Ireland's only dedicated building services engineering journal
Ireland's only dedicated building services engineering journal

Boiler turndown – aim to ‘optimise’ rather than ‘minimise’

When specifying burners it’s important to ensure the turndown is aligned to the working capacity of the boiler, says Bernard Dawson, Technical Director, Riello.

A case in point is boiler turndown capability for heating systems that, on the face of it, seem to make perfect sense. As the heat demand of the system reduces, the output of the boiler reduces to match, thus aligning supply to demand in the best traditions of energy efficient building services systems. Therefore, an infinite turndown capacity should, in theory, enable total load matching whatever the heat requirements of the system.

In practice the situation is very different because the boiler will have a minimum working capacity, governed by water flow rate and temperature difference between the flow and return, such that potentially harmful condensate collecting in the boiler heat exchange and flue system is prevented. This minimum working capacity limits the turndown capabilities of the boiler and, therefore, the potential for turning down the burner. Or, at least, this should be the case.

To put this into perspective, it is not uncommon to see turndown ratios of 8:1 or even 10:1 specified for a boiler that is limited to a turndown of only 2:1 or 3:1. If the burner was actually set to achieve the specified turndown then the boiler exhaust gas temperature would not be maintained above the 130-140°C required to avoid condensation in a typical low temperature hot water boiler.

As a result, the exhaust gases exiting the boiler would condense at lower burner firing rates, thereby potentially damaging the heat exchangers and flue system, which is not ideal. When we see a high turndown capability requested for a large commercial steel or cast iron low temperature hot water (LTHW) boiler, it sets alarm bells ringing as this indicates a potential lack of understanding of the operation of such systems, sometimes all that’s left is to find a boiler rental while we determine how to fix the original problem.

For other boilers the situation may be different, of course. For example, As per experts at Trusted Boilers,   condensing boilers are clearly designed to allow for – and encourage – condensation so that the extra latent heat can be used in the system, rather than being exhausted to the atmosphere. In these instances a high burner turndown is a positive advantage.

Similarly, lower minimum operating outputs can be achieved for commercial and industrial high temperature hot water (HTHW) and steam boilers, compared to LTHW boilers, so that higher burner turndown is possible. However, there are very few cases where a burner turndown of 8:1 or 10:1 is justified or beneficial.

End users rely on the expertise of the boiler room maintenance to deliver solutions that will provide the required performance with optimum energy efficiency and cost. It is therefore incumbent on specifiers to start including meaningful boiler and burner turndown requirements to avoid confusion and potential problems. 

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