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Old 07-31-2011, 06:36 PM   #1
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House pressurization & fireplaces


I have a smoking fireplace that the chimney people want to install a exhaust fan on the top of the chimney. My dad says that I need to add another inlet from the outside to the cold air return. I have two units in my house. One up and one down. Each unit has only one cold air return. If an outside door or window is opened, the outside air rushes in. If the damper is opened, then the same happens. Is my dad correct? And if so, how can I do this with return duct in a crawl space? Thanks in advance.

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Old 07-31-2011, 07:09 PM   #2
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House pressurization & fireplaces


won't help any. need to get what is called "combustion air" near to the fireplace(s) to feed the fire. putting it in the cold air duct is for fresh air if your house has odors or too much humidity. adding a fan to the top of the chimney I have never heard of and it will get plugged with soot and need cleaning too. you need to find a way to get more air into the house for the fireplaces. that is why they are very rare where I am as that air is cold and needs reheating. there are modern airtight fireplaces that have 2" pipes that go from outside underneath the hearth and feed air to the fire and have sealed glass doors. otherwise they are a huge waste of time and NRG IMO and more for cosmetic purposes especially if you have to buy the wood.

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Old 07-31-2011, 07:25 PM   #3
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Glad to hear your thought about the chimney fan. It had a 3k price tag. My dad says that the house is too "tight" and that to pressurize the house so that air is pushing out instead of just recirculating, we need to add air. Your thought about the combustion air close to the fireplace is a food one, but I have opened a window next to the fireplace with little impact on the smoky fp.
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Old 07-31-2011, 07:34 PM   #4
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Fireplaces can be a HUGE pain in the arze to get working properly. I/we went thru a long drawn out series of problems with them here last Winter. Too bad I did not save some of the links. (you could reread all my posts if you have lots of time). We had a real tricky/bad one and after many years of experience with downdrafting chimneys for furnaces and fireplaces I have learned a few secrets. One problem can be how high the chimney extends over the roofline. The bad one I solved was related to one I had with one of my customers units. Turns out that if the prevailing winds are strong and in the right direction and consistent and you have trees and valleys in the roof with several changes in pitch etc you can get some weird swirling winds around the chimney and this can cause it to not draw properly. Only solution is to add 5 or more feet to the top of the chimney to give it more drawing power. Our poster did that temporarily and it solved the problem. There is no substitute for real world experience. His problems occured on certain days when it was cloudy and there were changes in the barometric pressure which can and does affect chimneys and their ability to draw.
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Old 08-01-2011, 08:18 AM   #5
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House pressurization & fireplaces


In most cases if the chimney is not drawing adequately it is a function of the design of the fireplace, smoke chamber, and chimney, not the ventilation into the house. Chimney fans are, as you found out, very expensive and they are usually employed to aid proper drawing in poorly performing fireplaces, often found older houses where the fireplace and chimney were originally designed for burning coal, not wood.

In general, there is a design relationship between the diameter of your chimney flue, the shape and size of your smoke chamber (out of sight, between the fireplace opening and the flue), and the size of your fireplace opening. Too large a fireplace opening for the area of the chimney (the size of the cross section of the chimney flue) and it won't draw properly.

There is a good chance that he size of your fireplace opening is too big for area of your chimney. The least expensive way of testing this is to light a fire and block off the top part of your fireplace opening with a board. If the fireplace draws properly then that is your problem. You can move the board up and down to find the optimal resizing of your fireplace opening and then buy a smokeguard of the appropriate size. This will also solve some draw problems born out of a poorly designed smoke chamber. A lot cheaper than a fan or masonry changes.

http://www.amazon.com/Chimney-Plus-8.../dp/B002TNW8UG

Last edited by Ironlight; 08-01-2011 at 08:21 AM.
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Old 08-01-2011, 11:10 AM   #6
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Thanks for your post. I thought about the size of the fp, but it is a prefab unit. Dimensions are good. Right now, I can open the flu and withe a/c on, outside air rushes into the house from the chimney to the point of a tissue held in front of it is blown at a 90 degree angle. That's what makes us think there isn't enough air in the house. Steve
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Old 08-01-2011, 11:25 AM   #7
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Something may be drawing air out of your house. Do you have exhaust fans running continuously or the furnace fan running continuosly? Radon exhaust fan running non stop? Where I am they have one central exhaust fan that does all the bathrooms and kitchen and people forget one of the switches on and it depressurizes the house. If you have a HRV or ERV and it is unbalanced then same thing can happen. HIGHLY unusual that a large amount of air is escaping on its own.
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Old 08-01-2011, 01:19 PM   #8
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The room with the fireplace is large. On the opposite wall is the cold air return for the downstairs a/c unit. It is 24" in diameter. Just upstairs from this return is the ceiling mounted return for the upstairs unit. 12" in diameter. I believe the fp is the easiest source of air and that the output sources of air are too far or too few, which is impacting the draw on the fp.

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