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Cause of boiler short cycle?

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HelpInAK
HelpInAK Member Posts: 41
I’m in the process of adding heat teansfer plates to improve a staple-up radiant system to get temps into condensing range. Installing plates as I get them, approx 70% finished with the project. As I’ve been working I’ve also been slowly lowering SWT from an original 166F to see how the system reacts. All seemed well until today when I tested out 145F, which causes the boiler to short cycle - SWT shoots up to 155 before shutting off for a minute and starting over. I’ve currently got it running at SWT 150 with no short cycling issues but it isn’t condensing with return temp at 136. Any idea why it isn’t liking 145? Thanks

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  • Zman
    Zman Member Posts: 7,588
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    The lower the water temp, the less heat is transferred. This is great for boiler efficiency and thermal comfort but bad for short cycling.
    It sounds like you have a Lochinvar Noble combi. Combi's are almost always over sized to the heat load. You may be able to mitigate the problem by adjusting the on/off differential or activate anti short cycle settings on the boiler. A buffer tank would also help.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • HelpInAK
    HelpInAK Member Posts: 41
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    Thank you @Zman it is a Noble NKC199. Unfortunately I don’t know my way around a boiler other than changing the setpoint temp. I looked at the user’s manual and don’t see anything regarding either an on/off differential or anti short cycle settings. What/where are these? Is this something I should call an expert for?

    Boy I’m really going to kick myself if I’m already $2k into this upgrade just to find out I won’t be able to improve efficiency anyway because the boiler is oversized.
  • SuperJ
    SuperJ Member Posts: 609
    edited March 2019
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    I think Lochinvar nerfed the differential settings on the combi's. The off temp is fixed at setpoint +10f.

    Well, there are other things like space heating rate limiting, a buffer tank, zone combining, ensuring corrects flows, turn up your anti cycling time (try 10-15 minutes) to make sure things cool off substantially before it kicks on again.

    An unwired inexpensive electric DHW tank might be a good place to start, connected in a 2 pipe configuration to minimize the flow thru the smallish ports.

    If you have small zones you may want to combine some them so that they can handle the minimum output of the boiler.

    Thermal mass can make up for a multitude of sizing sins. I'm about 5x oversized (100kbtu boiler/20kbtu load, free boiler from a coworker), and run one or two cycles per hour because I have a decent sized buffer.

    Rate limiting can help, it will prevent the boiler from cranking up to high modulation even when it has some catchup to do. Set this to the lowest % that keeps your house warm.
    It's ok if it has to run for a long time to heat things up.


    Make sure your system sensor is correctly installed in the proper location.

    Make sure you have adequate flow (check the temp delta across your zones). If the delta is too high, you may not have enough flow in your floor, to deliver all the BTU the boiler is supplying.
  • HelpInAK
    HelpInAK Member Posts: 41
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    @SuperJ thank you, the rate limiting adjustment seems like a good (and the cheapest) place to start.
  • HelpInAK
    HelpInAK Member Posts: 41
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    Another question on heat rate limiting - I’m not sure where a good place to start is in terms of limit %. Work has been hectic so I haven’t been able to test any adjustments to it, but I did notice last night that the boiler was short cycling again when the garage slab calls for heat at a SWT of 150. It would fire up to about 60% and immediately hit set point and anti cycle. I had to bump SWT back up to 160 to keep it running steadily, If 60% fire rate is too high for SWT 150, I’d guess I’d have to shoot way low if I hope to run SWT 140 or lower. I don’t know much of anything about HVAC, but intuitively it seems pretty drastic to have to throttle the flame rate limit to 50% or lower. I know the space heat rate limit CAN be adjusted as far down as 10%, so am I just being paranoid, or is it ok to be throttling that far back?
  • mikeg2015
    mikeg2015 Member Posts: 1,194
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    Key to effeicincy is return water temp, not supply temps. You might see if you can reduce the flow rate to increase delta T.

    150F Supply, 125F return is more efficient than 140F, 130F return.

    Add mass. It will never make the issue worse. If you add a pump, buffer can become a hydronic separator and you can sometimes better optimize flow rates independent of load.
    HelpInAK
  • SuperJ
    SuperJ Member Posts: 609
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    HelpInAK said:

    Another question on heat rate limiting - I’m not sure where a good place to start is in terms of limit %. Work has been hectic so I haven’t been able to test any adjustments to it, but I did notice last night that the boiler was short cycling again when the garage slab calls for heat at a SWT of 150. It would fire up to about 60% and immediately hit set point and anti cycle. I had to bump SWT back up to 160 to keep it running steadily, If 60% fire rate is too high for SWT 150, I’d guess I’d have to shoot way low if I hope to run SWT 140 or lower. I don’t know much of anything about HVAC, but intuitively it seems pretty drastic to have to throttle the flame rate limit to 50% or lower. I know the space heat rate limit CAN be adjusted as far down as 10%, so am I just being paranoid, or is it ok to be throttling that far back?

    Don't get too hung up on the water temperature when rate limiting. Worry more about the space temperature. I would probably start somewhere between 25-40% (50-80kbtu). If it can't quite keep on on cold mornings bump it a percent or two at a time. Each percent is roughly 2000btu, which is a significant amount of heat.
    HelpInAK