Good catch Joe. A pressure balancing valve won’t give you 100% hot water with low flow. Here’s the reason: These valves keep the hot and cold water pressures balanced. They have an internal part that equalizes any pressure differences. Because of this, you can’t just cut down the water flow while making it all hot.

With a single handle, turning it all the way to hot opens the hot water inlet completely and restricts the cold inlet. But how much water comes out still relies on the system pressure. If you lower the water flow with the handle only, you’re lowering both hot and cold water together, not just the flow.

At very low flow levels, most pressure-balance valves can’t guarantee only hot water. The balancing part might still add some cold water to keep things equal.

To get 100% hot water at very low pressure, you’d need one of these:

* A thermostatic mixing valve (controls temp instead of pressure).
* A separate shutoff or volume control trim to adjust the flow, while the main valve handles the mixing.”

I don’t know what to call this simplest trim, but it’s on the right in figure 6, and I can’t attach a file here.