Midway · Wasatch County
Caps, Crowns and Dampers on Midway Chimneys
Wide decorative chimneys are common here, and many of them carry two or three separate flues under a single shared crown. That arrangement has its own problems, and a standard single-flue cap does not address them.
Several flues, one crown
When a fireplace, a furnace and a water heater vent through separate flues inside the same stack, the crown becomes a large flat surface with multiple penetrations through it. Every one of those penetrations is a joint where the crown meets a liner, and every joint is a place water can enter if it was not detailed with a bond break. On a wide stack the crown also catches and holds a great deal of snow, which then melts against it for weeks. Crowns on multi-flue stacks fail earlier than crowns on narrow single-flue chimneys for exactly this reason, and they take more of the chimney down with them when they do.
That is a bigger deal at this elevation than it would be in the valley. Midway sits close to 5,600 feet, and snow stays on a crown here for months rather than the day or two it lasts on the Wasatch Front, so whatever soaks into the concrete freezes hard before it has any chance to dry back out.
Crossover between flues
The other multi-flue problem is smoke crossover. If two flue tiles terminate at the same height on the same crown, exhaust leaving one can be drawn down the other, particularly when one appliance is running and the other is idle. What the household experiences is a smell of smoke or of combustion gases coming from a fireplace nobody has lit. The standard fix is height separation, extending one liner above the other, and the right multi-flue cap can be built to do that as part of the same piece of work.
What a cap is doing the rest of the time
Keeping snow out of an open shaft. Keeping rain off the smoke shelf and out of the mortar joints. Keeping birds and squirrels from getting into a warm vertical cavity and building in it. Breaking up the wind that gusts across the top and briefly stalls the draft. It is a modest piece of stainless steel doing four jobs, and it should be stainless rather than galvanized, because galvanized rusts through in a handful of seasons of acidic condensate and snowmelt.
Top-sealing dampers, and the season length
The heating season in this valley is long, and every hour of it a poorly closing throat damper is letting conditioned air up the flue. A top-sealing damper sits at the crown with a gasket and shuts the chimney at the top instead of at the throat. It also keeps snow entirely out of the stack, which a throat damper never did.
Waterproofing that lets the masonry breathe
Use a vapor-permeable siloxane. Do not use anything that leaves a glossy film. Trapped moisture inside masonry in a freeze-thaw climate is the mechanism that spalls stone, and a sealed-in surface makes it worse, not better.
Questions
Caps & Dampers in Midway — common questions
We smell smoke from a fireplace we never use. What causes that?
Most often crossover between flues that terminate at the same height on a shared crown. Exhaust from a running appliance gets pulled down the idle flue and into the room. Extending one liner above the other, usually as part of a multi-flue cap, resolves it.
Can one cap cover all the flues on our chimney?
Yes. A multi-flue cap is fabricated to cover the whole crown with a single lid, which protects the crown itself as well as the openings. It is also the opportunity to build in the height separation between flues if crossover is an issue.
Is a chimney cap worth it if we barely use the fireplace?
It is worth more, not less. An unused flue still collects snow, rain and nests, and the damage from water in the masonry happens whether or not anyone lights a fire. Caps are among the cheapest preventive items on a chimney.
How often should the masonry be resealed?
Typical breathable sealers last several years rather than decades, and exposure decides it. The weather-facing side of a stack in this valley needs attention sooner than the sheltered side. We would rather look at the condition and tell you it is fine than sell you a reseal on a schedule.
Also in Midway
Our other services here
Chimney Sweep Inspection Chimney Repair Fireplace Repair Installation Gas Fireplace Dryer Vents All Midway services
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