Better to vent furnace out wall or chimney?

hollanNovember 2, 2010

We are getting a new 95% efficient gas furnace and it has been mentioned that there are two options for venting it out of the walk-in, unfinished basement. It could either go out the unused, walled-in chimney that the oil furnace was vented through, or they could cut a hole in the concrete block wall and vent it out the side of the house. The basement ceiling is about 6-1/2 or 7 feet high. If it went out the chimney, it would leave more headroom. Since they say they could do either, which should we choose?

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if they say they can do either safely within code make a desition. We don't care if your flue goes out the wall or
the chimney as long as it is safe and within code.

    Bookmark   November 3, 2010 at 9:13PM
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My thoughts--

I know gas does not have the same exhaust as oil.

I have an 90+% efficient oil furnace installed by the previous owners. It is vented out the side of the house. If you choose this option consider: the exhaust from the furnace is then below the level of the windows.

In the summer, when our the furnace kicks in to heat water the exhaust is blown into the house as the prevailing breeze blows toward the side of the house where it vents, and into the windows. it does not help that the doofuses who installed it vented it beneath an existing deck and sunroom, where the fumes invade the deck space and float into the sunroom, living room and kitchen via the windows. Occasionally, the smell will come in via a window on the side of the house next to the furnace. When I hear that sucker kick in, I shut windows like a crazy person or the house might end up smelling like bus station. Yum!

I know they are telling you that this won't happen if you go through the wall, and I know that they are saying that it is gas so it "there is no smell" or whatever. They may be right, but if they are wrong, it is a very big wrong indeed, and you won't be able to change it. I would go through the existing chimney even if it cost you a bit extra to make any chimney repairs to do it!

I hope this helps!


    Bookmark   November 5, 2010 at 12:26PM
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They have the vents that go on the outside of the house that contain the exhaust and intake in the same unit that look nice. I would go this route and leave the chimney in case at some point you decide you want a pellet stove or something else to vent through it. There are quite a few different ones out there to choose from.

    Bookmark   November 6, 2010 at 12:19AM
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I have one furnace telling me that the new high efficent furnaces will only last 10-13 years because they had to make them get to hot in order to get the high efficienty rating. He things high energy furnaces are a work in process and that you should make your old furncace last as long as possible because the new ones will get a lot better made in the next few years. Any thoughts. My Lennox 78% furnace is 15 years old and I kind of wanted to take care of the credit, but if the furnace only lasts 10-13 years, the energy savings will not offsetthe cost of replaceing the furnace after such a short time.

    Bookmark   November 8, 2010 at 5:49PM
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I think don't it is possible to vent a 95% efficiency furnace up through an existing chimney. The exhaust gas is too cool for it to rise. I believe high efficiency furnances must be vented horizontally through a wall. I would be very leary of a HVAC contractor would not do this.

I have not heard of high efficieny only lasting 10-13 years. This sounds like a myth from an HVAC contractor who doesn't want to install high efficiency furnaces.

It is important to interview multiple HVAC contrators when shopping for new equipment and installations. Be prepared to ask these type of questions and see if you get a consistent response. Hopefully you will eliminate the hacks.

    Bookmark   November 9, 2010 at 10:21AM
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LENVT. Your venting is HIGHLY ILLEGAL. If i was the inspector on that job i would shut off your oil until the venting was proper. MICK HOME. A 95 percent furnace can be vented up an existing chimney providing the chimney is insulated and packed with purolite or vermiculite with the proper cap.

    Bookmark   November 9, 2010 at 11:26AM
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I am taking down the chimney from my house. When I called a reputable pipefitter HVAC guy to come take a look if I could run the high eff furnace vents out the wall instead of thru the chinmey he was puzzled why it was installed in the Chimney to begin with. Also he noticed the faulty black PVC pipe that was recalled awhile back. Nice. My Furnace was installed by the previous owner with Byrant which explains alot. They are clowns that soak people for every penny, which seems to be the way with franchises.

Others had said the same thing. There is no reason to go all the way up the chinmey especially with the condensation problems it creates.

So I'm going out the wall and saying goodbye to the leak prone Chimney.
Anyway if that helps.

    Bookmark   March 24, 2011 at 3:07PM
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Did the contractor who wanted to vent through chimney mention anything about direct vent or 2 pipes?

While some HE furnaces can be installed with an exhaust pipe only, having outside combustion air makes for better comfort (less/no static shocks, reduced infiltration).


    Bookmark   March 24, 2011 at 4:53PM
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After reading this entire posts and seeing no definitive answers, I'd figured I would chime in. As an architect I see a lot of projects where we replace an existing boiler with a Direct Vent (DV) boiler. 90% of the replacements use a horizontal, thru-wall vent. This is done typically because it is easier and less costly than going through a roof. Note on the attached link that there are maximum distances for vents and you are limited to where the outlets are in relation to windows, doors, A/C condensers, air intakes, overhangs, etc. You say you could use the 'unused, walled-in chimney' as a vertical vent. I assume this means a wood framed chimney enclosure. You could run PVC vent pipes vertically through this chimney but you would have a difficult time accessing the pipe joints and brackets during installation. DO NOT THINK ABOUT connecting to an old masonry chimney, even if it is terracotta lined. The direct vent gases are cool and will condense inside the chimney fast leaving a pool of highly corrosive water at the bottom of the chimney. This will eventually degrade the chimney and create possible CO leaks. For new construction I typically specify running the pipes vertically with a roof termination. This eliminates the steam plume shooting through the house wall while the boiler is on. Note that Weil McLain requires the pipes to slope back toward the boiler. I believe they collect the condensate and reheat it. Not certain how this works. I know that in a vertical installation there will be less vapor exhausted and more condensate generated as it rises through the pipe. This is typically collected at the bottom of the vertical stack into a condensate pump. This water is corrosive so be careful where it is pumped to (i.e. keep it away from landscaping). As long as you can find an inconspicuous location to vent the gases, the sidewall is the way to go for a retrofit.

Here is a link that might be useful: Venting

    Bookmark   April 15, 2012 at 1:36PM
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