Real, High Turndown Condensing Boilers

There are many articles written about and we’ve previously covered, the limitations of high turndown, condensing boilers (just Google “turndown & condensing boilers”).  If you haven’t read any of those articles, here is a brief summary:

• Turndown Ratio is the ratio of the lowest firing rate of a boiler to its high fire rate. i.e. 5,000MBH low fire for a 25,000MBH boiler = 5,000/25,000 = 20% minimum fire or 5:1 turndown.
• In standard efficiency steam and hot water boilers, higher turndowns (greater than 3:1, originally) could show efficiency gains, as the boilers needed to cycle less during low loads.
• However, there was (and still is) a practical limit, due to the fact that as the burner is turned down, excess air goes up.  As the stack O2 increase, efficiency decreases.  On standard efficiency boilers, that may be a worthwhile trade off, depending on the severity of the cycling.
• As parallel positioning controls became more common and better control over the air-fuel ratio was realized, traditional boiler turndown increased to ~10:1, with good control of the stack O2. (As a side note, actual turndown will also be affected by boiler type, furnace dimensions, draft conditions, etc.)
• Most modern condensing boilers use a zero-governor style gas regulator.  The flow of gas is induced by the flow of air past this regulator.  Increase air flow induces more gas flow, reduced air flow decreases fuel flow.  The boiler modulates by increasing or decreasing blower motor speed.
• Many manufacturers claim a 5:1 turndown.  Based on our experience, in real world installations, this is a very achievable and reliable setting.
• Some manufacturers claim turndown ratios of greater than 20:1.  Both the high and the standard turndown companies are using the above mentioned control scheme and same basic control components, the burner designs are also very similar.
• We work in many locations that have high turndown condensing boilers and do not find them set to these extreme turndown settings.  We believe it is possible, that these turndowns are achieved in a controlled environment, but do not see them in the real world installations where temperature, pressure, draft conditions, etc. are not under the tight control of laboratory conditions.
• The turndown limitations consist of control of fuel flow (all control valves have control range limitations) and mixing limitations within the burner design.
• Even if the high turndown ratios are achieved, the published combustion values all report more excess air than at “normal” turndown ratios.
• In a condensing boiler, the higher excess air decreases the required flue gas (i.e. return water) temperature needed to achieve condensation – in some cases to ~100^oF or less (not practical). Not operating in a condensing state loses ~10% from the boiler’s efficiency.
• In practice, it appears more efficient to cycle with condensing efficiency (losses of a couple to a few percent), rather than run continuously with non-condensing operation (during warmer/shoulder seasons).

In the last article written on this topic, we also mentioned upcoming control strategies that appear to be ready change the above assessment.  The new control strategies are now available, the following is a quick update on these controls.  The new control scheme is powered by Siemens parallel positioning controls – the same as used on larger, traditional boilers – now fitted to condensing boilers.  This allows for more precise control of the air-fuel ratio and at least as importantly, according to one boiler designer, they can maintain a higher fan speed and improve the ‘mix energy’ for the air-fuel

The image above shows the servo motors.  You can see the air servo (top) and fuel servo (bottom) circled in red.  These are positioned independently of each other, during commissioning, when a technician “sets the curve”. This is the same style of control system commonly found on larger, traditional boiler designs.  As control costs have come down, this is sort of control scheme is now a practical option on the smaller (relative to traditional boilers), condensing boilers. Per the manufacturer (Bryan Steam) this control scheme allows for precise, high turndown control, improved reliability and increased efficiency (controlling excess O2 – consistent flue gas dew point throughout the range).  Their final, published turndown ratio will be 20:1 – though they were able to comfortably achieve more in their lab – this ended up being a safe, reliable, repeatable set point.

This control is currently available on the larger sizes (3.5-6MMBH) of their FreeFlex boilers and will be an option on the smaller units (1-3MMBH), in the coming months. Without the parallel positioning control, the standard turndown remains 5:1.

All of the FreeFlex boilers still enjoy the benefits previously highlighted on our site. Head to https://bryanboilers.com/free-flex/ to learn more about this rugged, efficient boiler.

We’re excited to continue our great relationship with Bryan Steam and represent this awesome boiler! If you have questions regarding anything in your boiler room, please call us at 336-299-3035, email at sales@wcrouse.com.