can't reduce flue temps

[michael-p]

I have a sauna woodstove connected to selkirk DSP double wall pipe. I have a condar probe thermometer mounted about 18" above the woodstove. I start the stove according to manufacturer instructions - with the damper all the way up, 6 pieces of wood or so. Then I reduce the damper down to 20% or so after 15 minutes.

From 1 - 3 hours into the burn, I cannot get the flue gas temp below 900. This is with the damper all the way down and most of the logs burnt - no roaring fire. If I open the damper even the slightest bit the flue gas temp gets to 1000. What could be causing this?

 

[ABMax24]

How tall is the flue? Overdraft is probably the top culprit.

 

[michael-p]

I'd say 5' from the top of the stove to the ceiling, then 6' of chimney - 1' in the ceiling joists, 5' above. I have a 1' length of chimney pipe on top of my stack I could remove and still meet the minimum distance above the roof if that would help.

 

[stoveliker]

11 ft does not seem very tall.
Is the stove properly sealed/airtight? Did you do the dollar test on the door gasket? Ash drawer, if any?
No cracks (I presume and hope...)?

 

[michael-p]

The stove is virtually new - so I hope it's airtight. There is a designed air gap around the front glass window. I'll do the dollar test tomorrow and see how tight it is. But it seems like maybe everyone is thinking too much air is getting in, so I need to damp down sooner after startup(?)

 

[stoveliker]

Yes, you can try to damp down sooner.
Sometimes doors need adjustment after manufacturing and shipping to be tight enough.

 

[begreen]

This is the Kuuma, right? I thought it would vent ok. It may not be very efficient. I'm not sure if it does a secondary burn. If not, it may need a damper in the stove pipe too. What size is it, sm, med, or large?

For now, try loading less wood, like 4 splits. After 10 minutes on high, start turning down the air fairly quickly.

 

[michael-p]

Yup its the small. I do have a damper in the flue pipe, but hadn't been using it because I thought you are only supposed to use that after the fire is out. I am having problems getting my room up to temp, so the manufacturer said I should load the fire box all the way up and keep the stove damper open 50% for longer. But I think this will just increase the problem with the flue temps?

I'll try loading less wood, and see how this impacts both flue temp and stove/room warmup time. Should I also remove a 1' section of my chimney to reduce draft? Do you think this would help at all?

 

[Todd]

Not sure the design of your Kuuma but I have a Nippa sauna stove and it has a single spin knob type air control, no door or ash pan gaskets. I also have double wall pipe with a probe thermometer to monitor temps. The only way to control this beast is to use a pipe damper. My air control stays closed the whole time or it will really take off. I’ve tried installing gaskets for more control but it just creates a cooler dirty burn. I think sauna stoves are designed to burn hot and fast to get those rocks cooking hot. I usually fill my stove up about 3/4 full, light her off and watch the probe thermometer till it starts climbing into the 200-300 range then close the pipe damper almost all the way. In about a half hour temps are up around 1000 then start to fall. After an hour the sauna temps are getting close to where I want them depending on outside temps. Sometimes I’ll add a few more splits if I want it real hot in there.

 

[michael-p]

Thanks Todd - are you able to control your flue temp using the pipe damper? I also have the issue that if I turn the stove damper down too low I get a dirty burn, so it seems like I'm stuck between either having a hot fire with excessive flue temps or a colder dirty fire that takes forever to heat the room up.

 

[Todd]

Thanks Todd - are you able to control your flue temp using the pipe damper? I also have the issue that if I turn the stove damper down too low I get a dirty burn, so it seems like I'm stuck between either having a hot fire with excessive flue temps or a colder dirty fire that takes forever to heat the room up.

My pipe damper somewhat controls the flue temps but I still see an occasional 1200. I’m not too worried since it’s more of a spike and not consistent temps. I usually only have a fire going in there for 1-2 hours then it dies down. Once up to temp you can just throw a stick or two in the stove to maintain temps. I believe double wall can take 1200 for short periods and 1000 consistently but I’m not sure and may depend on manufacture specs?

 

[stoveliker]

I think the "either hot fire+excessive flue temps or colder dirty fire" suggests that you need to close the air to the fire sooner (and not close the damper so much that it'll be a dirty fire).

If you have secondary air tubes up top for a secondary burn, what do you see there for the different settings?

 

[michael-p]

@stoveliker - There isn't a secondary burn on this stove - just the one damper. I'm going to try a smaller load and a quicker close down today as per everyone's comments. I'll have to play with how much I close down to balance flue temp and cleanliness of burn. I think part of what is going on is I don't 100% understand the theory here and/or what to be looking for for how quickly to close down and how far to close down.

 

[Eman85]

Are you sure thermometer is accurate? You say you're closing down the air to 20% and it remains hot?, do you see any temperature changes when you close the air down?

 

[michael-p]

its a new condar probe thermometer so I think yes. If it's open 20% (yesterday at least) the temp reads 1000 (into the red) but if I shut it down to 0% it goes back down to 800 / 900. As the wood all burns up it slowly goes down.

 

[begreen]

Did you watch the Kuuma demo video for the stove to see how they recommend running it?

 

[michael-p]

@begreen - Yes, and I'm doing it exactly that way up to now. Going to experiment with a new process based on their advice to leave the damper more open (50%) for longer.

 

[begreen]

@begreen - Yes, and I'm doing it exactly that way up to now. Going to experiment with a new process based on their advice to leave the damper more open (50%) for longer.

The flue temps will be high during that initial phase. Try to keep it under 900º. After the fire is going well, start closing things down in stages, watching for the amount of smoke generated as a visual guide.