Mr Heater Big Max garage heater for my shop-venting

I bought a Mr. Heater Big Max 50,000 btu garage heater for my winter time small projects work area. Have a few JD carburetors and planter units to do this winter. Reviews on youtube are generally positive. My problem centers around the exhaust venting. I should say I am venting hortizontally. The Mr. Heater company wants owners to use the single wall stainless steel exhaust venting. Besides being non corrosive the joints are sealed with high temperature gaskets. I have sort of come to the conclusion that this due to 1) the exhaust is corrosive and there is moisture produced in the pipe, and 2) the furnace has a exhaust inducer which could possibly leak exhaust gas into the work area as the two wall class B galvanized vent pipes could leak with the positive pressure created by the inducer. My problem is that I don't like the stainless steel pipe. It doesn't fit through the wall thimble well at all. There is quite a gap between the outside of the pipe and the thimble. I am not talking about the air space or gap inside the thimble - just how the pipe slides through the thimble. Secondly, there are not the options on the shelf. With the class B double wall you can find just about anything like the part that swivels on each end to make any degree or angle. Lastly, the price is about 3 times higher for stainless steel. I think double wall would be safer going through the thimble as well since the outter pipe temperature should be less. I know people are venting these furnaces with double wall. In fact on Youtube several show how they are using the galvanized double wall. Like I said before, I think it is mostly about corrosion and not leaking exhaust fumes. Anyone have personal experience with this? I guess it was OK a few years ago to use the galvanized double wall hortizontally and the code changed. It is still OK to use it for vertical vents. I guess I sort of feel like if it is inspected annually for corrosion and there are no restrictions in the pipe or outlet it is probably OK. But, I do generally go by the book and safety is important to me. Ideas appreciated.

Morning Andy,


You chose a fine unit. I have had my Mr. Heater 12 years now using the galvanized piping with minimal corrosion ,mostly light surface corrosion. Your hvac guy will guide you with current code requirements and guidance. Propane or Natural gas? Annual inspection is paramount! CM

I have had one of those for several years now. My shop is steel sided and lined. I just vented it straight through the wall with a section of exhaust pipe that must be galvanized as it does not seem to be rusting. I did slope it down hill a wee bit away from the heater. I used to get alot of water coming out of there. Not so much anymore for some reason ? Only issue I have had is every fall when first starting it up I have to clean out the small hole in the blower where the hose goes on. The small one that goes up to the gas valve.

Digital CO detectors are cheap. Get one.
I would use a double wall SS chimney.

That's what I put in my shop about 12 years ago, using horizontal galvanized piping, it's working fine. We have natural gas, there might be a slight bit of corrosion, but it's insignificant. I did all the installation and gas piping myself, I had a furnace man look it over this fall, he said everything was fine. I do have a CO detector in the shop.

After I posted last night I read the Class B flue pipe (double wall, outside is galvanized/aluminum on the inside) installation booklet that I picked up at Menards yesterday. I could probably use the double wall, but it is totally outside of any and all codes for a hortizontal application. According to the pipe company booklet there can not be an inducer on the furnace as it can cause a positive pressure in the vent which could leak into the work/living space. An inducer could be added to the hortizontal vent, but it would have to be located at the outlet which would cause a negative pressure on the vent which is OK. Looks to me like this double wall pipe is to be used primarily on furnaces or water heaters with an exhaust hood where all of the exhaust gasses collect which a vertical vent is attached. The Mr. Heater Big Max heaters are 80% efficient which probably means the exhaust temperatures are fairly high. As soon as the exhaust leaves the furnace it cools quickly causing the condensation. Condensation must be fairly high as they even sell a section of pipe to drain the water away. According to Mr. Heater they want the vent pipe to ever so slightly go up hill leaving the furnace and going through the wall. I guess this is to expose the condensate to higher heat to vaporize it and push it out the terminal end. Remember I want to use a hortizontal vent. You can use class B double wall with a vertical vent for this furnace, but sounds like not with a hortizontal vent. I guess I am thinking of keeping my stainless steel special gas vent with the EZ seal gaskets.