No hot water flow unless cold water is turned on.

I have a tankless water heater for a bathroom that produces hot water to the sink but when I turn on the shower, the cold water has to be turned on for the hot water to flow. In other words, the hot water will not flow unless the cold is turned on. I don't understand why it works fine at the sink but when it comes to the shower, it does not flow. I ran the shower to see if the lines were crossed some how, but when I touch the pipes near the shower valve, the hot is hot and the cold is cold. I tried turning off the cold water ball valve behind the shower in the access panel and leave the hot water valve turned on to see if the hot would run but it doesn't. I'm confused.

 

Welcome to the forums.

Just a few questions.....
1) Is this a two handle shower valve or a one handle anti scald type setup ?
2) Which cold water do you turn on.... at the sink or the shower ?

If it's a single type valve with anti scald it may need to have the cartridge replaced.

 

Thank you for your response.

1) The shower valve is a one handle style. When I adjust the screws on each side of the valve and turn off the cold side there is still no flow from the hot side. (shower). If I leave them both open, there is flow from both.
I never thought it would be the valve because I bought it new.

2)The sink works fine. I was just using it as an example of how it is supposed to work.

 

That is correct. That is a balancing type valve. Based on temperature it changes the valve from hot to cold water to supply constant temperature. Without both sources..... it closes.

 

Thanks PJ Max;
I just adjusted the anti scald valve and the hot runs better.
Thanks again!