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Limitations of BMS

building systems

Boiler Controls and their place in Building Systems Automation

Firstly it is important to establish the controls already being used are working. It is quite common for commercial boiler plant to use a number of control elements. These may include simply time clocks, boiler sequencing, weather compensation, demand control, modulating burners and ensuring the correct air to fuel ratio and firing rate is being achieved.

Many of these are delivered through routine maintenance and commissioning, standalone controls or via a building management system (BMS). When installed, commissioned and maintained correctly each of these control strategies can deliver significant energy savings without compromising comfort conditions.

However, it is rare for such controls, or even a combination of several controls, to be capable of managing every aspect of boiler efficiency performance. This is where ‘fine-tuning’ through the introduction of additional boiler controls can pay dividends.

Understanding The Limitations of Your Existing Controls

There are several ‘layers’ of boiler control available and it is important to understand what each can achieve, their role and how they operate and finally how they can interact with one another.

Just as important is to understand the limitations of each type of boiler control or BMS strategy, as this will help in deciding whether additional controls will improve performance further and introduce additional energy and carbon savings.

This can be illustrated by considering two common types of existing controls: weather compensation and BMS.

Weather Compensation

Weather Compensation seeks to match the performance of the heating system in relation to outdoor temperatures. Simple systems will just measure the outdoor temperature and the supply temperature of the heating system, more complex system may include monitoring of indoor temperatures as well. Some systems also include night set-back and Eco functions.

Weather compensation (also known as weather optimization, outside air reset or variable temperature control) can be very effective but is not without some limitations. For example, where the boilers are supplying constant temperature systems such as fan convectors or air handling units, the variable temperature approach of weather compensation would not be appropriate. Also, most buildings will have domestic hot water (DHW) requirements, that often require constant water temperature and the heating boilers can be underutilized to meet these “part load” demands, resulting in boiler/s standby / short cycling issues.

Consequently, many buildings will have separate heating circuits for variable temperature, constant temperature and hot water systems and the boilers serving each will need to be controlled separately and under these conditions, the boilers will be subjected to very transient load conditions.

Remember, weather compensation is only one aspect of control that will cause the boiler/s hydraulics (boiler flow rates) to be changed. Controls such as boiler sequencing is a very effective way to increase boiler efficiency, however, if the boiler are not hydraulically isolated when not firing, will cause increased boiler standing losses and boiler standby cycling.

Building Management Systems (BMS)

BMSs are typically configured to control the buildings heating system as a whole. To control the boilers directly from the BMS, this is commonly done by measuring the common flow and return header (the blended temperature of all boilers), rather than monitoring and controlling each individual boiler. Therefore, while a properly commissioned and maintained BMS provides excellent control of the heating system as a whole, it is very unlikely to be monitoring or managing the performance of individual boilers or take into account the natural hydraulic boiler imbalances that can cause boiler standby and short cycling.

For example, it is impossible to identify which boiler is standby cycling from the common blended header temperature so standby cycling at individual boiler level often goes unnoticed, resulting in unnecessary energy consumption.

Ensuring existing controls are routinely checked and optimized and to fully understand what level of control you have within your building and your wider estate is highly recommended. In fact, there are a number of key actions that we would recommend are implemented see Top 10 tips for boiler room efficiency – www.FlameControls.com

The Need to Control Individual Boilers

Underlying all of these control strategies is a fundamental requirement to prevent the boiler from firing when there is no demand for heating or hot water from the building(s) the problem of boiler standby cycling is an example of this. Even the latest boilers with modulating burners, sequencing and BMS control are likely to suffer from boiler standby cycling.

Boiler standby cycling is a symptom of standing losses (heat lost via the boiler casing and flue). It is exacerbated with oversized boiler plant – which is still the case for most commercial boilers as specifiers seek to ensure the boilers can cope with extremely cold conditions that are only experienced very rarely (Please visit www.FlameControls.com for more detailed information about boiler standby cycling).

Setting priorities

Some people argue that if you have set up and maintained your BMS correctly there is no need for additional boiler controls. For the reasons explained above, this approach fails to take account of the various aspects of boiler performance that impact on overall energy consumption and will not be addressed by the majority of BMS configurations. Realistically, the majority of today’s existing buildings have ageing boiler plant and have had limited investment such as motorized valves and advanced BMS configurations to enable demand control, sequencing etc.

Clearly, if you have already invested in a BMS, and/or other controls, it is essential to ensure they are all working correctly and are maintained regularly. It is also important to be aware of any external factors, such as changes in building usage, which might warrant re-configuration and re-commissioning of the controls.

Common misconceptions

It is not unusual for some BMS providers and consultancy’s to suggest making better use of the BMS by further programmed or upgrades to replicate the M2G. However, the cost to upgrade the BMS is very likely to be prohibitive, require on-going maintenance and commissioning and longer payback periods. It is also very unlikely that the exact features or benefits of the M2G will be delivered, for example M2G not only addresses standby cycling, it also provides 1st and 2nd stage firing control for two stage burners to deliver additional savings.

A common misconception is that as a result of the M2G preventing the boilers from firing the circuit or system will drop and as a result the BMS will enable additional boilers to restore the situation. This is factually incorrect as M2G does not begin to control the boiler until the boiler has reached its set point or the set point established by the BMS – therefore the circuit/system temperatures are not artificially lowered and the BMS will not bring on additional boilers.

As the M2G is monitoring the boilers temperature profile every second it is able to identify when there is a genuine load being placed on the boiler – as soon as the boiler’s return temperature drops 3 degrees from the point it last fired the M2G will immediately allow the boiler to fire.

Unfortunately, old control strategies used in some retrofit boiler controls still available today do not have the ability to identify a genuine load in real time and as a result the boilers set point is suppressed which may result in the BMS bringing on additional boilers. When considering retrofit controls it is vital to understand how the retrofit boiler control makes its savings our useful crib will help you establish the control strategies and what the benefits and issues are.

Temperatures are never compromised

If the plant is used for the generation of hot-water as well as space heating, as is often the case, it is important that the primary circuit is maintained at a sufficiently high temperature to ensure that water storage temperatures are high enough to comply with the Approved Code of Practice (ACoP) on Legionella. As explained above, the boiler has to reach its set-point before the M2G begins to monitor and control the boiler, therefore stored water temperatures are never compromised.

Involve all stakeholders

The easiest way to determine whether you should install retrofit boiler controls is to involve all stakeholders including those responsible for the BMS and any consultancy to discuss what your objectives are, what controls are being considered, the demarcation between the controls and how they will work together. It will become very apparent very quickly that the BMS is not replicating M2G and it will not be commercially viable, both from a capital and operating expenditure to attempt to replicate M2G.

The challenge is to reduce energy costs and carbon emissions using technology which provides the best return on investment with the minimal impact on your operations and integrates seamlessly with your existing controls or future control upgrades. Just because the control is retrofitted and standalone does not mean it cannot be integrated with the existing BMS/building controls to enable building operator to view status of each M2G. When sourcing retrofit controls ensure this is possible.

M2G has become the market leading retrofitted boiler load optimization control. With over 7,500 being installed across the private and public sectors and favored by many of the UK’s leading Facility Management companies.

So when challenged to whether M2G is required in your estate then use the above to provide the commercial and technical reasons to why you are making the best decision for your organization.

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