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Combustion Air vs. Size of space

SpeyFitterSpeyFitter Posts: 420Member
I work doing Gas Fitting & Heating maintenace on a number of buildings that have boilers of various vintages from the 40's to recent models. I've been in this position for over a year now. On a number of our buildings from the 70's and 80's we have Hydrotherm Modular Atomspheric Boilers. These Boilers are relatively simple to work on, and fairly reliable, but inefficient too (running about 75-78% combustion efficiency accross the board). Most of these boilers are continuous pilot, and some of the later models have electronic ignition.  Here is a link to the manual:

Anyways, according to the Gas Fitter who is senior to msyelf and who has been here a lot longer than I, we seem to have a recurring theme of certain buildings that contain these boilers sooting up regularly. By regularly I mean the heat exchangers need to be cleaned every 2-3 years. The common belief held by him and the person above him who retired (and they are both pretty good gas fitters) that I replaced is that the sooting up is a result of the DDC having the ability to turn the boilers off and on as well as the Aquastat (DDC Solid State Relay - SSR is in series with each boilers Aquastat). So as long as the Aquastat has closed contacts, the SSR from DDC can turn the boilers on and off as needed to achieve a certain setpoint in the common building heating loop. This leads to "extra" short cycling in some cases but I'm not sure if I entirely believe it's the entire issue. The reason I say this is because why aren't other boilers in other buildings sooting up eventually as well?  Some schools that have these boilers dont soot up or rarely soot up. So I did a little bit of digging.

First thing I checked was combustion air on one of the plants I am dealing with that is starting to soot up. According to the Canadian Natural Gas Code, for anything over 400,000 BTUH on atompsheric appliances that require dilution air, we size it so the combustion air opening is 7000 BTUH per square inch up to 1,000,000 BTUH, (143 sq in) plus 14,000 BTUH per square inch for everything over 1,000,000 BTUH. Relief/Ventilation air is to be 10 sq inches or 10% of combustion air surface area opening, whichever is greater.  So on the plant I am dealing with which has 4x 900,000 BTUH MR boilers plus 1x 250,000 BTUH hot water tank (3,850,000 BTUH) I sized the combustion air and I am within specs according to code, BUT, if you check the manual above which I did, they require either 4000, 2000, or 1000 BTUH per square inch of combustion air surface area, depending on how you bring the air into the mechanical room, AND the combustion air opening and the relief air opening must both be the same size meeting this formula for EACH opening. So in this case my combustion air in this one particular boiler room does not meet the specifications, not even close (less than 50% basically, or more). Note that in the instructions it says if your mechanical room is a confined space you must meet the above combustion air requirements (4000, 2000, 1000, per sq. in, etc.). I measured the Mechanical room that these boilers sit in and it is BY  A LONG SHOT definately considered confined space as per instructions even though it looks like your average sized boiler room.

But we have another building with roughly the same load as this, and it roughly has the the neccessary required combustion air, or at least more than enough, and it soots up about every 3-4 years. The thing about this Mechanical room is it is quite small - similar in size to the above boiler room so the boilers are in similar proximity to each other in how they are arranged, the pads they sit on, etc. to fit in this boiler room. And I visited it the other day for the first time just to check it out and something I noticed that I found interested was the boiler farthest from the combustion air was starting to soot up the most. I also noticed in the original room above that the boilers farthest from the (undersized) combustion air seemed to be the dirtiest. This may just be a concidence, or it may not.

I plugged my analyzer into one of the boilers closest to the combustion air, something not many guys do on an atomspheric, and I read the oxygen, CO levels, with the boiler room doors closed (doors to outside). I would then open the boiler room doors and the oxygen in the boiler would increase by about half a percent and the CO levels would go down.

I have briefly looked at draft - most of our buildings with these boilers are vented relatively similarly - there are slighty variations but they all seem to have the common theme (common vented in a similar orientation for the most part into a chimney) - this is where I will go next obviously as draft can have an effect on how the flame/burner performs and lifts, as well, etc.

But what I'm getting at is, it's one thing to have enough combustion air, but how critical in your mind is it to have combustion air in the right place? I look at these boiler rooms and the cubic footage of air that they contain as a "buffer." The boilers pull their combustion and dilution air from this space and the space in an effort to create equilibrium pulls air from the outside to balance pressure. But if there is less buffer/cubic footage then I'd imagine you'd have more of a direct pull from the boilers to directly from outside, which creates some sort of resistance potentially to them getting design combustion air. Can boilers closer to the combustion air opening, effectively "starve" other boilers farther away from the combustion air source of air if the boilers are in close proximity to each other? (and, if this is true, it would be compounded by having a lack of combustion air opening...). How critical is it to balance your combustion air, in smaller spaces, relative to your fuel burning appliancees in your opinion? How critical is size of room vs equipment? Or do you think short cycling, or draft may more or less be the culprit.

I have measured draft on other boilers of this vintage in ohter mechanical rooms with an analzyer on the boiler, and with turning other boilers common vented on and off and I've seen slight variations in draft and combustion analysis as other boilers turn on and off, obviously creating more draft that "pulls" a bit more on the boiler I was checking - in some cases on dirty burners creating a bit more CO, probably causing a bit more flame "lift off" ultimatley tickling the bottom of the heat exchanger, or something along those lines. Gas pressures are always within spec - it's one of the first things I usually check and adjust if needed.

The other issue I look at is these plants do see some flue gas condensation in the mornings as these buildings aren't 24/7. But since the plants are well oversized most of the year this usually isn't an issue as the plants come up to temperature very quickly (within half an hour or better most of the time). When I clean the stainless steel burners I usually seem some "crud" which I liken to perhaps tiny bits of heat exchanger metal sacrificing itself and/or burner impurities falling back on the burners ultimately causing the burner ports to get dirty which can cause issues with CO production certainly, but so can soot falling back on the burners fouling them as well.

Anyways - it's amazing how wonderfully simple these burners are to work on, but, they seem to have some complex issues at times.  Any of your thoughts are appreciated.
Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
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  • icesailoricesailor Posts: 6,860Member ✭✭✭
    Questions & Answers:

    You've made some very good observations.

    I'm not the resident authority but I do know from experience that if you have multiple boilers and appliances venting into one or more (especially multiple) flues, very strange things can happen. One being what you describe. If opening the door to the boiler room causes the numbers to change, it pretty well says that there isn't enough draft/make up air.

    Those of us who have spent a lifetime pursuing the enjoyment of wind on a sail, know how little it takes to move you along.
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  • Tim McElwainTim McElwain Posts: 3,152Member ✭✭✭
    First and foremost

    I commend you for being interested enough to look beyond just the cleaning and try to find the cause.

    Second in looking over the attached manual there are several necessary installation requirements which must be met. In addition to making sure there is adequate air for combustion, venting and chimney size are also important. If sufficient air is available, and draft readings will indicate that in all scenarios. Yes distance from the air source for multiple boiler applications can cause the so called last boiler to be starved for air. Tandem connections of boilers for venting can also be an issue. I would go back and review this entire installation to make sure all the requirements have been satisfied. In the end it may be necessary to provide mechanical means of supplying all the air needed for combustion.

    It is not normal for boilers to be sooted up as you describe. there is definitely something wrong and keep in mind soot means carbon monoxide. You mentioned these being in some schools, I hope they have CO detectors installed and working. The school issue is more reason to pursue the investigation and find a solution.
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  • HenryHenry Posts: 472Member ✭✭
    Confined space

    Does the boiler room have combustion air and 10% ventilation air from the exterior? Does the combustion air duct end within 6 to18 inches above the floor as required by 8.4.2? We do not approve appliances over 400,000 BTU not having combustion air from outdoors! BTW, B149.1 takes precedence over the installation manual which dates from 2003. There is a new code slated for 2015. We have nearly completed the revisions and will vote on the final in June.

    If the boilers short cycle, there can be condensation forming on the heat exchanger and fouling. If the return temperature is below 130F, the same thing can happen.
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  • SpeyFitterSpeyFitter Posts: 420Member
    edited December 2013
    Code vs MFR's instructions

     Henry I'm assuming if you are not a Provincial Gas Inspector somewhere in Canada, you are obviously part of the industry somehow (mfr rep, instructor, etc.?) and well involved in the revised 2015 CSA Canadian Gas Code panel based on your response. Thank you for your input, I appreciate it.

    I just got off the phone with the local Provincial Gas Inspector and he told me that technically the gas code supercedes all MFR's instructions as more often then not what happens is an appliance is brought into Canada that has less strict requirements so the code would bring those standards up effectivelly (I've become so used to just following the code and knowing it's more strict that sometimes we take for granted the fact that it works the other way).  So the more strict code Canadian requirements would take precedence. He said in my case because  we are experiencing related issues, and the MFR's instructions are more strict than the code we should follow their instructions, on top of double checking all the other things (draft, vent sizing, etc.). 

    It's one thing to know the code, but it's another thing to understand the rationale for each code clause. Going further looking at the bigger picture, as a "First line responder" in the field, it is my responsibility to further and protect my career and the gas industry as a whole to ensure these appliances are running as safely as possible, without incident. If we start having CO poisoning injuries, or deathes, or explosions, the gaseous fuel burning industry will be viewed in a dangerous light. I take this responsibility very seriously.
    Post edited by SpeyFitter on
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
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This discussion has been closed.


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