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Old 11-04-2010, 09:49 PM   #1
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Combustion air


I have a high efficiency Armstrong furnace in an 8 y/o home. When the furnace was installed they put an outside air duct into the cold air return instead of running a separate combustion air duct direct from the furnace to the outside. The problem I'm having is, the outside air duct enters the cold air return next to a vent in the living room making the floor cold. I live in Iowa where we have cold winters. Is this common practice? I understand their intent to allow fresh air into the house, but doesn't that defeat the purpose of the high efficiency furnace? Should I run a direct combustion air duct?

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Old 11-04-2010, 09:56 PM   #2
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Combustion air


That may be make up air instead of combustion air if it is going into the return.

Some houses are tight and to make air changes fresh air is piped into the return.

What is the effec of your furnace?

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Old 11-04-2010, 09:57 PM   #3
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Combustion air


This isn't combustion air. It's allowing fresh outside air into the home & in some areas is required by code


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Old 11-04-2010, 10:22 PM   #4
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Combustion air


I think it a 95%. Its an energy star house. Right now the combustion air is drawn directly from the air in the basement. Why do I see other high eff. furnaces with combustion air plumbed to the outside?

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Old 11-04-2010, 10:47 PM   #5
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Combustion air


As said that is fresh air for the house. If it is to close to a register and is causing problems you may try moving it to another location on the duct.

A few reasons you should have the combustion air from outside are that it could put your house into a negitive pressure. The air that is being vented out has to be made up someplace and will infiltrate into your house as cold unconditioned air and you will need to pay to heat it up again. Another reason is you may be losing the efficiency you paid for with your furnace. A 95% may be running at 94% or so. Some 95% furnaces require the combustion air be taken from outside. Check your installation manual.
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Old 11-04-2010, 10:50 PM   #6
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Combustion air


depends on the furnace. Some are designed to have 2 pipes while others aren't. I have a 93% that's not designed to pipe combusiton air into the unit. It isn't a sealed combustion chamber
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Old 11-05-2010, 07:02 AM   #7
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Combustion air


If your unit has 2 plastic pipes for venting then it gets it's comb air from outside and you do not need a comb pipe unless you have a gas water heater and chimney. The other pipe is for fresh air/ventilation. We use a motorized damper by the Hoyme co. to allow it to open only when the heat is on and let air in then.


http://www.hoyme.com/
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Old 11-05-2010, 10:07 AM   #8
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Combustion air


Is the outside air duct a separate smaller pipe inside all the way through the return air duct, or is the outside air commingled with the return air?

The place on the rim joist where the outside air duct enters the house may not have been adequately insulated and this c ould cause the floor to be cold at that spot.

If the furnace can be walled off form the rest of the basement and ducts run to the outside for combustion air, then a furnace without a specific combustion air port can draw primarily outside air without making the whole basement cold.
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Old 11-05-2010, 02:11 PM   #9
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Combustion air


The furnace has two pvc pipes coming out the top. One goes to the outside and is exhaust. The other isn't connected to anything and has a vacuum on it when the furnace fires. This is the one I think should have been run outside. Am I right?

I'll look into putting a damper on the outside return air. Right now its just ducted with 4" flex duct.
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Old 11-05-2010, 03:06 PM   #10
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Combustion air


May want to check with your codes department. Here in Lincoln Nebraska's we are not allowed to put any kind of damper in the fresh air.
I would have the other pvc pipe run outside even though it may not have to be. There are rules as to how that should be done also. As I said regs the installation manual.
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Old 11-05-2010, 04:41 PM   #11
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Combustion air


Ditto. It will be more efficient taking the comb air from outside and I have not seen any where you have to take it from inside the house but we don't sell Armstrong. If the water heater is under 50,000 BTUs I believe then you may not need comb air for it either. Air naturally leaks into all houses for that. Depends on your local AGA gas code and building codes though.
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Old 11-05-2010, 09:45 PM   #12
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Combustion air


Right now the duct ending in mid-air with the vacuum effect is drawing basement air for combustion. If you continued it to the outside then it will draw outside air for combustion and not "waste" the semi-heated basement air.

You may need to put a cap or dome with sheltered vent holes over the horizontal end of the intake pipe outside so when the wind blows it doesn't adversely affect the furnace through the intake pipe. Or put a downward elbow on the end of the pipe and also put a few one inch holes in the pipe just before the end to break up adverse wind effects (the venturi effect).

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