# Stumped by a furnace



## beenthere (Mar 5, 2009)

All latent heat exchange is suppose to happen in the secondary. Some of course does happen in the flue pipe.


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## flashheatingand (May 3, 2008)

O,k, I can buy that. But, there is negative pressure in the heat exchanger? What about the condensate drainage at that point? Granted, it may be no big deal if heat cycle is fairly short, but what about longer run times? Surely the engineers have factored this in.

Well, in both cases, it wouldn't have hurt to have the instruction manual available, so as to verify that all tubes are where they are supposed to be.


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## beenthere (Mar 5, 2009)

The primary heat exchanger is too hot for moisture to condensate in it. Providing that its working right to begin with. As in not under fired, or air flow set up so high that temp rise is lower then the factory speced.

If you have a 90%er that has a min allowable temp rise of 45°, and you move so much air that the temp rise is only 30°, you will condensate in the primary heat exchanger.

An induced draft 80%er's heat exchanger is also in a negative pressure.


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## flashheatingand (May 3, 2008)

Yeah, I get the neg pressure thing. I get why you don't want condensate in the primary. But the drainage system through the secondary is limited, so I imagine, you don't want too much condensate in that area. 

Too much focus on why the water shoots out as soon as the inducer is turned off. For the others on the B-team, the reason for poor water flow when you open the drain into atmosphere, is because you are actually sucking in air.... Hence the reason for a trap

It would be nice to know how much condensate one should expect to pull from a unit. Better yet how much condensate per running minute? Anybody?


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## beenthere (Mar 5, 2009)

There is a math formula that will tell you that. I forget it off hand. And am not sure where I have it, or what file name its under.

How much does vary slightly with the temp and humidity of the combustion air.

Yes, a trap is needed for it to drain. kinda like a trap is needed for an A/C coil that is on the return side of the blower.


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