Chimney Cap Needed?

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Hi, I recently to get an old fireplace up and running. It's a brick and mortar fireplace with a newly installed gas log set in it. Prior to installing the gas logs, I had the flue closed up to keep the cold air out. Once I opened it, I got cold air coming into the house.
I'm a little concerned that the air is going in the wrong direction, especially if it gets windy (we get a lot of strong wind in my area). It doesn't seem like a strong draft, but it's enough to get me concerned about the chimney not doing its job correctly.
I went on the roof and looked at the chimney. I noticed that there was no cap on it, whereas just about every other chimney that I could see from my roof had one.
I looked down the chimney, and it was clean. There's no cracks or anything I could see in the chimney inside or outside the chimney itself.
The chimney is on the side of the house where the roof is at its lowest point. There is nothing blocking the chimney in the wind's path...no rooftop, no trees, nothing.
My question is, should I install a cap on the chimney? I would think this would do the job, but I would like to get some advice before I tinker with something I don't know much about.
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mike wrote:

Basically, it's not unusual for cold air to come down a chimney when the damper is open and no fire is in the fireplace -- that and to prevent losing warm air the other way is why dampers are there to begin with. :)
Does the chimney draw when the fireplace is in use? If so, I'd not be concerned. It's not possible to tell for sure, of course, whether the situation is now different than before, but one possibility is if the house is older and has been updated/remodeled that included weatherizing so what used to draw well doesn't any longer. With air tight houses, it's quite unremarkable that there isn't enough inleakage to provide adequate draw. If you open a window a crack, does the chimney then draw? If so, you've just demonstrated the above.
A cap _may_ help a little, it may not. Their primary function is keeping the weather and birds/vermin out, with a corollary function of some spark collection and draft control. I'd probably add one if you plan on using the fireplace simply to avoid the swallows building nests (as they do in mine every year and every year I say I'll fix it before next spring and forget until they've already got the nests established. Maybe w/ your post reminding me I'll get the lift out and get up there and do something about it this year!) :)
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My comments are in-line below:
dpb wrote:

It looks like it does normally. I lit some newspaper to get a smoky fire going and the smoke went up the chimney. This was when it was not windy though. I will try that test again when the wind kicks up.

The house was built in the 60's and has been through a few major earthquakes, so it's definitely not airtight. It passed inspection when I bought the house in 2000.

I see. I haven't had a problem with pests getting in yet, but it's probably a matter of time before they do. If it doesn't hurt, I'll probably put a cap in, just for that reason alone.
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mike wrote:

Cap won't help draft, but some sort of screen/damper could help keep wild critters out. Raccoons have made it down chimneys.
Draft occurs when column of gas in flue is sufficiently lower in density (read: warm) compared to outside. What happened when you heated the flue? How did you close/open the flue? Damper at throat, or up top? (Note that closed damper is _good_thing_ when fire is out.)
Common causes of reversed draft: negative gauge pressure in house caused by various discharges- such as another fireplace, open attic window or exhaust fan; chimney top too low for surroundings in wind.
Short chimney doesn't help, either.
You might consult a local expert, who can put eyes on it, and suggest action.
J
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On 28 Nov 2006 09:47:58 -0800, snipped-for-privacy@sme-online.com wrote:

Actually, a cap causes the chimney to warm up faster when you first start a fire, and thereby improves the draft.
CWM
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My comments are in-line below:

A damper at the bottom (is that the throat?). It looks original, and it's not airtight, but it certainly does a better job keeping the cold air out when it is closed.

That's great to know! I have a whole house fan, but I only use it when the flue is closed. The fan is great at demonstrating how non-airtight my house is. :)

The chimney is about five feet taller than the point at the roof where it is resides. It looks about as tall as all the others in a neighborhood of tract housing.

The inspector didn't have a problem with it, and I do remember seeing him measuring it (not sure why). I'm not sure if he would be considered an expert on the matter.
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The first time you get a bird or a squirrel down the chimney you will have the answer. It does not happen often, but it does happen. Be sure to get one made from stainless steel. The painted ones will rust and stain the chimney as it runs down.
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Gas logs are nice.
You can usually operate the in a "ventless" mode. The ones I have are like that.
IF your logs can be operated in "ventless" mode (i.e.: they shut off when the O2 level falls and they tend to burn with grossly excess oxygen) then there is little or no point in opening the flue.
If you want to flue to start "working" you have to get it warm/hot. That means you operate you gas logs at maximum output and gradually open the flue. When the flue "works" you are wasting most of the heat the gas logs produce. Fireplaces aren't particularly efficient; they are just for looks and feel. You get the same effect from the ventless logs.
You should get a CO detector (preferably "digital") just in case but Gas Logs just don't put out ANY CO.
The main reasons for the cap is to keep critters and rain out. Some of then can take on the extra job of completely closing the flue but you can already do that by other means.
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I would investigate this claim thoroughly before I trusted it.
Bob
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I can't use ventless logs. I'm in California. I believe they are illegal here.
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That's too bad.
Nonetheless, they do work. Modern "ventless" heaters are quite safe.
But it still a "good idea" to get a CO detector.

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I would think they would work, but I don't want to get any problems from inspectors.
I do have a CO detector on the opposite wall from the fireplace. I might put some in near the bedrooms and near the garage.
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Once you have an approved installation, I just don't see how the "inspectors" would know about your not opening the flue.
If you get at least one battery powered "digital" CO detector and move it about the house you will discover where CO usually comes from.
Incense, for example, generates a good amount of CO. (Families have been wiped out by bringing a charcoal grill into the garage.) Gas stoves generate essentially no CO. But burning food on any kind of stove will generate a reading. By a "reading" I mean something on the order of "10" which is just on the range of detectability. With the exception of the charcoal or coal burning the most likely source of deadly amount of CO come from VENTED fuel burning appliances that have failed. Even then, it often requires more than one failure mechanism: the burners are set too "lean" AND something has permitted the combustion gasses to enter the living space.
(Some years ago I had a run in with some stupid women from the gas company. They had turned off the gas for some outside repair work and I got lazy and decided to let them re-light the pilot lights. Showing that a little knowledge is a dangerous thing this silly woman stuck her CO detector in the top vent of the water heater. All those gasses went up the stack. But she read a 40 on her meter (which is low enough that had it filled the entire house we likely would have had no ill effects) and she "red tagged" the water heater. I took the tag off and complained to the gas company. Never heard back.)
Bottom line is that VENTED appliances normally produce a small amount of CO.

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Well, you can "investigate" all you want.
IF you use a ventless appliance (including a gas stove) then you should have a CO detector. You will soon find, however, that you generate more CO by burning dinner in an ELECTRIC oven than heating the whole house with ventless heaters.
Gas burning appliances that are designed to be vented are also designed to minimize the amount of hot gas that goes up the stack. Among other things this means that the amount of air introduced to the gas is kept at a minimum. A miscalculation means either excess heated air goes up the stack or you generate the deadly CO.
But there is no reason not to permit a ventless burning all the air it could posssibly use and then some. All the heat stays in the room. And that's why ventless heaters are very safe regardless of what the authorities in California think.
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Not only do you need a cap but you should also cap the top of the top bricks so moisture has less ability to soak them.

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Art wrote:

thousand dollars damage.
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There's mortor on top of the bricks. Does that help? From what I'm hearing, it sounds like a cap is the way to go, but for different reasons than what I was expecting.
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The top brick should have a metal cap on it. Most don't and that is one reason why people have chimney problems.

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If your concern is that cold air comming down the fireplace chimney and you need a cap anyway you may want to think about a lock-top chimney top damper for around $550 installed. Or if you are just wanting to insulate your damper and help it do its job better look into a chimney balloon for $40. Either will help you limit the cold air infiltration and I have seen some people use both.
A stainles cap would be good for keeping the weather elements and vermin out, but wont help you with cold air infilttration.
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Yes. Here's why:
http://www.milmac.com/BlockedFlue.JPG
--
Regards,
Doug Miller (alphageek at milmac dot com)
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