Condensing Problem ??

Andrew Stern

Hello...

I have a new gravity conversion (180 gallons in the system) with a cast iron 125,000 BTU boiler, a 4 speed circ pump (pumping away), an adjustable system by-pass, 1-1/4 near boiler piping and temp gauge and throttling valves on the return. The circ pump comes on/off with the burner. A single zone system.

When the t-stat call for heat the system comes on, but often satisfies the t-stat even though the boiler only comes up to 100 degrees or so. Delta-T is always less than 20-degrees with the boiler bypass closed. With the bypass fully open and the pump speed turned down, I can get the boiler up some, but nowhere near 140 under the mild weather conditions now. For me to build the temp in the boiler above 130 quickly, I’d have to by-pass and throttle back flow form the house to the point that I’d be concerned I wouldn’t circulate enough water to carry the BTU at design load.

I have read a lot about the effects of condensing and have enough experience to recognize the importance of the issue, but I’m having trouble understanding how any condensate could form around the boiler sections that are engulfed in flue gasses that are clearly well above 130 degrees. I understand that the water on one side of the cast iron is relatively cool and it definitely will cool the flue gas, but clearly the other side is still very, very hot. A temperature gradient must exist across the thickness of the cast iron, and I have trouble thinking that the low water temperature is seen across the metal on the outside of the casting. If condensing would occur, wouldn’t it happen where the system passes thru the dew point somewhere up the stack? I can see issues if the water ran back down the stack somehow to damage the boiler, but I have trouble seeing how you could condense within the boiler at these temperatures generated above the burners. I need help with this one as far as the theory goes.

Thanks all,
Andy