Forced air Fresh air intake


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Old 01-25-03, 11:19 AM
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Question Forced air Fresh air intake

I had a new heating system installed in our new house and it has a fresh air intake from the outside of house to the return air. I can feel cold air coming in when the blower is not running.
Isnt there supposed to be a check valve on the cold air inlet to prevent this?
 
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Old 01-25-03, 04:51 PM
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Hello: redvett

The simple answer to your questions is yes. There should be a one way baffle. The difficult part is why is there a fresh air intake?

To promote energy efficiency, outdoor fresh air is not required in residential structures. Only in public buildings and gathering places, etc. Codes vary from local to local but not often vary that much based on new heater installations.

To determine the need for a fresh air intake you can contact the heater manufacturer and your local health & safety departments within your area.

The heater manufacturer may or may not be aware how the installer is installing the units. However, they will be aware of the loss of energy efficiency provided by their heating unit.

Another possiblity is the entire ducting system may not have been replaced for some reason. Most likely as a result of cost cutting to provide lower bids and or by owner requests. Read your contracts and the exclusions.

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Old 01-25-03, 05:18 PM
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Thumbs up Re:Forced air Fresh air intake

Thanks for the reply. Just what i thought. The code here in Michigan requires you to have a fresh air intake because the houses are so air tight. But only if you have gas stoves,driers or water heaters which i have none all are electric. I have blocked off the vent for now because of the cold - 15 to 20 below at night. I will be getting with the contractor about this problem.
Thank again.
 
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Old 01-25-03, 09:13 PM
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I was asked a similar question concerning having a louvered door on his furnace room. The area he apparently lives does not experience that severe of winters and homes are not built that tight. Almost all new systems today have an inducer fan which eliminates the need for a draft hood. This cuts the air needed for operating the furnace in the example below in half to 1,500 cubic feet per hour. However, if you home is built tight, you would still be required to supply that amount of air to the system. Hence the fresh air supply. The following is the answer I gave him.

[QUOTE]
Yes, it is true and it is there for your safety. Depending on the size of the furnace, water heater and/or gas dryer, you will require a considerable amount of air for combustion. For example a natural draft or atmospheric furnace uses 15 cubic feet of air for combustion and 15 cubic feet of for dillution air (air that goes up the draft hood) for every cubic foot of gas. This same rule would apply if you have a gas water heater and dryer.

Let's assume one cubic foot of gas is equal to 1,000 BTU's. The natural draft furnace is 100,000 BTU/hr. output. To determine the amount of air needed for the furnace is 100,000/1,000 x (15 + 15) = 3,000 cubic feet per hour. If the water heater is another 40,000 BTU/hr. and the dryer 40,000 BTU/hr. That's another 2,400 cubic feet required per hour.

Since the room is 11 x 15 x 8 = 1,320 cubic feet, even if there is only the furnace, the cubic feet of air in the room is less than 50% needed per hour. As you can see, this is a lot of air needed for combustion. You have a few options here. You could install a louvered door, Put vents on the wall, one high and one low. The rule here is one square inch per 1,000 btu's required, high and low. For a total of 2 square inches per 1,000 btu's. So if you had a 100,000 BTU/hr. furnace, you would need a high vent equal to 100 square inches and a low vent 100 square inches on the wall. Since the louvers in the vent reduces the actual size of the vent for air to pass, the rule is the vent with louvers should be twice the size of the desired vent opening. In other words, the vent with the louvers should be 200 square inches. Most people just oversize the vents, rather than calculate it.

Your other choice is to duct in a fresh air supply from the outside to the furnace.
[QUOTE]
 
 

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