Overview Summary
A chimney tee is the fitting at the bottom of an exterior chimney run where the horizontal connector pipe from your stove meets the vertical chimney. It is shaped like a capital T turned on its side, and the leg that points down is not wasted space. That leg is a collection chamber. Soot, fly ash, and creosote flakes knocked loose during sweeping fall into it instead of dropping back toward your appliance, and a removable chimney clean-out tee cap at the bottom lets you empty the chamber in a few minutes without taking a single section of pipe apart.
Key points at a glance:
- A base tee assembly has three parts: the tee body, the clean-out tee cap, and the tee support that carries the weight of the vertical chimney.
- The tee support is the structural piece. It transfers the load of every section above it into the building framing, so it has to be anchored into solid structure, not just siding.
- Chimney elbows and tees do different jobs. Tees create a serviceable base with a cleanout. Elbows offset the pipe around eaves and overhangs and give you no access point at all.
- The cleanout cap should be reachable while standing on the ground. If you have to drag out a ladder to open it, it will not get opened.
- Tees, caps, and supports are listed as part of a specific chimney system. Mixing brands or diameters voids the listing and can void your insurance coverage.
What a Chimney Tee Actually Does
Most people picture a chimney as a straight vertical tube. In a through-the-wall installation, it is not. The stove sits inside, the chimney runs up the outside of the building, and something has to make the turn between them. That something is the chimney tee.
The tee has three openings. One faces the building and receives the connector pipe coming through the wall. One faces straight up and carries the flue gas into the vertical chimney. The third faces straight down and does nothing at all in terms of airflow, which is exactly the point. Gravity pulls debris down into that third leg, where it sits in a dead pocket well below the path of the exhaust.
Compare that to a system built with two elbows instead. The flue gas still makes the turn, but there is no low point and no access door. Anything that comes loose inside the chimney either clings to the elbow or drops into the horizontal run, which means it eventually works its way toward your stove. The base tee moves that problem to a place you can reach with a shop vac.
The Three Parts of a Base Assembly
The tee body
This is the fitting itself. On an insulated chimney system, the tee is double wall and insulated just like the straight pipe, because the base of an exterior chimney is the coldest part of the system and the most likely place for condensation and creosote to form. A bare or thin-walled tee at the base is a common cause of a chimney that stinks in the summer and drips in the winter.
The clean-out tee cap
The cap seals the bottom of the tee. Good caps come off by hand or with a single band clamp, seat against a positive stop so they cannot be pushed on crooked, and seal tightly enough that the chimney does not draw room air through the bottom. That last part matters more than it sounds. A leaking cap is a hole in your flue that pulls cold outside air straight into the stack and steals draft from your appliance.
Some installations use a drain cap instead of a solid one so that condensate has somewhere to go. That is more common with gas appliances than with wood, but it is worth knowing the option exists if you are seeing moisture at the base.
The tee support
This is the piece that holds the chimney up. Every section of pipe above the tee, every storm collar, every bit of cap and flashing, plus the weight of any snow or ice that builds up on it, lands on the tee support. It bolts to the wall and cradles the tee body, and from that point up the chimney is carried by the building rather than by the appliance below.
We sell tee supports in several sizes, including our 8 inch tee support and our standard tee support, and they are consistently two of the parts customers come back for when they are replacing a base assembly that was never sized correctly in the first place.
Chimney Elbows and Tees: When to Use Which
Both fittings change the direction of the pipe, but they are not interchangeable.
Use a tee at the base of any exterior run. It is the only fitting that gives you a cleanout, and it is the only one that pairs with a support bracket designed to carry the vertical load.
Use elbows above the tee, and only when geometry forces your hand. The usual reason is a roof overhang or a soffit that the chimney has to step around before it can continue straight up. A pair of matched elbows offsets the run, then the pipe resumes vertical.
Every turn you add costs you draft. Flue gas wants to rise in a straight line, and each change of direction introduces resistance and creates another spot where creosote can accumulate. That is why the general guidance is to keep total offsets to a minimum and to keep the chimney as vertical as the building allows. If you are trying to work out how many turns your specific layout can tolerate, our guide on how many elbows a chimney can have walks through the limits.
Getting the Tee Support Right
This is where installations go wrong most often, and it is the part that is hardest to fix later.
Anchor into structure. The support has to land on studs, a rim joist, a header, or a concrete or masonry wall. Lag screws into siding or into sheathing alone will hold for a while and then they will not. If the stud spacing does not line up with the bracket holes, add blocking behind the wall before you mount anything.
Respect the rated height. Support brackets are listed to carry a maximum height of chimney above them. If your run is taller than a single support can handle, you add wall bands at intervals up the run to share the load and keep the stack from flexing in wind. Wall bands are not optional decoration. They are what keeps a tall exterior chimney from working its joints loose over a few seasons.
Set the height before you drill. The tee support determines where the tee sits, and the tee determines where your connector pipe passes through the wall. Measure from the stove flue collar, account for the rise in the horizontal connector, and confirm the cap will clear the ground, a deck rail, or a hose bib before you commit.
Leave the cap accessible. A cleanout tucked behind a propane tank or eighteen inches off the ground behind a shrub is a cleanout nobody will service.
Installation Details That Decide Whether the Base Works
Slope the horizontal connector upward. The run from the appliance to the tee should rise toward the chimney, commonly about a quarter inch per foot. Flat or downward sloping connectors trap condensate and slow the draft.
Keep the horizontal run short. The less distance the gas travels sideways while it cools, the better the system performs.
Maintain clearance at the wall pass-through. Insulated chimney requires a listed clearance to combustibles, typically one to two inches depending on the system, and single wall connector pipe requires considerably more. The wall penetration uses a listed thimble or wall pass-through assembly to hold those clearances. Confirm the exact figure in the instructions that shipped with your pipe.
Know where the transition happens. Connector pipe and chimney pipe are different products with different listings, and the changeover point is at the wall, not somewhere convenient inside the room. Our overview of chimney pipe components breaks down which part goes where, and our guide to planning a through-the-wall installation covers the full layout sequence.
The Five Minute Annual Check
Once a year, ideally at the end of the burning season:
- Put a bucket or a bag under the tee and pull the cap.
- Vacuum out whatever has collected. A shop vac with a fine filter is the right tool. A small amount of dry, flaky material is normal. A large volume of shiny black tar is not, and it means your burning practice or your draft needs attention.
- Look up into the tee with a flashlight. You want clean metal, not buildup and not rust scale.
- Check the cap seal and the clamp. Replace a cap that no longer seats tightly.
- Look at the support bracket. Rust streaks running down the siding below it, visible gaps, or any sag in the chimney above are all reasons to stop and reassess before the next season.
Match Your Parts to Your System
A chimney tee is not a generic plumbing fitting. It is tested and listed as part of a complete chimney system, with a specific inner diameter, a specific insulation thickness, and a specific locking method. A tee from one manufacturer will often physically fit pipe from another and still be an unlisted assembly, which is the kind of detail that surfaces after a chimney fire when the adjuster asks who installed what.
Buy the tee, the clean-out tee cap, and the tee support as a set, sized to the flue outlet of your appliance and matched to the chimney pipe running above it. If you are unsure which diameter or support you need for your layout, send us your appliance model and a photo of the wall and we will confirm it before you order.
FAQs
What is a chimney tee used for? It is the fitting at the base of an exterior chimney that connects the horizontal pipe from your appliance to the vertical chimney run, while creating a low collection chamber with a removable cap so debris can be cleaned out from ground level.
Do I need a cleanout if I have a chimney sweep come every year? Yes, and your sweep will thank you for it. The cleanout is how soot and creosote get removed from the system. Without one, that material has to come out through the appliance or through a disassembled section of pipe, which turns a quick service call into a much longer and messier job.
How often should I open the chimney clean-out tee cap? At minimum once a year at the end of the heating season. If you burn heavily, checking it mid season is a good habit and gives you early warning if creosote is building faster than expected.
Can I use two elbows instead of a tee at the base? It is possible in some layouts but it is not recommended. You lose the cleanout entirely, you lose the structural support point that the tee support provides, and you add resistance at the coldest part of the system where deposits form most readily.
What does a tee support actually hold? The full weight of the chimney above it, plus wind and snow load. It transfers that load into the building framing so the appliance below is not carrying any of it. On taller runs, wall bands above the support share the load.
Can I mount a tee support to siding? No. It must be anchored into framing, masonry, or added blocking. Siding and sheathing alone will not hold the load over time.
What size tee do I need? It matches the flue diameter of your chimney system, which in turn matches your appliance flue outlet. If your stove has a 6 inch outlet, you are almost certainly in a 6 inch system throughout. Confirm against your appliance manual rather than measuring the old pipe, which may have been wrong to begin with.
Why is water coming out of my tee cap? Usually condensation forming in a cold exterior chimney, sometimes rain entering through a damaged cap or an unsealed storm collar. Either way it is worth diagnosing, because sustained moisture at the base accelerates corrosion and combines with creosote to form an acidic residue.
Disclaimer
This article is general information and is not a substitute for the installation instructions supplied with your chimney system, the instructions for your appliance, or the requirements of your local building and fire codes. Chimney installations are governed by standards including NFPA 211 and by state and municipal amendments that vary by location. Always follow the listed clearances, support intervals, and termination heights specified by the manufacturer of the exact system you are installing, and have your installation inspected by your local authority having jurisdiction. If you are not confident working at height or assessing structural attachment points, hire a qualified installer or a CSIA certified chimney professional. Shasta Vent is not responsible for installations performed contrary to the manufacturer instructions or applicable code.
