30

    fitter30 Well-Known Member

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    What are u using to control zone pumps and boiler? What is your sequence of operation? Radiators, fin tube don't start heating till 120* water. That oil fire boiler minimum return water temp 140-150*. Hows the system piped primary/ secondary loops? Whenever new controls are added baby steps need to be taken. Don't know if the nest stats are working properly making changes in outdoor reset constantly never work. Set water temp to a constant temp first and see how the stats are working and give it a few days because they have a learning curve.
     
    Last edited: Nov 19, 2020
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