I am watching the 5 part HVAC training video on NACHI tv and I just finished the first part. He kept talking about the 90-95 % furnaces capturing the latent heat and I had no F****** idea what he was talking about the whole time. I think it just hit me. So since it has an induced draft and a serpentine heat exchanger for more heat exchange surface area, then the air passing across it has enough time to “capture” all the latent heat and it condenses back to water. Am I understanding this correctly?

Yes - you’ve got the idea.

latent heat = heat that cannot be measured, i.e., does not cause a thermometer to rise.

Whenever a substance changes state from a solid to liquid, liquid to gas, etc., there is a certain amount of heat that has to be added (or removed) to accomplish the change of state. While the change of state is occuring, heat is continuing to be added (or removed), yet the temperature remains unchanged.

This is how it was graphically represented in school: We made a glass of ice water, which was 32 dF. We then put an immersion heater in the ice water and plugged it in. The water remained at 32 dF until all of the ice was melted (while stirring to avoid currents and stratification). Once all the ice was melted, the temperature rose as more heat was added (this is the opposite of latent heat - sensible heat) until the boiling point was reached. Once boiling, the water remained at 212dF even though heat was continuing to be added. If we could have continued to run the heater dry, all the water would have boiled away, and the heat would have started to raise the temperature of the water vapor that was created.

To apply this to a condensing furnace, think of the exhaust gasses of combustion as being water vapor. The primary heat exchanger will transfer heat to the airstream, reducing it’s temperature to somewhere approaching 212 dF, the point where water vapor will turn back into liquid water. The secondary heat exchanger will then continue to remove heat from the 212 dF water vapor through longer contact time (like you said) and increased surface area (a secondary heat exchanger looks similar to an evaporator coil with fins and tubes). Once enough heat is removed, the flue gasses will then be a mixture of liquid water and air. The furnace has condensed the water vapor from the combustion process back into liquid water.

Air conditioners also work on this principle - there is a huge amount of heat absorbed (or released) in changing refrigerant from a liquid to a vapor state and back again.

Thanks man!! I love when things just click

I commend you for sticking with those videos, great information but shooting the expert in a talking head video is so painful to sit and watch. Really just a cheap way to make a video what Inter NACHI need to do now is take that video use the info develop a story board and do a real professional video in a lot less time.

Thanks. There is so much “fat” that needs to be trimmed in those videos. I don’t care to hear those two shoot the ****

Here is a link to an index of the time stamps for you if you want to reference a specific topic.