High efficiency, condensing gas furnaces or boilers are capable of achieving combustion efficiencies near 95%. The flue gas stream, typically vented to atmosphere in older mid-efficient heating equipment, is further cooled through proper heat exchanger and system design, to further condense the exhaust gas stream sufficiently to strip a further 10-15% of latent heat before being exhausted. Vented to the outside using CPVC or PVC gas approved vent piping, for example IPEX System S636, they are replacing older chimney aspirated heating appliances.
Many condensing boilers utilize a circular heat exchanger design, similar to the NTI Tx series of boilers. Made of stainless steel, which is better suited to withstand the low PH of the condensate. Fluid will flow through tightly spaced stainless steel tubing to allow for maximum contact with the hot combustion gases during the combustion process. The burner design allows for even distribution of the air/gas fuel mixture for consistent heat distribution through the heat exchanger.
There are boilers utilizing a stainless steel vertical tube heat exchanger design consisting of a downfire burner. Boiler fluid returning to the boiler circulates vertically through the heat exchanger. Similar to heat exchangers found on the IBC , VFC and SL series of boilers. This design can also referred to as firetube. This design allows for the returning condensate to be drained vertically down the firetube and out to the condensate trap. This allows for a measure of self cleaning of the burner side of the heat exchanger as the returning condensate will help rinse down any induced combustion contaminants.
Other boilers use a mono block cast aluminium silicate heat exchanger like the Buderus GB142 series, Weil McClain Ultra, among others. Made with an aluminium silicon heat exhanger, this material is far less forgiving to adverse boiler fluid conditions than their stainless steel counterparts. System PH and boiler fluid conditions must be closely monitored throughout the service life of the unit.
When a high efficiency condensing gas boiler is operated at a low output temperature, ideally for low temperature radiant applications, the more the boiler will condense exhaust flue gases and extract the latent heat contained therein. Ideally one would like to see system water temperatures returning to the boiler at less than 120F. Instead of the flue gas condensing in the atmosphere after it leaves the chimney, as with mid-efficient chimney aspirated boilers, the condensing happens within the heat exchanger of the high efficiency boiler. Once the latent heat is extracted from the condensate, the condensate then runs out to drain. This condensate can be acidic, typically in the PH range of 3 to 5. Some jurisdictions require that this acidic condensate be neutralized prior to disposal through the sanitary drain. This is done with the use of a calcium carbonate based retention bed that allows the condensate enough contact time to neutralize to a PH of of roughly 7.
With the emergence of condensing boiler technology a number of years ago, the evolution of the technology is such that an efficiency rating of close to 95 or 96% can be achieved when the boiler is used in low temperature radiant applications.
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