In recent years, there has been a surge in the popularity of Modulating/Condensing or Mod-Con boilers. The combination of modulating firing rates and condensing heat exchangers has lead to the development of boilers with thermal efficiency up to 98%.
Mod-Con Boilers
A condensing boiler features a second heat exchanger or secondary heating coil mounted in a chamber where hot flue gases will provide additional heat transfer, utilizing more of the overall Btu/hr input. The heat transfer process in the secondary coil is best achieved with low temperature inlet water. Low temperature inlet water increases the heat transfer by condensing the water vapor in the flue product and releasing the latent heat.
While condensing technology increases heat transfer rates, modulation improves performance by meeting heat demand with greater accuracy. As heat demand increases and decreases throughout the day, the modulating boiler will increase and decrease the Btu/hr input rate to match. Boiler can routinely deliver turndown ratios of 5:1 (20% to 100%) or 10:1 (10% to 100%) depending upon their size.
Non-condensing, atmospheric boilers in installations around the country operate at thermal efficiencies in the 80% range with some older boiler operating well below 80%. With new Mod-Con boilers delivering up to 98% thermal efficiency, the annual cost savings in gas consumption can pay back the investment in new equipment in just a few short years. Not to mention the benefit to the environment of reduced gas consumption.
Water Heaters and Lime Scale Buildup
Water heaters have a potential that reduces thermal efficiency not normally seen in boilers, the problem of lime scale buildup. Lime scale is both an energy thief and the certain killer of tank-type, under-fired water heaters.
When water is heated, it undergoes a chemical reaction that causes the dissolved minerals to “precipitate” out as solids. The amount of precipitant (scale) is directly proportional to the volume of water used and the temperature of the water. The more water you heat, the more scale is produced.
Scale has a low thermal conductivity. In fact, it acts as a better insulator of heat transfer than a conductor of heat transfer. Scale that is one sixteenth inch thick will cause a water heater to use 15% more fuel, one eighth inch of scale requires 20% more fuel, and one quarter inch of scale increases fuel consumption by 38%. Increased thicknesses can reduce thermal efficiencies in surprising amounts as seen in the graph on the right.