Setting the K Factor

Discussion in 'Boiler Forum' started by theMezz, Feb 28, 2011.

  1. theMezz

    theMezz New Member

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    11
    Location:
    Central NY
    Just had a Navien CR-240 installed. Any suggestions as to what methodically I should use to properly select a K Factor ???
  2. Tom Sawyer

    Tom Sawyer In the Trades

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    look up the k factor chart online. It will give you the design and degree day for your area.
  3. theMezz

    theMezz New Member

    Messages:
    11
    Location:
    Central NY
    I have the chart .. but how does that help?
    I have to change the K factor every day ??????

    help !!
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,055
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    You could set it for your worst day. Then, it would always be able to produce the hot water you wanted. Not the most efficient, but probably easiest.
  5. Tom Sawyer

    Tom Sawyer In the Trades

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    3,184
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    Well you can but you should be able to find the average K factor for your area either online or call your local oil company who use K factor and degree days to schedule fuel delivery.
  6. Dana

    Dana In the trades

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    Methinks you're talking past each other due to Navien's definition of "K-factor". It is NOT fuel use per heating degree day in their parlance, but rather it selects the outdoor reset curve. The manual isn't all that enlightening, is it? The curves are on page 38 of the .pdf pagination. (and of course they're all in Celcius, just to make life fun for US Yanks. ;-) )

    K must mean "kurve" in "korean" or something, but whatever, the number selects curve for what 'mericans in the hydronic heating biz call "outdoor reset", varying the output temp in response to the out door temp as a crude model of the heat load. Running lower output temps increases the average efficiency of the system, but you need to run temps high enough that the radiators or other emitters can deliver the heat when it's wicked cold out.

    The K-setting that works best will depend on your radation type & quantity, flow rates, and actual heat load, but you can skip the hard math and do it empirically. To get there systematically means could start at the bottom (lowest temp curve K=0.5) and work your way up if it doesn't seem to be keeping up. If you start out with a mid-range and it's keeping up you'd have to work backward from there, observing the operation as you go. What you're looking for is the curve that gives you decently long burns at the lowest possible temp and still manages to keep you warm.

    If you know how much radiator/baseboard you have out there and know the water temp that kept it up previously (assuming this was a retrofit), you can be a bit more methodical. Look up your outside design temperature for your area (google "outside design temperature" and the name of the nearest bigger city (Syracuse NY would be +2F, or -17C), then find the curve that shows a temperature close to your old boiler's temp at that outdoor temp. If your old boiler ran 180F/82C and kept up even when it was 2F outside, the curve for K=1.5 crosses pretty close. Since there was probably a lot of margin built-in, you should start at least one curve lower, so try it at K=1. If it doesn't keep up bump to K=1.5, but if it keeps up at K=1, back it off to K=0.5.

    If you have radiant floors or somethign you can probably start off at K=0.5 and it'd probably keep up. If your emitters are fin-tube baseboard you may have poor response when it's above 40F out unless you bump it up to K=1.5 or K=2. I can't imagine any system that had previously been run off a cast iron boiler in central NY that would require a curve higher than K=2. Taking just a WAG at how old heating systems were set up in NY, start at K=1. If it's a new system, hopefully somebody designed the radiation decently for balance and low operating temp, and you can start at K=0.5.
  7. zl700

    zl700 DIY Senior Member

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    Location:
    Texas
    Dana is right on, if you needed 180 degree water at 0 outside, your setting would fall between 1.5 and 2.0 (changeable in 1/10 increments). We always suggest start high for comfort and turn down 1/10 every 4-6 days, when your house no longer warms to desired indoor set point temp, turn back up 2-3 tenths and you have found your homes "sweet spot".
    This is assuming you are operating your unit on supply water temperature operation, if set up for return water operation (more accurate reading of heat emitter performance and requirement) then the settings would be different but same principle.

    BTW if you have "K-Factor" capability I think you mean you have the CH-240 model, combi boiler
  8. theMezz

    theMezz New Member

    Messages:
    11
    Location:
    Central NY
    yes I do have the CH-240 - - THANK you .. your information is helpful.

    It's just weird because I have my house thermostat set at 70 and the house temp actually goes from 68 to 72 at times.
    My old system kept it exactly at whatever I set it.

    The installers tell me start off at 3.7 .. but I have no idea if that is causing my temperature fluctuation. (I have big old fashioned radiators)I

    I also notice my domestic hot water varies in temperature a lot the first 3 minutes of use or so - but that doesn't bother me as much as the house temp variation.

    I really appreciate your replies.. they are great. Thanks again.

    A sidenote for those thinking of switching to this type of heat. My floors are cooler now because my basement is colder. The reason is that my old 1974 boiler leaked heat into my basement,
    the new CH-240 does not .. therefore my basement is colder and my floors are colder. Something I never thought of !!! No bog deal - just a point of information. I have a temp gun
    and my floors are 5 degrees colder than normal.
  9. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    "The installers tell me start off at 3.7 .. but I have no idea if that is causing my temperature fluctuation. (I have big old fashioned radiators)"

    It probably is causing the wide indoor temp swings as the hotter than needed water super heats the radiators and continues to radiate long after the pump/boiler has shut off. Besides water temp, the thermostat anticipator or on digitals it is a form of sensitivity, often called cycles per hour may need to be adjusted. Try turning down K-Factor also.

    It is a trade off getting rid of the beast that kept the basement so warm, the benefit being the fuel savings. Many end up adding some heat via a extra zone to basement when needed.

    DHW on the CH has no adjustment valve like a tankless, so flow changes can result in temp changes. You might try programming the DHW preheat option for quicker and smoother DHW delivery.
  10. Dana

    Dana In the trades

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    Running a lossy jacked high-mass oversized boiler hot enough to keep the basment hot is one EXPENSIVE way to make radiant floors, eh? ;-)

    If you haven't already, consider...

    A: Air sealing and insulating the foundation sills & rim joists, and sealing up all unused flues into the basment, fixing all the window & door weatherstripping, etc.

    B: Insulating the basement walls n such a way that the foundation continues to dry toward the interior rather than forcing the moisture higher in the foundation to spall on the above-grade exterior, but with enough foam between the foundation and any interior wood or wallboard that the mold hazard stays low. Some suggestions on ways to go about it live here. (The thermo-hygric analyses are for Minneapolis, so you could get away with slightly thinner foam in the foam/fiberglass stackups, but don't go less than an inch of XPS, or less than 3" if EPS. Don't use any rigid foam with foil/poly/vinyl facers, since they're too vapor retardent.)

    When I air sealed the basement and put ~ R20 on 90% of the foundation walls it reduced my heating fuel use ~ 15-20%, and the basement stays 65F or above even after having converting to a low-mass tankless for a boiler. The uninsulated slab is still on the cool side, but the basement feels GOOD, and the first floors are quite reasonable in bare feet- always 65-67F in a 68-70F room. (Except for my radiant-floor zones, which are of course even BETTER in bare feet. :) )

    With old fashioned high-mass radiators setting your K to 3.7 would be a ridiculously high ramp which is almost certainly leading to the high hysteresis in your room temps. It's starting out with a huge delta-T between the mass of cold water coming from the cold radiators with a high firing rate, then the radiators end up dumping too much heat in into the rooms overshooting the setpoint, the whole place is then too hot for long enough that the radiators are then dead cold again, repeat. With the curve set correctly the radiators will warm up and cool down, but in a much narrower temperature range, and they'll never be super-hot or room-temp-cold to the touch when there's a moderate heat load.

    Crank the K-curve down to 1 or 1.5 then bump it up or down depending on how it's behaving. Again, you're looking for long, even continuous burns between calls for heat from the thermostat. If your thermostat has a programmable hysteresis or anticipation, as zl700 suggests, you might want to adjust that for a more-even heat, once you're close to the right curve. It might be worth spending a few bucks on a Tekmar 500-series T-stat that uses a PID algorithm for locking the room temp dead-on. (They're often useful for to managing the thermal mass of concrete slab radiant floors with low-mass boilers to reduce under/overshoots on the room temp.)

    Also, DON'T use deep overnight setbacks- if you do it takes forever to heat up in the AM unless you set the curve unnecessarily high, in which case you're blowing any savings that you might have gained in the setback period by spending more time with the burner running higher than condensing temps. You'll be more comfortable and use less fuel if you either keep a constant temp and dial in the curve.

    Also, high mass radiators are GREAT for running low-temp water- far better than fin-tube baseboard. Fin-tube craps out and becomes unpredicatble at out temps below ~120F, so you end up having to set the temps higher during the shoulder seasons and running ~88-90% efficiency when you COULD be getting 95% out it. With the curve set right those old-school radiators should be able to deliver even better comfort than it did with the high-mass boiler running in bang-bang on/off mode. The installer that told you to set the curve to 3.7 either knows something I don't, or is completely clueless and just picked a number toward the high side of the middle of the programmable range figuring you at least wouldn't get too cold.
  11. theMezz

    theMezz New Member

    Messages:
    11
    Location:
    Central NY
    thanks

    zl700 and dana - you guys should write mini book about the k factor - - great information that is very helpful - - thanks again ..i will let you know what happens ..
  12. Dana

    Dana In the trades

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    Navien should write that book, put it in their manual, and stop calling it "K-factor", since that term has already been in use referring to a common method of characterizing oil-fired heating systems. (The ratio of heating-degree-days/gallons-of-oil-used.)

    To normalize it with north American vernacular they should be calling it "outdoor reset curve" or something.
  13. Dana

    Dana In the trades

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    deleted double-post
  14. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    K-Factor besides the oil industry is also used as a term meaning graph factorization, usually involving two scales. In this case the scales are outdoor temp factorized with water operation temp.

    K-Factor graphing to determine operation
  15. theMezz

    theMezz New Member

    Messages:
    11
    Location:
    Central NY
    So is a low k factor setting better or a low # ?????

    thanks
  16. Dana

    Dana In the trades

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    The lower the curve, the higher the annualized efficiency will be, but if you go too low the temps might not keep up. Experiment to see how low you can get it. At 3.7 it's obviously super-high, to the point where you under/over shoot the setpoint temp like crazy. It's like trying to ease through a parking lot in a custom hot-rod- it's either stalled cold or screaming out ahead, with not enough middle-speed range to run nice and steady at the speed you'd like.
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