Why can't cast iron boiler run at 150 or 160

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Old 06-14-07, 03:54 PM
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Why can't cast iron boiler run at 150 or 160

I was speaking to a WEIL-McLAIN tech and was told boiler should operate at 180,but no lower than 170.I said that I wanted to run at 150 or 160 and I was told the cast iron would condensate.If you operate one of those reset controls on the out side of the house, doesn't the temp drop on a warn day. What am I missing?
 
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Old 06-14-07, 05:58 PM
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For a gas boiler, condensation starts at about 135F. For an oil boiler, it's about 105-110F.

You do not want prolonged exposure of cast iron boilers to condensing temperatures because the condensate is acidic and will corrode the cast iron. So keeping the coldest water (the boiler return) above these temps is necessary. A typical space heating system is designed around a temperature drop of 20F from supply to return. So if you set your aquastat for a 150 supply, you might be seeing 130F (or less if 150F is the high limit because most of the supply water will be below 150F) on return to the boiler. Enter condensation....

Outdoor reset comes in a few flavors.

"Partial reset" would be using temps between the lowest return temp your boiler can tolerate (see gas and oil temps above), and the typical 180F supply. So you might have an available range of, say, 160-180F for gas, and 135-180F for oil.

"Full reset" would use a range of basically room-temperature (70F) to 180F. Using these low (condensing) temperatures requires a boiler protection scheme, or a boiler designed to handle condensing. A decent, though technical, summary of various boiler protection schemes is at

http://www.tekmarcontrols.com/literature/acrobat/e021.pdf
 
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Old 07-09-07, 04:36 AM
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Originally Posted by BOB1934 View Post
I was speaking to a WEIL-McLAIN tech and was told boiler should operate at 180,but no lower than 170.I said that I wanted to run at 150 or 160 and I was told the cast iron would condensate.If you operate one of those reset controls on the out side of the house, doesn't the temp drop on a warn day. What am I missing?

The critical temperature is the return temperature; it is kind of hard to relate that to the boiler supply temp (which the tech was referring too) unless you know the delta T the boiler is operating at. Large mass cast iron radiators can flood the return with cold water, causing the condensation issues inside the boiler chamber. What is important is how long it takes the return water temperature to reach a minimum temp. that prevents the condensation and that the boiler reaches the idea temp in a short time.
 
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Old 07-09-07, 08:25 PM
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With primary/secondary piping the boiler could run at 180 but the system can be reset to whatever it needed to be.
 
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