short cycling

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Thatguy, Jun 29, 2009.

  1. Thatguy

    Thatguy Homeowner

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    Let's say a normal cycle for some system is 45 min on, 15 min off, duty cycle is 75% and total cycle time is 1 hour.

    What is short cycling?
    30% duty cycle?
    Total cycle time of 30 min?
    55 min on, 5 min off?
    5 min on, 55 min off?

    TIA. . .
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    I think the key here is how it achieved the required run-time. You could get a 50% duty cycle by running alternate minutes, or even seconds, but you'd quickly fatigue the system. Many things like a minimum off time, and if you turn it on again before that period, you probably aren't using it efficiently.
  3. jimbo

    jimbo Plumber

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    Vaque question. But to use an HVAC example> A system which is too large for the area will "short cycle". Meaning it will run for a fairly short time, perhaps only a few minutes. The thermostat will be satisfied and the unit will turn off. But one function of cooling is to dehumidify the air, which makes people more comfortable. Having the room air circulated through the cooling coils for a longer period of time accomplishes this, whereas in the short cycle mode, less dehumidification takes place. ]


    So it all depends on whether you are talking about HVAC or the booster on a Saturn rocket....what do you mean by cycle time??
  4. Thatguy

    Thatguy Homeowner

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    This getting clearer to me. What's proper also depends on the "inertia" of the system.

    A motor with a large flywheel, or a cooktop element, could have power applied for one second and no power for 10 seconds and still the system could work OK, but a light bulb with this duty cycle would be visibly flashing.

    And the on time could be limited by, e.g., how fast you can reasonably remove moisture from air.

    So short cycling is system-specific.

    For HVAC, and for a well-pump, what would most people say is short-cycling? Minimum times, minimum duty cycles, some other abnormal behavior, what would it be?
    Last edited: Jun 29, 2009
  5. John in herndon

    John in herndon New Member

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    Heating short cycling

    To expand Jimbo's reply to heating, the same principle applies.

    If the heater is oversized it will overshoot the thermostat because the blower turnoff is normally controlled by the plenum sensor. In a normal installation you account for this by the anticipator setting on the thermostat, but if the furnace is too large this doesn't work well and you overshoot the setting. The user turns the stat down below the desired temp to prevent the room from getting too hot due to the overshoot and the end result is big swings in temperature.

    If the unit is properly sized it will run nearly continuously (95%) on the coldest design day.
  6. hj

    hj Moderator & Master Plumber Staff Member

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    short cycle

    In a heating system, for example, if it normally takes 30 minutes to go from burner on, to shut down, and it starts taking 5 minutes for it to happen, that is short cycling, and it could happen for any of several reasons, but one result is usually that the customer is uncomfortable because it is NOT heating properly.
  7. Thatguy

    Thatguy Homeowner

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    My oversized furnace runs 1/4th the time on probably the 95th percentile day.
    My AC, maybe 50% of the time once the setpoint is reached.

    And with the owner messing with the controls, it could be. . .
    "Hunting is a self-exciting oscillation of a system, and is common in systems which incorporate feedback. It is an important phenomenon in many fields, including engineering, economics and biology.
    en.wikipedia.org/wiki/Hunting_(engineering)"
    Last edited: Jun 29, 2009
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Both are oversized. Neither will provide maximum comfort and probably won't last as long as they could.
  9. hj

    hj Moderator & Master Plumber Staff Member

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    hunting

    Hunting would occur in an "automatic" setting, when the A/C cools too fast and too far, so the heat comes on to restore the temperature but then also overcompensates so the cooling has to be activated again to try to restore equilibrium.
  10. jimbo

    jimbo Plumber

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    Let us know what the professor in your theoretical physics class thinks of our answers.
  11. Dana

    Dana In the trades

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    Sounds like your furnace is at least 3-4x oversized, and your AC 1.5-2x oversized. (On design day your heating system only sees the full load for a handful of hours, but the daily average load on design-day will typically be well over half the full-load.)

    Short cycling is about more than duty cycle. There's a fixed amount of loss that occurs on every cycle, so maintaining a reasonably long minimum cycle is as important for efficiency as increased duty cycle. For a typical hot-air furnace it won't hit it's full steady state thermal-efficiency until it's been burning for at least a coupla minutes, and the first 30 seconds are about half it's full-rating. (For cast-iron boilers it's more like 6-10 minutes, due to the higher thermal mass.) On AC compressors it's a similar issue.

    Minimum cycles can be stretched by the amount of hysteresis in the controls (thermostat, humidistat, other), or by adding thermal mass to the system (hard to do when air is the heat-transfer fluid.)

    An ~80% AFUE- type hot air furnace running a 25% duty cycle on design day is really only hitting about 60% actual as-operated AFUE efficiency, even if it's not short-cycling (meaning all burns are at least 8-10 minutes or 4-5x the ramp rate to steady-state.) See:

    http://simulationresearch.lbl.gov/dirpubs/42175.pdf

    Look at figure 5 for derating your AC. If you're at 50% duty cycle on design day, you're running between 90-95% of steady-state efficiency, on THAT day, but most of the season you're much further to the left on the curve, something like the 10% of full-load zone.

    For the furnace, see fFigure 6. Unless it's something special (sealed-combustion & forced draft condensing, etc.) , assume it's part-load performance is more like the SDL-C111 curve. If on design day you're at the 25% of full load mark, you're efficiency will be about 85% of full steady state thermal-efficiency rating. AFUE is typically 3-5% below steady-state efficiency assume steady state is 83-85%. 85% of 85% is 72%. But over the season most of the fuel is going to be burned during 5-15% of full-load region, so figure ~55% as-used AFUE if you have a standing pilot, ~60% if you have electronic ignition. If it's a fully sealed-combustion system (condensing or otherwise) with electronic ignition & automatic flue dampers, etc, it'll look more like the Bonn 85 Induced Draft curve, which isn't bad.

    FWIW: Bonne was the guy who did the best most-accurated baseline measurements of this stuff back in the '80s while studying short-cycling issues, which is why the DOE regression analysis models try to fit his measured data. But if the equipment is actually short cycling a lot of the time you can toss those curves out the window- assume it'll be at least 10% worse, possibly much more.

    Matching the equipment output to the load is the first most-important piece of designing for efficiency. The tendency in the trades over the years has been to always err to the overcapacity side to avoid the mid-winter morning call from the irate & freezing customer, but it's almost never the right thing to do. That's changing slowly, but there's still a wealth of ignorance out there- 3x oversized heating systems are almost the NORM in my neighborhood. AFUE presumes a 1.7x oversizing factor. By right-sizing it you can beat the AFUE numbers in a well designed system. But at 3x oversized and up you're slipping over an efficiency-cliff. (AFUE has many arbitrary and usually wrong assumptions- don't get me started! :) )

    Even if undersized slightly, most of the design-day heating hours occur while the occupants are in bed, and they never actually get cold even if the output lags the load for a few hours between 3-6AM. Similarly, if the AC is slightly undersized, even if it doesn't keep up with the full sensible-load on design day the fact that it's running at 100% duty cycle dries the air sufficiently that it's rarely a comfort issue, even if the temps run a few degrees high for an hour or three.
  12. Thatguy

    Thatguy Homeowner

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    That depends if you include an 'underdamped' human in the feedback loop:D

    Thanks for your answers. I'll try to come up with
    the broadest definition
    that works for the most cases
    for HVAC and pump systems.
  13. Thatguy

    Thatguy Homeowner

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    From a link Dana posted-
    "The maximum thermostat cycling rate is assumed to be 2.5 cycles per hour for the “Typical AC” – the
    average measured at 30 Florida homes (HEND 91). By comparison measured values of 1.5 to 3 cycles
    per hour and 3 was recommended as the “worst case” (MILL 85). Parken (PARK 85) measured values of
    1.6, 2.0, and 2.3 cycles/hr in the cooling mode at three test homes. Therefore, the “Poor AC” is expected
    to have a cycling rate of 3 cycle per hour. "

    so one def for HVAC short cycling would be "substantially over 3 cycles/hr".

    And for a submersible pump
    "Motors should run a minimum of one minute to dissipate
    heat build up from starting current. Six inch and larger
    motors should have a minimum of 15 minutes between
    starts or starting attempts.
    Max 50 to 300 starts/24 hrs, depending on motor size and hp and # of phases.
    Last edited: Jul 8, 2009
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