PVC furnace vents

[prowannabe]

4500 sq ft house, 5 yrs old, the builder actually lives in this house. 3 HVAC systems, 2 of them have galvanized vent stacks and the 3rd has 2 pvc 3" pipes. Why did he do this?

[kurt]

One Cat I and 2 Cat IV units?

What's going on in the last photo; it's so tiny I can't get any details?

[Mike Lamb]

Not sure what you are asking but the two PVC pipes are a direct vent Cat IV furnace set up. They join together into one concentric vent to the outside to provide exhaust and combustion return air in one pipe.

[prowannabe]

You're both correct, I was just wondering why the other 2 furnaces, 2 water tanks, and 2 chimney stacks all have galvanized vents but he went with pvc on this one?

[Garet]

Because you can't use PVC to vent a Cat I appliance.

[John Dirks Jr]

You're both correct, I was just wondering why the other 2 furnaces, 2 water tanks, and 2 chimney stacks all have galvanized vents but he went with pvc on this one?

Cat IV furnaces extract more heat from the exhaust which makes the exhaust much cooler when it's flowing out the pipe. This produces alot of condensate which would corrode metallic vent pipes. This is why PVC is used as it won't corrode from the condensate. The PVC is also sealed at the joints to allow the condensate to drain back down the pipe to be directed out the condensate drain. Cat IV are also commonly referred to as condensing furnaces. One thing to check on Cat IV's with PVC venting is that the horizontal runs slope slightly back towards the furnace in order to facilitate condensate drainage.

[prowannabe]

gotcha, thanks! guess I need to do another HVAC cont ed class soon! This old age memory is getting the best of me.

[AHI in AR]

You've got another issue there also, and it's one I see more often than not with high efficiency furnaces. I guess because they're relatively rare around here the guys who put them in don't always know what they're doing. And we all know that actually reading the instructions is too much work, what with having to tear open that little plastic bag and all. But if you look closely at your first photo, you will see that the exhaust vent is glued to the fitting on the furnace itself. This isn't right; you're supposed to use RTV silicone sealant there. The silicone allows you to pull the exhaust vent off without damaging anything if you ever need to for servicing, but it still maintains a watertight joint. It's not the end of the world as far as being a serious issue, but it's still wrong. (Well, it's wrong according to all the instructions I have seen previously. If some other manufacturer allows it I imagine someone else will correct me.)

If you run into it again, I recommend that you look around the furnace for the installation instructions. A lot of times the installer will leave them nearby. They will always have a diagram and instructions for this connection.

P.S. Being the "builders personal home" is absolutely no guarantee that things are all done correctly!

[kurt]

P.S. Being the "builders personal home" is absolutely no guarantee that things are all done correctly!

It's almost always a guarantee that there's at least a few things that are done incorrectly, or more likely, a few things that just plain aren't done......yet.....