High efficiency furnace question

[Robert Jones]

New construction, HE furnace located in the garage. Shouldn't the lower pvc pipe be run to the exterior? It did have suction while running, which to me, would be a no go due to the garage/carbon monoxide etc... Am I off base?

Sorry for the sideways pic, was in a hurry.

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[hausdok]

Hi Rob,

It would be nice if it were getting its air from outside but there's no requirement for that. It would be interesting to see what would happen with a car that had a fuel line leak, though.

That system is equipped with a sensor that will shut it down it if backdrafts and there's no way for any fumes from the garage to get into the air of the house. You just have to make sure that the makeup air rules for that furnace are met so that it can combust cleanly without struggling for air. If it doesn't meet the makeup rules, write it up.

ONE TEAM - ONE FIGHT!!!

Mike

[Terence McCann]

CO is not the problem - enough combustion air is however.

You have a hot water tank and a gas fired furnace in the garage and both of them are pulling combustion air from the garage. Unless it is one huge garage or they have brought in makeup air from someplace you will be short on air.

Did you notice makeup air anywhere?

On a side note they can pipe combustion air, for the furnace, in from the outside which you already knew. When I installed my new 95% furnace this past spring I did not pipe in combustion air from the outside. Something about bringing in 0 degree air into a hot furnace didn't sit right.

[Richard Moore]

http://www.docs.hvacpartners.com/idc/gr ... m05-56.pdf

See page 4, figure 2. I really don't know why someone would prefer to install it that way over the 2 pipe system, but is is allowable as long as there is sufficient airflow for the combustion air.

I wouldn't worry about CO from a car in the garage being used for combustion.

It would be interesting to see what would happen with a car that had a fuel line leak, though.

Why would it be any more of a concern than an regular 80+ furnace in a garage?

[Robert Jones]

Thanks guys.

There were 2 cold air returns inside the home, one on each level, so I am sure it is getting enough air. I just usually see it installed with both pipes to the exterior.

Thanks again.

[hausdok]

Hi Rob,

When you say "cold air returns inside the home" are you referring to the air that's going back to the furnace to be re-heated or are you referring to a fresh air intake source at the garage that's letting fresh air into the garage to meet the makeup air for combustion requirement?

OT - OF!!!

M.

[Robert Jones]

Hey Mike,

Back to the furnace to be re-heated. I did not see any vents in the garage to let in fresh air. I can see I need to do a little research on these HE units. What caught my eye was the fact the lower pipe was not connected to the exterior, and as Terry mentioned, there is a gas water heater right there as well. I have added another pic, but it basically shows the same thing.

[Terence McCann]

Hey Mike,

Back to the furnace to be re-heated. I did not see any vents in the garage to let in fresh air. I can see I need to do a little research on these HE units. What caught my eye was the fact the lower pipe was not connected to the exterior, and as Terry mentioned, there is a gas water heater right there as well. I have added another pic, but it basically shows the same thing.

Gas fired equipment needs adequate air for proper combustion (remove oxygen and the fire goes out or burns badly). Typically you would see two 6" sheet metal ducts entering the area. The requirements are 50 cubic feet for 1,000 BTUs. Some measurements and a little math will clear it up.

Good catch though....

[hausdok]

Hi,

Terry, the 50 cf per 1,000 Btu's applies for indoor combustion air. If there are two vents to the exterior of the garage as you describe, then they need two net-free openings totaling 1sq. in. per 4,000 Btu (2407.6.1). If there is only one opening - which is the way it normally is around here - it needs to be within a foot of the ceiling, and the net free opening has to be not less than 1sq. in. per 3,000 Btu and has to be larger than the cross sectional area of the vent connectors (2407.6.2).

ONE TEAM - ONE FIGHT!!!

Mike

[MechAcc]

350MAV Installation Instructions

on page 22 left column at the bottom it states that the vent pipe and combustion air must terminate in the same atmosphere zone.

Users Manual

On page 4 left column of the User's Manual the following statement is included.

"Your gas furnace uses air from

outside the home for combustion

and vents flue gas to the outdoors. It is

not to be installed using in-house air.

The vent and air-intake pipes must terminate

outside the structure and must

not be obstructed in any way. Do not

block or obstruct air openings on furnace

or spaces around the furnace."

The combustion air pipe, according to the installation instruction for this furnace MUST be installed outside next to the vent pipe. Period.

The furnace is incorrectly installed.

[Robert Jones]

Thanks guys for the responses. So basically, there needs to be a net free opening in this garage within a foot of the ceiling. Does that sound about right? The water heater alone was 40k Btu.

[hausdok]

350MAV Installation Instructions

on page 22 left column at the bottom it states that the vent pipe and combustion air must terminate in the same atmosphere zone.

Users Manual

On page 4 left column of the User's Manual the following statement is included.

"Your gas furnace uses air from

outside the home for combustion

and vents flue gas to the outdoors. It is

not to be installed using in-house air.

The vent and air-intake pipes must terminate

outside the structure and must

not be obstructed in any way. Do not

block or obstruct air openings on furnace

or spaces around the furnace."

The combustion air pipe, according to the installation instruction for this furnace MUST be installed outside next to the vent pipe. Period.

The furnace is incorrectly installed.

I think you'd be right if it were a 350MAV furnace but I don't think it is. Builders around here usually go with a less expensive model. I think it's either a 340AAV, 350AAV, 352AAV, 353AAV or 355AAV series and those can be configured with either an outside intake or an inside intake.

From the owner's manual:

"Your new gas furnace may have been installed in 1 of 2 ways, as

a direct--vent (2--pipe--Fig. 1) application or as a non--direct vent

(1--pipe--Fig. 2) application.

In a direct--vent (2--pipe) application, your furnace uses air from

outside the home for combustion and vents flue gas to the

outdoors. This type of application will have 2 pipes running

from the furnace to the outdoors. (See Fig. 1.) In this

application, the vent and air--intake pipes must terminate outside

the structure and must not be obstructed in any way. Do not

block or obstruct air openings on furnace or spaces around

furnace.

In a non--direct vent (1--pipe) application, your furnace uses air

from adjacent to the furnace for combustion and vents flue gas

to the outdoors. This type of application will have only 1 pipe

running from the furnace to the outdoors. (See Fig. 2.) The other

pipe will terminate in the same space as the furnace and is the

source of combustion air for your furnace. Therefore, the furnace

must not be enclosed in an airtight room or be sealed behind

solid doors. It must have adequate airflow for efficient

combustion and safe ventilation. Do not obstruct the

combustion--air pipe in any way. The vent pipe must terminate

outside the structure and must not be obstructed in any way. Do

not block or obstruct air openings or space around furnace."

Rob,

Use Terry's method to determine whether you've got enough volume in that garage to meet the 50 cu. ft. per 1000 btu rule. If not, figure out what size intake opening you need through the wall of the garage to the exterior to meet the requirements for either one or two openings.

Take the sum of the Btu/Hr input of both appliances, knock off the last 3 digits, divide by 3 and you'll have the size of the opening you need. Assume a loss of 25% net-free area for wire mesh, so you'll need to increase that by 1/3 to get your minimum allowable opening.

Here's an example.

Assume a 120,000 Btu/Hr furnace plus a 45,000 Btu/Hr water heater = 165,000 Total Btu hr.

165 divided by 3 = 55 sq. inches

Assuming 75% net free for metal mesh you need a round opening of not less than 73.35 square inches. They don't make them in odd-sizes, so you'd have to use a 10 inch inlet.

Or you can go with two openings.

165 divided by 4 = 41.25 sq. inches

Assuming 75% net free for metal mesh, you need an opening of not less than 56 square inches. Divide that by two is two 28 square inch openings. Since a 6 inch duct has a cross-sectional area of 28 square inches you'd need two six inch ducts - one within a foot of the floor and one within a foot of the ceiling.

WARNING: DO NOT TRUST MY MATH - DO YOUR OWN; I'M THE WORLD'S BIGGEST MATH MORON.

ONE TEAM - ONE FIGHT!!!

Mike

[Terence McCann]

WARNING: DO NOT TRUST MY MATH - DO YOUR OWN; I'M THE WORLD'S BIGGEST MATH MORON.

ONE TEAM - ONE FIGHT!!!

Mike

Right behind you.

[hausdok]

Very kewl new avatar, Terry.

OT - OF!!!

M.

[Robert Jones]

Mike,

You were on the money concerning the model. 340AAV.

Thanks for all of the help.

[MechAcc]

The users manual applies to the 340MAV

[hausdok]

The users manual applies to the 340MAV

No, it actually applies to the 340MAV, 350MAV, 352MAV, and

355MAV models.

It's academic at this point; it's a 340AAV and can therefore be configured with the intake inside the structure as long as the garage is of sufficient volume or has appropriately-sized intake openings directly from the outside. Most HVAC contractors around here install a single large inlet hood on the exterior wall but a few will use the other formula and install one vent through the wall and one vent with a duct to near the floor.

ONE TEAM - ONE FIGHT!!!

Mike

[Robert Jones]

Hey Gary,

Thanks for the responses. What are the main differeb=nces between the 340MAV and the 340AAV?

[Jim Katen]

The users manual applies to the 340MAV

The user's manual and the installation manual contain conflicting information. I'd tend to trust the installation manual with regard to this particular issue.

As for combustion air issues, I've yet to see a garage in my area that didn't provide adequate combustion air. I don't know about other regions, but around here, we have large overhead doors on our garages with gaps large enough to throw a cat through. Installing combustion air vents in a garage here would be like installing vents in a screen door.

- Jim Katen, Oregon

[Terence McCann]

The users manual applies to the 340MAV

The user's manual and the installation manual contain conflicting information. I'd tend to trust the installation manual with regard to this particular issue.

As for combustion air issues, I've yet to see a garage in my area that didn't provide adequate combustion air. I don't know about other regions, but around here, we have large overhead doors on our garages with gaps large enough to throw a cat through. Installing combustion air vents in a garage here would be like installing vents in a screen door.

- Jim Katen, Oregon

On an older home maybe but with new construction their pretty tight (in this neck of the woods).

[Brandon Whitmore]

I'm in the process of installing a 95+ in my house right now. Here's what my furnace manufacturer says to pay attention to in regards to where the combustion air should come from:

The high-efficiency combustion process allows venting with 2 or 3 inch PVC without the need for a traditional chimney flue. And because it can be direct-vented to the outside fresh air can be used for combustion. This is helpful if the air in the vicinity of the furnace is corrosive, such as around paint or chemical storage, and will protect the heat exchanger from the harmful effects.

[Jim Katen]

I'm in the process of installing a 95+ in my house right now. Here's what my furnace manufacturer says to pay attention to in regards to where the combustion air should come from:

The high-efficiency combustion process allows venting with 2 or 3 inch PVC without the need for a traditional chimney flue. And because it can be direct-vented to the outside fresh air can be used for combustion. This is helpful if the air in the vicinity of the furnace is corrosive, such as around paint or chemical storage, and will protect the heat exchanger from the harmful effects.

Unless the air from outside is corrosive too.

- Jim Katen, Oregon

[Brandon Whitmore]

Unless the air from outside is corrosive too.

Maybe Oregon has leakier garages because we can actually use the outside air...[]

[Denray]

Installation manuals alway say keep clear of laundry chemicals:

The following types of furnace installations may require OUTDOOR

AIR for combustion due to chemical exposures:

• Commercial buildings

• Buildings with indoor pools

• Laundry rooms

Sometimes the laundry equipment is nearby in garages?

[Brandon Whitmore]

Sometimes the laundry equipment is nearby in garages?

Thanks for that. Quite a few of the pre- 90's homes in this area have the laundry areas set up in the garage.

Who knows what else people will be storing in a garage as far as chemicals go. Just to be safe, I will just recommend pulling combustion air from the exterior when a 90+ is installed in a garage.