Originally Posted By: John Furr
This post was automatically imported from our archived forum.
Just in case someone wonders why a furnace would have two heat exchangers, they are the primary and secondary heat exchangers. the enable furnaces to reach such high efficiencies through the utilization of latent heat extraction.
The primary heat exchanger is like the standard heat exchanger on any older low efficiency or newer mid efficiency furnace, larger steel with wide openings to provide larger easy passage of the flue gasses. The heat exchange is strictly gas to gas between the hottest flue gasses and the preheated house air.
The secondary heat exchanger extracts the latent heat from the moisture that is a product of combustion. Latent heat has a much higher heating value and efficiency. As the latent heat is extracted from the combustion gasses, the moisture condenses in the secondary heat exchanger. This is where the term condensing furnace comes from.
The condensate is slightly acidic and needs to be collected, drained from the furnace and neutralized before entering most waste water drain systems.
The secondary heat exchanger is at a much lower temperature than the primary one. For this reason it is located below the primary exchanger, so that the airflow from the home first contacts the secondary exchanger... the ensures the highest delta T or temperature differential between the secondary exchanger and the house air, thereby maximizing the heat exchange between the air and the exchanger surface.
This preheated air then passes over the much hotter primary exchanger. The primary exchanger is what first contacts the flue gasses since the burner ports are located in the end of the primary heat exchanger.
Natural gas or propane condensing heating equipment is excellent and I highly recommend such products.
On the other hand, condensing oil equipment is trouble prone due to the high sulphur content in heating oil. When the flue gasses condense they form sulfuric acid that destroys venting components and heat exchangers causing premature equipment failure.
mid efficiency non condensing heating equipment is 84% efficient already and moving to a high efficiency model only improves that by 4% or so, and adds dramatically to the services costs associated with a condensing oil furnace/boiler.
I would wait until sulphur levels in heating fuels are regulated to the point they are well below todays standard or the invent a foolproof and sulfuric acid proof furnace.... don't hold your breath...
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John Furr
Residential Efficiency and Training Resources of Ontario
