What to consider when specifying boiler load optimisation

With growing pressure to reduce energy consumption and CO2 emissions, there is a renewed focus on increasing the efficiency of existing boiler plant. Retrofit boiler controls are often discussed and considered to deliver these objectives.

However, careful consideration is required when specifying these controls as some may have a negative impact on ambient temperatures and could conflict with exisiting controls e.g. BMS. This article provides an overview of the different controls strategies and some key questions to ask suppliers

The primary reason for installing boiler load optimisation is to remove the inherent inefficiency of boiler "dry cycling" which is found in virtually all boilers regardless of existing controls or the type, age, size, and application of the boilers. Boiler dry cycling is caused when boilers fire only to recover the standing heat losses (from the boiler itself) when there is no true demand for heat from the building load i.e. hot water and space heating.

Unfortunately, past attempts to prevent boiler dry cycling via retrofit controls have resulted in products/technologies being implemented with flawed control strategies that will directly conflict with existing controls and compromise ambient room and hot water temperatures. Many of these products are still being marketed today. It is only recently that newly developed technology has become available, which analyses the boiler thermodynamic load and controls the boiler in real time. These newer technologies complement and work in harmony with the existing controls, always maintaining the boilers’ designed set points/temperature with no impact on comfort levels.

How to determine the best solution?

So, how do you determine which retrofit boiler controls will provide the best solution? The first thing is to scrutinise the control strategies used in these products.

A historical approach is to artificially lower the boiler’s designed boiler thermostat/set point based on the boiler return temperature, to calculate the current boiler load from the system. This method has serious limitations on applications with variable temperature control and systems with constant temperatures from the same boilers (boilers providing space heating and hot water). These applications are found in over 90% of commercial properties. Clearly, lowering the thermostat of a boiler will result in energy savings. However, the same could be achieved by lowering the boiler thermostat setting manually to get the same result, but only at the cost of comfort levels and conflict of services.

The majority of today’s buildings have integrated "real time" control systems and using the approach described above can cause conflicts with the BMS and impact comfort levels. For example, the BMS or weather compensation may be applying a variable set point to the boilers i.e. in the morning the boilers could be operating at 80oC; then during the afternoon, due to solar and internal heat gains etc, the BMS may reduce the set point to 65oC. The purpose of this is to provide energy savings and optimise the boilers/heating system to maintain the desired space and hot water temperatures.

Avoid lowering temperatures and time delays

A control that overrides the boiler’s set points, therefore, will conflict with the BMS and the variable set point control. This could lead to the BMS strategy bringing on additional boilers to over compensate for the decreased boiler temperature, resulting in unnecessary energy consumption. Furthermore, ambient room and hot water temperatures will be reduced, resulting in potential complaints from the occupants.

Another approach has been to use predictive (time delay) boiler firing control. The control analyses the historical firing pattern to assume the boiler load and then physically reduces the number of firings over a predetermined period of time. This approach does not use real time data or true temperature profiles of the boiler load, therefore the boiler could be restricted from firing under a true heating load. Again the boiler’s required operating temperature will be artificially lowered, resulting in comfort levels and hot water temperatures being compromised.

I would suggest that a more desirable scenario is for the boiler load optimisation to integrate with and complement the existing BMS, something that is now possible using newly designed, patented technology. By measuring the flow and return temperature of each individual boiler every second and analysing the data every 10 seconds, this modern technology can accurately determine if there is a genuine boiler load from the system or if the boilers are firing as a result of dry cycling.

By identifying and preventing dry cycling average savings of 12% - 15% across an estate can be achieved over and above the existing controls including BMS. Capital cost is low, typically giving a payback of two years or less, while maintenance costs are non-existent as the technology is self-learning and requires no calibration or seasonal commissioning.

Retrofitting the wrong boiler controls, then, can not only compromise comfort levels and hot water temperatures, it may also prevent other controls from working effectively with a net increase in energy consumption. However, this isn’t a reason not to retrofit boiler controls; it simply means that more robust vetting of suppliers and their products is required to avoid being saddled with an inadequate solution. So it makes sense to understand what technology you are being offered, check its track record and talk to other people that have used it. Then you can be sure that the selection process is based on the true merit of the product and not just what the salesman wants you to hear!

Key questions to ask when specifying boiler controls

  1. How does the technology actually deliver the savings. Avoid controls which suppress the boiler’s set point or a variation on a simple time delay – this will reduce ambient temperatures within the building and lead to complaints.
  2. Boiler controls should have real time control by analysing the thermodynamics of each boiler i.e. avoid controls which use predictive firing by simply removing the number of boiler firings based on the historical boiler firing patterns.
  3. Understand the impacts of the boiler control on your existing controls – will they conflict with your BMS strategies? This can lead to increased costs.
  4. What is the supplier’s track record? What organisations have installed the products? Ask to see the case studies, data and ask to speak to their clients.
  5. What support can the supplier provide? Do they have the infrastructure? Can they implement the controls across your estate within time and budget with limited impacts on you?