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Old 07-29-2008, 10:22 AM   #1
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venting furnace residing in attic-how to do so safely?


We have a furnace/ac combination in our attic that has been running solely as an a/c for about 10 years. We recently discovered it was an ac/furnace combination and were told all we need to do to use the furnace portion is to have a gas line run to the attic and put in a flue(or so the HVAC guy said-it is really that simple?). Would it be safe to vent it through our chimney(wood burning though never been used(50 years and untouched-weird huh?)-it's right next to the unit in the attic) or will we need to run it straight through the roof (if it goes straight up it would be very visible front and center on the front of the house-which I would prefer to avoid)? We have a gas water heater and a soon to be removed boiler that are already vented from the basement. Also, how much should it cost (very roughly) to run a new gas line from the basement to the attic and to put in a flue? I also know a new thermostat must be installed to run it, but that's nothing I'm too worried about having done, I have been wanting to upgrade to a new digital one anyhow Also, are permits required for stuff like this?

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Old 07-29-2008, 04:26 PM   #2
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venting furnace residing in attic-how to do so safely?


Permits will definately be required.

The furnace might be able to be vented through the fireplace, but that's asking for all sorts of trouble, given that it has the potential to be used as a wood burning fireplace.

I'd go through the roof with the type B vent. You should know there are a lot of requirements for venting fuel burning appliances, and they'll affect the efficiency, performance, and safety of the unit.

Adequate combustion air is also important. That's fresh air for the unit to burn. Lots of requirements there, and they're definately life safety issues.

As far as cost, can't help you with that on this site.

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Old 07-29-2008, 05:17 PM   #3
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venting furnace residing in attic-how to do so safely?


Everywhere I know of, it is illegal to vent a gas burning appliance in the same chimney as a fireplace. Do you have a double chimney? If you do, then you can vent into the other side of the chimney.

Keep in mind that you definitely will need a permit for the gas pipe, and the vent, and if its an 80+ furnace you may need a SS Liner.

I remember your post. Cant help you with the gas line, but its probably gonna be ugly.

Sure hope your clearances to combustibles is within manufacturers specs.

Last edited by 8 Ball; 07-29-2008 at 05:19 PM.
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Old 07-30-2008, 07:16 AM   #4
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venting furnace residing in attic-how to do so safely?


A few thoughts'

For combustion air we have an huge attic fan which basically leaves a gaping hole in the roof for air. Plenty of air.

Heat rises, so you are starting with a disadvantage.

Our A/C runs through our furnace which is in the attic. Our A/c works GREAT, our heat for lack of better word sucks.

We do not have a "flue" per se - our furnace is "power vented" (I think - in the winter you can clearly see that the exhaust is being pushed out, it is not a natural draft) - so we have a regular old galvanized pipe sticking straight up through the roof. It was in the house when we bought it so I don't know if it's single or double wall or what, but it does not really get that hot.

Running a gas line is cheap. I would get a flex line and run it through a closet. The materials there will be a few hundred bucks.

How do you heat your house now?
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Old 07-30-2008, 11:25 AM   #5
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venting furnace residing in attic-how to do so safely?


thank you all for the replies I haven't a clue when it comes to gas anything, I grew up in a purely electric house. Right now we heat our home with a 50 year old natural gas boiler and baseboard hot water heaters. It's noisy, not very warm (except at the floor level right in front of the baseboards) and expensive to run (not to mention down here in the deep south, most people don't use boilers and the HVAC guys don't really know how to deal with them-they all tell me to replace it with a heat pump)! The furnace is much newer than the boiler and we didn't even know we had it until a few weeks ago(we just thought it was an air handler). We were planning to add a heat pump to provide heat, though I was fairly against it ( I grew up with one-it ran 24/7 and the house was never very warm without cranking the thermostat way up, which is just wasteful in my opinion.) but we know a new boiler is WAY out of our budget.
The best heat we ever has was in a house we rented that had radiant heat in the floors from a propane burning water heater. I'd love to do something like that in this house, but I think that's a bit of a stretch to attempt as well. I'm about to the point I'm just going go buy a space heater for each room. I just thought, "hey, we have a furnace and all the ductwork and such, why not use it?". But it appears it's going to be like pulling teeth to make it a viable option The HVAC guy that came out just acted like it was something super easy, oh well. We are currently working to improve our attic ventilation to help the a/c and lower our energy bills, but I don't know if it would be good enough for a furnace burn efficiently. The reason I mention the chimney is that it appears from the basement that the boiler and hot water heater vent through the chimney. I would love to use my fireplace though, and NOT cause an explosion...
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Old 08-01-2008, 06:42 AM   #6
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venting furnace residing in attic-how to do so safely?


Are your AC vents in the ceiling? I would worry that the heating would really not work very well like ours. Unless the heat is radiant (like those propane lamp stand heaters), heating from above does not work very well in my opinion. If you do it, I would forget about trying to put it through the existing chimney. If it's already in the attic, going through the roof is easy. Where are you and how cold does it get? How expensive is natural gas per therm (100,000 BTU) vs say, electricity per KW/h?


Last edited by pcampbell; 08-01-2008 at 06:48 AM.
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