Turning up Turndown for Better Commercial Boiler Efficiency

As an owner or as a design engineer, you want to be sure that your boiler selection will operate at peak efficiency, under all conditions. Thermal efficiency is simply the chemical energy added to the boiler, divided by the energy added to the boiler water. As more energy is transferred from the hot gas into the boiler water, the thermal efficiency increases and the temperature of the hot gas decreases. Turndown ratio plays a key role in this energy transfer. Many engineers and owners have been intentionally misled regarding turndown ratio’s and this article will dispense with the misleading information and down right nonsense about turndown ratios and efficiencies!

Patterson-Kelley, one of our most trusted brands of boilers, has published a blog post on the advantages and disadvantages of high turndown. We are sharing that post below as well as their explanation of how Patterson-Kelley will introduce 10:1 turndown without sacrificing system efficiency.

Commercial Boiler Efficiency

Placing system efficiency at the forefront of commercial boiler acquisition continues to be a trend in the heating and water heating industry. Manufacturers are racing ahead to push the limits of technology by offering aggressive turndown capabilities approaching, and even exceeding 20:1, at the detriment of system efficiency.

In an age where energy conservation is important and condensing boilers are increasingly being adopted, the dewpoint plays an important role in system efficiency because it determines whether the boiler will condense or not, as well as how much condensation will occur. The dewpoint is the atmospheric condition below which water droplets condense and dew can form, releasing heat that can then be absorbed back into the system. Condensation can occur at up to 130º F depending on pressure and humidity. The returning water in the heating system is used as the cooling medium; as the temperature of the returning water drops, the amount of condensate increases. The potential amount of condensate estimated at 100,000 BTU’s per hour is one gallon, if the boiler is operating at reduce temperatures.

Heating System Flexibility

Although heating systems are designed to meet peak loads, they spend most of their run-time hours off peak. Along with efficiency, flexibility is an important factor in boiler operation. As the building loads change, the heating system must be flexible enough to change with it. In the instance where the heating system is sized at a higher capability than is required, the flexibility of a boiler could accommodate the imbalance by turning down the input to match the load. This has become the key driving factor in the high turndown story.

Traditionally, turndown has been the limiter of efficiency. To maintain stable flames, excess air is introduced which depresses the dewpoint of natural gas. The dirty little secret associated with high turndown is that, in most cases, the boiler is no longer condensing.

Aggressive turndown capabilities usually lower the dewpoint and reduce the window of opportunity to condense due to excess air. The 970 BTU/lb. of condensate that engineers expect to get from the system, will continue to go straight up the flue.

Excess air to achieve highest possible efficiency:

5 – 10% for natural gas
5 – 20% for fuel oil
15 – 60% for coal

A Patterson-Kelley Solution

Due to technology improvements, Patterson-Kelley will introduce 10:1 turndown capabilities for the first time during 2019. Patterson-Kelley has achieved higher turndown with a unique process that allows the gas valves to reliably turn down without adding large amounts of excess air, resulting in flexibility and efficiency. This method delivers a 10:1 turndown rate while preserving the dewpoint of a 5:1 turndown rate, which helps maintain high condensation levels. This higher turndown capability will provide designers the ability to match input to building loads without sacrificing reliability. No more nuisance flame failures that have traditionally plagued high turndown boilers.