Combustion air and exhaust gas

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Old 01-03-13, 07:32 PM
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Combustion air and exhaust gas

Today's question is in two parts. When the exhaust from a high efficiency boiler vented out a sidewall hits outside air does it always naturally rise? Would a boiler with sealed combustion air reduce house air infiltration from the outside compared to an older open combustion boiler. Sometimes I think about stuff too much. Thanks.
 
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Old 01-03-13, 09:25 PM
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When the exhaust from a high efficiency boiler vented out a sidewall hits outside air does it always naturally rise?
If it's warmer than the surrounding air. There are many codes and rules as to where and how you can vent them.

Would a boiler with sealed combustion air reduce house air infiltration from the outside compared to an older open combustion boiler.
Yes, absolutely! Combustion air must come from somewhere, right? Take it from the room and the pressure in the room drops slightly which pulls air from any crack or crevice it can find.

Sometimes I think about stuff too much.
I don't think so!
 
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Old 01-04-13, 12:03 AM
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Hi gundi, I'm smiling as I often "think about stuff too much", as well. This will feed your curiosity.

Hot air cannot rise by itself. It is merely a lower density gas (as a result of the warming) occupying a space. Since we are calling it hot, it is usually surrounded by a colder/heavier gas which pushes under it and forces it up just like water floats a boat or a cork gets pushed to the surface, buoyancy. If 200 air, which we would think of as hot, were surrounded by 300 air, it would sink, not rise.

One of today's energy gurus, Dr Straube, speaking at a conference of experts recently blurted out, "Hot air does not actually Rise" and set the forums a-buzz with that revelation. I have his picture with that quote posted over my desk as this is a battle I have been fighting for well over a year now. So you definitely are not the only one who over-thinks a subject . And this is just the beginning. Try explaining to a HVAC contractor that the hot air going up that chimney is being pushed by the cold combustion air entering down low, assuming no fans.

Back to sleep, it's 2:00 am here .
Good morning NJ

Bud
 

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Old 01-04-13, 03:29 PM
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Evenin' Bud!

I like to think that the hot gases 'float', like a balloon without a skin.
 
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Old 01-05-13, 08:27 AM
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I've also been thinking about this subject lately. Someone mentioned to me that he noticed his house has been draftier since he fixed up all the drafts that were making his basement cold, which he noticed after he insulated all his heating pipes. I explained that, by making his basement air-tight he was forcing the combustion air to enter through the house and find its way to the basement. He said he was going to find and fix all those drafts too, and I told him he'd better do something to let in combustion air before he makes his whole house air-tight or the exhaust gases won't be able to rise up the flue.

Coincidentally, I'm installing an automatic vent damper on my atmospheric steam boiler, and last night it occurred to me that, if you wanted to make your house as draft-free as possible, you could run an air duct into your boiler room, put a vent damper on it, and wire it into your boiler controls just like the one on the flue, so both would open when the burner cuts in and close when it cuts out.

But I always get suspicious when something seems like a good idea but I've never actually seen it done before, so I wanted to get some informed feedback before I suggest this to anybody.
 
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Old 01-05-13, 12:37 PM
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Gas

Bruno
It's done all the time in comercial, and government buildings.
Sid
 
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Old 01-05-13, 04:16 PM
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Google "fan in a can" for one solution.

Also, it is possible to use a 'passive' system.

Run a duct in from outside, over near the boiler, then run the duct DOWN the wall, do a 180 and run it back UP the wall again, forming a 'heat trap'. The cold air will 'pool' in the trap and will only flow when there is a combustion device demanding air.
 
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Old 01-13-13, 11:53 AM
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That passive approach sounds great. It didn't even occur to me that, if everything else is tight, the vent damper should also prevent fresh air from coming in, so the second damper isn't necessary. I'm wondering if the heat trap is even needed.

One thing I need to be careful of is what's known as "spillover." My boiler has an open vent hood, like most atmospherics, so anything that impedes the flow of flue gasses up the stack results in it filling the vent hood until it overflows at the bottom. The hot gasses need to push a column of cold air out the top to establish a draft, so it has a bit of initial resistance to overcome, and if the heat trap creates resistance to fresh air coming in it has a little more work to do. But it's pretty easy to detect spillover, and I can always adjust the height of the duct after the "U" as needed.

Thanks for giving me some new ideas to play with.

Just one more thing. I assume the fresh air pipe doesn't need to be as big as the flue pipe since the hot air occupies a lot more space than cold air. Is there a sort of rule of thumb for the relationship between fresh air and exhaust duct sizing?
 
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Old 01-13-13, 12:27 PM
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Hi bruno,
I try not to jump in on these issue of cold traps and hot air pushing up the chimney, but it sound like you might want the input.

The cold air trap does not act like a plumbing trap as the vertical duct from outside, once filled with cold air will simply continue to supply more cold air. Replace the duct with some pvc and fill the trap with water. Then start pouring more water in from the outside. It will act more like a siphon than a trap. The one plus to the 180 bend at the bottom is it directs the air up where and mixes with the room air and thus does not feel as cold. Search "Saskatoon loop" or try the GBA link.
How to Provide Makeup Air for a Wood Stove | GreenBuildingAdvisor.com

If you are concerned about spillage, you need to do more reading. Spillage will initially occur when the combustion gas is being pushed by the colder air inside the utility room from one direction and from the cold air in the chimney from the other. Hot air has no ability to rise by itself. Gravity is pulling both hot and cold air down, it is just that the cold air is heavier and thus pushes the hot air our of its way, usually up if the path is available. Until that chimney is forced full of lighter warm air, spillage will occur. But the worst case scenario is when every exhaust appliance is also blowing inside air out. Then your combustion appliance will be competing with the others and the spillage will be at its worst. If you are concerned during normal operation, worst case may be a bigger problem.

Bud
 
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