Venting problem with Buderus G124 gas boiler

marc00

I'm having intermittent venting problems with my Buderus G124 gas boiler. When the weather gets cold and the unit kicks on after being off for a while, it will not vent, the exhaust will back up into the basement causing the CO detectors to go on.

The system exhausts through a lined chimney, two stories, approximately 20 feet.

Would it be preferable to power vent this system through the chimney? Looking for ideas.

Thanks!

Jamie Hall

It's a CO detector -- I hope -- not CO2.

Is this an outside chimney? That is, stuck on the side of the house? Or an inside one? Makes a difference, although even outside chimneys should draw if they are clear.

So -- the first thing I would do is find a good chimney sweep (almost as hard to find as a good HVAC person) and have the chimney thorouglhly cleaned and checked. Also, check the breaching from the boiler to the chimney. Is there a barometric damper on the breaching? Is there a powered damper which is supposed to open only when the boiler is called to fire? (I dislike the latter, especially for outside chimneys, as they allow the chimney to cool down, which can cause the kind of draughting problems on startup which you mention).

EdTheHeaterMan

There are many causes for a chimney to down draft from time to time. The obvious is a partially blocked vent. Have you looked at the base of the chimney where the liner connects the the vent connector? Inside... you need to take it apart to look inside. Have you any obstructions near the chimney outlet? Some chimney sweeps will come and test your chimney with smoke from a cigarette... then say "looks OK to me". But they are not there when you are having the problem. You need a real PRO that has experience with this type of problem.

I had a customer that had a pine tree next to the house that grew past the chimney top by over 10 feet. That chimney worked for years. Then "all of a sudden" it didn't! All of a sudden the tree was 30 years old! (if you can call 30 years "All of a sudden") When the wind was blowing in a certain direction the tree acted like an air scoop and made the wind blow down the chimney. On an oil boiler... That can be a real mess!

Robert O'Brien

Spill switch should trip, CO levels in flue gas shouldn't be high enough to trigger a CO detector with transient spillage

marc00

Thanks for all the responses..

It's an outside chimney and I do have a powered damper that opens just before the boiler fires. I replaced it when this started happening and watched it open several times when the chimney is not drafting so I dont think it's that. Would be interested to know more about what a barometric damper is and how it works. Sounds like an easy fix if that resolves the issue.

There's noting obstructing the chimney at the top and I've had a chimney company come and inspect the liner by sending a camera from top all the way down and open the pipe at the base where there is a clean out. if i remember correctly there was a little bid of oxidization that he swept out but it was basically all clear. Before he left he installed a cap at the top of the liner just to make sure nothing gets down there so I'm pretty confident there is no issue with a blockage or obstruction either.

Not sure what a spill switch is but it appears I don't have one or at least one that works. I will look into that. Would it just shut off the boiler if it detects CO escaping?

One last thing Im wondering about...the boiler exhausts out the back into a hood that is open from below so when the chimney's not drafting its like the boiler is just venting directly into the basement. Not sure if that is something that should be changed or if its normal for this system.

Jamie Hall

That arrangement for your boiler breaching is quite normal for atmospheric gas appliances. It's called a draught hood, and it is required. If you have one, you don't need or want a barometric damper.

Now that powered damper. Let us consider what happens. That damper is closed when the boiler is off. The chimney cools -- possibly quite a lot, since there is no air going up. Then that damper opens and the boiler fires. The chimney, being cold, can't draught (indeed it may actually be down draughting), so very naturally the combustion gas from the boiler spills out of the draught hood into the basement.

The solution is trivial. Lock that damper open. Yes, you will lose a small amount of heat -- but it's almost certain that the problem will go away.

EdTheHeaterMan

marc00 said:

Thanks for all the responses..

It's an outside chimney and I do have a powered damper that opens just before the boiler fires. I replaced it when this started happening and watched it open several times when the chimney is not drafting so I dont think it's that. Would be interested to know more about what a barometric damper is and how it works. Sounds like an easy fix if that resolves the issue.

A Barometric is explained briefly on page 27 of this booklet https://www.fieldcontrols.com/wp-content/uploads/2019/05/4338_ContractorReferenceGuide_2018.pdf

Usually used on Oil Fired boilers and furnaces to control the constantly changing draft conditions of a standard chimney. As the draft changes the Draft Control door constantly swings open and closed to keep the draft over the fire constant. Sometimes boilers are equipped with Gas Power Burners (unlike the atmospheric burners like yours) that require the use of a Barometric Draft Control to keep excessive chimney draft from causing poor combustion.


I do not believe that a Barometric Draft Control will help your situation that is more of a Insufficient draft or downdraft. They are to control excessive draft... the exact opposite of your condition.

marc00

Jamie Hall said:

The solution is trivial. Lock that damper open. Yes, you will lose a small amount of heat -- but it's almost certain that the problem will go away.

Thank you! I will try that. What is the best way to lock the damper open?

One other thing I just came to mind...I have a wood stove in the basement that vents out the same chimney, different flue. Is it possible the stove is exacerbating the problem by creating negative pressure in the basement when its burning and the boiler kicks on?

Ideally I like to use the wood stove along with the gas system when the weather turns cold here in Boston.

mattmia2

It would depend on the model but most have a switch on the side that selects automatic or always open.

If there is not enough combustion air for both appliances it could easily be back drafting the vent for the boiler.

Bob Harper

You have a lot going on.
The liner sounds like its aluminum because you reported a pile of oxidation removed. That means material is missing from the liner and thus should be replaced. Aluminum liners are approved but should be considered temporary just to pass inspection. They will corrode and fall apart from corrosion caused by condensation. If need be, replace it with a permanent stainless steel liner. Use a smoothwall liner and make sure its properly sized. An undersized liner can cause spillage from inadequate flow. Get it warmed up then shut off the gas with a draft gauge attached on the vent connector. If draft pressure strengthens, its undersized.
You have a built-in horizontal draft hood. It has a spill switch built-in to detect a flow reversal and shut it down by responding to high temperature- not CO. When the vent damper opens, it will entrain cool room air unregulated, which can kill draft pressure. Fire the unit to steady state and record the draft pressure. Now, block off the draft hood and record the draft pressure. It should strengthen. If so, install a double acting barometric damper with a spill switch wired into the same spill switch circuit then pan off the opening of the draft hood with sheet metal and observe the performance.
If the CAZ is depressurized, it will hinder flow and draft pressure. You can observe the draft pressure with and without the woodstove firing to observe the effect. If there are any powered exhaust fans, ensure they're off while you're testing. Ideally, a pro with a micromanometer so the pressure regimes can be measured. They can measure the stack effect in the house as a baseline. Next, the pressure gradient between the CAZ and the adjoining space and be measured static, firing then firing with the woodstove to note the effect and measure any depressurization. While this testing is done, the stack draft pressure is recorded along with ambient CO. Note, if a UL listed CO alarm is alerting, go to the ER because you should theoretically have CO poisoning. That's the alert level of listed alarms- waiting for you to get CO poisoning before alerting.
The last thing is that regardless of what you do you need combustion analysis to understand how your unit is firing and observe any effects from changes in draft pressure or sufficient makeup air.