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Combustion air for a large boiler


Bain
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I looked at a 960,000 BTU boiler--that conditioned an eight-unit condo--today that was seriously deprived of combustion air. There were also three gas water heaters in the same 20' x 20' partitioned portion of the basement to add another 100,000+ BTUs to the equation.

The only opening in the foundation was for a door, which had a ridiculously tiny make-up air vent in it. For you larger-city folks who see this more often than I do, what's the typical remedy? Logic suggests the exterior door could be removed and replaced with some sort of metal mesh, but that would create a slew of other problems when temps dropped below freezing.

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All good ideas. I was just wondering if there was an obvious fix I wasn't aware of that didn't entail ripping into the fieldstone foundation wall.

The boiler room was segregated from the rest of the basement--somewhat obviously--because the original boiler was coal fired.

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Looks like a boarded up window to the right of the door. Why not a vent there?

I'm with Rick. Vent through the window with a sign that says don't close this off. It's important.

Again, I was concerned with how the cold air could affect the plumbing pipes in the mechanicals room. All the ideas are good ones, and there was, indeed, a 4,000 square foot basement that could be used for make-up air like Bill suggested.

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There is more than enough door and boarded window to vent without going through the stone foundation. I would put two vents, one high and one low.

I put in a set up like Mike is describing, for a 500,000K boiler, it worked fine. The vents only opened when the system was firing. Grainger sells the motorized vent covers in a variety of sizes. They really aren't expensive.

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I don't think that freezing would be an issue with vents that are open all of the time.

I've worked in three buildings that had very large boilers in tightly sealed boiler rooms. All three had open vents. One was just a grill with mesh over the opening while the other two had short ducts that turned down and were open at the bottom. None of the rooms got even close to freezing in 0 degree weather.

The colder it is outside, the longer a boiler is going to firing, putting more heat into the boiler room. I'm not sure of the science behind turning the ducts down, but I'm guessing that warm air in the boiler room rising up into the duct helps counteract the cold air that's descending.

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You need what Steven T said; a high and a low vent. That allows convection air flow so you don't get stacking in the room. You don't have to set up actuators or dampers; just have free open air vents.

Set up a "trapped" inlet, where the duct extends in and down, to reduce hot air flowing out of the vent. That set up creates a sort of "dam", and the room will stay warm enough so pipes don't freeze. I see this in Chicago all the time, and our weather is a lot colder than Kentucky's.

Doesn't matter what has to be torn up, foundation or otherwise. You gotta have combustion air. Open up the wall, window, door, whatever.

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The high-low grills work well if the appliance is adjacent to an inside wall such as a warehouse or room sufficiently large enough to draw from.

If you are drawing cold outdoor air then a duct is needed, we call it a combustion air duct.

A bit of history.

As buildings became more energy efficient we soon realized we couldn’t rely on air leakage to supply our gas burning appliances. The problem was solved by providing a duct connected to the outdoors and terminating inside the furnace room usually attached to the side of the furnace cabinet.

Here is the first generation hand insulated hard pipe combustion air duct.

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It satisfied the appliances but made folks grumpy. Homeowners didn’t like all that cold air dumping in their basements so they stuffed the pipe with old clothing and insulation. A solution that seemed to work well was to place the end of the pipe in a 5gal plastic bucket. That would ‘trap’ the cold air and keep folks happy, the development of the termination boot was born. Needless to say we never patented the idea.[:-weepn]

Fancy gizmos were also developed such as the interlocked motorized damper termination boot, but they soon fell out of favor for the cheaper and trouble free modified paint bucket idea.

Whatever you recommend, you need to get outside air inside somehow.

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I agree that motorized louvres may not be necessary. Insulate the pipes, and keep an eye on them during exceptional cold periods.

The system I installed it on only operated during business hours and besides freezing during off hours, maintaining the heat was a concern.

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