Secondary burner project ... one final question (insulator)

[Dra2650]

A little history. My brother-in-law & I built my stove some 28 years ago. Modeled after his Fisher, so essentially an airtight box. The only differences are my primary air enters from the rear, the door is a hinge down, and mine is 3/16" plate (instead of 1/4", I think). It is still in really good shape. It was our only source of heat for the 1st 4 years. Since, it has been a used with our heat pump. Days below 50 and nights below 35 is my standard for when I burn a fire. I can get the stove hot, pack it full at bedtime, dial the air back, and still have large hot coals and heat radiating 8 hrs later.

Adding secondary burners caught my eye this winter (brand new to me). I watched a lot of Youtube videos, read many very long threads her and other sites, and even "white papers" from manufacturers. I have attached a drawing of my heater (black is the original stove layout) (blue is the added burners and baffles) (the fire brick is in brown just to make it easier to visualize and has always been there). The plumbing routing is not shown... just the secondary intake and outlet positions.

Finally, my question. Do I need an insulator on top of the SS baffle above the burn tubes? What is it's purpose? I have seen comment here and elsewhere, that it is to protect the stove top from direct heat. I have also seen that it is to absorb heat to keep the SS baffle hot as the "cooler" air hits it. Which is it? Maybe it is both.
The air gap above the baffle is in the 3/4 - 1" range. To me that seems sufficient for the protection argument. Am I correct?
If the reason is to properly maintain the temp of the baffle, my plan it to use cement hardibacker board. Someone posted that they had success using it. A ton cheaper than ceramic blankets. I would like to not need it just for the added weight on the tubes.

So, for my stove, is an insulator beneficial, needed, required, etc... and why?

Thanks in advance.

 

[begreen]

I wouldn't use hardibacker. it's not much of an insulator. Use ceramic fiberboard instead.

Not sure about the design. It would seem to be better to have the baffle lower and level with the secondary underneath the baffle, but that is presuming a front primary air supply. @Corey any thoughts?

 

[Corey]

My .02 -

You have two major challenges: and one thing for consideration:

First, as drawn, you have a very minimal area for heat transfer before the flue gas goes out the stove pipe - approximated it with some red arrows in the attached image.

If you are already building a secondary air system, seems almost trivial to move the primary air inlet to the front of the stove as well. Then you could set it up in a much more normal/modern fashion. Plus, you bring the secondary air in at or near the front, run all along the side wall, up the back wall, then come back forward. That gives the air a long time to heat up by the time it gets to the burn bars. Then the smoke / flu gas can enter a slot at the front of the stove and move toward the back...giving a lot more thermal transfer area. Depending on draft, you might be able to install some sort of deflector so the flue gas doesn't go straight up the flue pipe, but has to wrap around and come in from the back. That would give you most of the entire top of the stove to help transfer heat.

Secondly, with a solid door, (bit of an assumption, but you did say 3/16" plate - so assuming plate steel, not plate glass ceramic!) secondary burn is a guessing game at best. You can't see inside and the moment you open the door the secondaries will go out. So at minimum, if you have a way to put in a neoceram porthole, or even a full window, that will allow you to watch the light show.

A third consideration - if you put a large plate of stainless in the top it may be prone to warping. Especially if it has any solid connection with the stove/walls. I think most of the modern stoves just have exposed ceramic insulation up there. I've run exposed ceramic board in mine for years with no apparent degradation. So you might at least consider eliminating the stainless.

 

[Dra2650]

Thank you for your input. It will be a few days before I can do some more figuring.
In the meantime, I am using 2 separate runs of 3/4 black pipe. How many and what size holes would you recommend in each?

 

[Dra2650]

Update:
I woke up with a "bright" idea. My primary is 3" pipe and a butterfly valve. I found a plastic bowl that perfectly fit on the pipe and made a seal. I cut 2 holes to mimic the 2 secondary pipes IDs. I opened the butterfly wide open and lit a small cardboard box fire. Got it going good and closed the door. After a bit, I opened the door to a box completely full of smoke and the inrush of air ignited the fire. Reading that the secondaries need need as much air as the primary to match the flame intensity, my two 3/4" pipes can't possibly supply enough air.

Then I thought that maybe with a rear primary that I was losing some straight up the exhaust. I know that is a thing, because I had to add a shelf just a bove the intake pipe to force the air forward into the box 7.5 inches to get fire to burn hot enough. To test this I removed the burn tubes and replummed to get air to the very front and about 1" off the floor like was suggested. I closed down the primary so the 2 secondary runs would be the only air source. Lit another box fire, got it going, and closed the door. After a bit I opened the door. Not enough air to maintain a flaming fire, but glowing embers, and much less smoke. Best guess, similar air as I use when I dial the valve back to idle and maintain temp. So there was an increase of air to the fire by routing to the front. If I step up to 1" pipe I gain an increase of 60% more air.

Am I moving I the right direction?

 

[Corey]

I basically copied the NC30 layout detailed in their manual. (p17). They call for 3/16 holes in the front two burn bars and 5/32 in the back two. I put holes on ~1" spacing all across my bars.

https://pdf.lowes.com/useandcareguides/794909503068_use.pdf

If you have a chance to go check out a stove at a local shop, hardware store, etc, it would be worth the 'field trip' to see how it's set up. Or even google pictures of 'secondary combustion' to see what goes into the layout.

The cardboard box fire won't get enough heat in the stove to get real secondary burn, so I wouldn't worry too much about that experiment.

The main idea is to create a long, slow path for the secondary air to take so it has a lot of time to warm up inside the stove. I brought mine in at the front, run across the bottom / through the ash bed, up the back, then into distribution manifolds and finally into the bars. Not too hard to get the whole top section glowing orange - that is where the stainless really helps prevent the pipes from disintegrating.

If you can flip your whole set-up, it would give much more area to transfer heat. On the inlets, seems like yours may be way oversized. I get a roaring fire with about 1 square inch of primary air opening and that or less on the secondaries. Your 3" pipe is just a bit over 2 square inches. Excess primary air just flushes heat up the flue when the stove is burning - and pulls excess heated air out of your house. Excess secondary air just cools the bars and puts out the flame. Of course, conversely you can always close down a big opening... much harder to open up a small one!

My gut feeling is that if your whole secondary system could 'breathe' through one 3/4" pipe to fresh air, that would likely be plenty. If you have the 3/4 pipe on both sides come out to fresh air, that should be safely in excess and you may end up adding a cap or some way to slow it down a bit.