Huffing furnace

[woodburner forever]

I have a similar problem in that every time the temperature gets mild , above or near freezing, the furnace may backdraft which results in a " huff" noise and a momentary belch of smoke from any joint between sections of the black stove pipe which connects the furnace to the metal chimney. The metal chimney is sized properly for the size of furnace according to the manufactures instructions. If the temperature outside is well below freezing this does not happen. I suspect a pressure differential problem however I have no way of knowing for sure. Should I seal the sections of stovepipe with a sealant of some kind so then even if this problem continued the smoke would have no where to escape from or is ther another solution.

 

[begreen]

It might be negative pressure. Is there a nearby window or door that you could crack open to see if this stops the problem? This can also happen when the fire is starved for air and starts to smolder, then reignites the unburned wood gases. Is there a thermostatic damper on the air supply?

 

[Bad LP]

My joints are all sealed with oranges hi temp Permatex just for that reason because sometimes I’m impatient waiting to clear the gasses loading mid burn.

I have an electric damper for fresh air that powers open when the boiler switch is on.

 

[woodburner forever]

My joints are all sealed with oranges hi temp Permatex just for that reason because sometimes I’m impatient waiting to clear the gasses loading mid burn.

I have an electric damper for fresh air that powers open when the boiler switch is on.

begreen suggested I post our conversation to the boiler room for more input; see below

Hi begreen; To answer your question regarding the thermostatic damper on the air supply; My furnace is an RSF F-101 which has a thermostatic damper controlling fresh air intake ducted from the outside through a 5" duct to the front of the unit. This is a built in manufactured supply feature, not a homemade device. The downdraft problem I have is very intermittent. I can have the furnace going, such as today, with the outside temperature being mild, above zero, with no problems, and yet at another time under similar conditions the chimney/furnace will downdraft, " huff" I call it, and belch smoke though the black pipe connections as previously described. It is difficult to determine what conditions make it respond as it does! I see there are downdraft caps to fit on the top of the chimney which claim to eliminate this problem. What do you think of these devices?


Can you post this information to the thread in the boiler and furnace room? By posting there you will get a lot more diverse and hopefully helpful responses. There are a couple of reasons that could cause this issue, incomplete combustion or weak draft. It's important to determine which is the problem first.

 

[begreen]

You got it right and posted your response correctly to the Boiler room thread on your furnace.

This sounds more like puffback than a draft issue. When the automatic damper closes, the fire is going out and smoldering. Then when the thermostat opens the air supply the fire reignites in a firebox filled with unburn wood gases. They ignite with a mini-explosion.

Increasing draft may help. Is there a barometric damper on the stovepipe connecting it to the chimney? Describe the chimney system that the furnace is connected to. Is there a stainless liner? If no, what are the inner dimensions of the chimney? How tall is the chimney?

 

[brenndatomu]

I agree with begreen...fire is starved for air...can the idle air be adjusted (the air it gets when the thermostat controlled damper is closed) if so it sounds like it needs to be opened up a bit more...

 

[Bad LP]

It does sound like what is called "flash over" in the fire business. These unburnt gases collect to the point of exploding. My Tarm boiler has specific instructions on how to open the loading door and where to stand. I was in a rush and yes, I nearly pooped myself when it happened. Also, once in a while you get this while the boiler is running up to temp and that is why I sealed the joints as the smoke and ash escapes when it happens.

I'm running 6" round into 8X8 clay liner. My draft is well within range but I am beginning to think of dropping a 6" liner down thru the tile. My chimney runs thru the middle of the house.

 

[woodburner forever]

I have an 8" metal chimney extending approx 30' from the basement ceiling level up through about 25' of open space, vaulted ceiling, open concept, to the roof line ( no attic ) and then another 5' or so to extend about 2' or so above the ridge line. This chimney is a manufactured insulated stainless steel type . The chimney stays fairly clean with a sticky creosote build up on the section exposed to the outside. This coating never builds up enough to case a problem and mostly the buildup turns to the powdery type can be removed with the brush.
There is no barometric damper in the stovepipe from the furnace to the metal chimney thimble now. There was one when I first got the unit however it " huffed " even more so I removed it and it seemed to cut down on the amount it huffed.
I think you guys are on the right track with the air supply issue as it will huff almost exclusively when the temperature outside is mild and the furnace is damped down. I have taken to letting the fire go out during these times and going with my electric baseboard back up system, however this can get expensive fast. Fortunately this happens mostly on the shoulder seasons, spring and fall, with almost no issues when the temperatures drop.
Brenndatomu suggestion regarding the idle air is intriguing. I don't know if my furnace has this feature? Sealing the two sections of stove pipe such as Bad LP suggests might help however I take down these pipes for an annual cleaning and inspection and they are the type that slide over each other and collapse down slightly so that the pipes can be removed. If I seal this joint I will not be able to remove these pipe
sections. Where do I go from here folks? Suggestions?

 

[brenndatomu]

Brenndatomu suggestion regarding the idle air is intriguing. I don't know if my furnace has this feature?

Well, it gets air from somewhere, if not the fire would go out completely...its just a matter of how you adjust it.
In my experience sealing the stove pipe doesn't work...seems like the smoke will always find somewhere to come out during a puff event...better to fix the cause rather than the symtoms.
Do you load less wood when its warmer out? And use lower BTU wood during low heat load times? That can help too...Pine, soft Maple, Box Elder, etc, etc, can work real well for spring and fall heating...

 

[peakbagger]

It sure sounds like you have a classic puffback situation that is happening during shoulder season conditions. In cold weather with high heat demand, its going to puff less often as the air damper will not be closing. as often. A wood device is not like a car where when you let off the throttle the gas stops going into the engine. With wood the coals keep cranking out gas ready to be ignited as long as there is wood in the firebox. Since the damper cant control the amount of gas being generated it desperately tries to control the air introduced to the gases. So when the air damper closes you have excess gases with not enough air so the fire goes into partial or no combustion. In partial combustion its just smogs out the neighborhood or coats your chimney with creosote, with no combustion you have a plume of hot gases looking for a place to get enough air to burn. If you have a loose stack enough air may be sucked in for the gases to ignite or more likely when you get to the chimney enough air leaks down the chimney that the gases ignite quickly creating a small explosion (AKA puff). This typical happens when its breezy out as there is turbulence on the chimney and that tends to knock the stack draft around.

During shoulder season where the heat load is low, the only option if you want to run the unit is run small loads frequently. Dont try for a long burn and you should be good until the heat demand goes up.

 

[Bad LP]

To be clear I’ve never run my boiler in a shoulder season.
I firmly believe the air supply creating the event is either from my own doing and yes I did things outside the written SOG or the slow outgassing up the chimney allows some sort of back flow of air down.

This puffing is clearly visible at the chimney cap. It is nearly breathing.

 

[peakbagger]

I think its a rare occurence for a boiler with storage as boilers tend to be run full load regardless of current heating demand. A wood furnace or wood stove is different story where the owner is trying to match the combustion with the demand,

 

[woodburner forever]

Okay, here is what I am going to try; Small loads with hopefully higher burn rates as the furnace trys to keep up with the limited amount of heat demand. I have also built small fires, let them come up to full speed with the firebox door slightly ajar while I stand there and monitor it , ( meanwhile the air damper is not open due to the thermostatic control not asking for more heat to the house ) and then closing the door after the blower has come on. At that point the fire is damped down again but there is enough heat in the house maybe even for the rest of the day and then rebuild another fire in the evening after it has cooled off. This strategy works fairly well except for those occasions when conditions must be just right to get the " huff" effect. At that point I throw in the towel and let the fire die and go to baseboard heat again. If my problems are not able to be fixed with a mechanical solution then I will have to face the fact that this is just the way it is and except it. Thanks for your help everyone. I would like to stay in touch with all that have made suggestions and will continue to monitor this site as the subject interests me. British Columbia Canada thanks you for your input.

 

[Fred61]

I am not an advocate of burning anything but wood that is no higher than 20% moisture content but that could be part of your problem. Extremely dry wood outgasses more rapidly and produces a higher concentration of gas when a throttled down damper isn't allowing enough oxygen into the burn chamber. When it does ignite BOOM all the gas is burned at once and it could be self perpetuating if the wood is outgassing rapidly and the combustion air is limited. Like a diesel. I suggest you try some wood with a little higher moisture content for the shoulder season.

 

[hobbyheater]

This is your furnace. Only now it's made in Quebec and is called ARDENT ENERGY instead of RSF. I installed this furnace six years ago for my Mother-in-law and with the secondary air port on the back of the furnace, there has been no huffing problems; even with burning wood dry enough that it does not register on the moisture meter.

 

[woodburner forever]

Hi hobby heater; You are right , this is my unit. I just checked # 5, the secondary air and it seems clear. This is taking air out of the room and not fresh air from outside. I don't think it was meant to use outside air however. One thing I have noticed in the past, especially on my spring inspection, is that the small plate, possibly # 8 in the picture, can actually fall down and end up in the firebox. It just sits in its position on an angle and must deflect smoke or something, I am not sure what it does. It is intact now but it is not visible until the stove pipe is removed and I can see down into the unit. What does this plate do do you know?

 

[hobbyheater]

Does the huffing take place with the draft open or closed?

 

[woodburner forever]

It takes place with the draft damper closed.

 

[hobbyheater]

The boiler pictured below had a similar draft setup to the RSF the surface on which the draft butterfly closed would get a build up of creosote and prevent the butterfly from completely closing.

 

[woodburner forever]

Some other thoughts on my huffing problem. It did it again 3 times in five minutes today and then all is fine again. Temperature outside is cold ( -15 C ) which is unusual as problem usually occurs closer to zero. My daughter whom occupies a basement bedroom and sleeps with the window open a crack says she will wake up to a humming sound coming in through this small opening in her window. We speculate that the house is trying to pull fresh air into the house even though there is fresh air ducted to the front of the furnace and there is fresh air also ducted ( 4" dia ) to the return air. Is it requiring even more air? The manufacturer instructions state that flue gas draft should measure between 0.06"- 0.07" water column and plenum pressure should be at 100 PA ( 0.40" water column ) I wonder if this has anything to do with my intermittent " Huffing- back drafting " issue if that is what is going on? Thoughts?