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-   -   Outside air for furnace combustion? (http://www.avsforum.com/forum/15-general-home-theater-media-game-rooms/1195378-outside-air-furnace-combustion.html)

mtnsean 11-10-2009 12:33 PM

My furnace/hot water heater room has louvered doors that open into the rest of the basement. When the furnace kicks on it's pretty noisy for TV viewing, and wakes up guests sleeping in the room next to the furnace. I'd like to close off the furnace room with a solid door, which presents a combustion air problem. I had an HVAC guy take a look and he said it'd be easy to add two flexduct pipes (one near floor level, one near the ceiling) into this room connected to the fresh outside air, which would then allow me to close up the room. That sounds great, but two questions:

- Will these pipes to the outside make the furnace more or less efficient? My understanding was that they might actually improve efficiency, since the only fresh air being drawn in is the air being combusted, which today with the louvered door solution is being pulled in from the rest of the basement/house, which itself gets sucked in from the outside through cracks/leaks. Giving a direct line to the furnace room means I'm not combusting conditioned air, which theoretically would improve efficiency. But the HVAC guy said no, it'll probably make things slightly less efficient?

- Are the 2 required ducts basically passive inlet/outlet? i.e. when the furnace runs it pulls combustable air in through one, and exhausts the combusted air out the other?


Novex 11-10-2009 01:11 PM

I am curious about this as well...Would love to hear peoples input about using solid instead of louvered doors.

Colm 11-10-2009 01:35 PM

Combustion products already leave via the exhaust stack.

You just need a single duct to bring in outside air.

You need to check your local code. You may still require vents to the house even with outside air.

RTROSE 11-10-2009 01:47 PM

I am not an HVAC guy but I did do quite a bit of research on the subject during my build. Today as I understand it furnaces get combustion air from one of two sources. Inside air pulled directly from the house and ducted air from the outside. Typically the furnaces that pull combustion air from the home (like mine) have or are required to vent exhaust combustion air into or thorough a steel exhaust pipe. My neighbor just replaced his furnace (which was basically like mine) to a new high efficiency model and was required to pull the combustion air from the outside (even though his furnace was in the garage) and was also required to vent the exhaust out near the inlet both of which were PVC piping.

I can understand why there would be loss in efficiency. Right now your furnace does not have to work very hard at all to get its combustion air at all. If you start connecting piping to the inlets the furnace will have to work harder to pull its combustion air in. Think of it this way. Try drinking through a coffee stir, then through a regular straw. You still get your drink, but it is harder to do so.

As for louvered doors vs. solid this was the question I had when doing my basement. Again it all goes back to combustion air requirements. I had a setup where I enclosed my furnace into a little room, but I was able to keep it open to the back and the furnace could pull air through the unfinished portion of the basement. If I did not have this option I would have had to put in vents in the room to allow the furnace to more freely pull combustion air in. It is all about airflow my friends. I would definitely follow the recommendations of your HVAC guy on this due to his training and experience. You could always get a second opinion and you should. I contacted four HVAC companies and received three estimates for my work. Prices can vary widely too from one company to another so choose wisely.

Sorry for the ramble, I hope I helped.



ctviggen 11-10-2009 01:57 PM

The primary code for this is NFPA #31 (I have the same issue). NFPA 31 allows exactly what the original poster described, two vents, one near the floor and one near the ceiling, to bring in fresh air. This gives formulas for how many square inches are needed for each vent -- pm me and I get the info.

I don't believe the loss in efficiency will be that great, assuming the vents are large enough, but I could be wrong.

My specific problem is that I do want to not have any vents to the outside, except a single vent line going directly to the oil burner. Unfortunately, the NFPA #31 is silent on this. I need to convince the local inspector that this is OK, and I'm not sure how to do it.

cuzed2 11-10-2009 01:58 PM

I would NOT try and adapt an older furnace to "burn" outside air. These older furnaces were not designed for that application. It is also possible that the efficiency loss your guy mentions may have to do with the incoming air temperature?

I see 2 options:

Stick with the current arrangement making sure that you furnace is allowed to properly breath it's intake air (open basement or louvered doors). Your furnace guy should have a guideline that advises how much vent area is needed per btu of furnace rating. Your local codes will also likely have this data

- Or -

Go with a new properly designed and energy efficient system that is designed to combust outside air

ctviggen 11-10-2009 02:40 PM

I'm not sure whether you directed that comment to me or not, but my furnace is brand new and highly efficient. The manufacturer's website gives recommended venting and air intake requirements, and the air intake requirements include direct air intake to the burner.

cuzed2 11-10-2009 02:46 PM


It was not meant for you directly, and I should re-read my own post to check to make sure "what I meant" is "how it came out" :-)

I assume your unit being highly efficient; is designed to combust outside air. I assume it utilizes the PVC intake and exhaust..?

ctviggen 11-10-2009 03:04 PM

Well, I just typed a response that got lost. Here's a PDF of application for my boiler for direct "venting" (called air intake -- "venting" means exhaust). Boiler venting and air intake PDF

My boiler originally vented to the "home theater" room, and I want to change that. Unfortunately, the boiler room is bordered by the garage (no venting allowed there), the HT room (I don't want to vent there), a small hallway in the house (don't want the noise there), and a laundry room (can't vent there, as you can't vent to a room that exhausts). My options are to vent per NFPA #31 through the laundry room to the outside or use the technique in the linked PDF. However, I have over 15 feet I'd have to go, so I may need to use a "fan in a can".

I'm still researching the issue and will report back on a solution.

ctviggen 11-10-2009 03:05 PM

The technique in the linked PDF also has to go through the laundry room, but this is doable.

gitSehT 11-10-2009 04:52 PM

If you are going to bring in outside air, keep in mind your furnace is going to have to work all that much harder to heat that cold air. I would recommend a HRV Heat Recovery Ventilator.

Here is how it works.http://www.popularmechanics.com/home...s/1275121.html

mtnsean 11-10-2009 05:23 PM

Originally Posted by gitSehT View Post

If you are going to bring in outside air, keep in mind your furnace is going to have to work all that much harder to heat that cold air. I would recommend a HRV Heat Recovery Ventilator.

Here is how it works.http://www.popularmechanics.com/home...s/1275121.html

OK, but that's just it - my understanding is that the furnace isn't heating this outside air, it's burning it. The air it's heating comes from the cold air return ducts, which is basically (formerly) conditioned air (warm, if a little stale). The outside air is used for combustion only in a perfect world (I realize that there are losses and that you do end up having to heat some of it). However, this is theoretically better than combusting stale conditioned air inside the house since that creates drafts that pull in cold air through all the various cracks in the house, and that cold air has to travel inefficiently through conditioned air to get to the furnace to be combusted. So all else equal, I would think efficiency should go up, not down. In either case you're drawing basically the same amount of cold air into the house to burn; with the direct intake method at least that cold air has a relatively clean, low-loss path to the flame.

To the other question - this is a brand new furnace, the former owner put it in about 6 months ago (we moved in about 2 months ago). It's an 80% efficiency unit, but is designed to work with outside air or with inside air.


gitSehT 11-10-2009 05:32 PM

Ohhhhhhhhh. combustion air, I did not catch that part. good question, I have no idea...hahaha It kind of makes sense that using outside air to burn would work better and improve air quality 2 fold.

ctviggen 11-11-2009 03:29 AM

The only thing to consider is how cold it gets. Cold air can cause the oil to gell, so it's beneficial if the air intake runs a certain number of feet through conditioned spaces (read the PDF I linked to).

hanesian 11-11-2009 04:08 AM

I add a typically half informed comment.

I had a new high efficiency furnace installed last year and was told that by code (my county) all new high efficiency furnaces had to have an intake and exhaust vent via PVC as RTROSE mentions. The old furnace vent via the chimney was no longer allowed. Not only that, but to add insult to injury, I was required to add a chimney liner to my chimney even though I had to vent directly through the walls now. Go figure.

Anyway, back to the intake of fresh air: they installed an intake that at it's termination near the furnace is flex tubing about 4" diameter. When I asked the installer why - after spending all this money insulating my house - I now had to have a fairly large unobstructed inflow of outside subzero air all winter, and he said with a wink: "I am required by code to install it like this. What people choose to temporarily put into the end to keep the cold air out is beyond my control, but rumor has it that an old towel works just fine."

YMMV, and I'd love to hear more about how people in cold climates treat this large air intake near their furnace.

ctviggen 11-11-2009 05:28 AM

See this link, "Duct supplying outside air to floor in furnace area" (and the bucket idea):


I think the problem is that the new furnaces have outpaced the old codes. For instance, NFPA #31, which is supposedly THE code when it comes to oil furnaces, does not discuss direct air intake to the oil burner. The link I just provided does not discuss direct air intake to the oil (in this case, gas) burner, at least in small furnace rooms, which is what I have.

Mr_Mike_P 11-11-2009 07:43 AM

how about a picture of this furnace?

mine is a newer model, and uses PVC for intake/exhaust. HOWEVER, the intake is NOT required to be drawn from outside. Some installers just install a 1ft. verticle pipe from the "intake port" and put a 180 on it to prevent things from falling in.

a picture would speak a thousand words here.

Mr_Mike_P 11-11-2009 07:44 AM

what about installing the air intake low on the furnance room side and use the wall cavity to extract air from up higher on the other side, enclosing the "duct" with sheetmetal? you could even install baffles in the cavity to lower sound.

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