Outdoor reset curve Navien NCB 240e

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

  1. Dana

    Dana In the trades

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    Huh? Of course they do at lower temps! Fin-tube baseboard convectors aren't tall enough to deliver predictable amount of heat at temps much lower than 120F, but they still heat. Radiators taller than about 15" have predictable linear response down into the 90sF.
     
  2. fitter30

    fitter30 Active Member

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    Sure they do but why do their charts start at 110*-120* manufacturers ratings. Radiators are total different one they have mass and being made out of cast iron they don't give up there heat in a few minutes like aluminum.
     
  3. Dana

    Dana In the trades

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    Simply because at temps lower than that the output is very non-linear with temperature and can't be guaranteed to deliver some particular heat rate. The taller the convector, the more linear & predictable it is at low temp. The amount of stack-effect convection drive through a 7-8" tall convector isn't much even at 120F. But a finned convector in a 30" tall cabinet is pretty well characterized even at 90F.

    When the convection drive through a convector becomes too weak at some lower water temp the heat rate coming out of the fin tube is too easily affected by everything from dust-kittens to window drafts. Above 120F the temperature difference between the entering air at the bottom and the exiting air at the top of a 7-8" baseboard convector is high enough to produce sufficiently strong air flow to be reasonably consistent. Below 100F it's all over the place- too inconsistent to reliably design with, but it's still WAY more than zero.

    The output of 10" cast iron baseboard isn't very linear below 100F either, despite having a decent amount of thermal mass and a modest amount of direct radiation.

    They are not totally different- most of the heat from "radiators" is still delivered by air convection, with only a fraction of the heat delivered as direct radiation.

    Thermal mass has nothing to do with it, but with almost all hydronic radiators there's more thermal mass in the water volume than in the cast iron. A pound of water stores as much heat as 9lbs of cast iron- in volume terms that means a gallon of water stores as much heat per degree F as ~75lbs of cast iron.

    With modulating boilers tracking the load via outdoor reset there isn't much concern about whether they "...give up there heat in a few minutes..." since the heat is still coming in at a constant rate rather than duty-cycling. That is of course assuming the outdoor reset curve is reasonably well tuned and the modulated output rate of the boiler goes low enough. Even when the boiler is cycling on/off at low temp it's not going to appreciably affect the comfort levels even with low-mass fin-tube, as long as the delta between the turn-off and re-fire isn't super-wide.
     
  4. David Philips

    David Philips New Member

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    Tangentially related anecdote on baseboard with low water temps:
    NHB-80, main zone is 67' of Slantfin 30 baseboard. The current ODR curve raises indoor temp about 1º per hour while zone is calling for heat (on/off cycling is triggered by T-stat). At 48-50F outdoor temp, boiler setpoint is ~100F, supply/return delta-T is 2-4º, baseboards only feel slightly warm but it's been enough to raise/maintain the indoor temp as needed. I've been considering tweaking the ODR downward for longer runtimes to see what happens with the baseboard, though not sure if there's much to be gained.
     
  5. Dana

    Dana In the trades

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    If the NHB-80 is firing in the lower quartile of it's maximum (= less than 20KBTU/hr-in) with an entering water temp of 100F you're looking at about 95%-96% raw combustion efficiency (for most condensing boilers, anyway). Take a look at this typical curve- the red line is 25% of max firing rate:

    [​IMG]

    To bump that by even 1% to 96%-97% the entering water temp (EWT) at the boiler needs to be at or under 90F That might be do-able with 67' of fin tube without short-cycling, given that at minimum fire it's putting out less than 8000 BTU/hr, or (8000/67=) <120 BTU/hr per foot. At 90F EWT and an average water temp of 195F (100F out of the boiler, 90F back) typical fin-tube is probably delivering somewhere around 100BTU/hr per foot, and the efficiency is about 97%, which would mean an output of 7760 BTU/hr or (/67=) 116 BTU/hr per foot.

    It may still do a bit of cycling, but probably not much. With ~100' of 3/4" plumbing (including the 67' of fin-tube + distribution plumbing) you're looking at about 20lbs of water mass to work with- it's not a lot, but not nothing. Even if the zone radiation is only emitting 100 BTUhr per foot that's only about 1000 BTU/hr, or (/60=) 17 BTU/minute of excess, and the temperature would be slewing only 17BTU/20lbs = 0.85 F per minute. With the parameters set for a 10F differential between burner out & re-fire you'd be getting burn times in excess of 10 minutes, and fewer than 5 burns/hour.

    But if you were to go much lower than that to push it to the 98% combustion efficiency range with 80F return it will surely short-cycle.
     
  6. Dana

    Dana In the trades

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  7. David Philips

    David Philips New Member

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    Great, thanks Dana!
     
  8. FDNY/RETIRED

    FDNY/RETIRED New Member

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    [​IMG] [​IMG]

    Sorry been out of service again till yesterday, lucky it wasn't covid, was intubated for a couple of days. Got out of hospital day after Thanksgiving. I just figured how to load photos but I see I need to figure how to resize them to be smaller.

    I have a Taco SR502-4 switching relay. running 2 Taco 0015-MSF3-IFC on low. Zone 1 is my upstairs which controls 3 floors of 2 convection radiators in the kitchen 24 inch, Dining room 46 inch, Living room 54 inch and hall 36 inch. The next floor is 2 bedrooms with a 42 inch and a 56 inch convection radiators. Bathroom with a slant fin base/line 2000 36 inch. Next floor is master bedroom with 2 convection radiators both 46 inch and a slant fin base/line 2000 24 inch in the bathroom. Nest Thermostat is out of the drafts on a inside wall where the stairs are going to the second floor.

    Zone 2 only runs a family room with a 12 ft Slant find Base/line 2000. Nest Thermostat is on the wall opposite radiator, and there is the boiler room on the other side. Which also has my Washing Machine and dryer in it. the thermostat is not on the same wall in the family room opposite where the boiler is hung in the other room.

    I am on the Default #1 which is finned tube base board 120-180* supply and 101-147* return. Right now I have not touched anything but F setting which I put to 90% all other parameters are default. I have been using mostly eco mode till t-nite. Set at 65* down stairs and 66 upstairs. But it has been warm till today. I put the stats on heat and adjusted it to 67* downstairs and 68 upstairs. it is ow 69* in zone 1 and zone 2.
    The boiler ran for 1hr 40 min between 6-8. Down stairs ran for 1hr 6min 5 to 7:30

    My boiler started fast cycled with zone 2 only running , but once zone one cut on it stabilized at 14* supply and 140* return with no fast cycle. I reset my stats for Eco mode and raised them to 66*zone2 and 67* zone 1. It is 42* outside right now. 21mph NE wind. 0 % precipitation and 52% humidity @ 9pm.

    Again my problem is when only zone 2 is on. I have short cycle problems. When either Zone 1 is on, or zone 1 and 2 are on all seems fine. Except my return water temp is 140*
     
  9. Dana

    Dana In the trades

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    From a reset curve point of view the tall convectors on zone 1 can be treated like real radiators, and you have more than enough to run very cool water without short cycling. Without knowing the exact specs it's still pretty assured that they would be putting out more than 500 BTU/hr per foot of width, so at total 16' of total convector width would more than cover the minimum output of an NHB-80. It will probably do fine from a cycling point of view even with 110F output.

    With only 12' of low mass fin-tube on zone 2 it's a problem. At an average water temp of 120F (return water in the 115F range) it's only emitting 500-600 BTU/hr, and has very little thermal mass. If you can increase that to 35-40' of baseboard or replace it with 12' of tall convector or a suitable amount of panel radiator (which have at least some mass to work with) it would be fine, with very little cycling.
     
  10. fitter30

    fitter30 Active Member

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    Outdoor reset isn't perfect with one temperature sensor that may or may not be in a perfect place. (Among the factors that can be used to improve system efficiency is a group of building-specific factors such as how a building reacts to sun (e.g. amount of sunshine, time of day and time-of-year), wind (speed and direction), and “thermal inertia”, how a building responds to the heating system) i didn't write this but have been involved working on a leeds building with a ems programer.
     
  11. FDNY/RETIRED

    FDNY/RETIRED New Member

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    Thank you And there lies my problem the zone 2 with slant fin. I even reattached my other slant fin, Which is in the garage on ground level. That's another 12 ft. Thinking adding more load might help. It did, but still my zone 2 cycles to much when it is on alone. My zone 1 is running fine 138*f supply and 123*f return. keeping my house at 69*f upstairs. Running about 3 cycles per 3hrs. lasting 3 min to 5 min off. It is 41*F and raining here.
    But that zone 2 is the killer and I will take your advice on changing the radiator for that room. When I get ready to add the Hydronic garage heaters. That's why I figured a Navien NCB-240E wouldn't be over kill. With adding Modine Lodronic HCH 39K's if I can get them. One in the Garage and one in my work area. I have close to 70,000 BTU's added up between the 2 separate areas. Figured out by length, width and height for each room, and converted from watts to BTU's. It is insulated with fox block walls and I took care of sealing the garage door pretty good from drafts. Plus making insulation panels for the windows and my flood vents with 2" R-10. Again thanks for the advice. I am still going to have a company do a complete energy analyses. I am far from an expert on this stuff.
     
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