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RyanBruner 09-23-2009 11:19 AM

Fresh-air intake question
Two years ago, we moved into a brand new house. In the basement, there is a fresh-air pipe. It simply dumps air from the outside into the basement.

In the infinite wisdom of the builders, they placed this vent on the complete opposite side of the furnace. As a result, the basement is absolutely freezing in the winter. Even after finishing the basement walls (putting in fiberglass insulation with vapor barrier, and then drywall) the basement is better, but still freezing. It is because of this fresh-air intake. My kids play down there in the basement most of the winter, so I wanted it to warm up.

When I block the fresh-air intake vent, the basement temperature is significantly improved. But I've been told this is a mistake.

However, I've also noticed that in the cold-air return duct work in the basement that feeds back to the furnace, there is an insulated duct that leads directly outside. This vent is hooked into the cold-air duct work directly.

Wouldn't this vent, then, serve the same purpose as the fresh-air intake? Shouldn't it pull in the fresh air directly into the cold-air return, and therefore make the fresh-air intake redundant?

Ideally, I'd like to just remove the fresh-air intake entirely, relying on this other one. But perhaps that other vent serves a different purpose? I can't see what. My other option is to put in a whole new fresh-air intake that is actually near the furnace so that the cold air doesn't have to spread across the entire basement.

Any thoughts?

kok328 09-23-2009 01:55 PM

I have the same setup and agree that it makes the basement cold.
However, these are there for a reason.
The duct from the outside to the cold air return is the fresh air intake.
The duct from the outside to the basement is for fresh air combustion required of all high-efficiency furnaces. Notice that there are no cold air returns in the basement so there is no way for the fresh air to reach the combustion chamber of the furnace. The combustion fresh air intake should be ducted towards the floor to keep cold air from being blown into the basement versus being sucked into the basement. In addition, this prevents the home from reaching negative pressure thereby, sucking in cold air around the doors and windows.

RyanBruner 09-23-2009 02:02 PM

But doesn't the cold air that comes in from the fresh-air intake that is attached to the cold-air return duct work lead directly to the furnace? It seems like it should suck in the cold air from there, which then leads to the furnace for combustion in addition to the cold air from the cold-air ducts throughout the house.

Before cold weather hits, I either need to just remove the fresh-air intake pipe (that, yes, just dumps cold air to the floor, but on the complete opposite side of the basement), or remove that one and put a new one near the furnace so cold air isn't flooding the rest of the basement. There is probably a 15-degree temperature difference!

I'd love to just be able to close off the fresh-air intake pipe, and keep the fresh-air intake that's connected to the cold-air return duct work. It seems redundant (and inefficient) to have both.

travelover 09-23-2009 02:23 PM

Ryan, how is your furnace vented? Is it through two PVC pipes to the outside or through a conventional chimney?

kok328 09-23-2009 03:28 PM

"It seems like it should suck in the cold air from there, which then leads to the furnace for combustion in addition to the cold air from the cold-air ducts throughout the house"

You don't want CO2 entering the fresh air supply so the combustion air must come from a source other than the fresh air duct which, is distributed through the home.

RyanBruner 09-23-2009 08:04 PM


Originally Posted by travelover (Post 34950)
Ryan, how is your furnace vented? Is it through two PVC pipes to the outside or through a conventional chimney?

Um. Neither? There is a single metal vent that goes out from the furnace to the outside of the house out the wall. The water heater actually vents to this same outside vent. Is that what you are calling a "chimney"?

Let me back up and explain this better.

Our house has the main ducts going out from the furnace to the rest of the house. Then, there are cold-air returns in every room of the house. These all meet in the basement in a single, very large duct that runs directly back to the furnace, passing through the air filter, and back to where combustion takes place.

Connected to this same cold-air-return duct in the basement is a smaller (perhaps 6-inch diameter, maybe) duct that is insulated and leads directly to the outside of the house. During the winter months, even if I touch the insulation, I can feel it is extremely cold air coming in through that vent.

So, it seems to me that the purpose of that vent is to suck fresh cold air into the house via the cold-air-return ducts. If fresh air comes in that way, why would there need to be a separate fresh-air duct that simply pours cold air into the basement? The air from both sources end up in the exact same place.

Anyhow, if you're saying they are different (and I'm merely trying to understand WHY their different. To my untrained eyes, they appear to do the exact same thing), then that means I'll have to create a new fresh-air vent that is near the furnace so that the entire basement doesn't start growing icicles during the winter despite the $1000 I put into finishing and insulating the walls!

travelover 09-24-2009 02:46 AM

Ryan, thanks for the clarification. I thought you had a high efficiency furnace - thus the question about the PVC pipes.

Inside your furnace is a thing called a heat exchanger. It separates all the combustion gases from the air that is circulated through the ducting and the rooms. All the heat must pass through the metal surface of this heat exchanger from the flame to the room air.

The air that your furnace burns is taken from the basement and goes out the chimney exhaust pipe (through your wall). This exhausted air must be replaced. On older construction, it just leaks back in through many leaks. On newer construction, make up air is provided through a vent in the basement. If you had heat registers or cold air returns in your basement, this air could be made up from the outside air duct connected to your cold air return.

The amount of makeup air that your system is providing seems excessive, but I'll leave it to others to quantify how much is necessary to prevent back drafting (sucking in water heater exhaust).

RyanBruner 09-24-2009 06:16 AM

Ah. That makes sense. This means that I'll have to move the fresh-air vent then, rather than simply close it off. A bit of a bummer, but it is what it is.

I appreciate the help on understanding all this.

n0c7 10-30-2009 09:53 PM

Ryan, our home in Edmonton, AB Canada was built in 2003/2004 and has a near identical setup to yours - two 6" fresh air intakes, one to the cold air return portion and another to a combustion pot that runs to the floor and simply dumps fresh air into the basement. We also have a mid-efficiency furnace(sounds like you do as well) and a natural gas hot water tank with both "chimneys" tied together.

After developing my basement, I've come up with three solutions to our problem:

1) Install a motorized damper designed by Hoyme - Hoyme Manufacturing Inc - Motorized Dampers & Ventilators - North America. This closes the fresh air intake combustion pot and when the furnace comes on it opens up. Depending on who you speak to you may also want it to open it up for when your hot water tank runs(I would to be on the safe side to eliminate CO poisoning risks). You can install a heat sensor on the tank that will open the damper up. You can even go as far as installing a second damper for the fresh air intake that goes into the return air duct work as freezing cold air is not good for your furnace. You can e-mail the support contact on the website for more details, they're very informative and detailed. The site also contains a very useful FAQ. Downside - cost of the dampers and availability in your area.

2) Relocate your fresh air intake combustion pot near the furnace. Frame and close off the furnace room. Assuming your combustion air requirements are sufficient, install weather proofing/stripping around the furnace room door to contain the cold air within the furnace room.

3) Ditch the mid-efficiency furnace and go with a high-efficiency furnace. This eliminates the need for the 6" fresh air intakes and will require the installation of PVC pipes to the outside of your house for intake/outtake.

All three of these options are valid in my location but I would suggest consulting with your local authorities and building codes before making your decision.

I haven't decided on what route to take myself. With the basement being finished it's not too cold except I built an office attached to the furnace room so my feet get cold. I've got a blanket tucked underneath the furnace room door to keep the cold air in. :) For the first couple of years living here I remember very well how cold the basement can get.

I put a lot of thought and research into this so I hope this helps you, good luck!

Quattro 11-02-2009 05:07 PM

Question...why do builders still use this type of system? High efficiency furnaces have been around for what, 25 years? Combustion air comes in via PVC, and is exhausted via a matching vent pipe. No need to fill the basement with frigid air. It makes no sense to me.

I guess the only reason is to exchange air. Outside winter air is dry, so it helps to keep humidity low in new houses. Seems like a cheap way to avoid a proper heat exchanger (heat recovery type) that would be 80% efficient.

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