temp sensing fan control for pallet burner

[boostnut]

Correct me if I'm wrong but I think I recall reading a post about someone developing a programmable fan controller that adjusted fan speed in relation to stove temp. Yes, I've searched and have not been able to find anything. Can anyone point me in the right direction?
The reason I ask is because our shop uses a Central Boiler Pallet burner http://www.centralboiler.com/models.php If you take a look at the pics in the link you'll see the louvered panels on the front doors, they each have a fan in them that forces combustion air into the fire. They also have a damper door that closes when the fans shut off (at a set water temp of 180 degrees). We have been having problems with the unit "backpuffing", at times it can be somewhat violent. This typically happens when the damper doors open after being shut for a period of time. We would like to find a controller that varied the fan speed both up and down instead of just cycling them from "off" to full throttle to "off" again. Any help appreciated.
From what I remember the unit I read about here was not designed to control intake air fans BUT hopefully could be adapted to such a setup.

 

[leaddog]

The eko has a controler that might work for you.
http://www.newhorizoncorp.com/offer.php?id=4
Call and explain what you need and they might beable to help
leaddog

 

[atlarge54]

How long has this boiler been in operation? What does the factory or sales rep. say about the problem? Do you just burn skids or mix of skids and logs? A local sawmill installed a pair of OWB's last year and aren't very happy with them, seems the burn time isn't what was promised. I'm pretty sure they aren't Central Boiler units but I'll bet the controls operate like yours. My idea for improving performance involves operating the air supply and blower separately. Maybe using timers or temp. sensors to allow air before the blower turns on and continue to burn on air only after blower shuts down, with air supply shutting down when temp. is met. The basic theory is any time unit is burning w/o the blower the heat roaring out the chimney is reduced. I don't know if this would help with the explosive starts you've been experiencing. You could experiment by loading with fuel and allowing normal burn. Turn off power when boiler is not calling for heat and disconnect blower motors but not air solenoids, this shouldn't be more than a couple of wire nuts. Maybe even wire in a switch on blowers only and try running blowers after air only mode has allowed fire to ignite. These are untested ideas and shouldn't be tried unless some of the pros here think it is safe to attempt. Best of luck.

 

[boostnut]

How long has this boiler been in operation? This is our 2nd year
What does the factory or sales rep. say about the problem? Not much, it appears to be over the sales reps head.
Do you just burn skids or mix of skids and logs? Skids and misc. shipping lumber only

My idea for improving performance involves operating the air supply and blower separately. Maybe using timers or temp. sensors to allow air before the blower turns on and continue to burn on air only after blower shuts down, with air supply shutting down when temp. is met. That idea is similar to what we have in mind with 1 exception. When the unit gets up to temp and the blowers shut down the fire will "run away" to over 200 degrees if the intake air dampers dont shut down.

 

[Eric Johnson]

I suspect your problem is tied to burning wood that's too dry for either the current settings, or the design of the boiler. There is such a thing as too-dry fuel, and pallets fit into that category.

Sounds to me like you're on the right track, but if you want some additional help specific to a Central Boiler, here's a Yahoo Group devoted to that topic. They might or might not be able to help:

http://groups.yahoo.com/group/Central-Boiler/

 

[atlarge54]

My initial thoughts involved adding a second aquastat to operate the blower only. The temp on this set slightly lower than the other control. Sequence of operation boiler temp drops to 175 and opens air supply, if air supply alone cannot meet heat demand and temp drops to 170 blowers kick on, now raging fire brings temp up and blowers turn off, air supply is left open until temp rises to 175 or more and air is off with boiler in idle mode. These are arbitrary temp settings. I don't know what a Central Boiler uses for controls but a open on limit aquastat is pretty common on most OWB's. Please share how your experiments turn out.