Outside combustion air question- could use some advice

[dogwood]

I've been thinking about adding a 3-wire motorized damper to let more combustion air into the boiler room for our Tarm Solo Innova 50 gasifier. I currently have a six inch diameter round opening for combustion air on an outside wall of a 8' by 12' boiler room. That's about a 27-28 square inch opening, and what the manual calls for. There are no other window openings.

Even after a thorough cleaning of all boiler components, the flue, and the outside combustion air opening, I've noticed flue temps approaching 500 degrees Fahrenheit (260 Celsius). However, when the outside door to the boiler room is open, flue temps rapidly drop about 75 degrees F. The manual says I should be running flue temps of around 213 degrees F.

The boiler has a induced draft fan which is working fine. The chimney appears to be drawing well, and everything else seems to be working as it should. The wood I am burning (oak, locust, cedar, cherry) if anything is on the too dry side. Got moisture readings around 15 yesterday for my first burn this season. The flame in the lower combustion chamber is yellow and pale bluish as called for by the manual.

1. if you are letting in outside combustion air, about how many square inches of opening do you have?
2. Do you think it is a good idea to increase the size of my combustion air opening?
3. What size rectangular motorized damper (opening) would you recommend?

Thanks for your advice.

Mike

 

[Nofossil]

Outside air is good - manufacturer's recommendation seems like the place to start.

I have to question the 75 degree flue temp, though - assume that's a typo?

 

[jebatty]

I would follow the manual on area of combustion air opening, which you are doing with the 6" round opening. As to the drop in flue temp when you open the door, that could be caused by the sensor at the location that measures flue temp being impacted by colder air affecting the sensor and might not be an accurate indication of boiler performance as measured by flue temp. Too dry wood can cause an increase in flue temp. A thorough cleaning of the fire tubes should result in a drop in flue temp, which could be as much as about 100F drop, which than will slowly rise again as ash may collect on the inside of the tubes.

I can't speak for the Solo Innova, but I do have its predecessor, a Tarm Solo Plus 40 with forced draft, not induced draft, and homemade chain turbulators. "Sweet spot" flue temp as measured in center of flue about 15" above the flue fitting on the Tarm, and according to the draft damper setting that I use, is right around 400-435F, meaning that is the temp that produces the highest sustained output of the boiler, which is right around 140,000 btuh. Of course, that temp is maintained only during the high burn period, and it drops into the 300'sF as the burn progresses past high burn, and boiler output then also drops.

 

[maple1]

The manual says I should be running flue temps of around 213 degrees F.

Are you sure about that one?

EDIT: Also, on the topic, I am thinking that not enough intake area would drop flue temps (a 'choking' effect), and introducing more or the proper area would raise them to where they should be. Just my thoughts.

 

[dogwood]

Nofossil, the flue temp drops roughly 75 degrees from about 500F to about 425F when the door to the boiler room is opened. Sometimes more.

Maple 1, I double checked the Solo Innova manual. 213F is the figure they say flue temps should be. I'm wondering if that is an error and they meant 213C instead. I'll give Tarm Biomass a call.

Jebatty, I'll burn some wetter wood, around a 20 moisture reading, and see what happens. I did clean the tubes right before this first burn of the season, so that shouldn't be an issue yet. I just received a 2" boiler tube brush ordered for cleaning the tubes using a drill and an extension rod instead of doing it by hand. Maybe that will do a more thorough cleaning job and improve results.

As you say, the flue temp reading may be responding to colder air contacting the temperature probe sensor. Thinking along that line, maybe the temperature probe reading is too high due to responding to the 90+ degree F ambient temperature in the boiler room when the boiler is burning.

Maybe I should leave well enough alone. Thank you for your replies.

 

[jebatty]

The SoloInnova web manual states the following on flue temperature. Note that with dry wood temperatures higher than 450F may be reached.

" 3.4.8 Installing the 339N Probe Thermometer
The 339N Probe Thermometer (Part # 339N) is to be installed just downstream of the boiler’s flue
collar. Drill a ¼” hole in the side of the pipe and insert the probe all the way in (see Fig. 3.5). This
thermometer indicates of how well the wood is burning and when to clean the heat exchange tubes. If burning dry wood that is split, it should achieve temperatures of 350-450 ºF (176-232 ºC) or more on the probe thermometer."

 

[dogwood]

Thanks for finding that passage in the online Solo Innova manual Jim. That's exactly how my manual reads under the same heading, and how my 339N probe is installed as well. I've been looking at the 213F flue gas figure under 6.3 Specification Data: Operating Data: Flue Gas Temperature at Nominal Output. I called Tarm Biomass today. They thought that the 213F figure should probably have been listed as 213C instead, which translates to about 415F. That's a lot closer to the temps I'm getting at the high end of the Solo Innovas nominal output of 171,000 btuh, and the temps you quoted.

Thanks for questioning that flue temp number too, Maple1.

Interestingly Tarm Biomass indicated different 339N probes may vary up to 100 degrees in how they read out. We discussed substituting a thermocouple probe to get a more accurate readout. They also agreed with your and Nofossil's suggestion to leave the outside air opening as is. They said the combustion air opening should equal the flue diameter, which makes sense.

I'd appreciate If anyone might recommend a good thermocouple probe and complementary digital readout for the flue. Thanks again.

Mike

 

[jebatty]

For a probe, several different options. I like the K-type with a digital meter for stack temperature. The picture shows the readouts for my system. The boiler has burned out its load of wood in this picture, so the stack temperature is low. The digital meters, except for the stack, all use 1-wire DS18B20 sensors. This panel, which I made from an electrical box, with sensors has been in operation for several years.

 

[Hansson]

I have a motorized damper to let more combustion air in.
its 160mm pipe.
I have a Solo innova 30kW.

It opens when the Laddomt21 starts and close when the ladomat stops.

Works fine

 

[dogwood]

Thanks again Jim, and you too Hansson.

Hansson, do you have your motorized damper wired into the back electrical panel of your Solo Innova, or someplace else to let it know when to open or close? Is your's a spring loaded, normally open or normally closed type of motorized damper, or the three wire type.

I like how you installed the damper to keep the motor outside of the wall cavity too. I hadn't thought of doing it like that. If I upgrade my 6 inch spring loaded damper to a 6 inch motorized one, I think I'll add an elbow to the current wall opening and copy your installation. Be a lot less work.

Mike

 

[Hansson]

It is connected to the pump in the charging circuit. It is normally closed.When I start the boiler the pump starts running. At the same electrical circuit is the motorized damper. I believe the contact name is W11 or M7.
It is a solinoid 220v in the motorized damper.

 

[dogwood]

Thanks Hansson,

Mike

 

[Hansson]

I put tube upward to avoid cold drafts. It is not completely tight when closed.
I imagine that it can reduce cold drafts when it is closed.