Flue temp safety control on my wood boiler

[mpilihp]

I have a conventional wood boiler, older Logwood boiler that works great except for sometimes the flue temp races out of control (over 1000F deg) and I fear a chimney fire. This is expecially true when the boiler water jacket is cold and im just firing it up say in the morning. It takes the bi-metal draft door controller too long to start moving to keep the flue temp in a safe range.

So after seeing what another member did with his wood furnace controller I decided to do a similiar thing. Rob H on this site was willing to help and provided me the assistance and guidence to make it work.

The draft control on my boiler is a mechanical bi-metal device and works off of the water jacket temperature and controls a draft door on the bottom of the wood boiler. What I did was add a electronic temperature monitor/controller and a motorize draft door controller. The electronic temp unit has a sensor in the flue pipe.

Normally the chain goes directly from the bi-metal control arm to the draft door and as the water temp in the boiler jacket changes it pulles the door up and down.

Now I took and routed that chain through a pully that is connected to an arm that is connected to the motorized draft controler. see pic:

This is the PID controller (electronic temp monitor/controller) The top number is the current temp of the flue, the second lower number is the temperature that the motor will move to the fully closed position.

So as the temperature of the flue stack increases beyond what is a SAFE temp (It starts moving at 550F deg) the controller sends a signal to the motor and tells it to start closing, as it closes it moves the arm toward the left closer to the boiler. as it does it takes out the slack of the chain and lowers the door. As the temp gests back to/under 550 the motor will open back up.

In this last picture the temperature was high and the motor moved to the close position taking the slack out of the chain and closing the door lowering the flue stack temp. As it got back under 550d it opened it back up allowing the bi-metal controller to open and close the door as it normally would

So when the motor is in the OPEN position (top picture) the bi-metal controller regulates the temp of the boiler by the temp of the water jacket, the normal operation.

IF the bi-metal controller lets the flue temp get too high, then the electronic controller kicks in and moves teh arm to take the slack out of the chain. Between the two the boiler runs in a safe stack temp, usually 500ish. Now I dont have to baby sit the boiler in the morning before leaving for work and I dont have to worry if the kids add wood as this device will keep the flue temp safe.

Again many thanks to Rob H.

~ Phil

 

[rwh442]

Phil,

I control my stack temperature at 600 degrees F and have my "slam shut" limit set at 700 degrees. I see yours is set to 950? I have always wondered what the magic "creosote ignition" temperature is. The last time I researched this I think it ranged from 800 to 1200 degrees. I would like to bump up the stack temperature some yet.

Congrats on your control!

 

[mpilihp]

Thanks for the help Rob, because I only want it to begin to move when its over 550 deg and because I really dont know how to operate the Solo PID controller this is how I got it to do what I want. It completely closes before it reaches 950 deg but that was the temp it took to get it to not start moving until its above 550 deg.

Im sure theres a more direct way to program it so it makes better sense but I couldnt figure it out.

~ Phil

 

[leaddog]

I think what you have done is great but if you are fearful of a chimney fire eliminate the problem and make sure you chimney doesn't have a coating of creosote. The old timers always used to take a newpaper and put it in the flue when they lite the stove to keep any burnt out. when you were getting a high temp you were burning it out and if you did it regular you wouldn't have a problem. Make sure you are burning DRY wood and if you run into some warm days where you idle it alot check your chimney. Most chimney fires are after a very warm spell and then you get a very cold day. You burn it hot and the chimney gets heated up, You open the damper to fill the stove and sparks and very hot gases go up and lites it off. Just a couple of fires with undry wood and idleing and you have enough to burn.
It's wise to be afraid of it but if you learn what to watch for and how to control things it's not a problem. Just make sure you know how much creosote you have in your chimney and take care of it.
I know as thirty years ago I didn't understand creosote and learned first hand what a chimney fire is like. I had cleaned it just two weeks before. Busted the chimney to pieces, when the creosote burns it swells and the chimney was almost pluged afterwards. Only reason the house didn't burn was I have cathedral ceilings and the hot spots didn't touch them. The other problem is if they swell shut it will push the gases into the house and co2 gets you. Not trying to alarm you just wanting you to understand that keeping the chimney under control is better than just trying to keep the temp down. As sooner or later you WILL get a hot spark or high temp in your chimney and if you have a big coating it will BURN.
leaddog