Fly ash and/or residue insulating firebox?

While I had my furnace shut down yesterday with a grate issue, I did a total clean out. I was taking a really good look around at the firebox and noticed that the firebox walls had a layer of residue, (I assume from prior times of some wood burning) and also some fly ash build up. I thought to myself, I wonder if that layer of whatever could be insulating the steel, slowing heat transfer thru it to the air that the convection blowers are pushing around the outside of the firebox... Hmmmm...

SO, I proceeded with a wire brush in hand to scower off this residue on the inside walls of the firebox to see if it would make any difference. I just did a quick go around, just a few minutes worth. By now, you may wonder HOW would this suttle difference be measured other than by me just saying I notice more heat. I'm using this BBQ themometer monitor which consists of two probes. One measures my flue pipe temp and the other measures my warm air duct temp. Keep in mind to compare temps from before and after the wiring brushing I had a stable consistant coal fire burning for several hours.

Turns out that before the brushing of the inside of the firebox I would have a flue pipe temp of 160 with the warm air duct pushing out 110 degree air. NOW after the brushing of the inside of the firebox, I have a flue pipe temp of 160 and the warm air ducts are pushing out 115 degree air!! More heat means burning less coal in my book....

I would agree as well that ash build up in the coal bed around the firebricks would also insulate heat transfer around the bottom of the fire bed thru the bricks and steel. SO learned from this, I think a good clean out once a month and a brushing of the interior of the firebox once a week, while running a low fire is reasonable.

Has anyone else seen the same outcome? I thought this would be worthy of a new topic discussion, I would be very interested to hear any pro or con feedback, toughts or ideas

I believe you are correct, fly ash will insulate and deter heat transfer. I think it is illustrated by any horizontal run of flue pipe. As the fly ash build up on the bottom you can feel a big temp difference on the pipe under the fly ash versus the sides that are clear of the fly ash. Obviously this should be cleaned out, not for the heat transfer properties, but the blocking of draft issue!

Different stoves collect it in different areas. The Mark III accumulates fly ash on top of the baffle plate over time and this insulates the air tubes located there. When the distribution air flows through those tubes, it does not pick up as much heat if the baffle is full of fly ash. I would clean that off about 3 times per year, usually during a warm couple of days when I would let the stove go out after about a month of burning.

I totally agree. The coal ash will insulate the metal and cut down on the heat transfer. The cleaning of the plate steel inside the stoves is easy enough but the fly ash can get into certain chambers that surround blower pipes. These can be difficult to clean at times. On my stove for example, there is a top chamber where the hot exhaust gas surrounds blower pipes before exiting up the flue. Harman saw fit to make a small clean out door. But unfortunately it cannot be cleaned while the stove is running I would suspect. Have not tried it. Might work like a top load stove where the exhaust will still be pulled up the chimney.... but I digress.

One other point. If you are familiar with those flue pipe heat exchangers, they have a clean out method to scrape the pipes clean for this very reason.

For me like most people here, we run our stoves nonstop from November to April. So we are unable to clean it out during the heating season.

VIn.

I had another very interesting discovery two days ago while replacing a stove grate. It turns out that in the front and back of the furnace there is a gap between the inside wall of the fire box and whats called the front and rear liner. Now this gap is about half an inch wide and runs down to the ash pan area, where my combustion air enters. I thought to myself, suppose this gap is giving some of the combustion air a path of least resistance thru the fire box but totally bypassing the grates, where it should be pulled up thru to feed the coal... This bypassing air also would help cool the firebox since it wasn't helping the coal burn, hurting efficientcy (pardon my hillbilly spelling lol). I'm assuming these gaps help keep extreme heat from penetrating thru the front and back of the furnace. Thats the only purpose I see, maybe someone else here knows better.

SO, as an experiement, I pushed some small pieces of fiberglass insulation into these gaps to see if it would make a difference. Now, all the combustion air entering the furnace thru the ash pan door has no choice but to be pulled up into the bed of burning coal. It has no other route to take. I'm now getting about 5 degrees or more thru my duct with the same flue pipe temp. And I could close the air feed more to get the same heat thru my warm air duct meaning less coal is being burned too..

I also took the baffles out that are about 10 inches over the coal bed and wire brusher them clean so radiant heat could be transfered thru them to the top of the firebox more effectively..


I continue to learn more and more about the burning characteristics of coal and how these furnaces work.
Its all pretty exciting!!

Lightning wrote: I'm assuming these gaps help keep extreme heat from penetrating thru the front and back of the furnace. Thats the only purpose I see, maybe someone else here knows better.

SO, as an experiement, I pushed some small pieces of fiberglass insulation into these gaps to see if it would make a difference. Now, all the combustion air entering the furnace thru the ash pan door has no choice but to be pulled up into the bed of burning coal. It has no other route to take.

Are you sure these aren't for secondary air? I am not familiar with your furnace, but have seen gaps on some for the purpose of secondary air?



Eric

Secondary air? You mean like over the fire draft? From everything I've read so far, all combustion air for a coal fire should come up thru the coal bed for most effective burning, for burning hard coal anyways.. Maybe that secondary air you mention is needed for wood or burning BIT coal, So hydocarbon gases released could combust over the fire that otherwise wouldn't have a source of Oxygen..

Secondary air? You mean like over the fire draft? From everything I've read so far, all combustion air for a coal fire should come up thru the coal bed for most effective burning, for burning hard coal anyways.. Maybe that secondary air you mention is needed for wood or burning BIT coal, so that hydocarbon gases released could combust over the fire that otherwise wouldn't have a source of Oxygen..

Bottom line, I'm getting more heat in the house and less out the chimney..
I'll continue to monitor and see what happens, thanks for your suggestion!!

Secondary air is for your overfire air. you do need some to burn the gasses from the anthricite. If you have none you are looking for the potential for a nice puff Back



Eric

Yeah, Coalfire - I'm thinking that when re fueling there may be potential for a puff back. Already seen a few lol.. I'll leave my top door open just a crack so gases can burn off without too big of an explosive show lol.

To my amazement, I have 120 degrees on my pipe with 108 thru my warm air duct.. Before blocking those gaps I would have 120 degrees on the pipe and 99-102 thru the ducts!! And Its gotta be because that combustion air was bypassing the coal bed!

Lightning wrote:To my amazement, I have 120 degrees on my pipe with 108 thru my warm air duct.. Before blocking those gaps I would have 120 degrees on the pipe and 99-102 thru the ducts!! And Its gotta be because that combustion air was bypassing the coal bed!

You did the right thing. Routing primary air to serve as secondary air is the wrong way to go yet many makers do it. It guarantees that there will be air in excess of that needed for good combustion in the wrong place and since air is mostly nitrogen it just carries heat up the chimney.

A good secondary air system should be dedicated like those found in some antique stoves and modern wood burners.

Hi Lightning. Do your stoves' wall are vertical or not? Do they have some horizontal shelves to accumulate ash?
A very small ash accumulation on a vertical wall is normal but one needing a clean up weekly or monthly is not normal, well in my head!

Franco, yer my hero of the day. I knew I was on to something and I'm glad I payed attention in physics and chemistry in high school

nortcan wrote:Hi Lightning. Do your stoves' wall are vertical or not? Do they have some horizontal shelves to accumulate ash?
A very small ash accumulation on a vertical wall is normal but one needing a clean up weekly or monthly is not normal, well in my head!

Yeah the walls are vertical, they get a fine layer of fly ash.. The baffles are horizontal, they get covered on top with fly ash. My point was that keeping as much ash as possible off any of the heat transfering parts will optimize heat output to the house instead of up the chimney. I'm not seeing anything abnormal - at least I don't think so lol

When you shake down anything you can do to minimize draft through the coal bed will help. If you keep everything closed then close off the air intake also. You can also open the upper door to temporarily divert draft from going through the coal bed. Should lessen fly ash.

franco b wrote:When you shake down anything you can do to minimize draft through the coal bed will help. If you keep everything closed then close off the air intake also. You can also open the upper door to temporarily divert draft from going through the coal bed. Should lessen fly ash.

100% right. It helps a lot to do so.