I just had all of my cast iron drain pipes replaced with ABS (plastic). I’ve got one bathroom on the first floor and another directly above it on the second floor; the toilets share a drain pipe. About one minute after flushing the top toilet the bottom toilet refills for about ten seconds (the top toilet refills after flushing without problem). Every single time. Just to be clear:
flush upstairs toilet
upstairs toilet refills
downstairs toilet refills briefly
What is going on? What could be causing this? It did not do this before the repipe.
I will be telling the plumbers who repiped about this, but I would like to be armed with a little knowledge before they feed me some B.S.
I’m going to guess that a vapor lock is somehow siphoning water out of the lower toilet while the upstairs flush-water is rushing down past it. Although I don’t know how it could be possible unless they removed or blocked the vent stack when they did the repiping.
if the lower toilet has marginally functioning filling and emptying valves then it could do this. deterioration of either could be coincidental or unnoticed.
Have somebody flush the upstairs toilet while you watch what happens in the tank of the downstairs one. The lower toilet shouldn’t refill unless the water level in its tank goes down some. The only way for water to go out that wouldn’t be a leak to outside the toilet is the flapper, but flushing the upper toilet shouldn’t be able to jar it.
Maybe the downstairs toilet’s float valve is wearing out and responding to the changes in water pressure caused by the other being flushed.
What kind of innards does the lower tank have? Ball float on a long rod? Perhaps the upstairs toilet flushing causes small oscillations in the water in the lower toilet tank, and triggers it to briefly come on again.
Do you know if the bottom tank will do this more than once, unless it gets flushed again? So:
Flush bottom toilet: it refills.
<time passes>
Flush upper toilet: bottom toilet refills briefly
<time passes>
Flush upper toilet: bottom toilet refills briefly again? or not? Or it depends how long since the last refill?
This sounds like a classic case of inadequate venting. Essentially as the slug of water comes by where the lower floor toilet connects, it sucks out the water from the bowl. If thats the only symptom then it’s probably venting above the second floor toilet which isn’t large enough (or obstructed) to allow in enough air to follow the water down. If the lower toilet gets a large bubble, then there’s also a vent problem for that toilet too because the air can’t get out of the way of the oncoming water fast enough.
As for a connection between your change of pipes, since it was a retrofit, the only things I can think of is that they might have reduced the size of the stack so it’s easier to install. Maybe your cast iron was so plugged up with mineral deposits that it restricted the flow enough to mask the problem.
To connect two toilets, one on top of the other, most plumbers would use 4" pipe for the stack, which will accommodate say someone flushing the toilet, draining the bat and someone else using the toilet downstairs at the same time.
Another possibility is your stack is the right size…most of the way but they tied it into a reduced size vent where it penetrates the roof. As far as I’m aware the minimum size vent is 2" and in a lot of places even if you have a 6" stack you can never reduce it.
Other signs of inadequate venting are gurgling drains, drains which create too much siphon (an over powerful vortex which gives you that big slurp at the end, probably followed by a gurgle…that could also indicate an undersized drain), wet, moldy smell from lower floor fixtures even after you immersed everything in bleach.
As you can see there’s some confusion going on here. Can you (the OP) clarify that the downstairs tank is refilling (as opposed to the bowl draining)?
The bowl draining a bit would would very much be related to the drains being repiped, but if all that was done was the drains being repiped and the plumbers never touched the copper then the tank refilling when you flush another toilet isn’t likely going to have anything to do with them. The only way it could have something to do with them is, as some one else said, the flapper or float is responding to vibrations or pressure fluctuations in your plumbing system and I suppose if they removed the toilets and set them back in place it’s possible something got jostled around.
To borrow one of the suggestions from upthread, you need to remove your downstairs tank lid and open the seat. Have someone flush the upstairs toilet and see what’s going on. Is it going through a full flush? Is the tank draining a bit? Is the flapper bouncing up? Is it just running a bit? Is the float bouncing? All these things can be fixed (and on your own for less of a hassle then calling the plumber back in most cases). Or, is it possible that the PVC just makes a different noise and/or follows a different route then the cast iron and sounds like the toilet running?