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: www.athproducts.com/modules/lit_lib/download.asp?litFileID=1518
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