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twilightcall 08-23-2008 09:06 PM

Does a shower valve need to have both hot and cold connected to work?
I installed my new Moen shower valve today with riser and tubspout. It went well. I sweated all of my fittings and connected the hot and cold water lines. I only have cold water right now since I haven't installed the hot water heater yet. When I went to test the valve it only dribbled. Does it need flow from the hot side to work properly? Is this the reason I didn't get any flow? Thanks..

Nestor_Kelebay 08-23-2008 11:29 PM

That's probably it, but check with Moen to confirm.

Nowadays, shower valves have an antiscald feature built into them. Basically, what it does is that if there's a drop in the hot water supply pressure, it pinches off the flow of cold water to prevent you from a cold shock, and if there's a drop in the cold water supply pressure, it pinches off the flow of hot water so you don't get scalded.

Cuz otherwise, if someone flushed the toilet while you were having a shower, you'd come out with scalded skin.

When you connect the hot supply, the pressure should be equal in both, and the valve should operate normally.

buletbob 08-24-2008 06:51 AM


SD515 08-24-2008 09:03 AM

Exactly. I plumbed a Moen shower/tub valve last year with the same problem. Customer was also changing water heater at the same time, so I didn't have water on the hot side. I set the new water heater and just plumbed the water to it so I could get water to the tub, and the valve works fine when it has pressure on both hot & cold sides.

mstplumber 08-24-2008 01:32 PM

Everyone is correct. Moen valves feature a "pressure balance" cartridge which basically consists of a spool that is balanced between the hot and cold openings by water flow. The spool works by restricting the flow from the side with the most pressure, so if one side has no pressure it will block off the other side.

twilightcall 08-24-2008 07:00 PM

Thanks guys. You are all right. Since my water heater is installed yet I put a 3/4 to 3/4 fitting in and it worked great. Thanks Again.

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