Quick creosote buildup: Need Advice on Efficient & Safe Use of an Old Wood Furnace!

[Pinkalie]

I hope I'm in the right section, I am new here and this is an older furnace and it feels more like a stove to me than the oil furnace we had as kids, but feel free to move this thread if it's in the wrong place!

TL;DR: New to wood-burning as a primary heating source! Need advice on safe and efficient use of an old wood furnace. Please answer any questions you can, or read below for more context.

My Questions​

1. Is it bad to load the furnace with many logs at once?
   
   • I typically add 4-6 logs after getting good coals. I stake them in a pyre formation. I’m concerned that overloading might be wasteful or potentially unsafe.
2. Should I be worried if the temperature drops below the “safe burn” zone?
   
   • Once I close the door, the temperature only stays in the safe burn range (above 200°F) for about an hour. Is this normal, and how can I know when it’s okay for the fire to burn cooler?
3. Does creosote buildup on the door indicate buildup in the chimney?
   
   • After just a week of burning, I’m seeing heavy creosote on the door. Could this mean creosote is building up in the chimney too?
4. Is it dangerous to have small holes in the flue pipe where I occasionally see flame?
   
   • I sometimes notice light from flames through these holes. Could this be an issue?
5. Is it okay not to have a functioning flue valve or to rarely adjust the intake valve?
   
   • My flue has no working valve, just a knob with a spike. I also have the intake valve open most of the time. Are these issues I should address?

Background

We’re new homeowners using an old wood furnace for primary heating for the first time. I’ve used wood stoves before recreationally, but I’m new to serious wood-burning. Here’s my setup and routine:

• Furnace Inspection: We had the stove cleaned and inspected by a professional. The sweep recommended I burn very hot this year to avoid creosote issues since he noticed it was pretty bad from last year's use.
• Burning Process:
  • I start the fire with the door open and add 4-6 logs once food coals form. It usually reaches the top of the safe burn zone within 45 minutes.
  • After reaching 200°F on the flue thermometer, I close the door. However, the temperature falls below the safe burn zone after an hour, which led me to experiment with keeping the door open or feeding the fire more frequently.
• Concerns:
  
  • There’s heavy creosote buildup on the door after just a week of use, and I’m going through wood fast—about a fifth of a cord in a week.
  • Adjusting the intake valve doesn’t seem to help keep temperatures up; keeping the door open does. Also, the flue valve doesn’t have a working flap.
• Wood Quality:
  
  • I have 2-year-old hardwood stored indoors and a batch of newer wood cut last year, dried outside all summer and stored in the basement. The newer wood sometimes sizzles. I’m burning the older wood first and hoping the heat dries the newer wood gradually.

Any insights on my questions or advice on better burning practices would be very appreciated! Thank you!

 

[Pinkalie]

UPDATE :

Thanks to reddit, I was told that my dampener flap was missing on my flue valve. I could not see it anywhere near the stove so I dismantled the flue off of the stove and did not find it in there either. What I did notice was that there was a huge amount of ash up in the chamber bellow the flue. Since the chimney sweep came by only two weeks ago, I decided to call and ask him about all this.

He told me that the flap was removed long ago because it is illegal to have one on wood furnaces here in Canada.
That being said, he was baffled by the amount of ash that was forming bellow the flue, above the main burning chamber. He found that concerning since he did vacuum there.

He believes the creosote buildup may be because my wood is not dry enough, combined with my lack of experience building hot fires.

I will clean the ash out the top portion and arrange for my wood to air dry faster with a dehumidifier and fans. Heating the basement where my wood is stored using the dry wood I have left from a previous year should also help in the rest drying faster, I hope...

Any input or help about my situation is still greatly appreciated!

 

[Pbville CO]

First and foremost, the flue penetration through the roof/wall must be identified so excessive heat does not create a safety hazard. Ours is a Duratech triple wall flue for the roof penetration, but a single wall inside the cabin. FYI regarding this post, we are in Colorado and use a Hearthstone I manufactured in 1979.
• Is it bad to load the furnace with many logs at once?
This depends on the type of wood. IMO the flue temperature and roof penetration rating would be more of a focus. Also consider the manufacturer’s recommendations. Check the BTU rating of the wood you are using to define "hardwood" as this will help others who provide feedback. A quick search for “BTU rating of firewood” will work. Also, IMO lower BTU’s will need to have a lower moisture content.
• Should I be worried if the temperature drops below the “safe burn” zone?
If you can, find out the manufacture’s recommendations for the “safe burning zone” for the furnace as well as their recommended flue installation based on distance to transitions from vertical to offsets/ changes in angle. Low flue temperatures have caused me several issues burning just lodgepole pine which created creosote buildup. I am now using a mix of pine and hardwood; primarily mixing ash and honey locust after the fire is started with pine. Having the catalytic converter (at least 20 years old) I can get the temperature reading on the same t-stat as in the pictures posted up to 700 deg to “clean out” my flue occasionally, which vertically is about 8 meters high.
• Does creosote buildup on the door indicate buildup in the chimney?
To rule out any airflow issues, build the fire closer to the door if it is safe to do so. This will provide you with more information regarding how door buildup and flue temperatures are affected (if at all).
• Is it dangerous to have small holes in the flue pipe where I occasionally see flame?
I would recommend replacing the flue sections with holes in them unless you have a damper or catalytic converter installed. Our stove has a catalytic converter close to the stove (see lever in picture). On a side note, it will not “ignite” with pine but does when I use the hardwood mentioned above.
• Is it okay not to have a functioning flue valve or to rarely adjust the intake valve?
I would review the manufactures recommendations.
Also new here so hope this helps.

 

[salecker]

First and foremost, the flue penetration through the roof/wall must be identified so excessive heat does not create a safety hazard. Ours is a Duratech triple wall flue for the roof penetration, but a single wall inside the cabin. FYI regarding this post, we are in Colorado and use a Hearthstone I manufactured in 1979.
• Is it bad to load the furnace with many logs at once?
This depends on the type of wood. IMO the flue temperature and roof penetration rating would be more of a focus. Also consider the manufacturer’s recommendations. Check the BTU rating of the wood you are using to define "hardwood" as this will help others who provide feedback. A quick search for “BTU rating of firewood” will work. Also, IMO lower BTU’s will need to have a lower moisture content.
• Should I be worried if the temperature drops below the “safe burn” zone?
If you can, find out the manufacture’s recommendations for the “safe burning zone” for the furnace as well as their recommended flue installation based on distance to transitions from vertical to offsets/ changes in angle. Low flue temperatures have caused me several issues burning just lodgepole pine which created creosote buildup. I am now using a mix of pine and hardwood; primarily mixing ash and honey locust after the fire is started with pine. Having the catalytic converter (at least 20 years old) I can get the temperature reading on the same t-stat as in the pictures posted up to 700 deg to “clean out” my flue occasionally, which vertically is about 8 meters high.
• Does creosote buildup on the door indicate buildup in the chimney?
To rule out any airflow issues, build the fire closer to the door if it is safe to do so. This will provide you with more information regarding how door buildup and flue temperatures are affected (if at all).
• Is it dangerous to have small holes in the flue pipe where I occasionally see flame?
I would recommend replacing the flue sections with holes in them unless you have a damper or catalytic converter installed. Our stove has a catalytic converter close to the stove (see lever in picture). On a side note, it will not “ignite” with pine but does when I use the hardwood mentioned above.
• Is it okay not to have a functioning flue valve or to rarely adjust the intake valve?
I would review the manufactures recommendations.
Also new here so hope this helps.

Nice log house, reminds me of mine.But i don't have any stoves in mine

 

[stoveliker]

To check the moisture content of wood (indeed a large contributor to creosote problems), take a piece of wood, let it get up to room temperature, *then* resplit it in half, get your moisture meter and stick the pins in the middle of the freshly exposed surface, with the pins parallel to the grain, as deep as you can. That gives you the moisture content in the center of the split.
I use an mmd4e, which is $30 - but there are many others.

I don't know what moisture content works best for your furnace. Most wood stoves run best below 20%. If it's above 25% that will be a problem for sure. Maybe a furnace can deal with a 20-25% moisture content - but note that having 1/5 of the weight in water content means you're boiling a lot of water up the flue, using BTUs that could have heated your home.