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Posted by HeatPro on November 20, 2007 at 10:19:58:
In Reply to: Preheat boiler return water from stack exhaust? posted by Sean on November 20, 2007 at 00:50:55:
There are many manufacturing condensing boilers at over 90% AFUE. The problem for cast iron manufacturers is that they'd have to admit that copper isn't affected by gas fuel condensation; but water does corrode cast iron and steel. The question would arise - why not just use copper instead - which is now killing the incredible shrinking industries of residential cast iron and oil. When you get above 87% AFUE, you need an exhaust fan as there isn't enough heat for a chimney, so it is fan-assisted venting. That means you'd need an exhaust fan and a copper or other second-stage corrosion resistant heat reclaimer as you suggest which makes cast or steel as expensive as copper.