Stoker Hopper Fire

I guess this thread is an endorsement for a power vent with a baro over a DV unit.

ok.... why? lol

Maybe the direct vent fan is sized properly and does not need regulation.

TY Gambler, I hope thats right. I have never thought of this prob?

Here is a reply from Matthaus about this subject in an old thread.

I have an Alaska with a power vent. I have a barometric damper and manometer (see the pic in my ad for the stove which is for sale). I would never install any stoker coal stove without a barometric damper (as Jerry from LL stated). The only exception is the direct vent units on the Keystokers which mount directly on the stove and have calibrated air leak that acts kinda like a barometric damper. Also the Harman diret vent doesn't need one either.

I just noticed you have a Harman DV. You have nothing to worry about.

Thank you for posting that sir. Makes me feel much better.

The Harman stove is much more advanced than the Alaska stove I have. The manual alone is leaps and bounds better than what I got from Alaska. I was leaning towards the Harman but the dealer convince me that there was too much to go wrong with the Harmon. Also, the footprint of the Channing worked better than the Harmon

Anyway, I took this from the Harman manual: FSS
The FSS (Firebox Static Switch) is an electronic
pressure switch that senses firebox pressure. The
FSS works with the ESP Control to adjust the draft
blower speed to compensate for wind and changing
house pressures.

Is there a sensor that also adjusts the exhaust blower too? Why is the Harman safer?

THe Harman DVC is an air tight stove like a pellet stove . It has latches and a gasket on the hopper lid. It also has a temp sensor on the hopper that when activated it starts pushing coal at the max feed rate until it senses the fire has been fed out of the hopper area. Most hoper fires are caused by over draft situations and most commonly occur with the direct vent stoves which have no overdraft protection. One can aleiviate this problem by placing a baro damper before the direct venter to help prevent the overdrafting issue from occuring. In other words take the DV unit off the back of the stove stick a 6" piece of pipe a baro damper then put on the direct venter. As for positive stove pressure I think you will have a CO problem long before you get a hopper fire with too poor a draft.

I remember reading about the sensor on the hopper and how it pushes coal on to the grate. That must be a mess to cleanup. I would think a more elegant solution would be a snap switch on the hopper that cuts power when it hits a temp say abouve 120. Being an electronics geek I'm attracted to all the bells and whistles of the Harmon. How well is the built in thermostat? It's suppose to monitor the temp. in the room and make adjustments to the feed rate.

Wood'nCoal wrote:A hopper fire can start due to improper draft, the fan forced fire on the grate has to vent the products of combustion somewhere. If there's no or poor draft the stove has to vent somewhere, if a positive pressure develops inside the stove it has to go somewhere.

The other causes of a hopper fire is using the wrong size coal that allows combustion to vent back into the hopper and not having the areas around the grate properly sealed.

Also a stoker runs on a very low draft, a good chimney will provide more then enough drafting potential, hopper fires become a non-issue.

A properly installed stoker stove with a good constant draft and operated properly won't have hopper fires.

Actually, too MUCH draft is usually the cause of hopper fires - it was in my case (along with buckwheat size coal). I didn't recalibrate my draft when I tried out buck and between the more "porous" nature of buck and the extra draft, it allowed air to flow down through the hopper and coal bed and the fire followed the air (and fuel) right back up to the hopper like a dynamite fuse. Which, by learning the hard way, is why Keystoker says NO to buckwheat coal in direct vent versions of their stoves. The direct vent doesn't have a baro damper to control draft - it is controlled between the direct vent motor speed and combustion air supply. Any change to those (or the coal size) requires the draft to be checked and adjusted.

But, if I run straight rice, I haven't had any issues whatsoever.

My opinion - set up the stove for no more than .04 overfire draft. I run about .02 to .03. A stoker stove only needs enough draft to maintain negative pressure in the firebox at maximum firing rate. Any more draft than that and you're sucking heat up the stack (or blowing it out in my case..... ) You shouldn't have any problems or worries if you keep the stove clean and also the vent system.

Chris

Thanks for the info, Chris. I reread my post and I see a few places where I needed to be clearer.

Also a stoker runs on a very low draft, a good chimney will provide more then enough drafting potential, hopper fires become a non-issue.

A properly installed stoker stove with a good constant draft and operated properly won't have hopper fires.

In addition:

When venting a stoker into a chimney, a Barometric Damper is required to keep the draft to the stoker at a constant setpoint.

I also said in that post:

A direct or power vent system is more likely to cause this if the motor on the vent fails. I have no hands-on experience with these systems but I think they have a safeguard switch on them if this occurs.

Which is why this is a great forum. You have experience with a DV, thanks for your comments.

Just wanted to say thank you for all the input in this thread. Its a lil confusing sometimes trying to learn all you can.... and retain it. lol

I never stop learning, there's nothing worse then a "Know-It-All".

A less common cause for a hopper fire can be poorly/improperly sealed grates on some stove models. That was the cause of my getting a fire into the hopper My screw up so I did not have far to look for someone to blame. This can allow forced air from the fan to blow into the hopper. If your stove is new or you know someone has not replaced the grates you can be reasonably sure this will not be a problem.

The other causes of a hopper fire is using the wrong size coal that allows combustion to vent back into the hopper and not having the areas around the grate properly sealed.

Yep!