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Posted by HeatPro on December 09, 2004 at 15:12:02:
In Reply to: Help! Installers in Dispute: Hot Water Boiler Efficiencies - New Boiler Recommendations? posted by Cindy by the Sea on December 09, 2004 at 13:35:02:
You might have some information that can help answer the dilemma. If you know that the water temperature of the system was lower than 180F during the winter season, some of the efficiency argument against the higher efficiency unit is lost. When a boiler reaches above 87% AFUE efficiency, it utilizes the heat from moisture in the flue products so condenses water when the combustion gas is below the dew point. Usually when the boiler water temperature is higher than 150F the condensing begins to be reduced due to the heat exchanger characteristic. H2O turns from steam to water below 212F, so potentially, flue gas moisture can turn to water below 212F, but the difference from one side to the other of the exchanger can be high, especially thick cast-iron. As 92+ efficient boilers often use plastic exhaust tubing, the flue gases generally stay below 160F, so the combustion gas moisture can potentially condense at any time. The circuit water will usually only be above 160F during the few coldest weeks of the year.
Weil McLain's Ultra boiler can modulate down to half of your system's need for 100,000 btuh and go twice as high as you need. It has built-in efficiency modulation programming. With proper water flow control, you can arrange for the return water from your radiators to be 40 degrees cooler than when it leaves, so the boiler can be in condensing mode almost all the time, thus getting the efficiency you might want for nearly all the year.
"He said that the return water will be at a higher temperature in a cast iron radiator system compared to a floor radiant system, and it will not need to be heated as much" If the cast-iron radiator system MUST have 180F water, it will need to be at 180F for only one month of the year, it seems a waste to deny 7 months of the highest efficiency (and all the rest of the year if you make hot water with the boiler and a tank) for one month of ordinary bills in order to have a boiler that will be 8 percent more expensive to run all year round for the next three decades.
"He states that the high efficiency systems are really meant to be used in radiant floor heating systems, not the old big cast iron radiators like mine." It also seems that condensing boilers are a waste of a manufacturer's time if they will target radiant floor systems alone, as they are the least-installed systems of all. This installer needs to 'think out of the box.' New systems will probably progress to be mostly condensing boilers within the next decade, and older systems designed at higher temperatures can be adapted to make good use of them.