Sealed flue - am I going to have problems?

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chimneymac

New Member
Dec 1, 2020
5
New Jersey
Had the top of my 120yr old chimney rebuilt. One of the flues is only about a bricks wythe, unlined, and unused. This corridor is was open from the top all the way to the basement and not connected to anything but it is open in the basement.

The contractor took the liberty of ignoring the designs and closing this by sealing it at the top with brick and mortar. I was not pleased.
Does this flue serve no purpose and it is ok to be sealed or do I need to break it back open so that it acts as a vent? Lot's of information out there on not sealing old flues that have been burned in but does the same logic apply here (moisture/ventalation problems, etc)?


Cheers!
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
24,217
central pa
Had the top of my 120yr old chimney rebuilt. One of the flues is only about a bricks wythe, unlined, and unused. This corridor is was open from the top all the way to the basement and not connected to anything but it is open in the basement.

The contractor took the liberty of ignoring the designs and closing this by sealing it at the top with brick and mortar. I was not pleased.
Does this flue serve no purpose and it is ok to be sealed or do I need to break it back open so that it acts as a vent? Lot's of information out there on not sealing old flues that have been burned in but does the same logic apply here (moisture/ventalation problems, etc)?


Cheers!
I always try to leave a little bit of air movement but it probably doesn't matter that much in most cases.
 

Bad LP

Minister of Fire
Nov 28, 2014
1,563
Northern Maine
In an older home such as yours it’s likely that enough air leakage prevents any combustion problems.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
24,217
central pa
Wow, that makes sense now! I had not considered that this was for air/combustion. Sounds like it's not a catastrophic issue then.
It isn't for combustion air. It is about moist air from the house getting in there and condensing
 

gthomas785

Feeling the Heat
Feb 8, 2020
413
Central MA
In some cases having a small amount of air flow can cause the most damage. Imagine warm air from your house wafting up the chimney but not enough to keep the flue warm. That air reaches the cold exposed part of the chimney and it's condensation city. Water soaks into masonry, freezes, and causes spalling on the outside, and mold on the inside.

If you have more airflow, it is usually enough to keep the inside of the flue warm and avoid condensation, but at the expense of greater energy loss. Conversely, if you create a total vapor barrier from the inside, and ventilate it to the outside then you won't get moisture problems either.
 
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Bad LP

Minister of Fire
Nov 28, 2014
1,563
Northern Maine
It isn't for combustion air. It is about moist air from the house getting in there and condensing
Yours is a secondary thought but also correct. We have a fresh air chase in the chimney for a big house that has three boilers in the boiler room. Actually there are 2 fresh air chases in and 4 flues out.
 

chimneymac

New Member
Dec 1, 2020
5
New Jersey
If you have more airflow, it is usually enough to keep the inside of the flue warm and avoid condensation, but at the expense of greater energy loss.
Some darn good information in that statement.

Any way to measure this or be scientific? Seems similar to HVAC. Any calculation or guidance that takes into account ambient temperature , room temp, and temperature at the top of the flue?
 

gthomas785

Feeling the Heat
Feb 8, 2020
413
Central MA
Some darn good information in that statement.

Any way to measure this or be scientific? Seems similar to HVAC. Any calculation or guidance that takes into account ambient temperature , room temp, and temperature at the top of the flue?
Well the basic idea for a wide open airflow path is that you need to keep the flue temp above the dewpoint of the interior air.

When you damp down the flow to nothing it gets a little trickier because whatever small amount of moisture gets introduced could be diffused through the masonry so the dewpoint goes down over time. I think it would depend on the bulk moisture in vs moisture out rates. IMO It's easier to just seal off the thimble at the bottom, and put on a flue cap and be done.
 
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bholler

Chimney sweep
Staff member
Jan 14, 2014
24,217
central pa
Yours is a secondary thought but also correct. We have a fresh air chase in the chimney for a big house that has three boilers in the boiler room. Actually there are 2 fresh air chases in and 4 flues out.
You can only bring in combustion air through the stack like that if the appliance is vented through the chimney and the appliance is a fan assist appliance that lets you pull supply from that height.

If it is just an open chimney it would most likely depressurize the room not supply combustion air. Because the op said the chimney was unused it cannot be used to supply combustion air
 
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Bad LP

Minister of Fire
Nov 28, 2014
1,563
Northern Maine
You can only bring in combustion air through the stack like that if the appliance is vented through the chimney and the appliance is a fan assist appliance that lets you pull supply from that height.

If it is just an open chimney it would most likely depressurize the room not supply combustion air.
The 3 oil fired boilers were installed long before power venting but they are vented thru the same masonry chimney. The second chase takes heated air from the top of the 40' atrium and brings it back down to the same boiler room then back into the garden space. In the winter that solar heated air also runs thru a FCU and the boilers add more heat to the flow. Works perfectly as designed.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
24,217
central pa
The 3 oil fired boilers were installed long before power venting but they are vented thru the same masonry chimney. The second chase takes heated air from the top of the 40' atrium and brings it back down to the same boiler room then back into the garden space. In the winter that solar heated air also runs thru a FCU and the boilers add more heat to the flow. Works perfectly as designed.
What is pulling the heated air down the chimney?


Regardless of whether yours works or not it is a code violation and generally doesn't work well.
 

Bad LP

Minister of Fire
Nov 28, 2014
1,563
Northern Maine
What is pulling the heated air down the chimney?


Regardless of whether yours works or not it is a code violation and generally doesn't work well.
The FCU recirculates the hot air.

Somebody better go tell the architect and the builder. I've been on this site now for 33 years, 2 while under construction and 31 as the property manager and we have never had a single issue with anything boiler, air, CO, bad combustion or moisture related.
In fact the cold air rushes down into the boiler room so fast I slide a piece of plywood in front of the opening for the winter to slow it down.
I do however use a 8" duct booster fan in the flue that once had an oil fired hot water tank that was removed years ago to remove the excessive heat in the warmer months. In the winter I want that extra heat inside the envelope so I unplug it.
 

Highbeam

Minister of Fire
Dec 28, 2006
18,803
Mt. Rainier Foothills, WA
For the Op, I would want this void sealed top and bottom. I expect sealed voids in chimneys like those between clay liners and the outer brick.

Hard to imagine a natural draft appliance sending dependably sending exhaust up one flue and somehow sucking combustion air down another flue with the same inlet/outlet height.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
24,217
central pa
The FCU recirculates the hot air.

Somebody better go tell the architect and the builder. I've been on this site now for 33 years, 2 while under construction and 31 as the property manager and we have never had a single issue with anything boiler, air, CO, bad combustion or moisture related.
In fact the cold air rushes down into the boiler room so fast I slide a piece of plywood in front of the opening for the winter to slow it down.
I do however use a 8" duct booster fan in the flue that once had an oil fired hot water tank that was removed years ago to remove the excessive heat in the warmer months. In the winter I want that extra heat inside the envelope so I unplug it.
What is an FCU?


But regardless if I am understanding you correctly your second chase is pulling air from inside the building correct??? If so what does that have to do with the op's 120 yr old chimney that is unused? Your system which sounds like it was probably designed by a mechanical engineer is very unique and not typical at all. And you can't simply pull combustion air down through a chimney unless the appliance is designed to do so or an entire system is designed to make it work
 
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bholler

Chimney sweep
Staff member
Jan 14, 2014
24,217
central pa
For the Op, I would want this void sealed top and bottom. I expect sealed voids in chimneys like those between clay liners and the outer brick.

Hard to imagine a natural draft appliance sending dependably sending exhaust up one flue and somehow sucking combustion air down another flue with the same inlet/outlet height.
If one could completely seal the entire chimney that would be great. But generally I deal the bottom and leave a small gap at the top to allow moisture to escape.
 

clancey

Minister of Fire
Feb 26, 2021
708
Colorado
I do not know if this has anything to do with your sealed flu but I have a black pipe encased in brick coming from the bathroom and I think it was a venting system of some kind and used for the water pipes from a 1926 bathroom--its all surrounded by brick and goes up through the attic ( I made the attic into a room with walls and flooring and insulation)--so it changes into just a big black pipe that I think is now used for venting from the bathroom but the point is water gathers on it in a wind rain storm and slides down the outside of the pipe until it reaches the elbow--then drips on the floor--pretty good amount at times to where i have a pan underneath it ..Maybe that's why he suggested sealing the flu because of leakage or maybe it leaked before? I hope this is the right category of thinking here--clancey--I am trying...maybe..clancey
 

stoveliker

Minister of Fire
Nov 17, 2019
783
Eastern Long Island NY
your pipe is likely the drain vent - so that you can flush your toilet and the air in the drain line can escape. Old technology - and still needed and useful today.
The leak there has to be fixed on the roof with a proper (flashing) boot.
 

clancey

Minister of Fire
Feb 26, 2021
708
Colorado
Thanks stoveliker that exactly what it is and now that I know it can be fixed with ( flashing ) boot I will get that attended to--thanks again clancey