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sspeed 03-19-2011 10:14 AM

cold air intake from outside to furnace...
Our house was built in 1968. Yesterday a furnace guy was out to do a general checkup. He pointed at that we have:

1) Normal cold air return from all throughout the house
2) Cold air intake from outside piped directly in to the furnace
3) Makeup air vent in furnace room

He basically said #2, the cold air from outside directly in to the furnace, was "what they did 40 years ago" and was basically killing our furnace efficiency, since on the coldest days it was pulling in and having to heat outside air, possibly down to 10 below on our lowest days.

I'm inclined to believe him, however I also just wanted to get a second opinion before removing it and letting the furnace run on only return air from the house and the makeup air vent. Anyone know what the actual "code" is on that or where I can look to find it?

yuri 03-19-2011 11:32 AM

Some homes need that fresh air pipe to get rid of excess moisture in the house, cooking smells, animals smells, cigarette smoke etc. We use a motorized damper in it to open it when only the heat is on. Google: Hoyme damper. If you shut it you may create a problem. It should have a damper in it which you can close to try living w/o it. I would not remove it.

fabrk8r 03-19-2011 11:37 AM

Is your furnace 43 years old? I doubt it.

How is your furnace vented? If it's vented with PVC pipe it's a high efficiency furnace and the air supplying the furnace is called "combustion air" and has nothing to do with the air that is circulated through your home. Combustion air is supplied to the furnace from outside to allow combustion of fuel without pulling air from the structure and sending it out the flu. It is required by code is a lot of places now.

Now, having said that, it was common in older systems to have outside air supplied as "make-up air" to add air to the room where the furnace or other fuel burning appliance was located. This method also did not really affect the efficiency of the furnace a whole lot either, but it did add some cold air to the structure. Since your outside air is being piped directly into the furnace I'm guessing that it's combustion air and not make-up air.

Jackofall1 03-19-2011 11:43 AM

When you say, "Piped directly into the furnace", where exactly does it go, into the return air, or into the area of the combustion chamber?

Either way, this is bringing in air being used for combustion for all gas appliances, as well as exhaust sources such baths and kitchen.

Add a damper if there isn't one, but I wouldn't remove it.


danrb007 03-19-2011 11:43 AM

A trick I have used for the fresh air supply is pipe it into a 5 gallon bucket. have the end of the pipe sitting about 4 or 5 inches from the bottom. This will cut down tremendously on the cold air coming in from outside. The cold air stays at the bottom of the bucket since cold air sinks and warm air rises.

Marty S. 03-19-2011 01:15 PM

Outside air piped to the return is called fresh air/make up air. It's to equalize the negative pressure on the house from bath fans, drier vents,range hoods ect. By sticking in the return the air gets conditioned before dumping in the house. Without it the outside air will sneak in every crack and crevis,still the same amount but unconditioned.

Vents cut into the wall or a pipe just placed in the utility room is called combustion air. It supplies the oxygen for the fires to burn properly.

how 03-19-2011 03:06 PM

Are you are talking about a fresh air line that is passive (going from the outside and terminating within 18" of your burners) or active ( going from outside to the return air plenum with a vent opening at the furnace in the supply side plenum. You can't mess with either of these ( although the idea of adding a spill bucket to the passive system does help) systems without running the risk of co & co2 spillage into your house unless you have a mid or high efficiency furnace..

sspeed 03-19-2011 05:41 PM

Thank you for all of the replies. I'd have to guess the furnace is about 15 years old... Sounds like I should leave it...

To clarify:
What I was calling cold air intake comes from outside the house, through 4 inch pipe, in to the return about 3 feet from the plenum.

Then, in the same room, there is a 4in vent right in the wall that just dumps in to the room, I assumed that was the make-up air?

yuri 03-19-2011 07:26 PM

The one in the duct we call fresh air or ventilation air. The other bringing air into the room for the furnace burner is called combustion air. 4" into the return duct is quite small and just enough to bring in some ventilation/fresh air air w/o costing that much to reheat so I would leave it alone. Your guy was wrong. 40 yrs later we still bring in air for ventilation but we use a minimum 5" pipe and put a motorized damper on it.

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