Capping unused chimney

PC7060

I’ve got a dual flue chimney about 30’ high from foundation to top, bare brick, no masonry liner in either. One flue is for a old coal fireplace which now has a rarely used ventless natural gas log set. The other served the main heating boiler until a couple years ago when I install a modulating condensing that now vents out through a side wall.

A recent storm (a real tree killer) ripped the large copper cap off leaving the fireplace flue open to the weather. The boiler flue has a cap attached to the stainless flue liner and appears to be in good shape.  

My options are to: repair and reinstall the large cap which will take some work after a 30’ fall but is doable; 
Or install flashing to fully close off the fireplace flue. 

Is it a advisable to close off unused chimneys? I recall an inspector telling me chimneys need to breath to prevent moisture buildup and freeze cracking. 

Jamie Hall

An unlined chimney really should be able to breathe -- a little. But it doesn't have to be a lot. More important, a completely open flue will allow the occasional bird to get inside the house, which you do not want. I'd rather see you reinstall a cap -- or just a good cover (doesn't have to be raised -- wire mesh anchored to the chimney will do -- at the top and, to prevent losing too much air, a damper at the bottom. But that's me.

EBEBRATT-Ed

I agree breathing is good. You could have any good metal working or sheet metal shop make a new cap for that frame.

kcopp

I had two chimneys in my last home. Built in 1800.
1 was used and lined (SS) w/ an oil boiler.
The other totally unused and unlined... It would leak down the flue during heavy rain storms.
I had my neighbor ( a mason ) place a stone slab cap over it. It never gave me an issue again. 18 years later it's still good.

PC7060

Jamie Hall said:

An unlined chimney really should be able to breathe -- a little. But it doesn't have to be a lot. More important, a completely open flue will allow the occasional bird to get inside the house, which you do not want. I'd rather see you reinstall a cap -- or just a good cover (doesn't have to be raised -- wire mesh anchored to the chimney will do -- at the top and, to prevent losing too much air, a damper at the bottom. But that's me.

@Jamie Hall - fortunately the fireplace has a good damper installed and it closes snugly. The chimney guy recommended I seal it but other than buying the high-temp sealant the job hasn't made it further up the list. 

I'm adding a cap with small two-way breather vent as you and @EdTheHeaterMan recommended.  Not as brave as kcopp so don't want to permanent seal it!

Steamhead

@PC7060 , "ventless" heaters can be a major carbon monoxide source. I'd never put one in my house. That's what chimneys are for.

PC7060

Steamhead said:

@PC7060 , "ventless" heaters can be a major carbon monoxide source. I'd never put one in my house. That's what chimneys are for.

Yes, I’m not a fan either. The gas insert has been used once just to test the gas line. 

PC7060

I’ve reworked the original chimney cap/hood and am ready to attach to chimney. What is the groups preferred fastener for this application?

Hood is pretty large and the current plan is use tapcon masonry fasteners; 2 or 3 on the 3’ long sides plus 1 on each end.  

Last time it looks like 2-3 total were used. 

Bob Harper

The cap blowing off is an insurable loss. If your deductible is high or no other damage then it's on you. Yes, all chimneys 'should' have rain caps and mesh animal guards but they are not code required. However, if you're going to fabricate and install one, refer to NFPA 211. I agree with leaving the chimney open so it can dry. You also want 18ga. mesh to keep out the critters.
You can refer to chimneycapsdotnet for a good assortment of shapes and sizes. In your case, an outside mount with a lid overhang of about 3" would help keep your chimney dry. The outside mount moves the drip line further out but also allows for installing Tapcons from the sides, which is much stronger than through a top mount flange. Still, the integrity of the top course of brick should be assessed and repaired as needed.
ANSI Z21.11.2b allows 200 ppm CO-AF on ventfree logs. We don't sell them as no building scientist I'm aware of endorses them and ASHRAE is trying again to get them banned.

PC7060

Figured I’d update with current status to close this thread out. 

I called a local chimney sweep and described I chimney hood attached to my chimney. Once he found out the chimney was a single brick layer with no lining and over 100 years old he politely declined. The sweep said the liability associated with attaching a large chimney hood was very high as the hoods tended to blow off in high winds and cause significant damage to people and property. Which I thought was interesting because that was how this situation began. 

The sweeps advice was to put caps on both flus to limit water infiltration and allow proper venting. He advised i had proper flashing over the top with drip edge flashing 3 inches down the side. He also recommended that I use surface applied chimney waterproofing.  

I followed his advice and result was very good.  Just need apply the water proofing this afternoon to complete the job. 

JUGHNE

Handy roof hatch.....was that planned for this?

PC7060

JUGHNE said:

Handy roof hatch.....was that planned for this?

Yes, I snuck it past the wife during the renovations so I’d have easy access to roof. Usually without her knowing. Much easier than sneaking a 24’ ladder around the house. 😆