Does your boiler cycle too often? What is too often? Is cycling too often a problem? What issues can this cause? MOST IMPORTANTLY, how do we correct this?
What is cycling? First off, for on-off boilers, the natural state is to cycle, from off-to-on-to-off-to-on and so on, to attempt to satisfy a load. Easy enough, as the load drops either water temperature or steam pressure below the set point (plus offset), the burner will ignite and operate until the temperature or pressure setpoint is reached. At setpoint the burner will cycle off and this cycle will repeat. This is due to the output of the boiler being greater than the energy demand of the system. Overtime the average of the upper and lower temperatures in the system would equal the desired setpoint. If you take a ratio of the energy needed, divided by the energy supplied you get what is termed run fraction. This is the percentage of time boiler will run for any given hour.
Most modern boilers will operate with the principle of modulation, with the exception of some design types (many copper-fin designs, commercial steam generators, 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 will allow 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 the lowest output). All burners have a lower limit, a minimum firing rate, where they can safely operate. If the demand is lower than this limit, the boiler will need to cycle, in an attempt to satisfy the low load (in essence, a grossly oversized boiler will operate as if it were setup with on-off controls).
Another component that exacerbates the cycling is the fact that the burner cannot immediately light and start adding energy. The burner must perform a drive to high fire, pre‐purge, pilot trial for ignition 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 pressure or temperature will drop even further below the preset operating limit. This then causes the burner to ramp to a higher than required firing rate due to the proportional band set in the modulating controller. This causes the burner to cycle off faster than required since the heat load is not even as great as the low 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!
The general rule of thumb is, anything over 8-10 cycles per hour can be considered excessive. This can vary, depending on your facilities unique circumstances and more could be “normal” and fewer could be “excessive”.
Boilers can cycle for a variety of reasons. They can be oversized, their burners could be improperly setup, operating controls set incorrectly, improper controls selected for the service, system control issues producing load swings that fight the boiler’s natural operation, and on and on.
We often service boilers that are oversized. Many boiler rooms are designed, correctly, for possible future expansion. This is a prudent choice, as increasing boiler capacity to the next larger size is generally an inexpensive way to increase capacity and prepare for future demand – especially compared to future purchase and installation costs of additional boilers. However, this creates a (possible) temporary situation where you have more boiler than you need – the boiler is sized for the worst case, fully loaded, future scenario. Other times, it is a prudent engineering choice, due limited data from production equipment manufacturers or assumptions that must be made during designs can dictate over estimating boiler capacity. After all, you can work with too much capacity, too little is a definite and noticeable problem.
Modern burners can achieve higher turndown ratios (within reason) with good results and combustion efficiency. This can lesson excessive cycling and allow the boiler to better match the load. However, this requires that the burner be correctly setup to take full advantage of the available performance. They also should be specified with controls capable of the precision required. As turn down ratios go higher, the amount of control of the fuel and air sources must become more precise. The burner/boiler will become more prone to outside influences interfering with proper setup and operation. This can include associated items, like draft control, needing to be controlled to tighter tolerance to allow the burner to operate as safely and efficiently as possible. If the burner is not achieving its 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 experience rapid changes in demand, say from on-off control valves, pumps cycling on/off, etc. a simple mechanical (proportional) control will not provide adequate control. This can lead to constant over and undershooting of setpoint and increased 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 to cycling. Or, if load is below the lowest output the boiler is capable of and the operating (on/off) control is set with a 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 to overshoot, 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 RWF55 Controller to control your boiler’s response to load changes. This is a PID loop controller that can be tuned to best match your boiler’s response to system demand. The Siemens 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 proportional band setting in the modulating control. This integration feature helps to extend 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 heat losses 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 email@example.com,