Burning up grates on my Harmon stoker

i had the same problem some time ego and fixed it by cutting down on the airsupply to the combustion blower. u know, the manual disk on the air intake. close it up somewhat and see what happens..good luck

thanks for all the replies. When I took out the grates there was very little ash under them, not enough to block any air flow. I regularly open the "ash drain". As far as cementing in the grates, I have tried it both ways. These grates that are pictured were cemented in. As they warped, they broke the seal and it took very little effort to pull them out. the stack temp between the stove and the barometric damper is 400*. As far as the temp of the outside of the stove, i have yet to get a thermometer for it. It is quite hot.

I was told by the dealer rep that I may be runnning it too hard. however, as Matthaus pointed out, at 85 lbs. per day, that is only 45,000 BTU's, well below the 85,000 that Harmon advertises. If I lower the blower and turn down the flame, I will lower my output even more.

Here is a though I just had. The Harmon has a 135 CFM blower that blows air out the front of the stove and into the room or through a 6" plate that you can remove and connect a duct to it. I have it ducted about 20 ' and two 90* turns. That is at the limit that Harmon recommends. The air at the end of this duct is 190*. Is it possible that this duct is slowing the air too much through the heat exchanger on the back and top of the stove and causing it to trap too much heat inside the stove. If so, I could get a larger blower and force more air through the heat exchanger and duct and allow the stove temp to be cooler. Would this transfer into the grates being cooler or would they be just as hot? What should the outside stove temp be??? Any thoughts... Thanks..

Jerry

PS - I will be out for a few days and check back this weekend...

400* stack temp? Yes, that is way too high in my book. When I have a full-power fire on the grates (glowing coals to within an inch or so of the edge, then ash to the end), my stove temp runs in the middle 600's but the stack temp is only 200-225.

What did the fire itself look like, ie color and height of flame. My coals glow bright orange, but not so bright that I can't stare into the fire comfortably, and blue flames that roll and sort of swirl into the pent-roof above the grates but not out into the firebox. I'd try closing that restrictor plate down a ways and go from there.

jerry wrote:... I have it ducted about 20 ' and two 90* turns. .. The air at the end of this duct is 190*. Is it possible that this duct is slowing the air too much through the heat exchanger on the back and top of the stove and causing it to trap too much heat inside the stove.

I think we may have found the problem, also at 400* stack temp indicates that the plate above the stove is also overheating the flue gases. I forgot, are you measuring flue temp on the stack surface or in the flue? Since the duct attached to the stove is like a big pressure valve sitting on top of the stove plenum, the heat cannot wash off the top of the stove, since that is right above the grates the temps are all above normal including the grates. IMO the location of the burn is OK, except that it's too hot. The stove I bought that had the grates replaced after 10 years had grate damage in the same spots you have, so in essence I think you are putting 10 years of wear and tear on the grates in a few months by raising the temps to above design limits.

See if you can borrow a laser temp reader, measure the stove temp just above the door with the duct as it is hooked up, then try opening the front air passage all the way and let most of the air bleed out the front. If the stack temps drop, and the stove temps drop by more than a couple hundred degrees or so I think you may have found the culprit. an inline duct fan is not designed for temps over 150*F or so... this means you will have a problem dealing with the 190*F air you are producing, one possible solution is a 6" draft inducer made by Tjernland for wood stoves. They sell for around $149 and are rated for 400*F. You could use that within a few feet of the stove to carry the heat and push the air. A certain amount of bleed out the front may be needed just to reduce the temps in the pipe.

hmmm.....

I too run my Magnum stoker to 2 ducts that are over 20' away from my stove, BUT here is what I did.

From the stove. 2' of 6" duct into an 8" duct. The I do a run of ~15' or more into a TEE. I run 2 6" ducts off of that tee to the floor registers.

I also wanted some heat in my basement so I let a small amount of air come out of the front of the stove.

I don't know what my stack temp is, BUT I ran it that way for 1/3 season last year without issue. This is my first full season with this setup with the inline fans off (I fried a couple b/c the temp was too high).

So did I accidentally do a good thing by immediately letting the 6" duct open to 8". I would think that would allow MUCH more air flow through the 6" stack without as much back pressure.

Hey Jeremy, don't you have a water coil sucking up a bunch of btus? That would make a huge difference to the stove temp above the flam and in the flame over just using the duct work, IMO any way.

Matthaus wrote:Hey Jeremy, don't you have a water coil sucking up a bunch of btus? That would make a huge difference to the stove temp above the flam and in the flame over just using the duct work, IMO any way.

ahhhh.. yes I do. How silly of me to overlook that That for pointing that out Matt!!

Does my analysis above have any merit or am I just blowing "smoke"

OK, so the solution is you need to install a hot water coil to help reduce your heating costs even further

thanks for the input...

I plan to stop by the stove shop and see what they have as far as temp measuring equip. Just to be sure... Is the stack temp measured in the flu between the stove and the barometric damper? That is where I got the 400* inside the flu pipe. As far as the stove temp, is one of the magnetic stick on thermometers help, or do you need to measure inside the stove. Could I stick a probe in the hole used for checking the draft, or does it need to be measured in the fire. As far as the water coil, I was just ready to have two shipped to me, but I got busy with other things. I was going to have it circulate between the water jacket in my oil furnace and the coal stove. However, my oil furnace is only 45 gallons, so I felt I needed to have an additional holding tank in the mix. I also was thinking about installing a bigger distribution blower instead of the Harmon 135 CFM. Grainger Supply has one that is 326 and a bigger one that is 595. I would probably need to enlarge the opening into the back of the stove since the current one is on 2 " square. Question... If I would get more heat away from the stove through moving more air and/or a water coil, would the stove burn more coal and would I get more than the 45,000 BTU's that I now get???

jerry

Just to be sure... Is the stack temp measured in the flu between the stove and the barometric damper? That is where I got the 400* inside the flu pipe. As far as the stove temp, is one of the magnetic stick on thermometers help, or do you need to measure inside the stove.

Yep, I measure mine about halfway between the stove and the baro damper, and stove surface temps are from a magnetic-type thermometer on the upper left corner of the right-side (as viewed from front).

Disclaimer--- My stack temps are surface temps, also using the magnet, so if you're using a probe, then throw my stack numbers out, the probe will surely read higher, though 400 still seems high for that. Sorry-should have mentioned that before .

Question... If I would get more heat away from the stove through moving more air and/or a water coil, would the stove burn more coal and would I get more than the 45,000 BTU's that I now get???

Getting the heat off that stove would be the next step for me, I like Matthaus' idea- open the front of the distribution vent all the way up to let the air run free and check to see if the stove temps change and you get the heat off. If "uncorking the bottle" does the trick, then go from there- it would save spending the $$ on blowers and parts until you know that's what you need.

jerry, as Dutchman said you are measuring in the right place. I find that surface temps are around 100% to 150% less than internal temps. My magnum runs at around 300*F at max burn, so your 400* is high IMO.

Jeremy has the right idea with increasing to 8", that relieves the static pressure and allows more flow. Nice job Jeremy.

With regards to a larger fan on the Harman, I feel that will not work since the interior design of the plenum is for that fan, just enlarging the opening will do very little for improving flow. A better bet would be to try and eliminate the static pressure being built in the duct due to length of pipe, fittings, and vertical rise. Remember air is a fluid and needs a large enough pump, since the induced loss is exterior to the stove, the solution would be best exterior to the stove. after verifying lower temps by dumping the heat out the front you can try a metal bladed 8" fan at the top of the run, if you adapt the 8" fan the corresponding drop in static pressure at the fan should allow it to operate efficiently enough to reduce the duct temp to safe operating limits. Just remember that static pressure is the enemy to this type of set up, you want to open it up as much as possible and allow the fan to work.

And yes, IMO you would be able to increase the output dramatically if you get the air flow up and add a coil. Your current set up is basically causing all the btus to go to the path of least resistance instead of being put to work. The ironic side effect is that the grates are stying much hotter than they should.

Keep us posted as you solve this intriguing mystery.

Matthaus wrote:Jeremy has the right idea with increasing to 8", that relieves the static pressure and allows more flow. Nice job Jeremy.

sweet, thanks..... and I did it all by accident hehehehe......

Actually what I was trying to do was slow the air down coming out of the stove [I thought that would keep it hotter], then when it TEE's into 2 6" ducts the air going to each duct would be hotter......

But it looks like I avoided a problem without even knowing it

Well I was able to get some temp readings today. Unfortunately, I was only able to get a magnetic thermometer that sticks to the outside of the stove. Operating with the hot air going through 20+ feet of duct, the outside stove temp was 550* and the outside stack temp was 325*. The hot air duct just above the stove was 200*. Unblocking the front vent and allowing the air to come out the front of the stove as well made only a small difference. It must be said that I did not fully remove the blocker, I just pushed the rope to the side. the outside stove temp was 525*. Despite ath lack of a "smokin gun", I am still convinced that I need to move more air through the stove and hot air duct. Just when I will get around to doing that is uncertain. I will post my results when I do. Thanks again for all your input.

Jerry

Hi Jerry,

Here's what's bothering me. Many of us have Magnum Stokers, and although they can be touchy with the settings, I don't recall reading about anybody else burning up grates like you are.

So my question is: What's different?

Could you have a defective distribution fan? Perhaps it's not pushing the proper amount of CFM's out.

Could there be a blockage between the distribution fan and the air outlet on the top/front of the stove?


Are you sure the draft both inside the firebox and at the baro is correct?

Anybody else have any suggestions?

jerry wrote:...the outside stove temp was 550* and the outside stack temp was 325*...

The stove temp doesn't look unreasonable, but the stack temp does. At a stove temp of 550*F my stack is only around 190 to 220*F (outside)... so looks like we may be getting closer to the scene of the crime, but not there yet.... keep us posted.

isnt 325 a bit high for a stoker? i just checked mine......the efm is 134, the alaska is 126 and the kenmore is 350 (it's cookin this morning)