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Gas fire 4" Type-B vent replacement


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I have an older HVAC unit that has a double wall 4" Type-B vent. It's about 4.5" wide and there is a 4" vent inside that can slide out. Not sure if that is proper type-b vent? At the Heater there looks to be a small fan and it's only 28" from the roof so seems to be a short run. Is the double wall Type-B vent only required where it goes through the roof? It had two adjustable elbows right at the heater but the connection don't seem to have a good seal and they are single wall and rusty.

I wanted to reroute the vent a bit and have it go up then over as it's difficult to get by the heater. Can I use single wall from the heater and then double as it goes through the roof?

Also, looking for a good adjustable support at the roof as none was installed with the roof vent.

Something kind of like this:


Roof Vent

Heater Exhaust

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Since it's located in an unheated attic, you're not supposed to use single-wall vent material at all. If you do, moisture in the exhaust gases will condense inside the vent and cause corrosion, which is precisely what happened in your installation. Also consider that B-vents require 1" clearance to combustible materials, while single-wall vents require 6" clearance, which would be a pain in the butt where it has to pass through the roof. 

Use B-vent and B-vent fittings everywhere and make sure that all the parts are from the same manufacturer, who will have all the supports that you need. 

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Is there a brand you recommend that's easiest to use, with lots of options? I've seen some with their own custom locking design so then you have to get everything of their brand and then may have issues finding the right length or peace.

I've also seen some flexible offerings but I don't know how flexible a double walled vent could really be.  They also seem to come and fixed lengths with ends you can't simply detach and cut the vent to length and then reattach.

The small gaps where it connects to the heater are of no concern?

I'm guessing there's not but was wondering if there's any kind of insulation that can be applied after the fact to a single wall vent?

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All the manufacturers that I know of have the same array of parts - they just use different locking styles. DuraVent, Hart & Cooley, Amerivent, Selkirk, etc. They're more alike than they are different. 

Flexible double-wall B-vents are a good choice, they work fine, but they seem to need more support than rigid sections. Remember that the sections need to slope uphill at a rate of at least 1/4" per foot. 

There should be no gaps where it connects to the heater - I'm not sure why you think that's the case. You're not supposed to cut or alter any of them. If you need it, every manufacturer makes a telescoping section that can be extended as necessary to fit. 

Do not attempt to insulate single-wall vent sections. 


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10 hours ago, Jim Katen said:

There should be no gaps where it connects to the heater - I'm not sure why you think that's the case.

If you look at the pic, it looks like they just cut an elbow with snips to make tabs and it has gaps. They also just cut the top off the double walled straight part to make it fit. The unit looks nice but the install was maybe DIY.

I did read about the 1/4" per foot slope.

Now I just need to find all the correct parts. As you can not cut the double walled to fit it's hard for me to figure out the lengths that I'll need. I was worried that the flexible and telescoping would not have as good a seal or insulating rating.

I did see a telescoping section but it was like $180 and it seems the longer flexible sections are (COVID-ed) out of stock and or by a company with fewer 4" B-Type options.

I'll start with one company and see if I can get all the parts I need from one. 

Thanks for the info.

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