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Community and Q&A

Sizing a Modulating Condensing Boiler

Thomas Dresser | Posted in Mechanicals on

Have I analyzed a combination heat / hot water modulating condensing boiler sizing correctly?
Best options?

I am a homeowner looking at combination heat and hot water modulating condensing boiler options to replace an existing gas forced hot water boiler and a 40-gallon gas hot water heater. I have analyzed the combination systems using techniques from GBA for heating boilers. I am looking for advice whether my analysis and conclusions are valid and what my best options would be.

Located in southern Rhode Island (Zone 5A), the house is one half of a two-story duplex (1200 ft2) with an unheated basement. The current boiler system supplies 85 feet of fin tube baseboard in two zones, 45 feet upstairs, 40 feet downstairs. The 40-gallon gas hot water heater supplies one and a half baths, kitchen with sink and dish washer, and a clothes washer. Both units share the chimney along the adjoining wall.

I used Dana Dorsett’s article on 15-minute heat load calculations to calculate my heat load. Using my January 2018 gas bill I calculated the heat load at 25, 648 BTU/hr. at a 5-degree design temperature. This calculation is based on 79 therms usage, 82% boiler efficiency, and 663 degree-days sourced from I used a 50 degrees base because the unit was unoccupied with the thermostat set at 50 degrees, except for a few days. For the days it was occupied I added 20 degree-days per day to account for the 70-degree thermostat setting. I also performed an I=B=R calculation which resulted in 26,200 BTU/hr.

Using Dana’s “Sizing a Modulating Condensing Boiler” article I calculated:
1. 25-year extreme heat load of 36,000 BTU/hr. using 26,000 BTU/hr. and a 1.4 factor.
2. Average winter seasonal heat load at 13,000 BTU/hr. using a median temperature of 35 degrees from the Weatherspark site, about half way between the 5-degree design temperature and 68 degrees.
3. Radiant capacity of the first and second floor zones at 120 degrees AWT at 8000 BTU/hr. and 9000 BTU/hr. respectively using 200 BTU/ft.

Three different HVAC contractors have proposed Navien NCB 240 combination systems. I also listed the two smallest combination systems I could find.
Option 1 Option 2
Navien 240E Navien 150E or HTP UFT 140
Boiler Input Max BTU/hr. 120,000 60,000 140,000
Boiler Input Min BTU/hr. 18,000 12,000 14,000
Hot water GPM 4.5 2.6 3.2
Efficiency 95-96% 95-96% 95-96%

Here is my analysis and conclusions:

Option 1. The Navien 240E’s minimum input at 95% efficiency is 17,000 BTU/hr. some 30% higher than the average seasonal load of 13,000 BTU/hr. so the boiler will rarely modulate. The minimum input into the 85 feet of fin-tube radiation is at the maximum 200 BTU/hr. (17,000/85) needed for condensing. The boiler will likely short cycle, because the minimum input is approximately double each of the first and second floor zones. This system would appear to be oversized.

Option 2. The other two units’ minimum input is approximately equal to the average seasonal load and they can be expected to modulate only about half the time. The minimum input into the 85 feet of fin-tube radiation is 135-155 BTU/hr. below the 200 BTU/hr. maximum needed for condensing. The boilers will likely short cycle because the minimum input for the 150E is about 25% and 40% over the capacity of the first and second floor zones, respectively. The minimum input for the UFT 140 is about 50% and 65% over the first and second floor zones, respectively. These units appear to be better sized but only marginally.

Option 3. Per Dana’s article, Navien (NHB-80) and HTP (UFT-80) make boilers with no domestic hot water with minimum inputs of 8,000 BTU/hr. and maximum 80,000 BTU/hr. These minimum inputs are appropriately below the average seasonal load and the boilers will modulate most of the time. The minimum input into the 85 feet of fin-tube radiation is 95 BTU/hr. well below the 200 BTU/hr. maximum needed for condensing. The boilers should not short cycle because the minimum input is about equal to the capacity of each of the first and second floor zones. These units would be sized correctly.

My questions are the following:

1. Is Option 3 really the only correctly sized option?

2. If so, what do you recommend for domestic hot water:
a. A replacement 40 gal gas hot water tank?
b. HTP boiler with an indirect hot water tank?
c. Rinnai on demand hot water system?
d. Something else?

3. If not, which of the following are my best options:
a. Options 1 or 2?
b. Another type of combination boiler?
c. Keeping the current heat and hot water systems?
d. Something else?

Thanks very much for any advice that anyone is able to provide me.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm giving your question a bump by adding this comment. The best person to respond to your question (concerning Dana Dorsett's sizing methodology) is Dana Dorsett. Let's see if he responds.

  2. Expert Member
    Dana Dorsett | | #2

    Without tracking indoor temps more carefully there's the potential for a large solar gain error when using base temps as low as 50F.

    The 1.4x multiplier is an ASHRAE convention for sizing the boiler, primarily for being able to benefit from overnight setback strategies and has no bearing the 25 year maximum heat load. Sometimes 25 year absolute peak is higher than the 99% heat load x 1.4, sometimes not.

    If the 85' of baseboard has been able to heat the place adequately at an entering water temp (EWT) of 180F over the past 25 years it means the 25 year peak is probably less than 45,000 BTU/hr. But it could also be less than 30,000 BTU/hr.

    For heat loads as small as yours low mass combi-boilers are almost never good fit, especially with low mass radiation. They are best suited to homes with large heat loads and low to moderate domestic hot water loads, whereas you have a modest heat load and a moderate domestic hot water load.

    On the small side of the proposed solutions, the ~11K heating output of the NCB 150E is on the small side at RI incoming water temps (even for a 1-bathroom house) and with ~8000 BTU/hr being emitted from a single zone it would be dumping on 3000 BTU/hr, (50 BTU/minute) of excess heat into the zone than the radiation can emit. Assuming a best case 100' of 3/4" pipe + baseboard and a gallon in the boiler you're talking on the order of 25lbs of water, which will be slewing at 2F/minute, best case. If it's a 10F swing between the burner's turn-off temp and the restart temp that's a 5 minute burn which is fine. If the swing is tighter than that it would be getting into short cycling territory. If it's half-inch plumbing to the baseboards there will something like 15lbs of water mass in the zone and a 3 minute minute burn for a 10F swing. As a rule of thumb for short cycling 3 minutes is really the lower bound.

    On the other end of the proposals, with the zone emitting only ~8,000 BTU/hr The NCB 240E would be dumping 9000 BTU/hr or 150 BTU/minute into the zone, and with 25lbs of water in the zone the temp would be slewing 6F/minute and burn times the order of 100 seconds. This is the sort of short cycling you're seeking to avoid. The domestic hot water service is fine, but the space heating is problematic. It's pretty much guaranteed to short-cycle itself into low efficiency and/or an early grave if operated at condensing temperatures. A 10 second flue-purge on a 100 second burn blows away a sizable percentage of the heat released during the burn (mid single digits best case), which means most of the condensing benefits are being wasted during ignition cycles, while adding excessive wear & tear on the boiler.

    In situations like yours the solution is thermal mass, either on the hot water storage, the space heating buffer, or both. The range is a high-mass combi (like an HTP Versa, which is pretty pricey and complete overkill for your loads), a self-designed high mass combi with an external heat exchanger isolating the potable water in the tank from the space heating water (takes more design effort, but should be cheaper), or a mod-con that can modulate to less than 8000 BTU/hr and an indirect water heater operated as the priority zone. Ruling out the high mass combi option you're left with:

    A: Water heaters! Even with 25-35K of space heating heat load subtracted out, 76K burner in a modulating condensing water heater like the HTP Phoenix Light Duty, still has more burner capacity left over than a typical 50 gallon standalone water heater for serving up the domestic hot water. At EWT of 135F (what you might get out of a heat exchanger at a storage temp of 140F) the 85' of baseboard would be able to deliver ~22,000 BTU/hr. An EWT of 145F the baseboard will deliver about 26-26K. And there's no flow rate restriction based on burner size. There are other water heaters that can work, some will require more tweaking of the control to keep from short cycling when there is only a small space heating load. The Phoenix LD will at least modulate down to about 25K out, and will be less prone to short cycling, requiring fewer design compromises. With any of them it will require playing around withe water temperatures to find out the minimum temp at which the radiation still emits enough to cover the load would be necessary.

    B: Any small mod-con that can throttle back to 8K or less with an indirect fired water heater operated as the "priority zone" would allow you the best of all worlds, since 100% of the boiler output can be applied to the water heating load. With the UFT-080W that would be about twice the BTU/hr of a typical 50 gallon standalone, but even a 45-50K modulating condensing boiler + indirect solution serves up more domestic hot water performance than a typical standalone. Size the tank for the biggest tub you have to fill and call it a day. Most houses with only a single bathtub to fill will do fine with 35-45 gallons of indirect water heater.

  3. GBA Editor
    Martin Holladay | | #3

    Thanks, Dana.

  4. Thomas Dresser | | #4


    Thank you so much for your thoughtful, insightful response to my posting, and for the other related articles you wrote. I very much appreciate that you are able to explain and apply complex theory in a way that is both accessible to and actionable by a lay person. You have de-mystifyed combination burner sizing for me and now I have the confidence that I can work with the HVAC contractors to get to a good solution.

    Thank you again. Much appreciated.

  5. Dave Dave | | #5

    I've been reading thru the blogs seeking more information long after the horse got stolen from the barn.
    I replaced my 10 yr old boiler and hot water heater about 2 months ago, work from home, don't use hot water all day so I thought instant type unit would be more energy efficient for DWH and heating. Plumber (came highly recommended by a friend and has done other work for me, nothing of this magnitude though) recommended a Navien NCB-240E unit without even looking at my complete system. I have circa 1950 home, roughly 1100 sq. ft. Radiators in each room are like attached image, of varying length's all connected to a manifold in the basement, individual loops to each radiator, 1/2 tubing feeds, total length is ~37 ft. and the radiator core is ~5" wide. I have an extension with about 15' of slant fin baseboard radiator. That's the entire upstairs heating loop.
    Semi finished basement is my office and workshop, 2nd heating loop is ~25' along one wall, 8 ft. of it is bricked over by fireplace hearth, it's standard slant fin baseboard 3/4" feeds. I was trying to do the quick heat load calculation using gas bill and BTU output of the radiators but can't seem to find information on the type of radiators I have.

    It has been suggested that this unit is overkill for my home heating needs and I'm trying to solve that "suggestion" on my own. Any help would be greatly appreciated.

    I have a background in data acquisition and attempted to get the RS485 info on the Navien for monitoring, no such luck. Bought the APP, pretty much useless.
    Thank you for your time.
    Best Regards,
    Dave Adams
    Kings Park, NY

  6. Expert Member
    Dana Dorsett | | #6

    That's no radiator- it's a fin tube convector. Cabinet height, fin width & height and fin spacing all affect output.

    Most will be specified at 215F steam or for 190F water but the output at lower temps can be inferred. Measure it up, see if you can't find something similar to use for ball-parking it:

    For the heat emitted with 180F water, multiply the steam output by 0.7. For the heat emitted with 120F water, multiply the steam ratings by 0.2.

    A 24" tall convector with 4" x 3" fins will deliver about 1800-2000 BTU/hr per foot of cabinet width at 215F steam, so at mid-90s combustion efficiency figure 360-400 BTU/hr per foot with 120F water.

    If it's 37 total feet, the summed width of all convecters, with 120F water it could be delivering about ~15,000 BTU/hr which is fine.

    The min-fire output of the Navien NCB-240E at condensing temps is about 17,000 BTU/hr, and with the thermal mass of the water in the plumbing and only 2000 BTU/hr of excess heat going in the cycles should be pretty long (>3 minutes) and infrequent (maybe 5 per hour). Since you have the system installed, go ahead and program it for 125F fixed-temperature output, turn the thermostat way up and just time the burns and the off intervals.

    There's not much to be done about the 25' (or maybe really only 17', if 8' are covered) fin-tube baseboard in the other zone. It's good for about 200 BTU/hr per running foot, and to balance with 17,000 BTU/hr of boiler output without cycling too much you'd need at least 75' of baseboard. You may just set it up such that the thermostat for the basement is only enabled during a call for heat from the upstairs and see if that's enough to keep the basement comfortable.

    Alternatively you can find some craigslist SunRads (5" x 20" x ___ cast iron kneewall radiators) or similar and install about 10-15' of those. When mounted 3/4" away from the wall those are good for about 50 BTU/hr per inch of width @ 120F, or 600 BTU/hr per foot of width. While it would take 28' of SunRad to emit the whole 17K, they have a lot of thermal mass in water volume + cast iron, and half-that should be enough to suppress short-cycling. They're "architecturally appropriate" for a circa 1950 house (I've seen a number of higher-end mid-50s houses heated with a combination of SunRads + cast iron baseboard), and are more comfortable to heat with than those convectors. If previously used for steam they can be converted, but put a filter on the return side of the loop to keep any of the potential crud out of the water-tube heat exchanger in the Navien. They're nearly indestructable, and can be cleaned up and painted. (Even latex wall paint is fine if the water temps stay under 140F.)

    1. Dave Dave | | #7

      Good Morning Mr. Dorsett,

      Thank you for the information and correction about my "radiators" :-)

      I used the attached document as a rough estimate of my total BTU/Hr capacity for the convectors. Please let me know if my observations are correct. I opened one up and the fin portion of the unit is 5.75" wide and 2.5" high. Cabinet is 24" high and 48" wide, open bottom and louvers at the top. My convectors only have one pipe running down the center of the fins.

      I used the depth in inches of 6 on the chart, 24" height and 48" length which shows 12,455BTU/Hr for steam.
      Using table 3, I used 150 F for the temp and 65 for the EAT.
      12,455 X .43 = 5,535 Btu/Hr for the 48" convector. 5,535/4 = 1,383 Btu/Hr/Foot.
      Given that I have 37' of convectors that would be a total of 51,171 Btu/Hr???
      (I did not figure in the 15' of slant fin in the extension)

      I wasn't sure if any of the other tables were valid. Flow rate for the 240E is 4/5 gal/min.
      My system is currently running on outdoor reset and the heat load setting is radiators.

      As for the basement loop, I can just configure it to run with the upstairs loop, shouldn't be an issue if it contributes to the efficiency of the system, and I'm down here all day.

      I did read somewhere that there wasn't a need for night setbacks now that I have this hi efficiency system, is that true?

      Thanks again for your time and expertise, I appreciate it.

  7. Expert Member
    Dana Dorsett | | #8

    You'll overheat the basement if the baseboard runs whenever the upstairs zone does. A zone valve operated by a local thermostat in the basement to interrupt flow would work.

    ~1400 BTU/hr per running foot at 150F sounds about right, and 51K is probably more than twice the design heat load, which means you should lower the reset curve to match the load. You might even be able to get there with 110F water on design day.

    Overnight setbacks erase any saving by requiring higher temperatures which comes with lower combustion efficiency.You might need 150F to get rapid recovery from overnight setback, but that's running only ~86% efficiency. At 110F you'd be over 95%. Dialing the curve to the lowest temps that keep up with the without short-cycling will deliver the best overall efficiency.

    Are you pumping direct with the internal pump of NCB or is there a hydraulic separator and an external pump for the radiation zones?

  8. Dave Dave | | #9

    I guess I'll just leave the basement stat set the same as upstairs for now. I've already ditched the overnight setbacks and have the two thermostats set to 70.
    I'm not comfortable playing with the reset curve at this juncture. In theory, a guy that knows his way around the Navien equiptment from a company up in MA is going to stop by one day this week to have a look at the install. (just called, on his way here)

    Could I set up a test curve using one of the presets built into the machine? I've attached the chart from the manual. I've also included a picture of the hydronic setup, uses the circulator in the Navien to circulate the water in the pipe below the unit then sends the water out to the circulator pumps, or so I was told.

    Thank again,

    1. Dave Dave | | #10

      heat load suggestions.

  9. Expert Member
    Dana Dorsett | | #11

    The manifold & pumps look fine. The basement zone with the 25' of baseboard is probably WAY over-pumped even at the lowest speed of the pump, but it's sort of a "who cares?" scenario- it's not worth changing. The flow can be throttled back using the white handled ball valve above it's pump at the cost of being a bit noisier. Measuring the temperature difference from the supply & return to the baseboard might be useful. Ideally you'd have at least a 5F temperature difference across the zone radiation.

    If you want to try one of the pre-set curves, the #4 "Low Mass Radiant" will probably work just fine- give it a shot.

    Check to see what's currently programmed as the "Lowest Outdoor Temperature" and "Highest Outdoor Temperature" (see p58 of the manual: ). The defaults are +14F and +70F, which means it'll deliver the highest output temp of the curve selected when it hits +14F, and will back off to the lowest output temp at +70F.

    To monkey with the curve, move the end points.

    With the Low Mass Radiant curve the output range is 80-140°F. Set the Outdoor Low Temperature at -4F (the coldest it'll allow) and 96F for the Outdoor High Temperature. That way it won't put out 140F water until it's -4F outside, but won't turn it all the way down to 80F ever. If the curve were exactly a line (which it isn't) that would make the boiler increase the temperature by 3F for every 5F of temperature drop.

    Set up that way when it's 66F outside (when there's effectively no heat load), which is 30F below the Outdoor High Temperature the boiler output would be ~98F. At 98F the convectors are still pretty predictable, the baseboard, not so much, but it probably won't matter. When the temps drop to 36F (60F below the Outdoor High Temperature) the output temp of the boiler would rise to about 116F, which is still well into the condensing zone, with at about 95% combustion efficiency or a bit better. When it's +16F (80F below the Outdoor High Temperature, and approximately your 99% outside design temperature) the output temp will be 128F, which would still be delivering a combustion efficiency of about 90%.

    During milder weather when boiler output temps are low, if it short-cycles (=burn times shorter than 3 minutes, or more than 8 burns per hour) move the Outdoor High Temperature up to it's max. During colder weather if it doesn't keep up with the heat load and the indoor temps are dropping (unlikely, but maybe) move the Outdoor Low Temperature up.

    If it seems to keep up with no problem in the dead of winter you might consider changing the curve mode to High Mass Radiant, which would buy a couple percent more in as-used efficiency. At 120F out (the max in High Mass Radiant mode) the convectors should be delivering 600-700 BTU/hr per foot (if they really deliver ~1300-1400 @ 150F), and with 37' it would deliver ~22-26,000 BTU/hr. At tight 1100' insulated 2x4 framed house over 1100' of insulated basement would have a heat load less than 20,000 BTU/hr @ 0F. If the basement is finished but the walls aren't insulated it might need as much as 27-28,000 BTU/hr @ 0F, but probably not more than that. If the house is leaky enough to need more than that it's worth working on tightening it up.

  10. Dave Dave | | #12

    With regard to the basement loop, there is probably twice the length of 3/4" pipe feeding the baseboard than the actual length of the baseboard so there's a lot of water in that system. Unit is in the opposite corner of the basement. I turned the pump down to the lowest setting, no idea what the GPM is, I'll have to look it up. I can certainly attach a few thermocouples covered with some insulation to one of my data systems and plot it out the feed vs return over a few days to get some reasonable data.

    I've adjusted the curve as suggested as well as the Outdoor Low and High settings. My system allowed me to go up to 104 on the high side, but I left it at 96 per your suggestion.

    Last night I attempted to use the cast iron curve(my idea) and also changed position 1 of dip switch 2 to ON so the system monitored the return water instead of the supply water (suggestion of the Navien rep that came over). I'm not sure what this did but my shower was a disaster last night, ranging from burning to cold every minute or so.
    System was short cycling, heating was not on, jumping DHW up to 157 then shutting off, a minute later the same thing. Needless to say I restored all the defaults before retiring for the night. I just tested the DHW with the settings I just put in the system and it remained stable without any overshoot.

    House is well insulated I suppose. It's the home I grew up in, 2X4 walls had rock wool insulation blown in and the attic probably has a foot or more. Siding and roof are 1 X 8 tongue and groove boards. Basement walls are insulated on the finished side which is about 2/3 of the entire basement. All Anderson windows and insulated doors. Not too drafty.
    Game plan this weekend is to take the shop vac and compressor and blow the crap out of all my convectors. It's been a long while since I did that.

    Navien guy suggested that I should test house water, got the kit today. Also said it would be a good idea to run "sludgehammer" thru the upstairs system as parts of the convectors are cast and it would be good to clean out the system. Thoughts?

    I do not recall why he said it would be better to monitor the return temp instead of the supply temp on the system. Is that just the display or does the system do something differently? It's currently set on the default of supply.

    Thanks again for the help, I really appreciate it.

  11. Expert Member
    Dana Dorsett | | #13

    >With regard to the basement loop, there is probably twice the length of 3/4" pipe feeding the baseboard than the actual length of the baseboard so there's a lot of water in that system.

    So there's something like 75-100' of 3/4" (copper?) which would add up to about 2 gallons or ~17-20 lbs of water, give or take a little. That's not a lot of water mass. If we assume that at an AWT of 120F the 17' of exposed baseboard is emitting ~3500 BTU/hr and the covered 10' plus other plumbing is good for another 500BTU/hr it means the loop emits 4000 BTU/hr, so there is 17K-4K= 13,000 BTU/hr = ~200 BTU per minute going into that 20lbs of water. The temperature would then be slewing at (200 BTU/min)/20lbs = 10F per minute when just that zone is calling for heat.

    I don't know what the high/low differential is around the setpoint is for that combi boiler or if it's programmable (some are, some aren't- read the manual.) But let's assume it's default is to turn the burner off when the water is 5F over the setpoint, turning back on when it drops to 5F below the setpoint, a 10F swing. That would mean you're looking at a 1 minute burn. Even if the thing is programmable for a 20F swing it's still a fairly brief 2 minute burn.

    The heating side programming should not affect the domestic hot water side. Are you sure you hit the right DIP switch?

    I'm not sure what "sludgehammer" means in 'merican dialect (or any other)- is that Farsi? Magyar, perhaps? :-) The trademarked "SludgeHammer ®" product is something designed for treating septic systems, not heating systems, and would not recommend using it in your heating system. Fernox F3 is a product designed for removing sludge & scale from the innards of heating systems that might be worth running (once.) I'd be reluctant to run it through the Navien without getting sign-off from Navien first.

    A good filter (even a magnetic filter) is always a good idea, particularly with a water tube heat exchanger boiler (like a Navien) with old cast iron plumbing & radiators. Fire tube boilers a bit harder to clog, but for anything better than another cast iron beast some sort of filter on the return path from radiation is a good idea, as is a "dirt trap" element.

    1. Dave Dave | | #14

      I measured it at 87 feet of 3/4" pipe.
      I think we've beat the basement to death, I'll figure something out.
      Zone valve to heat it only if upstairs loop is heating, correct?

      Page 60 in the manual I received has
      Burner-Off Temperature:
      During space heating, the boiler turns off the burner when the
      space heating supply temperature meets or exceeds the burner-off
      temperature. Range 0-54F Default 4F

      Burner-On Temperature:
      During space heating, the boiler turns on the burner when
      the space heating supply temperature is below the burner-on
      temperature. Range 5-54F Default 5F
      I have not changed these settings

      Dip Switch: Initially yes, turned on SW1 of the 6 switch module instead of SW1 on the 8 switch portion.
      I corrected this mistake, re powered the boiler, ran some hot water and it still continued with the fast cycling on the domestic side. I then shut the unit down and put the reset curve back to 6-radiators and that stopped the issue.

      I'm currently running the system with all your previously suggested settings :-)

      SludgeHammer: That's what the guy called it, I wrote it down that way, obviously a mistake on his part?, not going to ask again, I was thankful he came over at all.

      Would a filter like this installed in the heating side work?

      I'm also going to install a whole house filter at the suggestion of the Navien guy. Currently just have it on the fridge water as that's all we drink. We have decent water where I'm at on Lawn Guyland.


  12. Expert Member
    Dana Dorsett | | #15

    87' of 3/4 plumbing has about 18lbs of water in it. The 27' of baseboard has another 5.5lbs, and the boiler's good for some too (you'd have to look up the water volume- it's probably in the manual), so you may have 25lbs of thermal mass to play with rather than the 20lbs used in the example.

    With the programmable burner off/on limits you will be able to suppress short-cycling without having to re-plumb any of it. Leave the Burner-On Temperature at the default -5F, but increase the Burner-Off Temperature to something like 25F, which will more than triple the minimum burn times when just the basement zone is calling for heat compared to the defaults. This means it will stray above the condensing zone at the end of a burn, but the efficiency hit from that partial loss of condensing is small compared to the hit taken when short-cycling. If the burns are still shorter than three minutes, bump up the Burner-Off Temperature by 5F at a time until it's at least close to that long. Three minute burn and 5 burns an hour would be fine- significantly shorter burns than that or significantly more burns per hour starts abusing the boiler, and is measurably less efficient.

    Those look like the right type of filters but I'm not familiar with that vendor. Some sediment traps have magnets to help out with ferrous crud from radiators too.

    1. Dave Dave | | #16

      Ok, thanks for that suggestion. That won't effect the main floor heating?

      I'm trying to figure out a way to track and record the cycling. Sure would be handy if I had the RS485 coms coming off the display board to the hub for the app. I'll contact the Navien guy and see if he can't get me some info.

      Do you have a preferred vendor for the filter?

      1. Expert Member
        Dana Dorsett | | #17

        The first floor has enough heat emitter that it probably won't cycle even with 120F water. The convectors put out 12,000-15,000 BTU/hr with 120F water. That means only about 3-5000 BTU/hr of "extra" once the water temperature rises to 120F (even if the curve is calling for only 105F) With the boiler's curve setpoint running 100F or 110F with the amount of thermal mass in the convectors & convector plumbing and the smaller amount of excess, the rate of rise is much slower, and would usually satisfy the thermostat before climbing above the condensing range.

        At 130F out it's still condensing (at about 90% efficiency) but it's emitting pretty much the full min-fire output of the boiler- the temperature isn't going to rise much above that unless the reset curve is pushing it higher. When the curve is calling for 105F, and the Burner-Off Temperature is set to 25F, it'll turn the fire off at 130F, and with the Burner-On Temperature set to -5F it won't re-fire until it drops to 100F. I'm sure there's at least 100lbs, maybe over 200lbs of thermal mass in your convector zone, so those cycles will be more than minutes long even during the shoulder seasons with the curve calling for 80F water (rising to 105F before Burner-Off kicks in).

        When you get the curve set up sort of reasonably (I'm thinking the Low Mass Radiant curve is still probably the most appropriate for your actual heat load) you can then start observing it as the outdoor temps drop, to see if it's spending much time above 130F out when it isn't actually needed. If it is, dropping the Burner-Off Temperature to 15F or 20F might be the best compromise, letting a few shorter cycles in when just the basement zone is calling for heat in exchange for more condensing on the main zone.

  13. Josh Durston | | #18

    This video is worth a watch:
    Some of the other coffee with Caleffi videos are great too.
    I do think that over pumping is an efficiency killer though. Ultimately the biggest variable in efficiency is RWT (not SWT). If you over pump your RWT is going to start creeping up on you killing efficiency. Technically you could be supplying 130-140 and still condensing quite a bit if your temp delta is large enough.

    I've got a 100kbtu Polaris tank heating about 6-9kbtu worth of panel rads and it works ok despite an almost hilarious oversizing factor, since it has a 34gallon buffer built in. The rads all have TRVs so the the load goes down almost to zero. I had to cut in a control to allow me to increase the temperature delta, but now I only get 1 or 2 run cycles per hour that are are minimum of 3-5 minutes. My point is high mass lets you get away with ridiculous things. (The plan is to eventually go with the smallest Lochinvar firetube Modcon, and an indirect tank, but the Polaris tank was free from a friend).

    1. Expert Member
      Dana Dorsett | | #20

      >Ultimately the biggest variable in efficiency is RWT (not SWT).

      This is true, but adds another layer of complexity to the setup.

      The Navien boilers can be set up to control for return water temperature instead of supply water output temperature- highly recommended in this once you have it basically working in condensing mode. Return water mode is selected with a DIP switch, the drop the setpoint temperatures on the curve by another ~10F or so from where they were when setting it up for supply temperature control mode.

      >"...heating about 6-9kbtu worth of panel rads..."
      >"...plan is to eventually go with the smallest Lochinvar firetube Modcon..."

      The low-fire output of even the smallest Lochinvar is about 8,000 BTU/hr. What's the point of installing a mod-con with so little radiation/or load that basically never modulates? A water heater is probably the right solution here (or a reversible air source chiller.)

      Sanden's CO2-refrigerant heat pump water heater (with the compressor outdoors, mini-split style) is good for about 15K BTU/hr, and would probably be cheaper to install than a KHN55 + indirect, and with the buffering capacity of the 83 gallon version should be able to supply both heat & hot water for a home with a heat load of 6-9KBTU/hr. (They have 43 and 119 gallon versions too.)

  14. Dave Dave | | #19

    Burner-On Temperature at 5
    Burner-Off Temperature at 25
    Will run like this and see how it goes. Thanks again for the assistance, I appreciate it.

  15. Dave Dave | | #21

    Cleaned all the convectors today with my compressor and shop vac. As it turns out, they are three tube convectors not single as I had mentioned earlier, not sure if it makes a difference.
    Also realized the bathroom has an 8 foot baseboard radiator, somehow I forgot about that.
    Hard to test anything, heat's been off the last two days, in the Hi 60's.

  16. Expert Member
    Dana Dorsett | | #22

    Is the bathroom radiator cast iron (if yes, how tall?), or is it a fin-tube baseboard convector?

    The fin size and spacing on the convectors matters more than whether it's a single fat tube running through the fins vs. 3 smaller tubes. The specs for comparable current products probably out there at the usual convector vendors' websites. But since you're not swapping them out it really doesn't matter- you'll be tweaking the reset curve to work for best efficiency & comfort with whatever ratings they have base on the system behavior.

    Doing the math on them and the heat load gets you to a reasonable starting point to start adjusting from, but from there it's all about lowering the temp to where it still keeps up but doesn't short-cycle. Starting from 140F @ -4F will likely take a lot fewer tweaks to find mod-con nirvana on this system than starting from 180F.

  17. Dave Dave | | #23

    No, the convector was ripped out of the bathroom during a remodel and an 8 ft. slant fin baseboard was installed. As we never really calculated in the 18' of slant fin baseboard in the extension, I didn't think it mattered compared to the convectors in terms of BTU/Hr output.

    I'm installing some thermocouples on the supply, upstairs return and basement return , connecting them to a datalogger. I'll have the 3 temps and the outdoor air temp as well.
    I haven't figured a way just yet to tell who is calling for the heat, but I have Zwave thermostats and might be able to something with those. Once I get it sorted out I'll have some real data. I'll see if I can add degree days as well. I like to prove out the theories.

  18. Dave Dave | | #24

    The thought crossed my mind to add some heat to my garage. It would be pretty simple to add some fin-tube sections along the wall and plumb them into the basement loop. Thoughts?

    Now that it's down in the 30's and 40's, I seem to have to jack the thermostats up a bit to keep warm. I don't have any hard data on the supply/return monitoring, waiting on a temp sensor delivery.

  19. Expert Member
    Dana Dorsett | | #25

    The heat loss characteristics of a garage are pretty different from those of a basement. Garages are also pretty air leaky- if the power ever goes down during a cold snap the risk of the heating loop freezing up is many times higher in a garage than anywhere indoors.

    Run a Manual-J on the garage before doing anything, but unless you're heating the garage 24/7 an oversized non-hydronic solution such as a wall furnace or rafter-mounted hot-air heater capable of heating the place up quickly usually makes more sense.

    1. Deleted | | #27


  20. Dave Dave | | #26

    I have a 10Kw natural gas generator, no worries about power fails. I was just thinking of a way to add to the basement loop and that was an obvious solution.

  21. LRRP71 | | #28

    Hello this is my first post here. I am so glad I found this forum and this post. I am in almost the same situation with a NCB-240E. I have a split-level Ranch that was about 1,2000 Sqft. It was raised after Sandy and I added a extra 1,200 sqft. Using Fox Blocks. My problem is my Family room which was the ground floor. Now it is the second floor 6ft higher, along with my boiler room. It is on a 2nd zone, with only a 16ft Slant/fin 2000 radiator. The rest of my house is Convectors. Except the 2 bathrooms, which are also slant fins at 4ft. The boiler was a oil fired Weil- McLain boiler. Switched to Natural Gas until Sandy hit. I had over 6ft of Salt water through my house.
    So I know where my problems lies with zone 2 and not enough element. I have tried using outdoor rest with no luck. Unless I keep my Zone 1 running, with Zone 2. I can get a stable run for almost 24 hrs. with 2-4 burns per hour. But I am at about 138 - 140F return water. That is on a 27-32f degree day, with blowing wind from the North. That is with keeping my upstairs Zone 1 set at 75F. My zone 2 set 73F. I thought the same thing adding more element. So I hooked up another 16ft of my old slant/fin 2000 into the zone 2. Which is in my garage. I plan on adding a Modine Lodronic HC 47 to the garage. Plus one to what use to be the crawl space. Which is now a 19ft x 26ft with a 9ft ceiling storage workspace. My Garage is the same but 6ft x 17ft is a crawl space with a 5ft ceiling. While 18ft x 17ft is the Garage. Now has a 13ft ceiling. the Garage is 4,488cuft. The Storage area is 4,446cuft.

    So anyway, I have tried my Outdoor reset on 1, 6, 4 and 5. But not messing with the the outdoor high and low temps. Or on and off burner times. What I have tried is playing with the F Parameter the (Max space heating capacity.) With no luck and in some cases made it worse. I tried a custom setting #7. but left the B to M settings at default. I played with the
    N Parameter (Supply Absolute Max Temp down to 115-138F)
    O Parameter (Supply Absolute Min Temp 80-102F)
    P Parameter (Return Absolute Max Temp 110-120F) and
    Q Parameter (Return Absolute Min 68-102F). I also did try using return water instead of supply water Temp setting with the dip switch. Nothing worked, because I know I just don't have enough element or high efficient low temp panel or convector elements. Which should help on zone 2. So, what I did was disabled outdoor reset. Changed my water temp to work between 120 to 135 supply. With both zones running, and I am getting about 120-132F return temps. No short cycle. But if Zone 1 is not running, and only zone 2. I am getting 40 second burns with 3 min off. then my return water goes to 148-158F.

    I know now the 240E is over kill for my house. Unless when I add the 2 heaters in the garage it will help. But that will be a Zone 3. If I do not fix Zone 2 it will always be a problem. My house has been almost totally rebuilt. I have stucco over the Fox blocks. I have new Certainteed siding with 3/4 poly board outside. Plus, new insulation in almost all the walls and ceilings. With 3/4 inch Drywall put up. Almost all my windows are new double pane Argon filled. A new rear Sliding Anderson door to the family room where my Zone 2 is now. I am using Gen 1 Nest Thermostats. Sometimes on Eco, but most of the time running on heat when it is below 35F outside. My house is at 73F upstairs in almost all the rooms. That is with keeping my stat set at 75 and supply water at 135f Except the 2 bathrooms. Which are about 70F and one room that I stay in gets up to 78 if I keep the door closed. I have 3 other adults and a child living with me. Hot water use is high, and so far, no problems using a shower, and having the dishwasher or the washing machine running.

    I am going to try putting my Outdoor low temp down. My outdoor High temp up. Then play with burner times first on low Radiant parameter. See if that helps with my problems. What I wanted to do was put radiant heat in the floor of the Garage and Storage area. But being short on money, and using New York Rising to raise my house. It was not a go. I also have 16 smart flood vents, but they are not allowing any drafts in with the special caps on them. Plus, I had trouble with the first contractor, with problems I can't address on a forum at this time. I hope to come back with some good news, because I am trying to save my money up to have the Hydronic heaters put in the Garage and Storage area. It was supposed to be done by now. This Covid mess and having a heart attack in Feb threw things off. With contractors, Plumbers you name it. But I think what I am also going to do is save up for the right low temp hydronic heaters, either convectors or panels to replace my old ones. Also use TRV's on the units if money allows this summer, and really try to fix this problem. Putting a smaller unit in is not a solution after spending money on it only 3 months ago.

    1. Expert Member
      Dana Dorsett | | #29

      Since you have a heating history on this place run a fuel-use heat load calculation. Assume 87% efficiency the temps you've been running. See:

      Adding zones won't fix any efficiency issues, but combining zones can. Your whole-house 99% design load is probably 25- 30,000 BTU/hr (which can be verified by fuel use), but individual zone loads are likely to be under the minimum output of the NCB. Your average winter load might be pretty close to the minimum-output of the NCB, but unless you have enough heat emitter on each zone it's going to cycle.

      Some amount of cycling isn't a disaster. As long as the burn times are 3+ minutes with much fewer than 10 burns per hour it's not going to take a huge efficiency hit or put excessive wear & tear on the boiler. The ignition stuff may wear out a bit sooner, but it'll still go a long time between component replacements. When it's going all winter running dozen or more 1 minute burns per hour it's in the short-cycling disaster zone.

  22. LRRP71 | | #30

    Dana that was one of the articles that brought me to the site. I have read it a couple of times. Along with getting answers from Terry Love and Heating Help. I have learned about Manual J, D design conditions. I read a couple of articles on I=B=R. I used the for the 65f HHD for actually 3 yrs. and I did a average. I used Kennedy Airport which is closer then Farmingdale. But they are really both the same. I also used Weather Spark to look some things up.
    But here is my problem since the end of 2013 I was living in a trailer in my backyard. While my house was being elevated from Super Storm Sandy. I only stayed in it from Jan to the end of March of 2013. Living in a destroyed house, and after my old boiler was destroyed. I put in has a emergency a QuietSide DPW120 A. BIG MISTAKE cause that unit was a POS and from a week after installation it had problems. But the guy who did it disappeared. He did about 5 houses on this block and within 2-3 yrs. Every person had to have them removed. Then we found out there was a recall on the units for a fire hazard and got screwed. Because no one new and put complaints in. At the time National Grid was giving discounts for putting these units in. They never took responsibility. Also, back in 2013 hardly anyone knew or serviced Tankless units. So I had to learn what to do. I lived in the trailer till This Sept 11th (of all days.) When I finally was able to get back in. So, the data I can use from 2013 is in a house with no ground floor, Inside walls or insulation. Plus, most of the second floor, had water damage in it. I had to tear the bottom parts of walls off. From May of 2013 till last year. There was no Gas, Water or Electricity in the house. I finally get back and the last storm that came Tropical Storm Isaias . Sent a power surge through my house, when the transformer and poles fell and destroyed my Boiler, AC units all my GFI sockets, Micro Wave, Clothes Dryer Handicapped Elevator in my Garage and dishwasher. Along with dropping a tree on my Ram 1500 and doing almost $9,000 worth of damage. So that's when I had the Quiet Side ripped out and a Navien NCB 240E put in. Enough about that.

    I really had no education on ModCon Units, cycling problems or that they had to work at low water temp for condensing to get 90-95% AFUE. I have only been getting an education in this since October. In reality the Plumber who put the unit in. Actually, tried to sell me on a smaller unit. But what did I know, and thought bigger was better. Mostly due to the Domestic Water side of the unit. So now that I know some more about these units, I wished I listened to him. Plus, I wish I had the money at the time to put Radiant heat in the Garage and Storage area. Since I had to have a new foundation laid. But stuff was not going the way it should have at the time.
    Now I have no choice but to figure out, how to remedy my mistakes. With the unit I have and try to get the unit not fast cycle on ODR. I have tried what I think is everything configuring the ODR. Today I tried what I said I was going to. Changing the high and low outdoor temp. Then playing with the high and low firing times. No matter what default curve setting I tried. Plus setting a custom one up. I am not able to get more the 2 min burns with 10 min off. So, I am hitting on 8-10 cycles per hour. That is with both zones working. When Zone 2 is on, I am fast cycling. 30 times if not more in an hour. If Zone 1 is only on. It is up to 15 cycles per hour.

    I turned off the ODR and am back to using the indoor temp. If I put my stats up to 73F. Water Temp at 125F. With both zones running, and my Stats set at 73F. I can get 2 cycles per hour Today was about 40's so my house stayed at about 70F with Eco mode on my Stats most of the day. Without the unit running in the afternoon at all. Once it went 35 I was able to play with it and get the burn cycles I am getting right now. But if Zone 2 goes on without zone 1. My cycles jump to about 8 to-12 per hr. If Zone 1 Runs with no zone 2. I am getting about 5 -8 burns per hr. No matter what I start at 114-125F supply water by the time it hits 131F It cuts off. But my Return stays between 117F and 121F. Till it cuts off and like I said it is running for maybe a little more the half an hour. Hits 130F and cuts off for 3 min.

    I really think my problem is my convectors and fin tube elements. I believe if I put low temp units at the right BTU for each room in and use TRV's. Move my zone 2 back to zone one. Most of my problems would be solved for ODR. Then if I put the 2 Hydronic Garage eaters Lodronic low temp's for ModCon units in. Put them in series on the 2nd zone and move the Stat into the garage. I has a layman think this might be an answer. Again without heat loss calculations done right. I am just guessing and could be way off.

    My one question I do have about ODR is, Parameter F (Max Heat capacity for space heating.) If I were to try a default curve or a custom curve. Then decrease the F parameter from 100%. Will this maybe decrease my cycle problems?

    I thank you for answering my post.

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