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Cat IV Combustion Make-Up Air


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Is it true that for a high efficiency furnace to make the most of its efficiency rating, combustion air needs to be drawn from the outside near where it vents? And not from inside as I see often.

I was taught this in a class and it makes sense but have never questioned it.

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Makes sense to me,

Inside, the volume of the space where that air is coming from can be reduced by bringing in autos or storing property. People can do dumb stuff like block any air inlet into the interior because they're bothered by the cold air. Inside, pollutants in the air from chemicals or cleaning compounds can get sucked into the unit. Inside, the temperature around the unit can fluctuate greatly as can the moisture content in the air. Clothes dryers, interior exhaust fans, fireplaces all can effect it too.

Most of that stuff doesn't happen outside, or if it does it doesn't happen as radically as it does in an indoor air environment.

ONE TEAM - ONE FIGHT!!!

Mike

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It's pretty common around here. It's allowable, but it's one of the cheesiest things you can do; it costs next to nothing to run the combustion air to the exterior, and with concentric venting heads, you only need one hole in the wall.

If the heating contractor is present when I see it, I make the point in front of his GC or his customer; they're ripping everyone off, they know it, and think they can get away with it.

I write it in my reports all the time.

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It's pretty common around here. It's allowable, but it's one of the cheesiest things you can do; it costs next to nothing to run the combustion air to the exterior, and with concentric venting heads, you only need one hole in the wall.

If the heating contractor is present when I see it, I make the point in front of his GC or his customer; they're ripping everyone off, they know it, and think they can get away with it.

I write it in my reports all the time.

I hear you and I agree, but since it's allowed, why write it up? Do you do it simply for the buyer's benefit, in case they want to improve the configuration in the future?

I'm not being contentious. I'm merely wondering if I should say something about it myself. Currently, I never do.

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Is it true that for a high efficiency furnace to make the most of its efficiency rating, combustion air needs to be drawn from the outside near where it vents? And not from inside as I see often.

I was taught this in a class and it makes sense but have never questioned it.

I can't imagine how it would effect the efficiency of the furnace. However, I can see how it would effect the efficiency of the house.

- Jim Katen, Oregon

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So ask yourself this . . . the intake and flue pipes can't be located in different atmospheric zones--i.e. on opposite sides of a house. I wonder why the manufacturers are okay with interior/exterior intake/flue configurations?

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So ask yourself this . . . the intake and flue pipes can't be located in different atmospheric zones--i.e. on opposite sides of a house. I wonder why the manufacturers are okay with interior/exterior intake/flue configurations?

Because it's not a problem?

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So ask yourself this . . . the intake and flue pipes can't be located in different atmospheric zones--i.e. on opposite sides of a house. I wonder why the manufacturers are okay with interior/exterior intake/flue configurations?

Because it's not a problem?

Indeed, but what kind of bizarre confluence of conditions would have to occur to affect intake- and flue-pipes on opposite sides of a house? I've angered more than a few people with finished basements when I busted them on the "atmospheric zone" issue.

I get the logic of the rule, mind you, but it really doesn't make a lot of sense to me.

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So ask yourself this . . . the intake and flue pipes can't be located in different atmospheric zones--i.e. on opposite sides of a house. I wonder why the manufacturers are okay with interior/exterior intake/flue configurations?

Because it's not a problem?

Indeed, but what kind of bizarre confluence of conditions would have to occur to affect intake- and flue-pipes on opposite sides of a house? I've angered more than a few people with finished basements when I busted them on the "atmospheric zone" issue.

I get the logic of the rule, mind you, but it really doesn't make a lot of sense to me.

Just speculating:

When the wind is blowing, one side of a house might be subjected to high pressure and the opposite (lee) side, low pressure. If you had high pressure on the exhaust side and low pressure on the intake side, the two might conspire to overcome the draft fan and cause the furnace to shut down.

- Jim Katen, Oregon

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So ask yourself this . . . the intake and flue pipes can't be located in different atmospheric zones--i.e. on opposite sides of a house. I wonder why the manufacturers are okay with interior/exterior intake/flue configurations?

Because it's not a problem?

Indeed, but what kind of bizarre confluence of conditions would have to occur to affect intake- and flue-pipes on opposite sides of a house? I've angered more than a few people with finished basements when I busted them on the "atmospheric zone" issue.

I get the logic of the rule, mind you, but it really doesn't make a lot of sense to me.

Just speculating:

When the wind is blowing, one side of a house might be subjected to high pressure and the opposite (lee) side, low pressure. If you had high pressure on the exhaust side and low pressure on the intake side, the two might conspire to overcome the draft fan and cause the furnace to shut down.

- Jim Katen, Oregon

Yeah, but it would take a lot of air swooshing into that flue pipe and wicked low pressure on the lee side to cause the draft inducer to have a hiccup. Like I said, I understand the concept, I just can't imagine there ever being a problem in real time unless there were a tornado, hurricane, or something. And then, pretty much the last thing on my mind would be whether the furnace was smoothly purring along.

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I'll provide a little clarity.

Building something to code is the crappiest way you're legally allowed to build. (Attribution to Indagator Bill, may he always be at peace in the Glory of his God.)

It's sucking air, heated air that someone paid for, out of the house. It's one of the many things creating a pressure imbalance that's drawing cold air into the house.

It costs almost nothing (compared to the total price of a Cat IV install) to do the job the best way it can be done.

When I explain it in those terms to my customers, I've never had anyone want it otherwise. I've never had anyone that didn't marvel @ how cheesey the heating guy was in cutting this little corner.

Everyone can write it or ignore it; I don't care. I write it though.

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Thanks all. I will contact a manufacturer or 2 and see what their take is on this, i.e., does the furnace efficiency rating depend on intake air coming from the outside, i.e., does it matter?

My guess is that is does matter.

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So ask yourself this . . . the intake and flue pipes can't be located in different atmospheric zones--i.e. on opposite sides of a house. I wonder why the manufacturers are okay with interior/exterior intake/flue configurations?

At least one manufacturer allows intake and exhaust at different locations.

From page 28 of Viessman - Vitodens_100 Boiler

The exhaust vent and combustion air intake system and terminations may be installed in one of the following type terminations (2-pipe system):

1. Horizontal air intake and exhaust vent pipes

2. Vertical air intake and exhaust vent pipes

3. Horizontal air intake pipe and vertical exhaust vent pipe

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I spoke with Walt, a tech from Carrier and he says some models require the 2 pipe set up and some don’t. It depends upon the model. The ones that can go one pipe, i.e., get combustion air from inside do lose some efficiency but it’s minimal, about 2%.

I was told that my own Cat IV Carrier is required to be 2 pipe unless it is installed in an attic or crawl space.

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