Ok, this is a head scratcher for me. I’ve got an apartment which was plumbed with pex and I installed a manifold as well. Got a tankless water heater (navien) which I use in almost all my rentals now. Every time the washing machine runs, and the hot water cycles off, I get water leaking out of the upstairs shower and tub fixtures (two separate baths). Sometimes even when hot water is opened on downstairs fixtures, it will leak a little when heater cycles off. I’ve never had this kind of problem, and took apart cartridge in bath fixture to see if it was a problem, but it was fine (relatively new). Any ideas what could be causing this water to push out through closed fixtures? --71.172.xxx.xxx

There are two types of pex. One the B type which is sold at the big box stores then the A type pex which does not use crimps which is commercial grade pex like Uponor or Rehau where the diameter does not go down with fittings. Unless the wall is opened to see where the leak is coming from then can diagnose. Fixtures need to be secured where there is no movement where in rentals tenants are harder on fixtures. In the apartment buildings the bathtub faucet was secured so there absolutely no movement where every possible opening a screw was put in to studs. --147.194.xxx.xx

Do you have an expansion tank? --50.5.xxx.xxx

My guess is there's a weakness in the cartridges or inserts in your upstairs shower and tub fixtures. They can't stay "stopped" with the abrupt change in pressure. Rare but I've seen this before. Please let us know the remedy when it's fixed and good luck. --73.215.xxx.xx

No expansion tank as the navien is a self contained unit. I’ve got these in several rentals, but never had this problem. This is the only apartment that has a manifold those (I wanted to try and distribute flow evenly in this big apartment). I can’t think of how manifold would have anything to do with it, so I’m thinking that the water heater must be dumping excess water in a burst or something that is pushing out through the shower and tub faucet. It’s very odd. --74.89.xxx.xx

Water Hammer from washing machine. You can buy water hammer arrestors for the machine for cheap. --157.131.xxx.xx

plus expansion tank somewhere on cold line would probably solve your problem. could just be how the plumbing is set up in that unit in conjunction with a somewhat high or quirky pressure in water heater. --67.49.xx.x

Those things are not allowed locally as there can be big sudden demand and then they can't meet the need. too much pressure on system here --71.104.xx.xxx

How about a water pressure regulator just before the manifold? --107.141.xx.xxx

Check the resting water pressure first to see if that is excessive. Shouldn't be much over 65 PSI if that. It could be your problem. --71.212.xxx.xxx

I’m on city water and it’s around 95 psi entering the building. I’ve not had this issue at any other apartments in town, so it’s strange that it only occurs here. I’m going to buy the hammer arrestors suggested above and see if that fixes the problem at all. Will report back! --71.172.xxx.xxx

Too high water pressure. --73.120.xx.xxx

A regulator can be installed in the building where the pressure is reduced. Maybe check out videos on You Tube. --147.194.xxx.xx

Try adjusting regulator down a bit. --172.250.xx.xxx