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#1
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why does a furnace need outside air?
I was doing some insulation in the basement, and I noticed that the
previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace. I assume that since the builder went through the trouble to put that in, that it's necessary.. but why is it necessary? Isn't there enough air in the house for the furnace to use? The furnace has been running for at least 4 years without it. I'm willing to repair this, but I'd like to know the reasoning behind it. I've found out that vent is a pretty major source of cold air coming into the basement. Another bit of information: The house is very drafty, so there's no concern about it being so sealed up that all the oxygen is gone. LOL |
#2
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why does a furnace need outside air?
outside COLD air is more dense so it burns better, more energy
efficent. Indoor air sometimes has pollutants like bleach that can rot out high effceny furnace heat exchangers, this is a verified issue burning indoor air means your pulling a vacuumn losing heated air and dragging even more outdoor cold air indoors making your gas bill go up. put the original vent arrangement back if you want your gas bill to be lower |
#3
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why does a furnace need outside air?
bf wrote:
I was doing some insulation in the basement, and I noticed that the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace. I assume that since the builder went through the trouble to put that in, that it's necessary.. but why is it necessary? Isn't there enough air in the house for the furnace to use? The furnace has been running for at least 4 years without it. I'm willing to repair this, but I'd like to know the reasoning behind it. I've found out that vent is a pretty major source of cold air coming into the basement. Another bit of information: The house is very drafty, so there's no concern about it being so sealed up that all the oxygen is gone. LOL Giving the furnace its own direct source of outside air will make you home less drafty. Really. All furnaces (non-electric that is) use air to burn their fuel. That air needs to be replaced. It will come from those cracks and holes all around your home bringing cold air in and moving it around your home. If it has its own air source it will mean less drafts. -- Joseph Meehan Dia duit |
#4
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why does a furnace need outside air?
I'm with you the OP said "...the previous owner had disconnected some
ductwork (4") which connected to the cold air return of the furnace...". That doesn't sound to me like it is being used for combustion. |
#6
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why does a furnace need outside air?
RayV wrote:
I'm with you the OP said "...the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace...". That doesn't sound to me like it is being used for combustion. Yes and no. I have such a connection in my home. There is a 4" or so duct that is piped into the return duct. That 4" pipe comes from outside. So anytime hot air leavs the house through the roof, or the bathroom fan, or furnace/hot water heater exhause, the cold air sucked in is through the furnace. (in theory) Its not directly used for combustion, but its for pressure equalization. The air used for combustion which is exhausted is replaced by air coming in from this pipe and into the return. -- Thank you, "Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 |
#7
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why does a furnace need outside air?
RayV wrote:
I'm with you the OP said "...the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace...". That doesn't sound to me like it is being used for combustion. Hi, They ducted fresh air like that in old days. Now thay don't do it any more. Combustion air intake is separate from cold air return duct. Just simply law of physics. |
#8
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why does a furnace need outside air?
"bf" wrote in message
ups.com... I was doing some insulation in the basement, and I noticed that the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace. I assume that since the builder went through the trouble to put that in, that it's necessary.. but why is it necessary? Isn't there enough air in the house for the furnace to use? The furnace has been running Consult your local building permits office. Many jurisdictions nowadays require an outside air duct to supply air to non-electric furnaces. The building code in Ontario, Canada, added this requiremennt about 20 years ago (and made iit retroactive.) If your house is appropriately insulated against draughts, the outside air duct will be more efficient and cheaper rather than less. -- Don Phillipson Carlsbad Springs (Ottawa, Canada) |
#9
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why does a furnace need outside air?
dnoyeB wrote: RayV wrote: I'm with you the OP said "...the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace...". That doesn't sound to me like it is being used for combustion. Yes and no. I have such a connection in my home. There is a 4" or so duct that is piped into the return duct. That 4" pipe comes from outside. So anytime hot air leavs the house through the roof, or the bathroom fan, or furnace/hot water heater exhause, the cold air sucked in is through the furnace. (in theory) Its not directly used for combustion, but its for pressure equalization. The air used for combustion which is exhausted is replaced by air coming in from this pipe and into the return. -- Boy, I'd disconnect that crap real fast. This is an energy disaster. What makes you think outside air is only coming in to equalize pressure when air is leaving via say a bathroom fan? In reality, the powrful furnace blower is gonna draw outside air in all the time. And the inside air will make it's way out, via things like the bathroom fan, whether it's turned on or not, or any other ways to exit the house. A normal house shouldn't need any air specifically brought in from the outside when the furnace is heating/cooling. Housed get enough from existing air leaks around doors, windows, opening/closing doors, etc. If you have a high energy efficiency house that is sealed very tight and you need more outside air, then an arrangement with a heat exchanger is used. Anything else is just wasting a lot of energy. IMO, the arrangement you have is similar to leaving a window partially open. Thank you, "Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 |
#10
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why does a furnace need outside air?
Don Phillipson wrote: "bf" wrote in message ups.com... I was doing some insulation in the basement, and I noticed that the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace. I assume that since the builder went through the trouble to put that in, that it's necessary.. but why is it necessary? Isn't there enough air in the house for the furnace to use? The furnace has been running Consult your local building permits office. Many jurisdictions nowadays require an outside air duct to supply air to non-electric furnaces. The building code in Ontario, Canada, added this requiremennt about 20 years ago (and made iit retroactive.) If your house is appropriately insulated against draughts, the outside air duct will be more efficient and cheaper rather than less. From his description, what he has is not for combustion air, which would make sense. He says the outside air connection is to the return duct. -- Don Phillipson Carlsbad Springs (Ottawa, Canada) |
#11
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why does a furnace need outside air?
wrote in message ... wrote: outside COLD air is more dense so it burns better, more energy efficent. Air burns? Nick Go back to sleep. |
#12
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why does a furnace need outside air?
Bud-- wrote: bf wrote: I was doing some insulation in the basement, and I noticed that the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace. I assume that since the builder went through the trouble to put that in, that it's necessary.. but why is it necessary? Isn't there enough air in the house for the furnace to use? The furnace has been running for at least 4 years without it. I'm willing to repair this, but I'd like to know the reasoning behind it. I've found out that vent is a pretty major source of cold air coming into the basement. Another bit of information: The house is very drafty, so there's no concern about it being so sealed up that all the oxygen is gone. LOL And yet another opinion- The outside intake to the air return runs the house at a slightly elevated pressure which makes it less "drafty" (all the drafts are going out). Of course that is only true for the short periods when the furnace blower is running. What about the rest of the time? And if the house is so drafty that it's noticeable, the drafts should be fixed. Disconnected, your house would probably be draftier but cheaper to heat since you are not forcing air out through cracks. Yeah, that's for sure. Plus, by drawing air into the house purposely to force air out cracks, you are doing exactly that. On a calm day with no breeze when there would not be much air flow through these cracks, and hence little energy loss, now you've got a blower creating air flow. Having a raw outside air intake into the return plenum is simply one of the dumbest things imaginable A new well sealed house might use an air-to-air heat exchanger but with your leaky house that wouldn't be effective. Having a source for combustion air that leaves up the flue is also an issue but not so much, as you said, in your drafty house. My house, drafty type, has a flexible insulated duct for combustion make-up air with the end bent in a U shape at the end - a block from cold air just flowing in. bud-- |
#13
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why does a furnace need outside air?
"Pop" wrote in message news:Xe0Qf.5567$eJ1.2166@trndny05... wrote in message ... wrote: outside COLD air is more dense so it burns better, more energy efficent. Air burns? Nick Go back to sleep. :-) |
#14
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why does a furnace need outside air?
air intake should go to furnace burner area not plenum.
4 inch sounds right for air for combustion |
#15
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why does a furnace need outside air?
dnoyeB wrote:
RayV wrote: I'm with you the OP said "...the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace...". That doesn't sound to me like it is being used for combustion. Yes and no. I have such a connection in my home. There is a 4" or so duct that is piped into the return duct. That 4" pipe comes from outside. So anytime hot air leavs the house through the roof, or the bathroom fan, or furnace/hot water heater exhause, the cold air sucked in is through the furnace. (in theory) Its not directly used for combustion, but its for pressure equalization. The air used for combustion which is exhausted is replaced by air coming in from this pipe and into the return. Pressure equalization? Don't think so. That cold air pipe is meant for a certain percentage of make up air from the outside. Required in many communities with newer homes that are built air tight and is likely to be 10 to 15 percent of the air. Seems like a bit much if in a cold place. But the purpose is to supply fresh air because much of the inside air is highly contaminated by the materials used in building, furniture, carpets, etc. |
#17
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why does a furnace need outside air?
On Thu, 9 Mar 2006 19:01:04 -0600, Mike Dobony
wrote: You obviously never heard of Sick Building Syndrome! Indoor air is highly polluted and needs a replenishment of fresh air. In the winter time the house is usually shut up more than in the summer and contaminants just build up. This is especially bad if someone has allergies to indoor contaminants, a growing cause of asthma! Also the air exchanged with opening a door is contributing unheated air to the house. The outdoor air supply is being heated before entering the house. I went through a building design construction course. Community college level, once a week for 12 weeks if I remember. On the furnace heating system picture it this way. Your house is an insulated box with insulation for the outside walls only and on the ceiling. The furnace fires up and the heated is recirculated within this tightly insulated box to bring the air up to your set (comfort) temperature. Heated air will cool on its own mainly conduction loss with some loss through leakage. The (heated) leakage air loss has to be replaced by outside air. The best route for this outside air to enter the house is to duct outside air into the return air duct so that it is heated first before it joins the circulation. It is very dangerous to over seal the house, ie make it too airtight, and not have this outside fresh air intake. The reason is that heated air normally rises up the chimney. But when the room air cools air contracts and it will suck air (in the reverse direction) down the chimney. This suction includes toxic burnt gases present in the chimney and can be blown back into the house. You breath this stuff. This is slow poisoning. Another reason for the fresh air intake is that in the absence of one the heated air inside the house will be drawn up the chimney together with the furnace gasses. You lose expensive heated air up the chimney every time the furnace fires up. The best solution is to have a separate fresh air supply to the furnace burners. This way outside air goes direct to the burners and goes up the chimney immediately after combustion. The burners do not use up warmed room air. Often all the contractors do is to have an open duct from the outside to an opening near the burner assembly. Of course in winter this lets in freezing cold air. Many homeowners plug this duct up with fiberglass or remove the duct altogether. It will work but you will be sending warmed room air up the chimney. I fabricated an enclosed duct to trunk the duct air into the furnace. Cold air is drawn in only when the furnace fires up, a per normal convection, and the cold air never gets into the basement. I understand newer furnaces already have this feature. To recap there should be a fresh air duct from the outside and connected to the furnace return air trunk. There should be a separate fresh air duct to supply fresh air to the furnace burners. |
#18
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why does a furnace need outside air?
mm wrote: On 9 Mar 2006 04:40:22 -0800, "bf" wrote: I was doing some insulation in the basement, and I noticed that the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. -------------------- Are you talking about the cold air *return* via which the air heated by the furnace returns to the furnace to be heated again. Or are you talking about a duct to supply combustion air to the fire, which then goes up the smoke stack or out the combustion air discharge? This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace. I'm curious how big this duct is. I'm sorry it took me so long to respond (Thanks for all the replies from everyone) It is a 4" circular ductwork that leads from the outside into the regular "cold air return" (inside air source) from the furnace. |
#19
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why does a furnace need outside air?
It is not combustion air. Air it is there to meet the fresh air requirement. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens) "bf" wrote in message oups.com... mm wrote: On 9 Mar 2006 04:40:22 -0800, "bf" wrote: I was doing some insulation in the basement, and I noticed that the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. -------------------- Are you talking about the cold air *return* via which the air heated by the furnace returns to the furnace to be heated again. Or are you talking about a duct to supply combustion air to the fire, which then goes up the smoke stack or out the combustion air discharge? This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace. I'm curious how big this duct is. I'm sorry it took me so long to respond (Thanks for all the replies from everyone) It is a 4" circular ductwork that leads from the outside into the regular "cold air return" (inside air source) from the furnace. |
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