National Weather Bureau, 80% of the heating season requires 50% or less of the BTUs needed for design conditions. In effect, the heating system, including the boiler and the installed radiation, is oversized for most of the heating season. Proper Application of Outdoor Reset… Outdoor Reset calculates the right water temperature for the radiation based upon the load that the house or building is experiencing. What has the greatest impact on a building’s load? The outdoor temperature! So by simply incorporating the outdoor reset function that comes with the Mod/Con boiler’s operating control, the boiler can start delivering the appropriate water temperature needed at the given set of outdoor conditions. As you look into design conditions, reset curves and Bin Data, you see that for a large majority of the heating season, the boiler can lower the water temperature so the return temperatures coming back to the boiler are below combustion gas dewpoint levels. The boiler flue gases are now condensing; the boiler is operating at or near its rated efficiencies and the apartment building or house is comfortable. When designing around 180°F water, 70°F indoors and a design outdoor temperature of between zero and 10°F, you will find that, until it gets down to 25°F or colder, the reset curve will calculate a water temperature that provides return temperatures down below the dewpoint of the flue gases, ensuring the boiler is operating in a condensing mode. Another benefit provided by this style of boiler is in addition to the condensing feature—the burner can modulate. This means that as the load changes, the boiler will consume only the necessary fuel to meet that load. Unlike traditional on/off boilers, where if they are firing they are consuming 100% capacity, the modulating boiler can fire down to as low as 10% of its capacity and then modulate all the way up to 100%. In the perfect scenario, when using a Mod/Con boiler, the radiation chosen should be able to provide all the BTUs needed with low temperature water. For that to occur, all of the exisiting homes and commercial buildings would have to change and/or upgrade their exisiting radiation. In some rare instances, that actually does happen, but for the majority of the time, it doesn’t. Will the boiler condense all the time? No, but it will condense the majority of the heating season. Also, it will modulate its firing rate to match the load the building is experiencing. All of these features add up to reduced fuel consumption and more comfortable heating systems. ICM If you have any questions or comments, e-mail gcarey@fiainc.com, call (800) 423-7187 or follow George on Twitter at @Ask_Gcarey. ICM/March/April 2017 19
ICM March-Apri 2017
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