Residential Condensing Boilers

Last month I wrote an article about boiler controls and maximizing efficiency. This month I thought I would write about the boilers themselves. There is no way that I could cover all the different styles of boilers in one article, so I am going to limit the discussion to residential condensing boilers. To define this a little bit more I will be covering boilers that are less than 300,000 BTUH. The reason I have chosen this size range is because that is the standard cut-off for the AFUE rating system (Annual Fuel Utilization Efficiency). There are a lot of things that need to be discussed in regards to how a condensing boiler operates, maximizes efficiency and how it differs in installation from a conventional boiler. I hope to highlight these things to give you a better understanding of residential condensing boilers.

I would like to start by giving a very simple explanation of condensing boilers vs. traditional non-condensing boilers. In all boilers we burn a fuel, Natural Gas in this case, mixed with make-up air, Oxygen, to heat the water in the boiler. The by-products of combustion escape from the boiler out the flue venting to the outside. In a traditional boiler the combustion gases are hot, around 350 °F. About 10% of the boiler efficiency is lost in the steam that is part of these gases. In a condensing boiler we try to lower the flue gas temperature to make some of that steam condense back into water. This hot water can then transfer more heat into the boiler water, rather than escape from the flue venting. By recovering some of this heat, we can increase our boiler efficiency beyond what a traditional boiler would give us. If conditions are right, we start to approach 98-99% efficiencies.

As the next step in this process you may be asking; how do we get the boiler to condense the steam in the flue gases? Well, there is one major key to this process, and that is getting the return water to the boiler at a temperature below the flue gas dew point. The dew point temperature has a lot of factors that determine what it is, we will not cover them in this article. I will simply say that the dew point is typically around 130 °F. So, if we can get the return water to the boiler at a temperature below 130 °F, we will get the boiler to start producing condensate from the steam in the flue gases and the efficiency will rise. As you have probably figured out by now, if we can get the boiler return water temperatures even colder, we will condense even more, boosting the efficiency even higher.

While all of this sounds wonderful and easy to apply, there are a few more things that you must consider. First off, to get the return temperatures low enough to condense the flue gases, the system must be able to run on low supply water temperatures. Radiant floor systems and snowmelt are two great applications for condensing boilers. Higher temperature terminal units like radiators and finned tube will not allow the condensing boiler to optimize efficiency as well, but by utilizing an outdoor reset curve you will be able to run in condensing mode for a good portion of the year. The key here is that your overall system is what will dictate the boiler efficiency so it is important that the entire system be designed properly.

The second issue to look at is that when we condense the water vapor in the flue gases, it is not just water anymore. Some of the by-products of burning the fuel gets into the condensate, which in this case, turns it acidic. The condensate produced by condensing boilers ranges in pH from about 3-5. This requires two things be addressed; the boiler heat exchanger and the condensate drainage. Condensing boilers need to be able to withstand the acidic condensate running across the heat exchanger. This is one of the main differences between non-condensing and condensing boilers. Typical condensing boilers utilize Stainless Steel or Aluminum heat exchangers which will hold up much better to the condensate than copper or cast iron. We also have to deal with the condensate as it drains out of the boiler and into the sewage drain line. This is easily addressed by adding a neutralization kit after the boiler condensate drain to raise the pH level to around 7.

The last major concern with these style boilers also has to do with the acidic condensate, but this time on the flue venting side. Since our flue temperatures are now much lower, typically 200 °F or less, we will be condensing within the flue venting also. This condensate will run through the venting and back into the boiler. Since this condensate is acidic we again have to compensate for that by using the correct materials to prevent corrosion of the flue vent. The most common materials are CPVC, Stainless Steel or AL29-4C. These materials must be used to ensure safe operation of a condensing boiler.

As you can see condensing boilers can be much more efficient under the right conditions that a traditional boiler would be. However, there are many things that you should be aware of before making the decision to use one. Your hot water piping system should be designed to maximize efficiency by allowing for low return temperatures to the boiler. You also have to make sure to handle the acidic condensate inside the boiler, at the outlet drain of the boiler and even in the flue venting. If all of these issues are addressed properly, you can have one of the most efficient heating systems available by utilizing condensing boilers.

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