Venting new garage heater through existing metal chimney

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ace400xs

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I am in the process of installing a 80,000BTU Mr. Heater garage heater in my shop. The manual calls for a 4" vent, either B-1 or single wall, horizontal or vertical. I have an existing 8" metal vent that was used with a wood burning stove that has been removed. I am trying to figure out someway to reuse the existing chimney without having to put a new hole in the roof or the wall.

I have been scouring the internet but so far have been unable to come up with a simple, legal way to reuse the existing chimney. It seems I could just run the 4" up through the 8" pipe but I need some way to secure the 4".

My other option is to put a hole through the wall and horizontally vent it. Clearances are tight since this is a low ceiling but I think it can be done. Would just prefer to use that existing hole if possible.

Any ideas?

J
 

Jadnashua

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An oversized flue won't draw properly, so you really do need to follow the installation instructions. I've never looked for a spacer to hold the smaller pipe in a larger flue run, but it shouldn't be all that hard to fabricate one. Depending on how it is run, it might be hard to piece it together, but you only need to retain most of it through the roof, assuming you don't want to make a new hole. But, it doesn't seem like it would be too hard to put a new cap on the smaller pipe and chuck the old one entirely.
 

WorthFlorida

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With any chimney design for any type of heating plant that burns gas or fuel oil is to prevent condensation inside the liner. To large there may not be enough heat to keep the chimney above the dew point. If a section goes through a cold attic it could cause condensation hence little or no draft. The condensate can drip back to the plenum and cause corrosion. You’ll notice that very high efficiencent furnaces or water heaters will have a vent fan to exhaust the cold fumes out.

Jadnashua gave good advice.

On a cold morning and a car engine starts you’ll always see condensation drip out of the exhaust pipe. It stops once the entire pipe is heated up.
 
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