The Eight-Per-Cent Solution


Manufacturers of direct gas-fired heating equipment make different, and seemingly conflicting, claims about the efficiency of their equipment. Some claim 92%, others claim 100%; so which is accurate? Actually both numbers are accurate, if you understand the context for the claims. There are two terms used when referencing the efficiency of gas-fired equipment: combustion efficiency and thermal efficiency. Understanding the basis for those terms will help you to better understand how to apply this equipment. Combustion efficiency is a measure of how effectively the heat content of the fuel is transferred into usable heat. With direct gas-fired equipment, since there is no flue, all of the energy generated from the combustion process is transferred into the space being heated. In that sense, the direct gas-fired equipment can be considered 100% combustion efficient. However, not all of the energy from combustion produces sensible heat, or heat that can be felt as a temperature difference and is applicable for offsetting the heat losses in the building. Another byproduct of the combustion process is water. The heat required to vaporize this water is referred to as latent heat. Approximately 8% of the energy from the combustion process is used in the vaporization of water. In that sense, the direct gas-fired equipment can be considered to have a 92% thermal efficiency. While the latent heat is not considered usable for convective heating, it can be considered beneficial. The heating equipment is operating during periods when the outside air is colder. The colder outdoor air has less capacity to hold moisture, which results in lower specific humidity conditions (i.e. drier air) both outdoors and indoors. This is why many people use humidifiers indoors during the winter. The water vapor from the combustion process increases the specific humidity of the air delivered to the building through the heater. Since most buildings without other sources of moisture indoors have lower humidity levels during the winter, the additional water vapor from the combustion process can be beneficial to the indoor air quality conditions. There are currently no standards for testing nor agencies certifying the efficiency of direct gas-fired heaters. The American National Standards Institute (ANSI) specifies in standards ANSI Z83.4 and ANSI Z83.18 that the conversion from sensible to total heat for direct gas-fired heaters is 0.92 (92%). This is based solely on the chemistry of the combustion process, since there are no other losses. So is direct gas-fired equipment 92% or 100% efficient? The answer is…Yes! It just depends on how you define it. If you have any questions or need to provide documentation on the efficiency of direct gas-fired heaters please feel free to contact the Applications Engineering department here at Cambridge and we will be happy to help.