Oversized overtall chimney?

captainco · September 7

I was on a job once with a contractor and it took opening a double garage door to overcome the leakage of a house. We didn't tell the homeowner to fix his house or tell him his chimney was too big for his orphaned water heater. Excessive house leakage causes down drafting not excessive up drafting.

Wet chimneys loose about 40% of their capacity when wet, which is why caps are called rain caps.

Open door, cold and wet chimney and door open. Which way will the air move initially.

There is not one scientific study that proves chimneys can be too big. I can come up with hundreds of field situations where big chimneys were never the problem. Pressure and temperature control chimneys not size.

Bob Harper · September 7

Install a smoothwall 316 ss chimney liner sized for the input rate. Incorporate a barometric damper and set it using a draft gauge btw the baro. and the flue. Seal the upper level air leaks in the home. Run combustion analysis to tune the burner.

A manual fixed blade stack damper is a flow obstruction. BTW the barometric damper and the burner compartment, it can cause sufficient flow disruption to cause flame impingement, which leads to CO production and soot. Use with caution and only guided by testing.

Too high draft pressure can cause CO production so it must be regulated and the only thing that does that reliably is the baro. damper properly set. In extreme cases you may need to install a baro. larger than the vent connector diameter or add a second. On gas, you would use double acting baros. with spill switches attached.

Jamie Hall · September 7

Really now. You are going to use a fan to bring in more air… in hopes that a little less air will come through the cracks? Please…

dandub1960 · 5:18PM

https://forum.heatinghelp.com/discussion/comment/1864233#Comment_1864233

So I've only seen barometric dampers used to limit draft on oil or wood stoves. They appear in my simple mind to break the pressure connection between the combustion appliance itself and the chimney by allowing dilution air into to stack (like a vacuum breaker of sorts). Whereas my atmospheric gas boiler has an open draft hood that forces the boiler and the chimney to function independently (up to a point).

My fixed blade damper is above the draft hood, so throttling the chimney. I would expect the boiler internal draft to carry on as before.

Maybe I'm picturing this wrong but if I put a baro on the chimney isn't it just going to kick open and suck up air like crazy until we mimic what is currently happening at the draft hood? Does the baro need to connect to an outside air supply?

Bob Harper · 6:40PM

A draft hood de-couples the flue from the appliance. A baro. allows the appliance to be connected to the chimney but with the ability to regulate the draft pressure, which a hood cannot do. A fixed blade in the connector is a flow restriction and not a draft control.

A baro. is pulled open by high draft pressure. It allows cooler room air to enter, thus reducing the draft pressure down to the set point as determined by a counterweight that has been adjusted using a draft gauge. It does not induce but rather limit draft. A hood entrains air up the stack unregulated 24/7. The baro uses room air. The baro. also allows the attachment of a spill switch, which the hood does not or does a poor job at.

EdTheHeaterMan · 7:02PM

https://forum.heatinghelp.com/discussion/comment/1864349#Comment_1864349

Maybe I'm picturing this wrong Probably not you have this pictured correctly but if I put a baro on the chimney isn't it just going to kick open and suck up air like crazy That is exactly correct until we mimic what is currently happening at the draft hood? Pretty much Does the baro need to connect to an outside air supply? No just connected to the vent pipe connecting the appliance to the chimney base. The draft hood would then be eliminated if you go this route. I have seen overdrafting conditions that require a second Barometric draft control in order to allow the appliance to operate within the proper range.

BTW Doing this will not solve the problem you have described in your original query. You need to eliminate the chimney if you do not want to or can't afford to do the other work sealing the structure. Eliminate the cause of the overdrafting and vent the boiler mechanically with metered air flow and in and out of the boiler room.

Or replace the boiler with a direct vent ModCon boiler that does not require a chimney.

Oversized overtall chimney?

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