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Modulating/Condensing Manual

Tim McElwain
Tim McElwain Member Posts: 4,556
<span style="font-size:16pt">Hi Folks,</span>

<span style="font-size:16pt">I have been doing research for a couple of years now on Mod/Con equipment. I feel much challenged in attempting to write a manual on the subject. So I am asking for help from my many friends in the industry. I have compiled stacks of manuals and data on this subject and look forward to finally putting it together in some understandable format. Many of you are much more qualified perhaps to take on this task than I am. It is with that in mind I am asking you for your input. </span>

<span style="font-size:16pt">I am inclined to write the manual from the control side of this subject rather than the installation/venting/piping/maintenance side although some of that will have to be a part of this.</span>

<span style="font-size:16pt"> As always I am looking at this from the perspective of diagnosing and troubleshooting these systems. I am interested in your truthful opinions of these systems from both a positive and negative perspective. It is never my intent to bash anyone’s equipment so please keep that in mind. I will give you and your company recognition for anything offered.</span>

<span style="font-size:16pt">One of the first things I am looking for is a few paragraphs defining just what you think Mod/Con is and how it works.</span>

<span style="font-size:16pt">I am looking forward to hearing from you and really appreciate your help on this project.</span>


  • SpeyFitter
    SpeyFitter Member Posts: 422
    My "input" - pun aside

    What is a "Mod-Con" according to me? I"m sure some can sum it up better than myself but I'll offer my input with a bit of history to direct the discussion as to why modulating condensing boilers are here.

    The first boilers (coined "conventional boilers) were fixed fire rate equipment that fired at one input, with cast iron heat exchangers. These boilers had several disadvantages that had to be mitigated for in the design of a system to allow them to serve different types of heat emitters.

    The main products of combustion are H20 (water/steam) and CO2. In order to protect the cast iron heat exchanger, a conventional boiler must waste some heat up the flue (with a subsequent loss in efficiency) to keep any steam in the products of combustion from condensing. If the return water temperature on the water side of the cast iron heat exchanger is too low (said to be "below the dewpoint"- typically 135 degrees  for most natural gas systems but what the actual dewpoint is needs definition in this manual for the various factors that contribute to what it's actual temperature is), these products of combustion can condense on the heat exchanger walls and cause premature failure. Worse yet, this condensation can eat through the walls of B & C vents, and destroy chimneys (if they are used for flue gas venting), which poses a serious safety hazard for occupants of the building the boiler serves. To mitigate for this, designers & installers must select components and controls that "protect" the boiler (protect meaning, to not allow the return temperature of the boiler to get below the flue gas dewpoint). 

    Another problem with conventional boilers is because they often only had one fixed firing rate, they were often oversized a large percentage of the time for the heating loads they were serving. Like any gas fired equipment, in order for a conventional boiler to run efficiently, it must get up to operating temperature. When heating loads are low, conventional fixed fire rate boilers often can not get up to operating temperature which severely handicaps their ability to run efficiency, and there are also side effects to this as well. Heat exchangers become dirtier quicker when a boiler short cycles further causing a reduction in efficiency if they are not serviced & cleaned regularly. Also, if a boiler can not get up to operating temperature, it can cause some flue gas condensation in the venting system. This is a safety hazard overtime.

    Enter the solution - the modulating, condensing, boiler. In the grand scheme of hydronic heating, the modulating, condensing boiler is just a baby. But it is a solution to many of the efficiency robbing quirks of the conventional boiler. First off, what does a modulationg, condensing boiler do?

    A modulating boiler has the ability to adjust it's firing rate between 2 set input values.These values are expressed often times as a ratio of each other, typically for many boilers it would be "5 to 1," meaning this particular boiler can modulate it's firing rate  at inputs from 20% up to 100%. By modulating the burner, assuming the boiler is sized properly to the heat load requirements, it can offer just the right amount of input to keep the burner running longer which increases thermal efficiency.

    A condensing boiler thrives where the conventional boiler fails. Where a conventional boiler requires the return water temperature on the water side of the heat exchanger to be above the flue gas dewpoint to protect the heat exchanger and venting as outlined above, a condensing boiler thrives in these circumstances. Instead of wasting heat up the flue to keep the products of combustion from condensing, a condensing boiler and it's venting system are made of corrosion resistant materials (typically stainless steel, or aluminum) that require and allow this condensation to occur. The more condensation that occurs, the more efficient a condensing boiler runs. Subsequently, the lower the return water temperatures, the more condensation occurs, the more efficiently a condensing boiler runs.

    As water in liquid form is heated from near freezing, to the point where it is close to flashing to steam, it's temperature rise is proportional to the heat that is applied to it. This is called "sensible" heat. If you had one pound of water, and applied 100 British Thermal Units to it (BTU), it would rise 100 degrees (1 BTU = the heat required to raise the temperature of 1 pound of water 1 degree fahrenheit). Now when water gets to the point where it wants to flash to steam, it requires a bunch of heat to actually flash to stream. This heat that is required is called "latent heat."  Anytime water changes states, it needs latent heat. Anytime it remains in one state but changes temperature, it is sensible heat.

    Now inside a condensing boilers heat exchanger, when the products of combustion condense on the heat exchanger walls, they give off this latent heat from the change of states from steam to water, and subsequently an increase in efficiency is realized over conventional boilers. And as you can see, the more condensation that occurs, the more efficienctly a condensing boiler runs.

    Modulating AND Condensing is really the perfect combination. You now have a system that can adjust its fire rate so not only does it match the heat load requirements precisely, but it can ensure that the temperatures returning to the boiler (depending on the heat load) are as low as possible to maximize condensation, and subsequent increased efficiency.

    There are however a few disadvantages to Modulationg condensing boilers, but it's late and I'm going to bed so we'll talk about those later... LOL


    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
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