Block off plate. Galvanized vs SS?

[Sisu]

Ahhh heck, if given the opportunity, we would argue about whether or not the wheel is round. :lol:

No it is not.....

 

[Retreadsme]

Speaking of .... I've got some square tires on my truck now! I've run Michelin for years and never had a set like these, but that's neither here nor there.
My insert has ceramic fiber blanket (CFB) surrounding it (not touching - still have 1 1/2" air space) on 3 sides plus CFB laying on top the block-off plate. My fireplace is on an outside wall; block/brick construction which I felt would be a heat sink. I used 1" thick 8 lb/cu ft density CFB - rated at 2300 deg F. It cost $86.22 including freight for 50 sq ft. I have some left and if you are interested give me a PM. Enjoy your day.

 

[THEMAN]

Yeah, but the difference in melting temp between zinc and steel is over 1900 degrees F.

I am just trying to be helpful and give back some of what I have gotten here. Galvanized metal doesn't belong near wood stoves, that is why you don't use gavly smoke pipe for your woodstove right?

Just because some or many people get away with doing a certain thing most of the time, why ever risk fate if it is unnessasary?

I have been following this post and I find it funny that no one has said anything about this https://www.hearth.com/econtent/index.php/articles/making_a_block_off_plate/
Seems to me that if its being advertised as a "how to" then it should be ok. I'm just saying.

 

[Ogre]

BINGO

 

[JJEGLBS]

I'm in the process of planning for a block-off plate too. Home Depot has galvanized steel sheets (26 gauge), but not in the size that I need. Lowe's has the size that I need but it is called "plated steel." Is that the same as galvanized? If not, will it still work for my purposes? I plan to put rockwool directly on top of the BO plate and then fill the chimney cavity with either vermiculite or thermix. Thanks.

 

[cycloxer]

I am doing the same thing right now. I am using 22 gauge plain steel for my plate that I am going to spray with stove paint. I just bought a 2' x 4' sheet at Lowes for $30. I wouldn't use galvanized sheet as it really does start to outgas at 400 degrees and is therefore rated for a maximum temperature of 212 deg F. If this plate is close to your stove or stovepipe it will get above that temperature. Steel or Stainless are your best options. Aluminum is good for 1200 degrees, which is okay, but if you ever did have a fire down there it would melt into a puddle.

Above my plate I am using 2" thick Roxul RHT 60 rockboard insulation which has an R value of 4.2 per inch. The board is rated for 1200 deg F and has a melting temperature of 2150. I also put a plug of this at the top of my chimney just below my top sealing plate.

 

[oconnor]

Hey all,

As the OP here I really do appreciate all the input... but boy I did not know this would open such a can of worms. Goes to show just how much there is to know (or assume) about this endevor. I'm going to take my chances with the Gal. If Idie I'll have my wife log in for me and pass along the news...

But since we are on the Backopff plate as a topic can we add insulation materails to this too. I hear everyone suggesting "Rockwool" what are the other alternatives for insulation above the backoff place and also to use just under the top cap to create a dead air space between the liner and the masonry chimney??? And where can I get it, I'm having a hard time finding it locally.

Thanks for all the input, from all sides and the passion. Nothing better than a good passionate debate and conflict to bring out the big guns (and old text books to verify these things...)

Before we talk about alternatives, I am always asking myself the question WHY insulate above the plate. I'm just not sure what the intent of the insulation above a well sealed block off plate is. Many do it, and I don't see any harm, unless you are using fibreglass pink to do it.

But from a temperature tranfer thru the plate perspective, I've always thought that the 15 to 25 feet of liner, insulated or not, would likely make the space inside the chimney and above the plate, hotter than the space below it, given the space below is able to recieve limited ventilation into the room the insert is installed in. So, following that logic, insulation above the plate would add to the heat held within the chimney, and discourage it being transfered back into the room. I do acknowledge that heat in the chimney is valuable for draft and performance, but adding insulation at the plate seems like 99th percentile improvements.

Does that make sense to anyone else, or am I out to lunch?

PS - in light of the total amount of heat at play here, we are talking about small amounts, but as I hear said from time to time,

Measure with a micrometer, mark it with a grease pencil, and cut it with an axe

 

[cycloxer]

The insulation provides a thermal barrier between the inside of your house and the outside. Sheet metal has virtually no R value. The higher the R value, the better. You want to keep the heat in the house both when the stove is running and when it is not running. The lower down the plate the better off you are too. You want to heat the least amount of your chimney and fireplace as possible.

I read back on old threads on this before I decided how to manufacture my block off. I think a steel plate (regular or stainless) is very important for one reason: it is a fire proof barrier between your house and your chimney. I would recommend a plate like this to anybody for safety reasons alone. If you experience a serious event, a steel plate screwed into the smoke box of your chimney will prevent any fire from getting into the house.

I am actually working on this today with basic tools. I don't have a sheet metal shop. It's not that hard to do if you have a few tools. I am going for a two piece plate as my opening is very long (48"). This also facilitates removal should I ever need to gain access to the chimney or liner. I have yet to locate the hole for the stove. Then the plate will be sealed along the perimeter with high temp silicone and tapconned into the mortar joints. All of this work is completely reversible should I ever wish to restore the fireplace (doubtful).

 

[cycloxer]

Here is my two piece plate. All I have left to do is cut the hole for the stove pipe, paint it all up w/ Rutland stove paint to prevent rust, and seal and screw it into place w/ the 3/16 x 1-1/4" Tapcon's. Not too bad. This makes a nice firewall for the house. The melting temperature of steel sheet like this is about 2,600 degrees F and there is nothing that will out-gas. I added a picture of some of the different types of sealants and mortars you can get you know where. This stuff is all $3-7 per tube and has usable temperature ranges from 500-2000 degrees F. They will all work depending on what you are trying to seal. I suggest the higher temp mortars for areas near the stove and stove pipe itself and the lower temp silicones for the perimeter of the plate. Sealing off the perimeter of the plate prevents air, moisture, and smells from entering the house, but if you have a tight fit of insulation above, this is not necessary.

I'd finish today, but it is now Turkey Time

Hope everyone has a great Thanksgiving

 

[JJEGLBS]

Great couple of posts cycloxer. I was planning on 1 sheet, but I may try 2. How are you going to attach the 2 sheets to each other (silicone sealant, screws, or something else)?

 

[brokeburner]

Hey man go get ya a 10 dollars peice of sheetmetal and be done with it. Im a pro welder and ive had metal fume fever it is terrible .Why not have peice of mind instead of thinkin i got a hot peice of galv up there.

 

[cycloxer]

Yeah I am using 5 Tapcons per panel to secure in place and then 2 sheet metal screws will hold the two panels together. This is totally overkill, but I tend to build things the German way - complicated, but also as solid as a Panzer tank. I am going to use silicone on the perimeter of the panels against the brick and at the seam between the two. I am cutting the liner hole Friday when I pick up the stove so that I can get the location perfected. I plan to use either furnace cement or this 2,000 degree high heat mortar that I have to seal the plate to the stove pipe.

If you have a smaller opening, you could certainly do this in one panel. I have also seen people do a center section of sheet in two 'C's so that you have some adjustability with the stove pipe. There are variations of the theme - use whatever works best for you.

You don't want galvanized sheet near your stove pipe. It's nasty stuff if it gets too hot.