Question about Cast Iron Heat Exchanger condensation?

They say the flue combustion gas will condense and corrode a Cast Iron Heat Exchanger if the water return temperature is below 130 deg Farenheit, and they adjust this temperature using a piping bypass and when the boiler's outlet temperature is say 150 deg Farenheit.
But then they say the circulator pump starts immediately....so that doesn't make sense since the outlet temperature will not have reached 150 deg Farenheit and therefore the water return temperature will be BELOW 130 deg Farenheit???
Why won't this cause corrosion since the return temperature will be below 130 deg. Farenheit and the burner will be running so the combustion gas will contact a colder return water temperature heat exchanger surface and should logically cause condensation and corrosion?
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On 30 Jul 2006 10:29:33 -0700, snipped-for-privacy@hotmail.com wrote:

Because rust never sleeps.
--
-john
wide-open at throttle dot info
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