Heat shield

[tbear]

I read that 36" from combustibles was recomended and that, by using metal spaced 1" from the combustible wall you could decrease the 36" rule by half. What thickness of steel are we talking? Would cheap tin siding work? Thanks in advance, Ted

[Bricklayer]

Insurance company made me put a heat shield behind my woodstove in the basement last year. The one I bought at a fireplace store was 18 gauge steel. And it came with 1" spacers. I've seen copper union fittings used as spacers also they are about 1" in length. I think you'd want to stick with steel. Not tin siding. At least 18 gauge. And evaporator stacks run a lot hotter then woodstoves do.

[maple flats]

When I was in the woodstove and furnace business, 24 ga or heavier was fine, I'm not sure about lighter weights. Galvinized (or SS) is good, aluminum will not do it. The spacers need to be at least 1" long and non combustible. I've made them out of 24 ga galvanized sheet metal, 1" pieces of copper tubing or have used ceramic insulaters (electric fence type) but they are longer than 1" and each layer of 1" or more cuts the clearance needed by 50%. So no layers=36" 1=18", 2=9" and 3=4.5". You need to realize that for example, the 2=9" means 9" from the combustible surface not 9" from that layer of non combustible.
As mentioned above, an evaporator runs far hotter than a woodstove, but the clearance requirements are designed to protect the combustible surface in the event of a chimney fire, when the stove pipe is glowing hot.
When I put up my stack originally, I made a triple wall pipe, it was inspected by our fire dept which is required in my area. They liked it, I then needed to have our insurance agent get a picture of it, he also liked it. When I replaced that stack I then used 1" ceramic insulation wrapped 2x around the stack, then covered with a layer of light gauge SS. Again, both inspections were good.
I like the looks of the last method, it is much neater. At the bottom edge of the SS outer shell I cut slits every inch and bent them in at 90 degrees to give a finished appearance. The insulation and SS extend down 3' below the ceiling joists and it goes up to touch the underside of the roof steel. If you have wood close to the stack where it goes thru the roof, that is vulnerable too, it needs to be cut back and made safe. Also, you should have a spark arrestor on the stack unless you have a non combustible roof.