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Boiler Short Cycling

Jerry L
Jerry L Member Posts: 5
The boiler is a NTI,(NY Thermal inc)- CT 90 Odyssey direct vent. Yes It appears to be a single run, some of the basement ceiling is covered in this 1951 cape. I'm actually firing a .65 W delavan nozzle at 140 psi, lowest recommended rate. No net IBR ratings listed just gross output of 89,000. Grunfos circ.is a UP 26-64 F

Comments

  • Jerry L
    Jerry L Member Posts: 5
    Boiler short cycles

    I just installed an oil fired(riello)boiler in my house- I'm using the smallest nozzle at .75 gal/hr-It's a text book install, pumping away into 120' of 3/4" fin/tube baseboard. Single zone with an outdoor reset control built in. The boiler reaches the setpoint temp.very quickly, however the returns lags fairly long in getting hot. I installed a grundfos UP-26 high head circ.thinking I'm not moving enough water. Spoke with the boiler rep.on pressure drop through boiler block,he said very minimal. How do I achieve longer cycles? Primary/secondary loop? Install high output baseboard? The load calculates to about 70,000 btu's-and boiler output is about 105,000...I understand I'm a little overfired or too much?
  • Mitch_6
    Mitch_6 Member Posts: 549
    Just a quick question

    is the 120 feet of SF 30 on a single 3/4" run.

    Also Riello is a Burner what is the Boiler.

    105,000 is the gross input of #2 fuel oil at .75 gal provided you are running 100psi.

    Do you have the Net IBR rating of the boiler output.

    Mitch S.

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  • Brad White_141
    Brad White_141 Member Posts: 21
    What I see

    First to basics:

    Your calculated heat loss in BTUH is______?

    Your total fin-tube output actually is_______?

    Without these nailed down it is all conjecture. Still, here are some thoughts:

    At 0.75 GPH your input is about 105 MBH and output at 85% maybe 89 MBH in round numbers.

    I am not sure if the fin-tube is in series or is it in split circuits, even on one zone? That will make a difference.

    The 120 feet of fin-tube, say it is at 600 MBH per foot, that is 72 MBH right there and on a design day.

    Is your Grundfos a 26-64? That is about 20 feet of head if the flow is at a 20 degree delta-T. I would expect 120 feet of tubing even in series and with another 30 feet (150 feet total) to get it there, to impose a head loss of 11.0 feet per 100 feet of pipe at 7.0 GPM in 3/4" type M copper. With 50% for fittings thrown in, you are right about there! Over 4.3 FPS velocity too.


    Now, because you are firing oil, your reset gradient is limited to maybe between 180 on the high side or more if you need it, down to 150 on the return side. This is to allow 130 degree water or more on the return side and is cutting it closely.

    Yes, I would definitely pipe it P/S. This way you can get deep reset on the radiation side where it counts. Of course, back to basics, you need to know how much radiation you have relative to your heat loss. The more the better and this will give you a starting place.

    You may also invest in a buffer tank and use that as your controlling medium, fire the boiler to maintain X temperature in the buffer tank and sip off that to your secondary. This hydronic piggy bank will save some cycling especially in mild weather.

    I suspect a combination of not too much radiation nor heat loss at this time coupled with limited reset on your boiler is a prime suspect, overall.

    My $0.02 and in random order :)

    Brad



  • Mitch_6
    Mitch_6 Member Posts: 549


    > The boiler is a NTI,(NY Thermal inc)- CT 90

    > Odyssey direct vent. Yes It appears to be a

    > single run, some of the basement ceiling is

    > covered in this 1951 cape. I'm actually firing a

    > .65 W delavan nozzle at 140 psi, lowest

    > recommended rate. No net IBR ratings listed just

    > gross output of 89,000. Grunfos circ.is a UP

    > 26-64 F



  • Mitch_6
    Mitch_6 Member Posts: 549
    Dans book

    The Golden Rules of Hydronic Heating page 69

    3/4" 67 Linear feet longest safe continuous loop.

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  • mp1969
    mp1969 Member Posts: 226
    Common Supply

    I would run a new 1"supply to the middle of the series loop and create two zones of close to equal length.You will then be reversing the flow in 1/2 of the system and thus shortening the loop length. You have the option of creating two seperate zones here for thermostat control(if so desired)

    This is known as common supply/separate returns. May not be practical due to conflicts of the structure but this will shorten the returns to rule of thumb constaints.

    MP 1969
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