Shower Valve Troubles

[smith1995]

I'm having a problem with my moen posi temp shower valve. So I have a tankless water heater, im in the process of fixing the elements right now so i can't actually test for hot water, but I can turn the water back on. I replaced the shower valve as it was giving me issues. Here's the problem though, I get full hot water at the bathroom sink but not at the shower. When I turn off the cold water isolation valve for the bathroom at the header I get pretty much a dribble of hot water from the shower (which isn't enough to turn the water heater on as it senses for flow), but I get full flow of hot water at the sink. With the shower valve still at full hot I turn back on the cold water isolation valve for the bathroom and the shower is flowing fully and turns the water heater on, even though all I did was turn the cold water back on. This confuses me so much , how could turning on cold water affect the flow of hot water. I already have the shower valve stopper set to max heat. Im not sure if this is normal or not. I guess I could try and clean the cartridge but the while valve is brand new.

 

[Twowaxhack]

The shower valve is pressure balanced. If you turn the cold water off, then you won’t get hot water flow out of the shower. You must have hot pressure and cold pressure, it’s pressure balanced.

Both sides of the shower valve must have water pressure in order for the shower valve to flow any water other than a dribble.

 

[RickFlorida]

Is it possible for the posi-temp cartridge to be installed upside causing this? I cant' remember if they can be installed upside or not or that would even cause this problem. But I noticed you said you replaced it so thought I would help rule that out.

the only thing I can remember of what they look like when you install them is there is a circlip you stick through the protruding end to hold it in place and I believe it's possible to install them upside down?