venting gas furnace in attic

[bman03]

Recently moved into home in Eastern MA(Zone 5?) and have a 2 story 3K sqft home. There are 3 gas furnaces and AC units. One of the furnaces is in the attic. The roof is a "T" shaped Gable roof. The "top" of the T has soffit vents and a ridge vent. The "stem" of the T has soffit vents and *no* vent at the top. The furnace is located in the stem. The top of the stem roof has a flat roof top. What's concerning is that there is no venting of the attic area where the furnace is. The furnace itself has a vent stack; label reads "American Standard Freedom 80 Single Stage". I don't see an intake pipe so I'm assuming it's not a sealed burner. Because it's not a sealed burner and I can't foam the underside of my roof deck, creating a "conditioned space", and I need to vent the stem attic. Roofer thinks a passive vent or a (side)gable vent is enough. I think the furnace is creating more heat than a passive vent can handle. What options do I have to vent this attic properly?
Top Attic measurements: 30' x 25'
Stem Attic measurements: 24' x 34'

Thanks

[Shophound]

Wait...the exhaust (flue, products of combustion) of this furnace is discharging directly into the attic and not through a vent opening on the roof itself?

Or is it combustion air (air provided to the burners) that you're concerned about?

If you have combustion gases blowing into the attic, that's terrible, unsafe, a carbon monoxide risk, a condensation risk to your structure, and so forth.

[bman03]

I apologize if it's unclear in my post...

"Fresh" combustion air for the furnace is coming from the soffit vents from both roofs. The furnace has a stack which penetrates the flat roof and is about 3'-4' above the roof plane. However the furnace is in an attic space which has no venting for warm, humid air (from house, from soffits, from hot roof, etc.) to escape. My concern is that the heat from the furnace itself and (assumed) leaking ductwork increases the amount of heated air in this space. The ductwork is wrapped in insulation wrap but, my reading makes me believe there's air leakage.

[ga-hvac-tech]

As I read...

You have an 80% furnace that has a metal flue vent, yet uses attic air for combustion.

If you want to foam your attic (bottom of the roof), which IMO is a good idea for colder climates...
You really have only one choice: Get a heating and AC contractor to change the furnace to a 'direct venting' condensing furnace.
Instead of a metal flue vent, you will see PVC pipes going to the roof. Depending on whether your contractor chooses separate roof penetrations or a 'combo' roof vent... you will have one or two PVC pipes going through the roof.
A professional roofer doing the penetrations would be a good idea in a snow laden climate.

The condensing furnaces come in various efficiency ratings 90%, 93%, 95%, and 96-98% high end furnaces... this means if you put 100 BTU's of heat energy in fuel into the furnace, you get xx% of the heat energy into your home (the rest goes up the flue). Currently, you have an 80% efficient furnace, or 20% of the heat is going up the stack.
We suggest minimum a 96% VS (variable speed drive) furnace, although a modulating furnace would be nice in your climate.

[vstech]

unless you are sealing the attic space with foam like GA mentions... I agree with the roofer...

combustion air will easily be achieved with gable venting.

and the heat from the cabinet of the furnace is inconsequential compared to the solar gain.

duct leakage is an issue, not for the attic, but for the living space. leaking ductwork places the living space in a negative pressure situation that will draw unconditioned air from the outside, depleting the effectiveness of insulation, and lowering the comfort in the house.

[vstech]

and, if the roofer is installing a ridge vent, but not opening up the soffits... he's a moron.

[bman03]

The ridge vent, soffit vents, location of the furnace, and exhaust stack are the existing state of the attic(s).
My concern is with the hot, humid air in the "stem" portion of the roof. I can't afford to change the furnace and foam the underside of the roof deck. This past(first time in house) Winter we had taller ice dams on the roof where the furnace is located, I'm assuming it's because the hot air had no place to escape. I'm also assuming putting a vent in that area of the attic\roof will make things better not worse. So the roofer and I are discussing putting a powered gable vent vs. a passive gable vent. Opinions?

BTW: Thanks for responding so quickly.

[vstech]

http://www.thisoldhouse.com/toh/arti...131346,00.html

from my understanding of ice dams, heat buildup is the issue, causing melt to refreeze as it drips down the roof surface.

keep the roof cold, and you're golden. VENTILATION is the key for this. how close to the furnace is the attic access stairs? THAT is the biggest heat gain for the attic...

also...
"3. Exhaust to the Outside
Make sure that the ducts connected to the kitchen, bathroom, and dryer vents all lead outdoors through either the roof or walls, but never through the soffit. "

[pecmsg]

Foamed roof joist will not prevent ice damming alone, the attic must be vented to keep the air temperature below freezing

[bman03]

The pull-down stairs is about about 1'-3' from the furnace. I put an attic stair cover in this summer (5 pieces of foamboard. Last fall I doubled up the batt insulation. There was an R21(faced) between the joists and put down unfaced R30 perpendicular. None of the vents are through the soffit.
My *fear* with a powered gable fan is that it would draw burned gasses from the furnace vent stack back into the attic.

[hvacvegas]

The pull-down stairs is about about 1'-3' from the furnace. I put an attic stair cover in this summer (5 pieces of foamboard. Last fall I doubled up the batt insulation. There was an R21(faced) between the joists and put down unfaced R30 perpendicular. None of the vents are through the soffit.
My *fear* with a powered gable fan is that it would draw burned gasses from the furnace vent stack back into the attic.

It's possible.

You need to pick 1 of 2 options.

1. Seal the space, foam everything, get a direct vent, high efficiency furnace.

2. Have a unsealed attic, pipe in combustion air.

Thats really about it.

either way, your going to need to run some piping.

So, the real question is, do you want to be energy efficient, or not?

[bman03]

I'd prefer to buy a new furnance and foam the attic. Heck, I'd prefer to move the furnace from the attic to the basement and foam the attic anyway. In both cases I don't have the money to afford the renovation.
There's soffit vents along two sides of the roof which are open to the attic. Isn't that enough combustion air for the furnace?

[hearthman]

You seem mixed up. Do you want to ventilate an unconditioned attic space or condition the attic space? Regardless, check with your local codes. For one, you will probably have to provide a second means of egress and both will have to meet the code requirements for stairs and railings.

You can install deep baffles in the rafter bays then spray foam over them and that will ventilate the roof deck (except where there's framing) but you will need to cover the foam with drywall or other approved interior wall sheathing.

An 80% furnace in an unconditioned space must have B-vent all the way from the appliance collar-out. With a foamed roof deck, you would need to box out a chase, install a listed firestop/ support that does not have any insulation inside it then the listed roof flashing, storm collar and rain cap or extend the chase up through the roof to look like a chimney. If you install a CAT IV condensing furnace in an unconditioned space you will have problems managing the frozen condensate. However, if you condition the space the CAT IV is the way to go. To better understand why you would ventilate a roof, check out Dr. Joe Lstiburek at BuildingScience dot com.

[bman03]

Based on the fact I cannot afford to make my attic a conditioned space, I'll have to leave the attic unconditioned.
That part of the attic has no passive ventilation for the soffit vents, no ridge vent, no roof vents, no gable vents. Add to that, there is an active heating system - gas furnace. What kind of ventilation should I use to exhaust the warm\hot, humid air? Powered or passive?

[jtrammel]

Passive vents, commonly referred to as "turtle vents". Powered attic ventilator's will make the problem worse because they pull conditioned air out if your house through cracks in the attic floor, thus creating a warmer attic which makes ice damming worse.
http://bradyroofing.com/ridge-vents-...-right-for-me/

[hvacvegas]

An 80% furnace in an unconditioned space must have B-vent all the way from the appliance collar-out.

Could you provide the code on that sir?
I need it .

[jtrammel]

Could you provide the code on that sir?
I need it .

International fuel gas code book. Don't remember the section or article... I had to show my dad because he didn't believe me.

[Shophound]

Passive vents, commonly referred to as "turtle vents". Powered attic ventilator's will make the problem worse because they pull conditioned air out if your house through cracks in the attic floor, thus creating a warmer attic which makes ice damming worse.
http://bradyroofing.com/ridge-vents-...-right-for-me/

Precisely.

Our OP needs to have house to attic air leakage assessed by an energy auditor or home performance contractor well versed in building envelope science. The furnace itself could contribute to an ice dam situation being that it not only warms the air around it in the attic, it gives off radiant heat from itself and flue piping, which will warm nearby roof decking. That said, how much the furnace might affect warming of surrounding surfaces would depend on how warm the attic is...if the deck is losing heat to the sky faster than the furnace and house air leakage can keep it warm enough to cause ice clinging to the weather side of the roof deck to melt (beneath the snowpack) then ice dam formation is reduced.

OP will want to address house to attic air leakage no matter what. It is a comfort, energy, and safety matter.

[bman03]

OK. Passive Vents. Gable Vent or Pot\Mushroom\Turtle vent(s)? Note that the roof for the attic area in question is lower than the roof in the adjacent attic area - in case that influences the decision. (see original image)
For Gable, note that one side of the attic does not have an outside wall (see original image). So the recommended practice of 2 Gable vents on opposite sides of the attic cannot be accomplished.
For pot vents, I'm concerned about snow coming into the attic.

[vstech]

gable should not be used with ridge...

ridge gets it's ventilating power from the surface of the roof. it needs the entire surface to adequately draw air out.

a ridge needs TOTALLY unobstructed soffit openings the entire perimeter of the house.