# Is outside air really needed for a furnace?



## berricks

I live in a 40 year old home in Boulder, Colorado.   It has a high efficiency natural gas furnace that's one year old.  We bought the place after this was installed.  I've found an 8 inch round air supply duct coming directly in from outside that empties near the base of the floor.  I noticed the lower level of the house was very cold and found a huge draft of cold air coming down this pipe.  

The house has original windows and plenty of cracks around doors.  Is this air supply pipe really necessary?  What else could be done to keep the cold air out?


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## MinConst

I'm no HVAC expert but I don't see the need or an 8" pipe emtying onto a floor. Yes your furnace needs outside air but not 8" of it and it should be ducted into the air chamber.
  Lets see what the pro's have to say.


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## CraigFL

It would be unusual to have a modern high efficiency furnace that doesn't use outside air for combustion-- usually built right in to the ducting(coaxial with the discharge pipe) or into an airtight enclosure around the furnace. The vent you speak about may have been put there earlier for the old furnace to utilize outside air when people first learned about increasing the efficiencies of their old furnaces. You should verify that the new one is receiving outside air from somewhere else and then close off that duct or build an enclosure around the new unit and duct the air into that enclosure so it doesn't get into the other parts of your home.


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## Hube

In all cases, a hi-efficient furnace NEEDS combustion air, usually a 2" or a 3" diam. is adequate, and normally is placed very near, or right within the furnace itself.
If this 8" vent you are talking about is terminating at or near the floor level you can put an "anti-spill' pail around it that is slightly higher than the lowest  level of the 8" vent. This will help eliminate any drafts and only allow any air to come in that is actually required for combustion purpose.

example; Let's say this 8" vent  terminates 6" above the floor. A pail or container with a diam. of approx 10-12" and a heigth of approx 8" will do the trick.


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## glennjanie

Hello Berricks:
Yes, the furnace does need outside combustion air. Some houses supply that combustion air through leaks but modern homes are sealed very tightly and a seperate pipe to the outside is necessary. This is mentioned in several local and national building codes especially if the furnace is installed in a closet that is also sealed. Furnaces are typically installed next to the water heater too which requires more combustion air. The 8" pipe is the largest I have ever heard of being required. Also, remember most high efficeiency frunaces and water heaters have fan forced exhaust which moves a lot more air than gravity does.
Hube has a very good remedy with his pail.
Glenn


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## kok328

I have the same drafty pipe (I think it's only 6") in the same location and same heigth off the floor.  This is for combustion and pressure equalization.  I also have a 4" pipe coming from outside to the end of the return duct manifold.  This is for fresh air intake.


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## a1jatt

i see lots of ice build up around this pipe .. and the end of this pipe with container attached to the inlet duct of the furnace, seems like it is making the incoming air even colder and making furnance work harder than it should be. 
Now its really cold (-40 Degree C ), thermostat is set at +35 degree c and its temp in the house is 10 degree c. Furnance is running non stop for last three days.

I was wondering if it will be ok if i move this thing couple feet away from the furance, since it will still be supplying enough oxygen. 

 I was wondering if i can put an electric heater inside the furnance room to eliminate this cold wind comming throuh this fresh air duct pipe. 

thanks.


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## Quattro

I don't understand this. High efficiency furnaces use outside air for combustion only...you don't need "extra" air just to heat your house. Air is circulated through the supply and return system. These big pipes dousing the basement in frigid air really sounds counterproductive.


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## CraigFL

Quattro said:


> I don't understand this. High efficiency furnaces use outside air for combustion only...you don't need "extra" air just to heat your house. Air is circulated through the supply and return system. These big pipes dousing the basement in frigid air really sounds counterproductive.



I agree with you! But the history was:

1. Combustion air was sucked in thru the air leaks in older homes
2. Next, people became concerned(read $$$) and put large ducts for cold outside air near their furnaces
3. Next, they realized this was bringing in too much extra cold air so they enclosed the furnace into a box and ducted the cold, outside air to it.
4. Next, furnace makers made ducts for combustion air conect directly to their furnace. 

So... Some people are still back at #2, which works and might be fine for unused basements but it could be better with a little work.


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## Quattro

I see! Thanks for the history lesson!


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## cbay

Hmm.  Outside air is required to feed the furnace.  I understand that.  However, does the requirement for ducting outside air into the space around the furnace change when there is also a heat recovery ventilator (HRV) in the house?  The HRV brings the fresh air into the house from outdoors, but warms, or tempers it before ducting into the supply side of the furnace?  
This seems intelligent, whereas bringing cold air into the house and simply dumping it (untempered) into the basement seems counter productive especially at -20C!  Would the continued use for this be simply for the domestic water heater?
My suburban house is only 15 years old, but since tract housing is done on the cheap, it doesn't surprise me much.


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## Trapperj

Does a high eff. furnace need air ducted from outside to the return air plenum?


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## mudmixer

The make-up air is for combustion and not to fill the plenum with cold air.

Cold air into a siphon break (pail) is very common and required by code in many areas.

I have one in my townhouse and keep the door to the utility room closed to prevent the furnace from sucking out the warm air from the house and burning it up. - That area of the house has several cold air returns drawing cool floor air into the furnace/plenum to be heated and recirculated.

The idea is to separate the fresh air intake for combustion from the interior air that is circulated through the furnace/duct system. Cold exterior air is not all that cold and it works quite well to provide oxygen for combustion in a "controlled" way.

High efficiency (90+%) is different and the outdoor intakes are piped directly to the furnace. I chose to put is an 80% furnace to avoid all the problems (exterior appearance, destruction of interior ceilings, etc.) that come with a pair of plastic pipes and could not justify the cost with a high efficiency furnace. - My heating costs with an 80% furnace in MN are much lower than my AC costs annually.

Dick


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## madison67

we have a large 8" fresh air intake in our furnace room in the garage.  we also have a tankless water heater in the same room.  this is the 2nd time my water pipes have frozen this winter.  It is -12F outside.  what the heck can i do to keep my pipes from freezing?  We have a cistern and the water supply comes into the same room to the pump.


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## kok328

Until the weather breaks, cap off the 8" intake and leave the furnace closet door open to use the air in the garage.  Wrap your pipes with heat tape.


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## Jeffh

Install a gas-fuelled garage heater.


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