04:54AM | 12/23/02
Member Since: 12/22/02
1 lifetime posts
We live in a 50+ year old home. The cast iron waste pipe is rusted through (a slight hole) leading to the septic tank. I've put a temporary patch on it. It needs to be replaced. It's a 4 inch cast iron drain - through the roof (for venting.) My question is when I replace the cast iron drain with PVC - can I reduce the 4 inch diameter to something smaller for venting up through the roof. Or does it have to be the same as the old>


05:05AM | 12/23/02
Member Since: 12/11/02
12 lifetime posts
It should remain the same size. You can leave it there and just use a no hub coupling(rubber) to connect the cast to the plastic.


12:20PM | 12/23/02
Member Since: 11/17/02
50 lifetime posts
Well yes you can reduce the size of the vent. Since the vent required for a normal home is 3 inch, you can reduce to that size. Make sure you use the proper transition coupling to go from cast iron to plastic. Since the outside diameter of cast iron varies depending on if its service weight or extra heavy (even varies between manufacturers and year of manufacture), you will need a transition coupling manufactured by Mission or by Fernco. The coupling you need has a stainless steel sleeve with a neopreme insert. One side of the coupling is slighly larger than the other side to allow you to connect different outside diameters. If you can stay with four inch all the better, but if you need the extra space go ahead and reduce. In addition, if you decide to reduce to the smaller 3 inch size, you will need a 4 x 3 bushing. You could buy a 4 x 3 transition coupling, but they are much more expensive than the combination of a bushing and a coupling.
Good Luck
Plumbing Prof
Anoka Hennipin Tech college


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