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Thread: Do I need a fresh air intake in furnace room?

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    slybry is offline New Member
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    Do I need a fresh air intake in furnace room?

    Sorry.... Just moved into the house and have lots of questions.


    We have a forced air oil furnace. In the furnace room there is a 4 inch duct coming in from outside. It comes down the basement wall and with the openning about 1 foot off the floor. The previous owner says it is required by code.

    When furnance fan runs there is ice cold are sucked in from outside through this duct. This seems completely nuts to me as I have spent all my time in my previous home trying to seal it and caulk it to keep cold air out and now I have a house sucking it in !!!!

    I am very tempted to plug it up.

    Questions:

    1) Why do I need this 4 inch hole in my house?

    2) My friend mention he has one as well but he has some sort of bucket or trap at the bottom that the cold air falls into. I do not know what it accomplishes. I have ask about it at home reno center but they do not seem familiar with what I am asking about. Does anyone know what this might be and what it might do.

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    HayZee518 is offline Deity
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    IF you're house is completely air-tite then outside air is allowed [inside] for the system to function properly. Forced hot air means cooler air is sucked into the heat exchange medium, heated and distributed throughout the dwelling. Eventually the dwelling becomes saturated with the heated air. Doors open windows open so this heat escapes and make up air is needed. That's why outside air is introduced into the system. 4 inch imo is small - 8 inch would be better.

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    LazyPup is offline Deity
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    Per code requirements any room which houses any solid, liquid or gaseous fired appliance must have 50cu.ft of volume for each 1Kbtu of the combustion burners.

    Typically a residential furnace will range between 100kbtu & 200kbtu and in most cases when they create a dedicated furnace room they place the water heater in the same utility space.

    For illustration let us say that the data plate on your furnace lists it at 100Kbtu and you have a 60gal water heater with a 40Kbtu burner. The combined combustion load in the space would then be 140Kbtu.

    Allowing 50cu.ft for each 1Kbtu we would then get 50cu.ft x 140 = 7,000cu.ft

    You then multiply the length x width x height of the room to determine the total cubic footage and it is less than 7,000cu.ft it would be classified as a "confined space".

    Again, for illustration let us assume you have an 8' ceiling. We divide 7,000cu.ft x 8' = 875sq.ft. Thus unless the furnace room is 25' x 35' or equivalent,with an 8' ceiling it is a confined space and requires a ducted in source of combustion air.

    I understand your concern that the incoming cold air might cool your house, but such is not the case. The incoming cold air is consumed by the burner and has absolutely no effect on the temperature of your house. Failure to have a sufficient supply of combustion air will result in improper combustion which produces excessive levels of carbon monoxide and it will result in oxygen depletion throughout the structure.

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    slybry is offline New Member
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    Thank Lazypup for excellent explaination. I understand the need for this now.

    I also have a heat pump. This means that 70% of the time I am heating with electricity and therefore no combustion. Only using oil maybe 30% of the time (live in Montreal by the way). But this pipe is pulling cold fresh air into house whenever furnace blower comes on (I have it set so blower only runs when furnace is running).

    So the result is that 70% of time I have cold air being drawn in when not really required as no combustion is happening.

    Is there any system that you are aware of that would open and close outside venting to correspond to when system switches to oil?

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    Hube is offline Journeyman
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    A 4 " fresh air vent will do just fine for an average size home (1200-1800 sq. ft. )
    This vent should be a few inches above the floor and placed within an"anti-spill "surrounding. A simple bucket or pail will do the trick as being in this surrouding it will only allow air from the outside to enter into the home only as it is needed.

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    When in stalling ducts into a confined space for combustion air we are actually required to install two ducts. One must terminate within 12” of the ceiling while the second one must terminate within 12”of the floor.

    The ducts must be sized to provide 1 square inch of cross sectional area for each 4000btu of the combustion equipment.

    In the above example we had 140Kbtu which means our ducts must be 140,000 / 4000 = 35sq.in

    The code gives us a table for computing the cross sectional area of round ducting.
    3” dia. = 7sq.in
    4”dia. = 12.6sq.in
    5”dia. = 20sq.in
    6”dia. = 28sq.in
    7”dia. = 38.5sq.in
    8”dia. =50.3sq.in.
    9”dia. = 63.6sq.in
    10’dia. = 78.5sq.in.

    Thus for the above illustration the correct size of ducting would be 7” diameter.

    The code prohibits installing any device, such as a bucket, which could impair the flow of air.

    So how do we prevent the cold air from coming in when you are operating on the heat pump?

    In general, the ducts will only draw air in when the air pressure in the machinery space is slightly lower than normal atmospheric air pressure. During combustion air is drawn into the combustion chamber and exhausted out through the flue, thus this creates a slight drop in air pressure in the machinery space and outside air is pushed in by normal atmospheric pressure from the outside.

    In general, state of the art HVAC systems have ducted return air from the living space to the furnace so there is no draw of air from the machinery space. With no draw, there is no corresponding reduction in atmospheric pressure in the machinery space and therefore airflow through the duct is minimal. In fact, during the heating cycle the air in the machinery space is generally warmer than the air outside, and considering that warm air rises, the actual flow is up from the machinery space to the outside.

    In many older homes, and sadly some cheaply installed new systems they do not have ducted return air from the living space, but rather, they rely upon infiltration from the living space to the basement where the air is drawn directly into the furnace. In this case when the furnace is running in the heat pump mode the return air to the furnace would result in sucking outside air into the machinery space through the combustion air ducts.

    The correct solution would be to install a complete ducted return air system, but where that is not practical you could at least install a duct from the furnace conditioned air return to the partitioning wall between the machinery space and the remaining portion of the basement, then install weather stripping on the machinery space door to minimize air flow. Keep in mind that the return air from the living space is passing through a heat exchanger in the furnace, so there is no direct contact between the returning conditioned air and the air required for combustion.

    In this manner the only air being drawn in through the combustion air ducts would be the actual volume of air required to support combustion when the oil burner or water heater is firing.

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