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Short Cycling Survey

Harvey RamerHarvey Ramer Member Posts: 1,932
I would be interested in hearing everyone's opinion on what boundaries or time limits defines short cycling as we know it. What is the minimum runtime on different combustion appliances that you consider to not be short cycling?
Ramer Mechanical
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Comments

  • STEVEusaPASTEVEusaPA Member Posts: 1,506
    Good question Harvey. I remember this discussion on another forum, with a different take by Jim Davis with some real world testing. I found it very interesting and different from conventional thinking (Not that I'm promoting another site) But here's the link:

    http://hvac-talk.com/vbb/showthread.php?2034341-Short-cycles-more-efficient&highlight=SHORT+CYCLE

    Another one, also good stuff (Jim's comments start on page 5):
    http://hvac-talk.com/vbb/showthread.php?2024991-Newer-Kenmore-5-ton-Gas-Pack-on-home-we-just-bought-Induced-draft-airflow-question/page5
    steve
  • delta Tdelta T Member Posts: 435
    Specific to Lochinvar, but at their factory training for the knight boilers (this was a while ago 2011 I think) they said that anything over 6 CPH was too often and would cause excessive wear on the boiler. So I guess that amounts to something like a 7 min on, 3 min off cycle?

    Now personally I think that is on the edge. If I found a system with 6 CPH I would probably try to increase the cycle length if it were possible. I tend to aim for cycles in the realm of 2-3 hours in mild weather and cycles that are days long in cold weather.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 2,505
    I have always felt that short cycling is very destructive to efficiency. Short cycling is hard on the equipment and increases the standby loss.

    More noticeable in air conditioning with chillers working with a 10-12 degree td lop volume must be a minimum 3gal/ton for comfort cooling and 6gal/ton for process cooling.

    I am all for buffer tanks if needed to increase loop volume to prevent short cycling. To me anything more than 3-4 cycles is short cycling
  • Harvey RamerHarvey Ramer Member Posts: 1,932
    edited September 29
    I have always tried to get a minimum of 5-7 minutes on time.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Jamie HallJamie Hall Member Posts: 6,708
    Now you've opened a can of worms!

    Before we get too far into the soup here, though, I think we need to clearly define whether we are referring to an overall cycle -- that is, in the case of a heating appliance, the time between calls for heat from the thermostat or other control device(s), and the relative on and off time within that cycle -- or whether we are referring to the on/off cycling within a call for heat, such as might be the case for a steam system with a boiler/burner combination which is oversized for the current heating load (which, incidentally, varies with room temperatures and heat loss, just to confuse things), or a hot water system with a buffer tank but limited (or non-existent!) heat source modulation -- just to name two.

    In the first case, one is looking at the overall heating appliance and the effect of cycle timing on it as a whole -- such as how much does the boiler cool down between on calls. In the second case, particularly with steam, the boiler does not, in fact, ever cool down during the on part of the thermostat cycle, so effects on the boiler (such as expansion) are minimal, but there is the question of the effect of multiple starts over a short time period (or just cumulative number of starts) on the burner -- particularly any pump motors or ignition components. A very different beasty.

    Then one also needs to consider the type of system -- at the extremes, a system with very low heating mass (such as a forced air system) and no modulating capability on the heat source may have to run quite short cycles (such as the 6 cycles per hour mentioned) to maintain any sort of comfort level, while a system with very high mass, a sophisticated modulating heat source, and a good predictive heat load algorithm and sensors, such as a radiant floor system, might be able to run for literally months at a time without ever turning off. And should. (I would expect such a system should not have to cycle at all if the mean air temperature is more than 10 15 degrees cooler than the desired space temperature) A steam system with big radiators should be able to maintain comfort quite nicely with one thermostat cycle per hour, even in the coldest weather -- but may well have a burner cycle timing of 4 to 5 minutes on to 1 minute off, once all the radiators are fully heated, if the boiler and burner are properly sized.

    How short a cycle is too short? I wouldn't care to express an opinion! But I look forward to the debate!
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Harvey RamerHarvey Ramer Member Posts: 1,932
    I just love listening to you talk, Jamie

    I am mostly interested in cycles within a heat call for this discussion. Obviously 1 minute cycles would be incredibly inefficient, given pre and post purge and flame stabilization. But once you are at 5 min on-time how much if any efficiency is gained with a 10 minute run time, or even an hour? There is a curve, but I have not seen it defined.

    Short cycling is never talked about with on-demand water heaters, I have noticed.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • HVACNUTHVACNUT Member Posts: 656
    edited September 29
    With winter looming, I've been thinking about my own boiler, (and some customers) and it's cycling habits.
    I have an oil fired boiler, 4 zones, no ODR...yet.
    I'm tired of hearing the burner turn on with a SH call, boiler reaches limit, then meets differential, burner goes into prepurge, zone satisfies, and all shuts off. Then 2 minutes later another SH zone calls, and on we go.
    So I drew this up. I haven't installed it yet, but I'll let you know.

    Delay on make. SH circs run, no burner until TD. DHW overide.

    Patent pending. Lol
  • STEVEusaPASTEVEusaPA Member Posts: 1,506
    edited September 29
    @HVACNUT I have (had) a similar situation at home, 6 zones (4 radiant). I've tried many ideas including something like yours. I went with a Tekmar 374, (2 boiler control) and an extra 501. Used 'Boiler 1' to power the 501 to run boiler loop circ. When that didn't 'keep up', temperature started dropping, 'Boiler 2' (the burner) kicked in.
    It was my best attempt of all my other attempts, but still didn't help. Plus the fact that oil/Tekmar/injection mixing/boiler protection/ODR just don't work together.
    My end result was something I posted in another thread, which was (is),
    -Boiler charges buffer tank or Indirect only. Boiler sensor for Tekmar in indirect per @hot rod 's recommendation
    -Zones run off of indirect, via injection mixing, ODR. Because the 2 high temperature zones have oversized baseboard, I just run them at the same temp as the radiant.
    -Boiler protection via 3-way mixing valve.
    We'll see if we get a real winter to take advantage of the ODR.
    steve
  • RayWohlfarthRayWohlfarth Member Posts: 212
    Fascinating discussion. I believe the outside air temperature and building heat loss will factor into the equation as well. I was taught a boiler is sized to operate continuously on a design temperature day and maintain 68 degrees F at that temperature. As the outside temperature warms, the building does not require the same amount of heat and the boiler will cycle. The cycles will be more frequent the warmer the outside temperature is.
    Ray Wohlfarth
    Boiler Lessons
    Click here to take Ray's class.
    Click here to buy Ray's books.
  • Mad DogMad Dog Member Posts: 3,183
    The essential question IS? How much time can you BILL....AND get PAID for to "study" a system you are walking in on that has been altered, neglected, abused? Mad Dog
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • bobbob Member Posts: 813
    I know when MH designed the T87 it was designed to cycle 5 times per hour if the heat anticipator was set per instructions. At 50% load say 30ºF one cycle would be 6min on 6min off. As load increases on times get longer and off times shorter but still 5 cycles per hour
    bob
  • GordyGordy Member Posts: 7,122
    Interesting. I would say the period of time it takes to reach stable combustion readings times 2 on initial cold start. Now cycles between the actual call once up, and running should be at least that initial combustion stabilization period time. But that is just an opinion.

    Isn't the most efficient appliance one that is off? In saying that so long as the setpoint of the space is held. Is there significant gain from one that cycles to one that burns for longer periods to maintain that same given parameter requirement?

    If one appliance cycles on, and off to satisfy a heat call for 20 min. At a lower efficiency maybe do to oversizing,or lack of modulation capability. While the other burns the whole 20 min. At a higher efficiency because it's properly sized, and has modulation capability, is it a wash? We have all heard the 10min rule thrown around until combustion stabilizes.

    Clearly there is not any efficiency numbers to digest that except for btu requirements.
  • hydro_newbiehydro_newbie Member Posts: 31
    @Harvey Ramer , how long does flame stabilization take? And what does flame stabilization mean? Forgive the silly question, I'm a homeowner who imagines that once I get the 'whoosh' of the burner lighting, the flame is stabilized. :smile:

    @Gordy, if you wait for combustion readings to stabilize, then it sounds like we're talking about the time it takes for the boiler to reach something close to steady-state temp? As the boiler is ramping from cold start, I imagine the flue gas temp will change (lower initially, then level off to something higher once the boiler is close to steady-state temp).
  • Jamie HallJamie Hall Member Posts: 6,708

    @Harvey Ramer , how long does flame stabilization take? And what does flame stabilization mean? Forgive the silly question, I'm a homeowner who imagines that once I get the 'whoosh' of the burner lighting, the flame is stabilized. :smile:

    @Gordy, if you wait for combustion readings to stabilize, then it sounds like we're talking about the time it takes for the boiler to reach something close to steady-state temp? As the boiler is ramping from cold start, I imagine the flue gas temp will change (lower initially, then level off to something higher once the boiler is close to steady-state temp).

    Neither of those questions is remotely silly -- they are both very relevant to the how short is too short question. In the first one, different burners and fuels will have different stabilization times -- and those will also differ with the temperature and thermal mass of the firebox, among other factors (such as draught). The second one is also very relevant, particularly in regard to the distinction between the cycles I mentioned during a heating call, once everything is hot, in steam systems.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • hot rodhot rod Member Posts: 7,094
    If you look at it from simply a cycle life of the components, regardless of efficiency, that 10 minute on/ off is the guideline I have used.

    The manufacturer of the product probably dictates a cycle life expectancy for components like gas valves, fan motors, relays, switches, etc.

    Parts suppliers can quote and supply a component, a micro switch for example, for a 100,000 cycle life or a 300,000 life cycle. So a boiler manufacturer may look at the cost analysis and expected cycles to select the component.

    Determining a "typical" system to design too would be a bit of a guess.

    We data logged on/ off cycles on a zone valve in a N Wisconsin application one season to get an idea of how many times the motor and end switch cycles. At about 5000 cycles, a switch designed to a 100,000 cycle life would provide about a 20 year life expectancy, for an example.

    Probably the highest cycling is tankless WHs, which can cycles dozens of times an hour, often for a few second duration. Hotel applications would be a good proving ground for tankless WH cycling.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • LanceLance Member Posts: 63
    Cycling rates are elusive and ever changing. Design and application and use are all critical to achieving the correct balance. Consider the log in a fire, its cycle rate is one and once it is done, we get another. Our challenge is to make it all work as best we can, but all factors must be considered if we are to succeed, including a total do over if necessary. And sometimes its just can't be done because it is simply "operator error".
  • NYplumberNYplumber Member Posts: 487
    Like @Hotrod mentioned, 10 on 10 off is about what I look for as a general rule.
    Looking at electronics and you will some times see three cycles per hr as the rating.
    For mod cons i look for continious burn rate. Buffer capacity and right sizing the boiler. Some of my systems run 20hrs per day (observed).
    For dry and wet base boilers, there is a stack wet time that should be observed. 3-5 min to get up to efficiency ratings if I recall learning by @Tim McElwain class on conversion burners.
    I see lots of 2 min on and 2 min off for the systems installed in homes that I come to service, This adds a bit complexity for combustion testing and adjustment.

    @Harvey Ramer I owe you a phone call about a job we spoke about before the summer.
    :NYplumber:
  • hot rodhot rod Member Posts: 7,094
    New controls on mod cons for example give you some control over cycle time. Functions like "ramp delay" can help extend the cycles.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • NY_RobNY_Rob Member Posts: 490
    You can also limit the heating capacity/max fire rate on mod-cons so that 80K BTU mod-con is only firing up to 40K BTU's... that helps extend cycles. They also include programmable CH Anti-cycle interval settings to prevent fire up after it just shut down.




    What is needed is a central intelligent controller that reads the temps of all monitored rooms and looks at the heatloss over time to establish patterns and can hold off individual zone calls for heat till at least two or more zones are calling for heat or a preset amount of time has passed or >1 deg. temp drop has occurred.
    Nothing is more frustrating then completing a call for heat, seeing the boiler shut down then seeing it start right up again two min later because a different zone is now calling for heat.
  • Harvey RamerHarvey Ramer Member Posts: 1,932
    With most installs it is relatively easy to get long run times. Not so easy on Combi installs. I know you can add a buffer tank, but with that additional expense, one may as well have installed the correct sized boiler and an Indirect Water Heater.

    When the budget only allows for a Combi boiler, the cycles have to be dealt with.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • ch4manch4man Member Posts: 17
    last week i was doing a supervised start up on a WM LGB-6 i think, the vaporstat was toast due to a really crappy job of a control tree design.
    anyway, after rebuilding the control tree and replacing the vaporstat, i got the 6-plex condo up to temp, vents closed and now was cycling on the vaporstat. no matter the cut in / diff settings the system would drop pressure fast enough to seem to me short cycling.
    maybe 3-4 minutes of, a few minutes off. IDK i didnt time it, just seemed "fast"

    i'm still in AC mode.

    thought?/ oh and does anyone make a vaporstat worth a pinch of poop these days, the HW's are too fragile, takes nothing to murder them....

    John,,,,,,,,,
  • hot rodhot rod Member Posts: 7,094
    Most Combis have the same turndown and limiting features. I just installed a 120 Combi that turndown to 11K. It keeps up fine with our DHW needs, it does have solar pre-heat some days :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Jamie HallJamie Hall Member Posts: 6,708
    It is to be expected that in many steam systems running with a vapourstat -- or pressuretrol, for that matter -- that the off time will be quite short. Surprising how fast the steam condenses in there and thus the pressure drops! It doesn't sound as though you are out of line at all on this one. The ratio of on time to off time is a rough guide to just how much too big the boiler is -- but a ratio of about three or four to one is to be expected, given that most systems are sized to give that infamous pickup factor (which really does need to be there...). If the ratio is closer to one to one, you might try downfiring the boiler, but not too much!

    There are pressure controllers which are sensitive in the same range as the Honeywell vapourstats (the old mercury ones were excellent if you should be so lucky as to find one), but you will find that they are pricey.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    When I bought my house, it had a 1950 GE oil-fired boiler heating my radiant slab. The boiler had its own thermostat set at about 160F and the limits were about +|- 2F (i.e, too tight). The boiler kept itself warm during the heating season. When the house felt cold, its thermostat called for the house circulator to run. 120V thermostat ran the circulator directly.

    The boiler would cycle on for about 60 seconds, and off for about 90 seconds. I considered that to be too rapid cycling. I adjusted the set points on the thermostat, so it kept the boiler at 140F +|- 5F. It did not cycle as often but was still really too fast. I did not know about condensation in those days; I probably got some.

    But when I retired that boiler after 55 years or so, it was still running fine (though low efficiency). No leaks, the controls did not need annual replacement or anything. I guess those GEs were pretty tough. I did replace the original low-pressure GE burner with a Beckett after a while because the GE burner need a replacement part, and they were no longer available.
  • j aj a Member Posts: 1,673
    > @Mad Dog said:
    > The essential question IS? How much time can you BILL....AND get PAID for to "study" a system you are walking in on that has been altered, neglected, abused? Mad Dog

    Wow u asked a great question....as hard as it was ya just have to do an about face and screw
  • KoanKoan Member Posts: 377
    edited October 11
    How short a cycle is too short? - I think for me it is definitely whenever my vapourstat shuts down the boiler at 8oz. This means my massively oversized boiler (750 EDR for 400 EDR of surface) is making steam faster than my radiators can condense and return water to the boiler. Normal operation is about 2 to 2&1/2 oz. However, i also think I am losing efficiency whenever the system is running above 4oz. Then, I am still making pressure instead of heat, but this isn't really short cycling because it keeps running.
    Because my burner cannot be efficiently down-fired, I plan on pulsing it to prevent overshoot. Working with @PMJ , his cycles were in the range of 20 min but I think I saw some at 7 minutes.. These cycles are intentionally shortened to create efficiency.
  • rbeckrbeck Member Posts: 25
    With mod/cons I like to see a minimum run time of 20 minutes. Is longer better? Of course, but I have seen so many under 4 minute average run times it is sickening I am ecstatic to see 20 minutes or more.
    With US Boilers (Burnham) adjustable 2 minute low fire hold, programmable zone panel to limit the firing rate per zone, adjustable PID changes from the display, independent adjustable diff above and diff below settings, ability to reduce boiler input for heat and DHW independently and now the 10:1 turndown K2 water tube and Aspen fire tube boilers hopefully we see much longer run times.
    When I talk to a contractor's no matter how long his run time is I always want to get even longer run times.
  • Leon82Leon82 Member Posts: 236
    I have heard less than 300 seconds is short.
    My whn averages 3.5 hr burn times
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