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Boiler in Finished Basement II (*quiz)


Jeff Remas
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You are inspecting a 20 year old home that is not considered to be unusually tight construction. The family has recently finished the basment and has framed around the utility area creating a separate, nice size utility/mechanical room of 12'x12' with 8' ceilings. The room is sealed from the rest of the basement. All combustion air comes from within the room. In the room is an electric hot water heater and a gas fired hot water boiler for the hot water baseboard. The boiler has a 60,000 BTU input rating and a 52,000 BTU output rating.

We now know that there is not enough combustion air available in the room. The boiler needs 3000 cu feet of combustion air and only has 1152.

The homeowner has decided to bring in fresh air from the outside for combustion air.

What size opening to the exterior is needed to accomplish this?

How can this be accomplished.

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Originally posted by MTL_Inspet_Man

A 6in diameter duct shoould do the trick

8in dia probably better

a duct with motorized damper should do it.

Am i right?

Nope, neither of the first two work and the third will work with either of the first two but still wouldn't be correct.Whoops, I was wrong; looked at the wrong reference. It sounds like he wants all combustion air from the outside and his reference to "opening" is singular so Rick's first answer doesn't work, his second does and I still wouldn't want to trust a damper and wouldn't see it as necessary.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by Jeff Remas

You are inspecting a 20 year old home that is not considered to be unusually tight construction. The family has recently finished the basment and has framed around the utility area creating a separate, nice size utility/mechanical room of 12'x12' with 8' ceilings. The room is sealed from the rest of the basement. All combustion air comes from within the room. In the room is an electric hot water heater and a gas fired hot water boiler for the hot water baseboard. The boiler has a 60,000 BTU input rating and a 52,000 BTU output rating.

We now know that there is not enough combustion air available in the room. The boiler needs 3000 cu feet of combustion air and only has 1152.

The homeowner has decided to bring in fresh air from the outside for combustion air.

What size opening to the exterior is needed to accomplish this?

How can this be accomplished.

There are several correct answers. They depend on which combination of the following factors the installer chooses to incorporate:

Engineered installation.

Mechanical installation.

Passive installation.

The two opening method.

The single opening method.

A ductless installation.

An installation using horizontal ducts.

An installation using vertical ducts.

Sole reliance on outdoor air.

Reliance on a percentage of indoor air plus outdoor air.

What type of grille he intends to put on the opening.

Given that the question includes the required air volume and the available air volume, and since you used the singular in referring to the opening size, I'm going to guess that you're pointing me toward a passive installation with a single opening that takes the volume of indoor air into account. Here's my guess, assuming that the opening could be located in the upper 12" of the space:

The ratio of indoor spaces is the available volume of air divided by the required volume is 1152/3000 = .384

The outdoor size reduction factor is 1 minus the ratio of indoor spaces: 1-.384= .616

For the single opening method, I need one square inch of opening per 3000 btu/h: 60,000/3,000 = 20 square inches

The full size opening times the reduction factor is: 20 X .616 = 12.32 square inches.

So, a 4" duct has 12.5 square inches and would work IF it were the same size or a larger size than the boiler's vent connector.

However I need some sort of grille or screen in front of the opening -- I can't have critters crawling in & out of the utility room -- so I choose 1/4" hardware cloth on the outside of the opening (no reduction in area is required) and an antique metal louvered grille on the inside. The grille doesn't have a net free area stamped on it. With this grille, I can only use 75% of its overall area.

The grille is 4" x 12" with an overall area of: 4x12 = 48 square inches.

The net free area is 48" x .75 = 36 square inches.

As long as I use a boot that transitions from 4" round to 4x12 rectangular, I'm fine.

Of course, since the grille has adjustible louvers I have to fix them in the open position. One minute with a mig welder fixes this problem.

So my answer is a 4" duct with a transition to a 4x12 metal grille.

But that's only one answer. There are several others that are also correct.

- Jim Katen, Oregon

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Originally posted by hausdok

Jim,

My reference says 1 sq. in. for every 4K Btu for a single exterior opening. However, in another place in the same reference it says 1 for 3.

Hmmm,

You must be using the Code Check HVAC that's based on the 2000 IRC. Figure 20 has a typo. It's supposed to read 1 square inch per 3,000 btu/h. In the Code Check '03 version, the drawing is now Figure 17 and it's been corrected.

- Jim Katen, Oregon

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Hi Jim,

Actually, I'm using Code Check Complete - Figure 40 versus Table 8. Perhaps Table 8 compensates for mesh reduction.

Nevermind, I was looking at the wrong thing; I should have been using Figure 39. It's 1 square inch per 3K Btu/hr as you've stated. Using that, Joe's 6-inch opening works.

OT - OF!!!

M.

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Originally posted by energy star

"There are several correct answers. They depend on which combination of the following factors the installer chooses to incorporate:

Engineered installation.

Mechanical installation.

Passive installation."

Could you explain the difference between each of these?

It doesn't matter; a home inspector has no way to know when he's looking at it whether an engineer was involved or not and the question says nothing about any mechanical ventilation so you're left with passive - a simple opening to the outside, which is what he asked about.
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Originally posted by energy star

"There are several correct answers. They depend on which combination of the following factors the installer chooses to incorporate:

Engineered installation.

Mechanical installation.

Passive installation."

Could you explain the difference between each of these?

The engineered installation is just what it says. You hire an engineer to run the numbers and give you a stamped drawing. It's invariably different from any of the provisions in the model codes.

A mechanical installation uses a fan to bring in outdoor air at the rate of at least .35 cubic feet per minute per 1,000 btu/h . There's also a provision to use the building's ventilation system to bring combustion and ventilation air into the mechanical room.

A passive installation is what we're all used to seeing -- basically a hole in the wall. The only trick becomes sizing it depending on which section of the code you feel like using.

All of this stuff is in section 2407 of the IRC.

- Jim Katen, Oregon

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Originally posted by hausdok

Jim,

My reference says 1 sq. in. for every 4K Btu for a single exterior opening. However, in another place in the same reference it says 1 for 3.

Hmmm,

OT - OF!!!

M.

It depends on whether you are using one opening or two openings. One opening being more restrictive.

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Originally posted by hausdok

I say Joe answered it correctly with his very first option - a 6-inch duct - 'cuz it's got an area of just over 28 square inches. Jeff?

OT - OF!!!

M.

Two 5" openings would also work, one within 12" of top and one within 12" of bottom of enclosure: if ducts are verticle.

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Originally posted by energy star

To be correct. We must know where he wants this combustion duct run. Vertical up through the roof? Or out the sidewall. Does he want one or two openings?

20 square inches, 12'' from ceiling, if he wants one opening.

Through the ceiling would also be acceptable if there is an open attic above.

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Jeez,

Why do you guys insist on complicating the question? He asked very simply

"What size opening to the exterior is needed to accomplish this?"

He didn't say openings, he didn't say opening locations, he didn't say mechanical ventilation and he didn't say anything about ducts.

All you have to do is answer the basic question; and then, if he wants to pose another question about size of multiple openings, ducts, etc., answer those.

It's a quiz, guys; you're not suppposed to reformat the questions or read into them. Just pick the simplest answer that can answer the basic question.

OT - OF!!!

M.

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See,

You are over-thinking it again. Imagine that you have never worked in the business and this question is put to you. All he wants to know is the size of the opening (minimum required) to obtain exterior combustion air for 60,000 Btu. It's that simple; if he wanted to make it more complicated he would have. Jim provided the answer above - it's 20 square inches and Joe was the first to correctly answer by stating that a 6-inch opening would be adequate because that has an area of just over 28 square inches, which is plenty large enough once you reduce it for any mesh.

This is why you, I, Jim and other experienced folks should stay off of quiz questions. We probably shouldn't even jump in until a day or two has gone by and nobody has correctly answered the question.

Quizzes are to help inexperienced folks learn to find and formulate answers. Questions that are not quizzes, which are what normal posts are, are entirely different - those are fair game to everyone.

We've been talking offline today about how to incorporate some sort of regular quiz feature into the board. Look for it in the future.

OT - OF!!!

M.

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OK time to put this one to bed. Simple question and simple answer.

What size opening (singular word btw)? 20 square inches for outdoor combustion air or 1 square inch per 3,000 BTUs.

How can this be accomplished? How about installing a grill with a net free area of 20 square inches installed within 12" of the top of the enclosure.

Don't read into the question.

There are several ways to accomplish this but the question was specific to what size opening as in one opening.

Jim K was able to nail it and give other useful and correct information first.

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