Thread: Venting Question

Venting is definitely not my strongest area, so see if this makes sense.

This is an older house, built in 1948. Every other house in this neighborhood that I inspected has a large 3" or 4" vent on the roof for the bathroom, and a smaller vent for the kitchen. This house had two 1.5" vents on the roof. One was above the kitchen, and the other was above the bathroom. There was no reducer visible in the attic, so it doesn't look the larger vent ever existed.

This is a duplex, and the other bathroom does have a 3" vent on the roof, as well as the smaller 1.5" vent for the kitchen. Both drains are tied together under the house, and there is one main line leaving the crawl space to the street.

I thought that for a toilet vent, the minimum size is 2". The toilet, sink and tub vents are all tied in at the same wall, so it has to be venting through the 1.5" pipe.

2" is what I have always considered to be the smallest.

Because it is a duplex and not two townhouses the plumbing systems are interconnected regarding the DWV system (possibly even the supply system is interconnected and water is included in the rent amount, not common on duplexes but I've seen it before).

That means, based on your description, the DWV system has a 3" vent and 3 - 1-1/2" vents.

Yes, the DWV does have a 3" as well as 3 x 1.50" vents total, as they are tied together before heading to the street. The toilets are probably about 16 feet apart from each other. I was sure I would see a 3" pipe in the attic that was reduced to 1.50" above the bathroom, but the pipe coming through into the attic looked original, and was indeed only 1.50". I'll have to look closer next time I'm over there, but I do duplexes all the time, and they all have a 3" for each half, except this place.

Something weird must have happened way back when they were building this one.

Unless it has recently changed, the Uniform Plumbing code requires the toilet to have a minimum 2" vent. That's for starters. If the cross section area of all other vents are included with the 2" they should be equal or greater than the cross section area of a 3" drain line. I have not verified this but it seams that if a 1 1/2" vent is used for the older (water waster) style toilets, it would choke down the vent air that allows water to remain in the trap after flushing. Perhaps with the newer low flush toilets it would not make a difference but it could. Some low flush designs have a larger Douglas Valve which allows more water to rush into the bowl causing a scouring flushing action and would probably require at least a 2" vent. Needless to say, anything that interferes with proper venting interferes with proper flushing, and could siphon water out of the trap making the trap seal no existant and be problematic.
If I am wrong, one of the young apprentices will correct me for sure...