CO problem today

While sitting in my family room this evening, I started to smell flue gas from my coal boiler. The coal boiler is vented in an 8" square terra cotta flue in a brick chimney. In the same chimney is a 12" flue for the family room fireplace. I was getting a down draft down the fireplace flue because of the warmer temps, rain and low barometric pressure. It was a faint odor but I stuck my co detector on the hearth at the fireplace and with a minute it was rising fast to almost 200. I didn't wait to see how high it was going to go. I opened all the windows and it dropped within a minute. The fireplace damper was closed but they are not air tight. So I climbed out on the roof in the rain with a piece of aluminum foil, removed the chimney cap on the fireplace flue, covered it with aluminum floil and reinstalled the cap. Problem solved for now.

coalkirk wrote:While sitting in my family room this evening, I started to smell flue gas from my coal boiler. The coal boiler is vented in an 8" square terra cotta flue in a brick chimney. In the same chimney is a 12" flue for the family room fireplace. I was getting a down draft down the fireplace flue because of the warmer temps, rain and low barometric pressure. It was a faint odor but I stuck my co detector on the hearth at the fireplace and with a minute it was rising fast to almost 200. I didn't wait to see how high it was going to go. I opened all the windows and it dropped within a minute. The fireplace damper was closed but they are not air tight. So I climbed out on the roof in the rain with a piece of aluminum foil, removed the chimney cap on the fireplace flue, covered it with aluminum floil and reinstalled the cap. Problem solved for now.

Hmmmm......Not sure if I'm following your problem here: Why would a downdraft in your 12" fireplace flue cause a back-up of coal boiler gases vented in a separate 8" flue? (Wouldn't just increasing the boiler fire temp solve the problem by increading the draft in your 8" flue?)

OK..Never mind!! I misread the part about smelling the flue gases when sitting in your family room, caused by the fireplace flue downdraft sucking in the boiler gases. Makes sense now.

It is not uncommon to have a backdraft in multiple flue chimneys, I have experienced it myself. The best cure is an extension of the solid fuel device's flue. A terra cotta chimney topper can add a foot or two and be installed easily. A flat, vertical plate across the chimney between the two will too, but probably isn't all that attractive.

Can you fit the cap to the coal's flue? That may cure it also.

I have 2 fireplaces with dampers and a furnace each with its own flue but in the same chimney and I have had wood smoke come back down the 2 flues that were not being used. One of the things that has helped was when I run either fireplace I crack open a basement window.

coaledsweat wrote:
Can you fit the cap to the coal's flue? That may cure it also.

Sounds like a great idea to me! (Sure beats climbing up on the roof in the rain!)

Something like this might help. http://www.woodlanddirect.com/Chimney/Top-Sealing-Chimney-Dampers/Lyemance-Damper-Energy-Saving

All the above ideas work, but the cause is a slight negative pressure in the house pulling the downdraft in the fireplace's flue.

How about a full time outside air source for the combustion air blower in the coal boiler's stoker?? this will reduce or eliminate the negative draft on the house,, reduce cold-air infiltration all winter long, and probably make the house more comfortable [fewer drafts around doors and windows]..

Sure glad you caught it in time,,,

Greg L

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Also an option.
http://www.google.com/url?q=http://www. ... u4_GcHCamw

Thanks for all the replys. I've got three CO dectectors, one on each level so if it had gotton really bad, I would have at least been alerted. I rememebr a post by Yanche awhile ago about the threshold it takes for these damn things to sound. I'm going to go back and reread that today.
I'm going to take alittle advice from several of you. My first thought was the top mount spring loaded damper on the fireplace flue as Dallas suggested. By installing that and keeping the fireplace damper open, the flue will stay warmer and be less likley to experience a downdraft. Adding an outside air source to the coal boiler will help somewhat but there are alot of things in a home that cause negative pressure. Bath and kitchen exhaust fans, clothes dryers, radon mitigation system, etc. In many commercial buildings, there is an outside air source into the building which is slightly pressurized. That keeps the buildings interior under postive pressure. I might experiment with that. If I could find an attractive terra cotta extension for the coal boiler flue, I would do that if the other measures don't work. Here's the offending chimney. Luckily I walk on roofs in the rain for a living. Now you've got to wonder. Is he a sniper or a home inspector? Maybe both

coalkirk wrote:Thanks for all the replys. I've got three CO dectectors, one on each level so if it had gotton really bad, I would have at least been alerted. I rememebr a post by Yance awhile ago about the threshold it takes for these damn things to sound. I'm going to go back and reread that today.

It's not only a problem with the level at which the CO detector sounds it's the reliability of the CO sensor itself. I'm not taking about the entire instrument but the tiny sensor within the instrument that converts CO gas to electrons. There are only two companies (FIGARO ENGINEERING and MST Technology GmbH) worldwide that make consumer level CO sensors and they are not very good. There is a large variation in unit to unit sensitivity and a short overall lifetime. Your CO detector is just about worthless for reliability detecting the level of CO that can harm you 5 years after first being manufactured. I've collected a lot of engineering info on the subject and when I get time I'll post my findings. What's needed is a simple test to verify the functioning of your CO detector. I'm thinking something like putting the CO detector in an enclosed box with a burning candle and timing how long it takes to go off. Any combustion chemists on this forum that can calculate the CO produced by a candle and provide a CO concentration vs. time graph for a specific enclosed volume?

There was an article in a recent edition of This Old House magazine regarding chimney pots, which were apparently very common in the early 1900's when more people heated with coal. There are apparently people who want to use them again as a decorative item or to use it for other things like table supports or yard fixtures. This would seem to be an idea for your problem. There were dealers mentioned for new and antique pots in NJ and PA. One website given was http://www.chimneypot.com

No affiliation obviously, but seemed like a good idea.

I really like the look they give also. Might not work archetecutrially on my home though. check out these copper chimney caps. http://copper-by-design.com/fs/cc.htm

coalkirk wrote:I really like the look they give also. Might not work archetecutrially on my home though. check out these copper chimney caps. http://copper-by-design.com/fs/cc.htm

Those look nice when they are new but what will they look like in 2 or 3 years when the copper gets oxidized?

I guess beauty is in the eye of the beholder. Aged copper is beautiful to me!