backdrafting caused by exterior chimneys

**Kristi Silber** · 02-15-2012, 03:07 PM

I thought this was an interesting, if long-winded, read about the air movement effects of having an exterior masonry chimney, the kind attached to the side of a house. It suggests one effect could be backdrafting of gas appliances. Sounds plausible in theory - anyone notice a connection?

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**Gunnar Alquist** · 02-15-2012, 04:36 PM

Originally Posted by Kristi Silber

I thought this was an interesting, if long-winded, read about the air movement effects of having an exterior masonry chimney, the kind attached to the side of a house. It suggests one effect could be backdrafting of gas appliances. Sounds plausible in theory - anyone notice a connection?

Long-winded is right. I didn't get the connection with gas-fired appliances. Sounded to me like that would be if the gas vent pipe was run up the exterior wall. But, I kind of got bored and started skimming, so I could have missed something.

**Jerry McCarthy** · 02-15-2012, 05:23 PM

ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ this guy is full of hot air and backdrafting badly.

**Kristi Silber** · 02-15-2012, 05:33 PM

Sounded to me like that would be if the gas vent pipe was run up the exterior wa

ll

Yes, I think that's just what he's talking about. Exterior fireplace chimneys are outside the house envelope, and they get cold and draw heat from the air in the chimney. The air can get cool enough that the pressure gradient in the rest of the house pulls it back in. That's how I understand it, anyway. There's more to it than that, but there's my little oversimplification.

**Kristi Silber** · 02-15-2012, 05:40 PM

Jerry, does that just mean he's a bore, or he's wrong?

OK, here's a snippet for those who don't want to slog through the rest (understandably!)

"...Meanwhile, the house is at a stable temperature from top to bottom which is higher than the average temperature in the chimney now that it has cooled. The negative pressure low in the house due to stack effect is more powerful than the draft being developed in the chimney and the chimney backdrafts. ...cold outside air gushes down the chimney onto the hearth and into the low pressure zone caused by stack effect in the house....

...Here is the harsh reality: When you combine an outside chimney with an appliance installation below the neutral pressure plane of the house, the system will suffer the cold hearth syndrome during cold weather. Period. The result is just as certain for furnaces and water heaters, only it's not called the cold hearth syndrome, it's called a cold basement. "

**Bob Harper** · 02-15-2012, 05:43 PM

He never identifies himself under his web site's "about us" section.

Here's part of what he's drawing from: http://static.hpba.org/fileadmin/fac...oodburning.pdf

This pdf is NOT copyright protected on purpose. The HPBA deliberately wanted this document copied and shared as much as possible. Feel free to copy and share it with anyone.
Enjoy!

**Kristi Silber** · 02-15-2012, 06:01 PM

Thanks, Bob, that's very interesting. Maybe this kinda stuff is old news to HIs, but it's not something I've thought about before. Sometimes I smell combustion fumes in basements; I guess any number of factors could be at play.

Stephen G · 02-15-2012, 06:52 PM

Good read, what I think he wants you to take away from his verbage is, that you have to keep in mind the 'Home as a System'. Inside chimneys versus one, two, three sided and fully exposed to the outside chimneys. He offers a reminder to pay attention to internal draft makers ie: kitchen fans, clothes dryers and the like. They will steal your draft.
Keeping in mind what creates draft and what conditions inside and outside the home can remove it. Barometric pressure, wind and temperature...
My stove is in the basement, about 4 feet below grade, my stack is all outside, and about 25 feet of it. This is the worst possible arrangment you can get.