By Jeff Lawley | April 23, 2019
Does yourboiler cycle too often? What is toooften? Is cycling too often aproblem? What issues can thiscause? MOST IMPORTANTLY, how do wecorrect this?
What iscycling? First off, for on-off boilers,the natural state is to cycle, from off-to-on-to-off-to-on and so on, toattempt to satisfy a load. Easy enough,as the load drops either water temperature or steam pressure below the setpoint (plus offset), the burner will ignite and operate until the temperatureor pressure setpoint is reached. Atsetpoint the burner will cycle off and this cycle will repeat. This is due to the output of the boiler beinggreater than the energy demand of the system.Overtime the average of the upper and lower temperatures in the systemwould equal the desired setpoint. If youtake a ratio of the energy needed, divided by the energy supplied you get whatis termed run fraction. This is thepercentage of time boiler will run for any given hour.
Mostmodern boilers will operate with the principle of modulation, with theexception of some design types (many copper-fin designs, commercial steamgenerators, for examples) or smaller, and less than ~30HP commercial boilers,which oftener are set to operate as on-off, low-high-off, low-high-low,etc. Theoretically, modulating willallow the boiler to run and match the load without needing to cycle. So what makes a modulating boiler cycle?(Hint: remember that turndown ratio is the boiler’s max output divided by thelowest output). All burners have a lowerlimit, a minimum firing rate, where they can safely operate. If the demand is lower than this limit, theboiler will need to cycle, in an attempt to satisfy the low load (in essence, agrossly oversized boiler will operate as if it were setup with on-offcontrols).
Anothercomponent that exacerbates the cycling is the fact that the burner cannotimmediately light and start adding energy.The burner must perform a drive to high fire, pre‐purge, pilot trial forignition and main trial for ignition (60 seconds total or longer in many cases)before the burner ever starts adding heat to the boiler, the boiler’s pressureor temperature will drop even further below the preset operating limit. Thisthen causes the burner to ramp to a higher than required firing rate due to theproportional band set in the modulating controller. This causes the burner tocycle off faster than required since the heat load is not even as great as thelow fire input.
What constitutes cycling too often and what problems does this cause? To answer these questions, a couple of possible considerations need to be taken into account. There is an aspect of how cycling affects the life span of different boiler designs and an efficiency component.
- The fire side of the boiler is being rapidly heated and cooled, every time there is a cycle. This creates a cyclic stress that affects the boiler length of service, more frequent cycling increases this stress cycle.
- Every time a boiler cycles, it must go through a pre-purge cycle and a post-purge cycle, to ensure the combustion chamber is free of unburn fuel from the previous cycle. This is a good way to help ensure safety, but every time air is moved through a hot boiler, it carries heat up the stack. A boiler’s efficient heat exchanger design works both ways!
Thegeneral rule of thumb is, anything over 8-10 cycles per hour can be consideredexcessive. This can vary, depending onyour facilities unique circ*mstances and more could be “normal” and fewer couldbe “excessive”.
Boilerscan cycle for a variety of reasons. Theycan be oversized, their burners could be improperly setup, operating controlsset incorrectly, improper controls selected for the service, system controlissues producing load swings that fight the boiler’s natural operation, and onand on.
We oftenservice boilers that are oversized. Manyboiler rooms are designed, correctly, for possible future expansion. This is a prudent choice, as increasingboiler capacity to the next larger size is generally an inexpensive way to increasecapacity and prepare for future demand – especially compared to future purchaseand installation costs of additional boilers.However, this creates a (possible) temporary situation where you havemore boiler than you need – the boiler is sized for the worst case, fullyloaded, future scenario. Other times, itis a prudent engineering choice, due limited data from production equipmentmanufacturers or assumptions that must be made during designs can dictate overestimating boiler capacity. After all,you can work with too much capacity, too little is a definite and noticeableproblem.
Modernburners can achieve higher turndown ratios (within reason) with good resultsand combustion efficiency. This canlesson excessive cycling and allow the boiler to better match the load. However, this requires that the burner becorrectly setup to take full advantage of the available performance. They alsoshould be specified with controls capable of the precision required. As turn down ratios go higher, the amount ofcontrol of the fuel and air sources must become more precise. The burner/boiler will become more prone tooutside influences interfering with proper setup and operation. This caninclude associated items, like draft control, needing to be controlled totighter tolerance to allow the burner to operate as safely and efficiently aspossible. If the burner is not achievingits full turndown, the boiler will cycle more than it should.
Controls also play a role in how frequently a boiler cycles. For example, if the load side experiencerapid changes in demand, say from on-off control valves, pumps cycling on/off,etc. a simple mechanical (proportional) control will not provide adequatecontrol. This can lead to constant over and undershooting of setpoint andincreased cycling. For a mechanical(proportional) control, the span (proportional band) could also be too narrow,resulting in an overly aggressive response to load swings, leading tocycling. Or, if load is below the lowestoutput the boiler is capable of and the operating (on/off) control is set witha small differential, the boiler can cycle more. Lastly, if you have a PID loop controller,for boiler load control, incorrect tuning of the PID parameters can lead toovershoot, undershoot and cycling.
What can be done? First a proper system/boiler review to assess the cause(s) of the cycling should be performed. Your service company should be able to assist with this. Is this happening because of boiler size/selection, the dynamics of the process, incorrect burner setup, or improper control selection or setup? Once the cause(s) are identified, start with the simple (less expensive) and easy to implement changes first. Simple things like reviewing operating and limit setpoints can yield a noticeable improvement, at a minimal cost. Note: do not make changes to your boiler or system controls or settings if you are qualified and properly trained to do so – when in doubt, call your service experts. If these simple changes do not yield the desired results, control upgrade options should be evaluated, based on the costs compared to potential operational improvements and costs savings. If the cycling is extreme, it could even be cost effective to look at smaller replacement boilers (keep in mind whether or not the facility is expected to need additional capacity in the future).
A simple,inexpensive solution that is worth consideration, is the use of a Siemens RWF55Controller to control your boiler’s response to load changes. This is aPID loop controller that can be tuned to best match your boiler’s response tosystem demand. TheSiemens RWF55 incorporates a function (Q) that will keep the burner at low fire(minimum input) even after the burner starts to add heat below the proportionalband setting in the modulating control. This integration feature helps toextend the on time of the burner and limit the number of on and off cycles.This helps to reduce fuel usage by reducing pre‐purge and post purge heatlosses that would otherwise go up the stack.
If you have a boiler that is cycling too much, not performing at its best, or if you have any other boiler equipment or boiler service questions, please give us a call, we’re here to help. We have the expertise to support you during the purchase, installation and startup, and experienced factory trained technicians to support you and keep your equipment in tip-top shape, long-term. Our technicians are available 24/7/365, for emergency services.
Call us at 336-299-3035 email at sales@wcrouse.com,
