Defintition for exterior masonry chimney

Bob Harper

Forget all this interior vs. exterior stuff. As far as venting heating appliances into masonry chimneys goes, they ALL need a liner. This is for function but also for code. We have hashed this one out so many times and it still keeps popping up. Reasons to reline:
-sizing
-masonry flue not capable of its intended function
-contain flue gases
-contain condensation
-rapid heat up for more stable draft
-required by every major code and standard
-makes sense

You guys better enjoy these few short yrs left as the code community will be coming down on you before long and begin enforcing this. With efficiencies going up as home ACH go down, you'll see a rise in litigation over contractors who should have relined but didn't. Problem is, more people will have to get sick until things change.

Boilerpro_5

Chimneys

I see this situation alot, new 80% gas fired draft hood boilers being installed into nearly 100 year old clay lined chimneys that are 30 to 40 feet tall, with 10 to 15 feet exposed in the attic and about 3 to 5 above the attic. The International code defines an exposed chimney as one exposed to the outdoors below the roof line (I believe) ...does this include exposure in the attic? I see lots of potential trouble with these chimenys and I often lose bids to other contrators because they are slapping in cheap chimney vent boilers into these chimneys, leaving the old gas fired water heater in place,and I recommend power vent units and an indirect.

Boilerpro

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mark_46

Boilerpro

I can only share my story as it relates to your comment about leaving the old water heater.

In my case, I installed a Munchkin (direct vent of course). When the borough inspector reviewed the plans, he asked what will be done about the water heater. We said what do you mean? According to code my borough uses, you can't vent just a conventional water heater out an exterior chimney. (FYI mine is about 30 feet tall too) I guess because condensation would be too great. So this forced us to take action on the water heater. I went with an indirect.

Boilerpro_5

Also add

84 % induced draft boilers being vented into these chimneys. See this too.


Boilerpro

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Constantin

Ditto

We had a chimney flue serve a 50 Gallon gas water heater and a space heater in the kitchen when we bought the house. While situated at one end of the house, the chimney was 100% inside the house save for a bit sticking out on top (and a short transition through a cold attic). The chimney was made of brick, single-wall, with no liner on the inside (i.e. just brick and mortar) and it ran from the basement up three stories.

The bricks were saturated 94%+ even on the second story (the heater was in the basement). In other words, even a chimney inside a home will fall prey to an "orphaned" water heater if it's not lined, oversized, etc. In the basement, we had fist sized holes in the chimney from where all the freeze/thaw had made the brick spall and disintegrate.

Based on my limited experience, I would try to talk a HO into adopting a properly-lined flue (size, material, and all) for their heating equipment so that their masonary chimney doesn't do the same thing that ours did. We had to remove ours and start over, it was too far gone.

Mark_46

Constantin

In my case, for starters my chimney has two 'chambers'. One for what was the boiler and water heater, the other for a fireplace. Both were built with clay liners (i think they're clay?).

But as far lining the chimney, in my case I chose the indirect mostly becasue its superior to a conventional heater but also because a stainless steel liner would have cost more money! Considering both of those facts, it was a no-brainer.

What I did struggle with a little was opting for the indirect over a draft inducer for the existing conventional heater which was definitely less costly. But I figured, "do it once, do it right", right?

Rodney Summers

Boilerpro

Manufacturer's instructions for today's gas-fired appliances specify very limited conditions under which the appliance is allowed to vent to a masonry chimney. The conditions include chimney size, state of condition, location and construction. Because many existing masonry chimneys are oversized, unlined, in poor structural condition or partially exposed to the outdoors as was reported in your post, they might not be suitable to serve gas-fired appliances. Masonry chimneys serving to vent fan-assisted gas-fired appliances are becoming rare. In many cases, a masonry chimney must be relined with a retrofit metal liner system, or vent such as Type B vent will have to be installed within the flueway of the chimney. Masonry chimneys have a large thermal mass that has to be heated above the dew point of the combustion gasses to avoid condensation. The flue gas temperatures discharged from many appliances will not be high enough to sufficiently warm the chimney mass, especially where a chimney is oversized and/or extrior.

If you look back into older posts on the wall you will find alot of info about this very topic. One I'm particularly fond of is about our ol' friend Goldie Locks, see...

http://forums.invision.net/Thread.cfm?CFApp=2&&Message_ID=118595&_#Message118595


You are correct when you say that the International Code defines the (exposed)"cold chimney", and the Code does convey this within its tables. As I've stated above, the manufacturer's limit the conditions to which these appliances can be vented and these chimneys are of particular intrest to me (and obviously you).

The tables (vent sizing) assume that the only portion of the chimney or vent thats exposed to the outdoors is the portion of the chimney that extends above the roof penetration. Even though an attic is technically not outdoors, the temperature in the attic can be very close to the outdoor temperature. In some climates, the actual attic temperature can be far lower than the assumed temperature used in the computer modeling on which the sizing tables are based. Therefore, the portion of a venting system that passes through the attic should be kept to an absolute minimum if your the contractor actually installing one new. The code maintains that the listed tables be applicable ONLY to vents and chimneys that are NOT exposed to the outdoors below the roof penetration. If the vent or chimney IS exposed to the outdoors BELOW the roof penetration, the listed sizing tables don't apply. The tables for exterior masonry chimneys obviously apply to chimneys that are exposed to the outdoors below the roof line, and the list of conditions intends to compensate for the cold masonry. Conditions such as requiring double-wall connectors and draft-hood equipped appliances are intended to put as much heat into the chimney as possible because it will be needed to produce draft and minimize condensation in the less-than-ideal chimney. Exterior chimneys require greater heat input to make them work. An alternate to the continued use of an exterior chimney would be relining the chimney (which you already knew) or abandoning the chimney and using high-efficiency appliances.


Robert O'Connor/NJ