We have a 10 year old vented gas fireplace. The fireplace, which has been cantilevered out from the foundation, sends the exhaust up a chase which extends several feet above the roof line. The chase is OSB covered with felt paper and cheap vinyl siding. Underneath the chase from the outside, you can see the uninsulated OSB bottom. The space directly underneath the fireplace is also uninsulated (between the first floor's joists).
I can see un-papered batting under and around the firebox (top and bottom louvers.) I also hear intermittent "pings" when it rains, as it has done steadily here for the past four days. I think it is rain drops hitting the top of the fire box.
Last January, there was an incredible draft coming through the fireplace, which made it impossible to raise the thermostat on the first floor. In order to heat the first floor, we covered the fireplace with taped Hefty bags which were most unattractive, but mildly entertaining since the bags moved in and out and gave the fireplace a life of its own.
We had two fireplace companies take a look and both recommended that we install a new direct vent fireplace (Mendota brand) and vent out the back, insulate the chase, cap the top, etc. This would require us to remove and re-cut our granite surround, which consists of one giant piece (no mantle). The full replacement would cost approximately $10,000 (tear out, install, granite, electrical, permits, etc).
I have scoured the internet but have come up empty with respect to finding a less expensive, less invasive, option. My wife would like to forgo the Hefty bags this winter.
I think you have 3 separate and often concurring problems.
2. A poorly constructed, non-insulated, and drafty chimney chase is acting like an “open window”.
3. A water leak(s) in the chase.
Diagram A Courtesy of Fireplacenetwork.
1. I believe your most serious problem is “Downdrafting”.
Downdrafts force the hot exhaust gases that are trying to flow up and out of your fireplace back down the flue liner and into your living area.
Downdrafts occurring when the fireplace is in use will cause carbon monoxide poisoning!
If you don’t already have them, get yourself several carbon monoxide detectors for your house.
Downdrafting: Downdrafting is usually caused by a chimney that is too short. That puts the flue opening directly in the path of wind vortexes. Wind vortexes are created as wind passes over roof ridge lines, trees, mountains, etc. The greater the velocity of the wind, the greater the vortex. This vortex wind creates downdrafts in your chimney flue.
Diagram B Courtesy of FireplaceNetwork.
The minimum (One often needs to go even higher) height a chimney should be is 2 feet above any roof surface or obstacle within 10 feet of the top of the flue. (See Diagram B)
Your chimney may be too short. If so, another section of flue would raise the height of the termination cap 3 feet, and I think that will help ward off down drafting, or you could replace the termination cap with a down draft proof one or do both, raise the height and recap.
2. Drafts: The chase appears to be poorly constructed and is probably the cause of severe drafts around the firebox inside your home. These drafts are not to be confused with the down drafting problem.
I would have the vinyl siding removed (carefully, so that it can be reused and reapplied), have several of the OSB panels removed so that the chase can be properly fire stopped (see image), the chase insulated, the firebox chalked with fireproof chalk from behind for draft prevention and then, put the chimney chase back together and have the chase wrapped with house wrap (such as Tyvek) to keep the wind out, then reside with the vinyl siding.
3. Water Leaks: You have a leak (s) in the chase that is evidenced by both the photo of the underneath of the chimney chase and the “pings” you hear. The photo shows water damage (rot) and shows that the leak has been occurring for awhile.
The leak (s) may be in the chimney chase itself or more likely, at the top of the chase where the chase cap (called pan flashing in the diagram) may have rotted away.
The cap appears to be made of galvanized metal and galvanized metal doesn’t last that long before it starts rotting away.
If you replace the cap, use stainless steel or aluminum.
The chimney termination cap collar could also be leaking. (See diagram A)
The water leak could also be coming from the area where the roof meets the chase.
There is supposed to be a saddle (also called a cricket) that sheds rain water away from the chase (see image below).
Courtesy: Licitra Roofing
You should hire a LOCAL professional, such as a Professional Home Inspector, Home Builder (General Contractor), or an Architect to look at these problems (and any I may have missed) and help you make decisions as to the best course of action. This should not be too expensive.
I don’t think that the cure for these problems, even if you had to fix all of them, would be anywhere near the cost of replacing your fireplace. A good carpenter for the chase work (including adding insulation) and a good HVAC contractor (for the flue, cap, pan, firestop and adding an additional 3 foot section of flue if necessary) should be the only people you need to hire. The cost of insulation and additional flue parts, and even a new pan flashing and termination cap shouldn’t be too bad.
Carl Heldmann, byoh.com