Jotul F35 is uncontrollable

[Richspar]

I have owned woodstoves before, and they may have heen older, but they were controllable. We moved house, and I installed a new Jotul F35. I am not impressed at all. It is totally uncontrollable. I want to be able to load it for the night, but with chimney damper almost closed, and the primary air shut it is raging. 900+. Not the slow burn I was looking for. And the joke is that the manual says that if it is too hot, out of control, then close the primary air. Good design, but it doesn't work at all. Today I had a close look at the stove, and see that even with the primary air closed there is still an opening of about 1/2 inch. I built a plate to sit beneath the slider, with smaller holes that would be closed completely. That helped control the fire with one or two pieces of wood, but still the fire would rage if loaded up. I do not feel safe leaving this stove alone, and going out. I will be changing it for a good old stove that has one air source that I can control.

 

[stoveliker]

Are you overdrafting?
What is the height of the chimney from stove top to chimney cap?

 

[all night moe]

Yes, if you're pulling a strong draft, try closing the key damper. Draft will still flow around and through the holes in the damper.

 

[Todd]

Block the unregulated primary boost air that comes in the lower front firebox from the doghouse!

This is a problem with most noncats, lack of control. If your chimney is more than 15’ tall then most likely you need to make mods to have it perform as Jotul tested it. On my F45 I blocked my boost air and 25% of my secondary air to get the control I wanted. Also look into flue thermometer for tool to know when to shut down the air sooner.

 

[all night moe]

@Richspar , Todd is our resident Jotul user. Just look at his sig. His experiences will help you along. Don't give up on your stove yet.

 

[Richspar]

Thanks for the quick responses. I will persevere a little longer to fix my draft problem. Here ate some answers/comments to your posts. My chimney is tall, about 30 feet. I have tried closing the chimney damper completely, but stove is still raging. @Todd I will reply to your post with some pics of my stove, and some questions.

 

[Richspar]

Block the unregulated primary boost air that comes in the lower front firebox from the doghouse!

This is a problem with most noncats, lack of control. If your chimney is more than 15’ tall then most likely you need to make mods to have it perform as Jotul tested it. On my F45 I blocked my boost air and 25% of my secondary air to get the control I wanted. Also look into flue thermometer for tool to know when to shut down the air sooner.

@Todd here are pics of what I have done so far. Taken from underneath the stove (looking up). I show pics of the primary air in Open, Half and Closed conditions. You can see in Closed there is still a big opening. I made a metal plate to reduce the size of the opening (to zero) when the primary air is closed - see the Modified Closed pic.

 

[Richspar]

Block the unregulated primary boost air that comes in the lower front firebox from the doghouse!

This is a problem with most noncats, lack of control. If your chimney is more than 15’ tall then most likely you need to make mods to have it perform as Jotul tested it. On my F45 I blocked my boost air and 25% of my secondary air to get the control I wanted. Also look into flue thermometer for tool to know when to shut down the air sooner.

I am not used to some of the terms. What is the doghouse? Is that the area under that plate which I had to take off to get access to the primary air flow? Wh

@Todd here are pics of what I have done so far. Taken from underneath the stove (looking up). I show pics of the primary air in Open, Half and Closed conditions. You can see in Closed there is still a big opening. I made a metal plate to reduce the size of the opening (to zero) when the primary air is closed - see the Modified Closed pic.

ere does the boost air come in? Is that the 3 holes in this pic, or are those where the primary air comes into the stove?

 

[stoveliker]

Yes at 30 ft the flue will suck so much that the EPA minimum openings will allow too much air coming in.
The solutions of Todd will be helpful with that.

Some folks use two key dampers too.

 

[Todd]

I am not used to some of the terms. What is the doghouse? Is that the area under that plate which I had to take off to get access to the primary air flow? Wh


ere does the boost air come in? Is that the 3 holes in this pic, or are those where the primary air comes into the stove?

That nose looking thing is the doghouse. On the F45 there are two holes on it. I plugged mine with two screws. Not sure where the F35 boost air hole are located underneath but the F45 has two small about 1/2” diameter hole right behind the primary air slide. You may have to drop your bottom heat shield to find that as well as the secondary air inlet. Mine is on the left side bottom and is about 1/2”w by 4” long. I used a magnet to block of 1/4 of that.

 

[Todd]

In the F35 manual there is an air flow diagram that gives you a rough idea how the stove works. Most of the primary air is you air wash for the glass but you can also see the air coming out the doghouse. That just directs to much air into the lower firebox in my opinion. It also looks like there may be some air directed into the lower rear of the firebox? The F45 does not have that. All secondary air in the F45 goes directly into the secondary air baffle manifold.

 

[Richspar]

In the F35 manual there is an air flow diagram that gives you a rough idea how the stove works. Most of the primary air is you air wash for the glass but you can also see the air coming out the doghouse. That just directs to much air into the lower firebox in my opinion. It also looks like there may be some air directed into the lower rear of the firebox? The F45 does not have that. All secondary air in the F45 goes directly into the secondary air baffle manifold.

@Todd Thanks for pointing me to the Air Flow Diagram in my manual. I missed that, which was a bit foolish of me! So do you recommend that I block the 3 holes at the dog house, and just allow the primary air flow to enter by washing over the glass? And also that I reduce the secondary airflow. I like the magnet idea as it is easily adjustable, allowing some trial and error! As for the boost air, the diagram does not indicate that there is any. But I will have a look under the stove (removing the base plate).

 

[Todd]

@Todd Thanks for pointing me to the Air Flow Diagram in my manual. I missed that, which was a bit foolish of me! So do you recommend that I block the 3 holes at the dog house, and just allow the primary air flow to enter by washing over the glass? And also that I reduce the secondary airflow. I like the magnet idea as it is easily adjustable, allowing some trial and error! As for the boost air, the diagram does not indicate that there is any. But I will have a look under the stove (removing the base plate).

Yes those three holes are the unregulated boost air. I would try blocking those first then go from there. It may be easier to block them from underneath if you find them after dropping the bottom heat shield. Then you can also see where the secondary air comes in near the back. Who knows, blocking the boost air may be all you need but with 30’ I’m betting you need more mods. I would try a step at a time and adjust as you go.

 

[Todd]

Another thing to look at is your fire building/reloading techniques and when you turn down your air. Not turning down the air soon enough especially on a large coal bed can cause the load to off gas all at once and run away.

I always like to rake my coals forward, load larger splits on bottom then fill in from there. I only use max air til I have flames established then set it at medium air. I’ll watch my flue temps to guide me when to turn it down further since they react more quickly than stove temps. My internal flue temps range from 400-600 when I’m cruising along at 500-700 stove top temps.

One other thing is to check your door gasket. Take a piece of paper and place it between the door and the stove. Latch the door and try and pull it out. If it easily comes out you need to adjust the door latch for a tighter fit. This could allow excess air into the stove.

 

[begreen]

Note that the burn cycle is quite different with a modern stove than an older stove. It was common practice with an older design that didn't have secondary combustion to just light and then turn it down, often to a smolder. That's how they earned the nickname 'smoke dragons'. A modern stove has a secondary combustion cycle which is where the real heat, and clean combustion happens. As the fire heats up the wood, it outgases rapidly. This increases the intensity of the fire which strengthens the draft. The draft creates a strong vacuum in the firebox which pulls in secondary air which is distributed under the baffle.

Tricks to maintain a reasonable burn temperature include burning thicker splits, packing the firebox tightly with minimal air spaces between the wood, and to close down the air quickly, in increments, so that secondary combustion occurs in a controlled fashion. With good dry wood this can take place within 15-20 minutes of a cold start and less with a hot reload. There is a learning curve to the process, but it soon become routine with 24/7 burning. A flue thermometer helps a lot in tracking the fire. The stove top temperature reacts too slowly to help much. Once the fire has settle down to cruising temperature there usually is no more need for adjusting the air as long as the wood is well seasoned. This thread illustrates the process:

Thread 'Starting a fire and running an EPA stove'

Jan 17, 2020

Questions about starting a fire and avoiding a runaway stove come up so frequently I decided to document starting our stove. This is a Pacific Energy Alderlea T6 with about a 3 cu ft firebox. The stove is connected to a straight-up, 20' interior flue. The outside ambient temperature is 34ºF. The blower was off all the time for these shots. The firewood is douglas fir at about 17% moisture content. This is a N/S loading. An E/W loading start will follow.

NOTE: as testing will show, the older Condar flue thermometer is reading low. It needs recalibration.

There are 3 instruments shown.

1. ...

• begreen
• Replies: 79
• Forum: The Hearth Room - Wood Stoves and Fireplaces

• 

I strongly recommend not closing the air all the way as modified. It can cause the fire to smolder. If a flame then reappears and ignites the hot wood smoke, one is going to be in for a very unpleasant surprise if there is a strong puffback. In the least a puffback can be unpleasant and at the worst it can damage the stove and be life threatening if it blows out the stove glass or blows apart poorly secured stovepipe joints. Blocking the boost air port is safer. If the draft is too strong then a second key damper may be necessary.

 

[Richspar]

Thank you to @Todd and @begreen for all your advice. I will try all options to tame my stove over the next few days. I will update this thread later in the week.