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Author: Javelin Controls
Date: February 21, 2022

Stop Losing Money And Reduce Boiler Cycling

If you do not work to reduce boiler cycling it can be extremely costly. But what is it and why does it happen?

Most conventional boilers cycle from on to off and vice versa continuously to satisfy a load. They run with a cold-water tank and hot-water cylinder for water storage. The expansion tank will draw in cold water to regulate the levels which helps mitigate any expansion that occurs when water is heated. Similarly, it replaces water lost by evaporation or leaking.

Combi boilers provide both water and home heating. They have two separate systems that connect to your radiators and hot-water supply. Because they do not have extra tanks like conventional systems, they are constantly on standby. When you use the hot water in your building, the combi system burns fuel which causes the correct exchange to heat water.

Most boilers like these generate heat efficiently. But sometimes a system can seem to turn on and off repeatedly. This is known as boiler cycling and whilst undesirable - it can prevent overheating when the contents are not used up fast enough.

The downside of this is that it poses unnecessary stress on the system itself and is highly inefficient. So what can we do to mitigate this?

Modulating – Reduce Boiler Cycling

A turndown ratio is the boilers’ max output, divided by the lowest output. All burners (regardless of type) have a lower limit and minimum firing rate in which they can operate safely. In any instance where the demand is lower than this limit, the boiler will be required to cycle to satisfy the low load. Thus a largely oversized boiler will generally operate in a way that is as if it was set up with on-off controls.

Furthermore, if the burner cannot immediately light and therefore start adding energy, this can also increase cycling. A burner must perform a series of tasks that can take up to a minute, or even longer in most cases before it can add heat to the boiler.

The temperature or pressure will then drop even further below the operating limit. From this point, the burner is then caused to ramp up to a higher than required firing rate. As a result, the burner will start to cycle faster than necessary - causing an inefficient output.

How do I know if my boiler is cycling too often and what problems can this cause?

Before being able to answer these questions properly, there are a few factors that need to be taken into consideration. Firstly we should consider how cycling can affect the lifespan of different boiler designs as well as their efficiency.

Firstly, the fireside of the boiler when cycling is being rapidly heated and cooled. This creates stress that can impact a boiler's length of service. This is then amplified by the frequency of cycling taking place.

A gas hob alight with a blue blame

Secondly, whenever a boiler cycles, it goes through a pre-purge cycle and post-purge cycle. This is done to ensure that the combustion chamber is rid of unburnt fuel from any previous cycles. This is beneficial as it helps ensure safety, however, whenever air is moved through a hot boiler, heat is then carried up the stack meaning that boilers heat exchanger design works for both ways.

Generally speaking, a boiler that does more than 8-10 cycles per hour can be considered excessive for most uses. However, the unique circumstances of your facilities will usually determine this.

There are a range of reasons a boiler can cycle. For example, they may be oversized or the burners could be improperly set up. Additionally, if operating controls are set incorrectly or if improper controls have been selected for the service, these can affect the rate of cycling as well.

Oversized Boilers

Boilers will require regular servicing to ensure they are running as efficiently as possible and not leaking any harmful gas. Boiler rooms are often designed so they can be used for future expansion. Doing so is well-advised as increasing the capacity to a larger size is a cheap way of ensuring you are prepared for future demand. This is especially true when compared to any purchase and installation of additional boilers which can be costly.

However, a concern that may arise here is that you have a boiler that is larger than necessary. You do have to remember though that the boiler is sized for the worst-case future scenarios. It can also be a smart choice as the limited data from production equipment manufacturers or assumptions made during designs can often fall above an estimated boiler capacity.

When it comes to boiler size, it is better to have too much than too little in most cases.

Higher Turn Down Ratios

Many modern burners can achieve higher turndown ratios with decent results and combustion efficiency. This is useful as it can lessen excessive cycling and allow the boiler to better match the required load. However, for this to work the burner must be set up correctly. Additionally, they also need to be specified with controls that are capable of the precision needed.

The amount of control for the fuel and air sources must become more accurate as the turndown ratios get higher. This is because the burner/boiler can become more prone to outside influences which can interfere with the set-up and operation. Thus, this can often include associated items such as draft control, needing to be controlled to tighter tolerance to allow the burner to safely operate efficiently. A boiler will cycle more than necessary if the burner cannot achieve its full turn down.


Another factor that can play a role in how frequently a boiler cycles is the system controls. For instance, a load size can experience rapid changes in its demand from control valves. These prevent hot water from circulating when it is not supposed to. However, if you are experiencing constant changes - it may produce the same on/off issue you faced before.

Furthermore, a mechanical control will not always be sufficient. This can lead to repeated over or underestimating of levels and increased cycling as a result. Where levels are hard to measure or are overshot, it can create the same effect as we previously mentioned. It’s not cost-efficient and can cause huge strain.

To ensure this doesn’t happen, professional HVAC control technicians like our own will install the systems and carry out servicing regularly. We can ensure the correct levels are set and the least amount of cycling is taking place to save you money in the long run.

Beep Box

Javelin Controls offer a product specially designed to save you money. The boiler energy efficiency product (BEEP) has been successfully applied to boilers all around the country. Thanks to this development, our clients can be sure that their boiler(s) only fire when they are needed.

Instead of cycling on and off with little or no load - they will be used efficiently, saving you money on running costs and repairs/replacements.

You could see yourself saving around 17% on energy bills when you call Javelin to install and set up the software strategy our own team designed. It requires little downtime to install and will see you making great savings in no time at all.

Take Action

Now we have covered the different causes of cycling you may now be asking yourself, what action can I take in order to reduce cycling? Firstly, you should consider getting a review of your system/boiler to accurately assess the cause of the cycling. Your chosen service company should be able to guide you through this process. You will need to find out the source whether it be because of the boiler size, the dynamics of the process, an incorrect burner set up etc.

Once the causes are identified, consider the simpler and less expensive changes first. For example, reviewing operating and limit setpoints can bring forth a noticeable change and improvement at a very small cost. However, note that you should not make changes to your boiler or system controls or settings without the proper training to do so. If these changes do not make enough of a difference though, you should evaluate your control upgrade options based on the costs in comparison to the potential improvements.

In extreme cases of cycling, you may wish to consider replacing your boiler with a smaller replacement boiler.

Another solution that will help cut costs is to consider the use of a PID loop controller that can be tuned to best match your boiler's response load charges. This allows your system to regulate many elements that will improve cycling.

From temperature, flow, pressure and speed, having a proper method of controlling these factors will keep stress to a minimum. Also, it gives you an extra layer of security, reassuring you that your boiler is running as it should at all times. This is also beneficial as it will help reduce fuel wastage - resulting in a more energy-efficient, cost-effective production.

More About Javelin

Javelin Controls Limited was originally formed 14 years ago and has subsequently developed to the stage where, in addition to supporting Hampshire and West Sussex County Councils, both directly and indirectly, in all things controls related, it has also successfully completed a £1.3M contract for Hampshire County Council providing their schools portfolio with Boiler Energy Efficiency controls packages.

Get in touch with the Javelin Controls team to transform your work environment with a high-quality building management system. We’ll be more than happy to discuss your control and HVAC system requirements and find a customised solution to suit your business needs.

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