Modine 75k Hot Dawg Vent Pipe

[Lonstar]

When I built my garage in 2004 I installed the Modine heater in a corner. At the time the walls were just studs, no insulation or drywall. Figured I'd get to that sometime later. It's now 2018 and I just got around to finishing the interior.

When installing it I upsized the vent pipe to 5" per instructions and vented it horizontality with a downward pitch. I used B vent pipe that exited through the wall behind the heater. A 3" to 5" adaptor connected to a 5" single wall adjustable elbow, then to a 20"~ long section of single wall pipe, and then to a 20"~ long double wall pipe that went through a thimble in the wall. Overall length is 3.5'.

The 3x5 adaptor, single wall pipe and vent cap are shot, the double wall pipe and thimble still look new. Plan was to replace the worn pieces and reinstall exactly as before. When hanging the drywall I cut a hole the same diameter as the one in the OSB and vinyl siding, which is between 8" and 8.5" depending on where you measure.

Now I find that I should use Category III pipe instead of B vent. Problems (in my mind) are:

1. Can't find an adjustable elbow in Cat III, only fixed 45 and 90.
2. Can't find a 3" x 5" adaptor in Cat III
3. If I change it to 3" for the entire run (which seems ok now with Cat III or am I wrong?), the interior and exterior holes will be too big.

The current heater location in the corner will need an adjustable elbow to line up with the hole in the wall. I can't move the heater to be better aligned, it'll be too close to the wall (the side wall is left of heater and the hole in back wall is also left of the heater).

Any suggestions on how I can use Cat III pipe? I guess I could use B vent again since it's pre 2011? It worked fine for 13 years, but I think it's best to do it to current code specs.

 

[thammel]

Look for z-vent cat 3 parts. This is the stuff i bought and there was a large supply of various adaptors.

Tom

 

[Lonstar]

I ran across Z-vent products when I was searching, that could be the brand I end up buying.

My main concern is pipe diameter. The original manual explicitly states an HD75 needs to be upsized to 5". When I took the heater down to insulate and drywall, I found that the 5" single wall sections of B vent pipe; the adjustable elbow, the single wall pipe, and the 3x5 adaptor, had rusted together on the inside. None of those 3 parts can be separated, they're rusted solid. I can't even turn the adjustable elbow. It's understandable why Cat III is now preferred.

Now my question is if 3" or 4" diameter is ok for this model. The 2004 manual states it has to be 5". I downloaded the latest manual and it states 3" or 4" Cat III, the section and table describing 5" pipe is no longer there.

I called Modine today. After an extended time on hold, the person I spoke with said they talked to a tech and was told I should do what the original manual states. So that's 5" B vent. My concern is current code is Cat III for horizontal venting. That means 3" or 4" pipe. I asked if it's ok to use a smaller diameter pipe with my 2004 model, or did they change something in the heater after 2011 to make it work with smaller diameter pipe. Their response was basically "I don't know".

I'd prefer to go with new 5" B vent and put it back in exactly how it was. But I now see exactly why they changed that spec - the pipe does indeed rust on the inside. I'll gladly pay the price to change to 4" Cat III stainless, but only if they can tell me 4" is ok with the model I have. They aren't saying that's ok to do.

 

[Showkey]

I am surprised tech support even talked to you.........their standard response is ......we don’t talk to DIY or what you got...... read the manual.

 

[SpaceDog5]

So how much was it ?

 

[Lonstar]

How much was what?

 

[Lonstar]

I could shoot the inside of the B vent with a rattle can of header paint. See what it looks like inside after 4 or 5 years and report back.