In the early days, boilers were made of two materials - steel and cast iron. Then, one day someone decided to create the copper fin type boiler. It was determined copper was a better heat transfer material, which allowed for smaller equipment and better efficiency. As regulations got tougher in Europe and the States, new "condensing" boilers flooded the market, using new materials like aluminum and stainless steel. Before we go too far, lets think about WHY new materials were needed.
ANY boiler can condense just like any car can drag race. But, I don't think a 1973 Pinto is going to win you many races! Condensation occurs when the boiler return temperature drops below 125-140 (depending on application). So, if you ran 100 degree return water to a copper-fin boiler, it will be very efficient and condense. At least for the three months that it runs because after that, the acid in the condensate will have eaten the burners; obviously not what we want. So, manufacturers decided to use stainless steel and aluminum, which in theory makes a lot of sense, right? Let's use a material that is more tolerant to condensate. But how will those materials handle other conditions --- city water treatment, glycols and system softeners all play a role in a boilers performance. Just taking a "condensing" boiler, dumping some tap water in and turning on power does not guarantee you will have a good working boiler system. In fact, I'd bet the opposite, especially in central and south Texas! City water conditions provide a number of hurdles for these manufacturers with pH and chloride being the biggest culprits. And, oh, by the way, since that's listed in the fine print of the operating manual that will go missing in the first six months of operation, it is not a warrantable item. Try explaining that to an owner..."we're sorry mister building manager, but that $40,000 boiler you just installed in November is not covered under warranty because you're chlorides are at about 42 ppm. We'd be happy to order you another one."
Ok, so then how do we save energy without worrying about whether or not our chlorides are below 25 or our pH is between 6-8??? We go back in time and learn from the past; CAST IRON! Now, before you click this window closed, give me three more minutes. I promise, I'll show you I'm not the only "crazy one" in the bunch. Hydrotherm introduced the KN series boiler in 2005; a condensing cast iron boiler that can heat ice cubes if you can supply them giving you up to 99% efficiency. The counterflow design of the heat exchanger and the top mounted burner literally blows the condensate off the pins and into the stainless steel catch pan. If it doesn't stay on the pins, it can't corrode. And think about it, if water quality was an issue, don't you think their lawyers would require the same small print??? (by the way, there is no small print regarding water quality).
Now, I mentioned there are others that have come around to this idea. The 2012 ASHRAE Handbook (yes, THAT ASHRAE) states the following:
"The condensing portion of these boilers may require special material or operating techniques to resist the corrosive effects of the condensing flue gases. In the past, typical cast iron, carbon steel and copper were not suitable materials for the condensing section of a boiler. Certain stainless steels and aluminum alloys were suitable. However, advances in design, controls and manufacturing have allowed materials such as cast iron to be used where they previously could not be; as with all products, consult the manufacturer for the proper application."
So there you have it; ASHRAE's gone mad, too! Or, they know Hydrotherm has manufactured a product that takes something old and revamps it for the current conditions of today's mechanical room needs. Kind of like soupin' up that '73 Pinto for Friday night races!!!
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