Non-condensing versus condensing boilers

Brad White_9

The magic number of condensing and non-condensing seems to be about 90%, some say 92%, the differences being how the efficiency is interpreted.

Let's state for the record that ALL appliances condense at some time during their operation. The issue is, for how long and should it be encouraged? Condensing equipment is built to take it. Cast iron can tolerate it for short periods absent specific condensing designs, but it has to dry out and be kept above gas dewpoint for most of it's operation.

Let's also stipulate that conventional (meaning non-condensing) appliances are in the 82-85% efficiency range for gas and oil respectively and with modern equipment, vent dampers and electronic ignition as applicable. Stack temperatures would be in the range of 350-450 degrees more or less depending on a few variables.

The "near condensing" offerings squeak past this mark and get to the 87-89% range.

One telling distinction between these marginal differences often shows up when the chimney is exterior to the building. What does not condense within the appliance itself condenses in the chimney. Even if the chimney is internal to the house but is not lined, it may still occur. Conventional 82-85% appliances tend not to have this issue.

Condensing on the other hand requires specific venting materials and systems. Some are plastics, some are super-ferritic stainless steels such as AL 429 C.

Aluminum versus Stainless Steel: Stainless, specifically 316L (low carbon) or 316Ti (titanium stabilized) seems to have a higher preference among specifiers. Alumimum is more conductive and a lower cost but is also more prone to degradation by acids and alkalais. The pH of both the water side and flue gas side are critical. The water side can be dealt with (sealed system). The combustion side is essentially at the whim of the gas and air quality coupled with the burner. In other words, it is what it is.

Keep in mind that ALL metals degrade. Thickness makes up for disolvability. Aluminum castings tend to be thicker as a result.

Debbie_2

condensing vs. non condensing boilers

I have been doing some preliminary research and need to clarify some questions about boilers and efficiency.
The info. I read states that you should avoid a too efficient mid-efficient furnace because of problems with condensation. They said don't go above 82% in a mid-efficient furnace. Is this correct? Does this apply to boilers as well? I saw some burnhams (revolution) that were 88% but non-condensing. Cast iron. Our current cast iron is 43 years old, that Burnham sounds like a good thing but not if it will come with problems.
Secondly, when looking at the high efficiency condensing boilers some have aluminum heat exchangers some stainless steel. Which is better? What does it mean when they have an aluminum block but a stainless steel burner?