No water flow

[Jerry Simon]

Water heater supply line valve shut-off. Got cold water flow to all sink faucets and to toilets. No hot or cold to tub or shower. Valve turned on, and flow everywhere. Why no cold flow at tub and shower when water heater supply line valve off?

Is this a quirk of anti-scald devices at tub & shower?

[John Kogel]

Yes. They often need pressure on H and C to work at all.

[Jim Katen]

Water heater supply line valve shut-off. Got cold water flow to all sink faucets and to toilets. No hot or cold to tub or shower. Valve turned on, and flow everywhere. Why no cold flow at tub and shower when water heater supply line valve off?

Is this a quirk of anti-scald devices at tub & shower?

It's a pressure balancing shower valve. They've been required for a long time. I'm not even sure if you can get one that isn't pressure balancing anymore.

It equalized the pressure on the hot and cold sides in real time. So, if you're showering and someone flushes a toilet, the reduction in pressure in the cold side causes the valve to match the same pressure on the hot side. As a result, you don't get scalded. Instead, you just see a reduction in flow on both sides. If the pressure on either side drops to zero, it brings the pressure on the other side down to zero as well.

[Jerry Simon]

Grazie.

It's amazing that after almost 20 years inspecting, and almost as much time building, one learns new stuff on almost a daily basis. Gratifing as well.