Boiler Facts... George R. Carey, Jr. High Efficiency Boilers and High Temperature Radiation? Vice President, Fluid Industrial Associates gcarey@fiainc.com twitter: @Ask_GCarey Over the past few years, there has been a tremendous amount of Mod/Con (modulating and condensing) boilers installed all over “Hydronics Alley.” These boilers operate with natural or propane gas as their fuel source and are referred to as high efficiency boilers because they have efficiency ratings in the 90s—some as high as 95%. To attain such high efficiency numbers, they intentionally condense some of the flue products that are formed as the result of combustion. Normally, we would make sure that the boiler never condensed its flue products because, if allowed to, the condensate would rot out the boiler and vent piping. When combustion occurs, energy in the form of heat is transferred through a heat exchanger (pinned castiron sections, copper-finned tubes, cast-aluminum and stainless steel) to flowing water which is on the other side of the flame. When the flame is produced, combustion gases are formed. These gases contain water in the form of vapor. Generally speaking, we want these vapor gases to vent out of the boiler and up into the venting/chimney system and out to the atmosphere. Mod/Con boilers are designed to allow the flue gases to condense right inside the heat exchanger. They even provide a condensate drain to allow the condensate to escape from the heat exchanger and drip through a neutralization kit into a drain or condensate pump. The benefit of condensing these combustion gases is that they contain heat or energy that we normally lose up the chimney. Just like in a steam system, when water in the form of vapor (combustion gases) condenses, it gives off a lot of latent heat. How much? Well for every pound of water vapor that condenses back to liquid, 1,000 BTUs of latent heat are released. The heat exchanger “catches” this heat and transfers it over to the system water—and this is how these Mod/Con boilers achieve the higher efficiency ratings. More of the unit of fuel goes into heating the system water rather than up the flue pipe. Just how are these flue gases condensed? In the exact opposite method of how we prevent these same gases from condensing in a non-condensing boiler. Condensing occurs naturally—when the combustion vapors cool below their dewpoint, they will condense back to liquid.Water temperature has the greatest impact on whether the flue gases in a boiler will condense or not—it’s all related to the dewpoint of the combustion gases. Oil-fired systems want to keep the water above 140°F to prevent the gases from condensing while gas systems generally want to be above 130°F to prevent condensing. Using a Mod/Con boiler, the water temperature in the return needs to be 130°F or below for its flue gases to condense. With return water in that temperature range, the flue gases will be condensing and the boiler will be operating at or near its published efficiency rating. Whenever the return water climbs above 130°F, which is higher than the dewpoint of the combustion vapors, those gases will not condense in the heat exchanger. The most commonly asked question is, “Can I use a high efficiency modulating and condensing boiler in a system that incorporates traditional high temperature baseboard?” Some people in the industry say, “No, you can’t! The water needs to be hot enough to satisfy the baseboard’s requirements, which are too hot to allow any condensing of the flue gases!” If you aren’t condensing the flue gases, why use a condensing boiler? A large portion of these Mod/Con boilers are sold in retrofit applications for both residential and commercial buildings. The boilers are usually cast iron oil-fired or atmospheric vented gas boilers and they are serving high temperature terminal units such as copper baseboard or cast iron radiators. When these systems were first installed, the radiation was sized so that on a design cold day, with water circulating through the radiation at 180°F, the room temperature could be maintained at 70°F. Because of those design conditions, that is where the comments are made about compatibility issues between Mod/Con boilers and baseboard radiation. Yet, how often during the heating season do we actually encounter design conditions? In the Northeast, design conditions make up about 3-5% of the heating— the rest of the year, the conditions are somewhere less than design. In fact, through Bin Data collected by the 18 ICM/March/April 2017
ICM March-Apri 2017
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