It seems that galvanized steel is taboo for block off plate material. This is odd considering the outer shell of my Jotul c450 is made of galvanized steel, significantly closer to the heat source than the block off plate, not to mention other sheet metal parts on the insert including the blower housing, snap stat housing, air inlet pipe, and others. I can only assume that since the outgassing temp of galvanized steel is so high that the manufacturer thinks you have bigger problems to worry about by the time you reach it.
Galvanized block-off plate is just fine
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Gotcha. I read through several threads on making block off plates and galvanized was discouraged when mentioned. As I was about to push in my insert into the fireplace I realized how much of it was galvanized. A block off plate seems like a great application for it, no rust and it doesn't have to be painted.
FYI
Galvanizing is a zinc coating. Zinc melts at about 900˚F and vaporizes at about 1650˚F
You block off plate shouldn’t be hitting those temps
Galvanizing is a zinc coating. Zinc melts at about 900˚F and vaporizes at about 1650˚F
You block off plate shouldn’t be hitting those temps
stoveliker
Minister of Fire
Vaporization is not something that happens at a specific temperature as melting does.
Vapor pressures are a continuous function of temperature. Zinc does evaporate at 400 F. And it evaporates faster at 750 F.
Apparently the temperature of a block off plate is not high enough to create dangerous concentrations of zinc vapor. (But it does add zinc to the air; at 750 F its equilibrium vapor pressure is 0.1 mm Hg.)
So roughly what temps can the block off plates reach with a very hot woodstoves running 24x7?
Any rough estimates?
Galvanized is forbidden around our forges. But that's mainly in place for any metal going into the fire.
stoveliker
Minister of Fire
I think @bholler is more qualified to answer that.
My uneducated guess is that it should remain below 500 F. However, that depends on so many issues that a single answer is unlikely to be have much value. (E.g. how well does it connect to the flue pipe giving thermal conductive input into the plate, how well is the firebox insulated, how far is the plate from the insert (radiative load), how much does the air between insert and firebox get refreshed, how well does the plate itself conduct (because there will be a gradient in its temperature), etc. etc. etc.)
At 500 F, the equilibrium vapor pressure is about 0.0003 mm Hg. (See how quickly that rises, from 500 F to 750 F (250 to 400 C) from 0.0003 to 0.1 mm Hg. That's what an exponential increase is/does - something we have all learned with the pandemic, I hope. Hence it's tough to give a good answer, given the temperature gradients in the plate.)
My uneducated guess is that it should remain below 500 F. However, that depends on so many issues that a single answer is unlikely to be have much value. (E.g. how well does it connect to the flue pipe giving thermal conductive input into the plate, how well is the firebox insulated, how far is the plate from the insert (radiative load), how much does the air between insert and firebox get refreshed, how well does the plate itself conduct (because there will be a gradient in its temperature), etc. etc. etc.)
At 500 F, the equilibrium vapor pressure is about 0.0003 mm Hg. (See how quickly that rises, from 500 F to 750 F (250 to 400 C) from 0.0003 to 0.1 mm Hg. That's what an exponential increase is/does - something we have all learned with the pandemic, I hope. Hence it's tough to give a good answer, given the temperature gradients in the plate.)
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I don't know the temp but I can tell you I have never seen any evidence of the plating breaking down on block off plates
Don’t they make class a chimney outer walls out of galvanized? On the cheaper versions?
I know I run 1200 degree exhaust into my galvanized exhaust pipe and it looks great.
I know I run 1200 degree exhaust into my galvanized exhaust pipe and it looks great.
stoveliker
Minister of Fire
Huh, 1200 exhaust? Seems to me you could gain some efficiency...?
Also 1200 F is 650C and even a non trained eye will see glowing then. Your galvanized parts have never seen that, because that's "call the FD"--territory.
Also 1200 F is 650C and even a non trained eye will see glowing then. Your galvanized parts have never seen that, because that's "call the FD"--territory.
Automotive exhaust. But the exhaust pipe never sees that unless something is very wrong
I'm assuming that is the temp of the gases in the inner jacket of the class A pipe. I have had an occasional spike in that territory, but only from a spaceout for turning down the air.
Academics meet reality.
1200 is coming off of aluminum pistons! When you work an engine you need to feed it. It's designed to run all day long at 1200 with most authorities actually specifying 1250 as the acceptable continuous temperature. Many older trucks have an exhaust gas temperature gauge in the middle of the dash instead of a speedometer since those operators actually drove by EGT.
Now, yes, I admit this temperature is measured at the tail of the exhaust manifold right before the turbo which extracts at least a little bit of heat before hitting the exhaust pipe. Also, the exhaust pipes might not have zinc galvanization. Aren't they aluminized?
stoveliker
Minister of Fire
Well looks like it's time to pull out our IR temp guns this winter. Should be interesting to see who's seeing what temps.
Wife got me an IR gun and a moisture meter for my birthday.
Better late than never!
Wife got me an IR gun and a moisture meter for my birthday.
Better late than never!
I would think that as soon as the surround is pulled, the cavity above the insert would cool down a bit. For an accurate reading, some folks with a digital probe thermometer could try putting the thermocouple temporarily up near the block-off plate. Testing should be done with the blower on and the blower off.
Yes egt can absolutely be that high at the manifold. That's not what we said though we said the exhaust pipe doesn't see that temp
Yep, internal gas temps vs. skin temperatures. Not unlike flue temps above the stove.
