Is 3 GPM OK for a valve for shower and tub?

I recently moved into a 2-story house built in 1994. One shower and tub in a bathroom upstairs has very weak water flow (same for cold and hot water). The faucet over sink in the same bath has OK flow/pressure. The water is clear. The tub pipe with the faucet removed is only at 0.5 gallon per minutes.

It is the one knob just turn system (Posi-Temp?). I removed the knob, disc, and cartridge. And after I turned the water to the house back on, the water flow at the valve (with both hot and cold water I guess?) seems OK to me - it is around 3 gallons/minutes. I studied the hole and did not find any stop like the one explained in Visual inspection at the valve show no sign of calcium deposit to me, some one who had never seen such thing before.

My question:

Is the flow at 3 gpm OK with both hot and cold water? If so,

- could the pipe/valve be clogged? The water heater that serve the bathroom was recently replaced. But I don't know if the shower/tub water flow was good before that. How can we flush it?

- could it be some stops/shut off somewhere nearby? how can I find out?

Any suggestion/comments? Thanks!

A puzzled mom

3gpm is about half of what a typical tub/shower valve can supply, given 'normal' pressure and 1/2" pipes. It would take a LONG time ot fill a tub at that rate! Is this the only bathroom with that problem (i.e., do you get good flow and pressure at other places in the house)? Some valves have screen filters on the inlets, and those may be clogged. Since taking the cart out doesn't improve things much, it is likely the supply, and not the valve.

Is it the same on both hot and cold, or is the cold volume much better? When they replaced the WH, they had to turn the water supply off. It's possible that valve is not fully opened. If it was a gate valve, it may be broken, and the flow you are getting is that which can 'slip' by. If it is a broken gate valve, the handle will (usually) just keep turning, and never stop at the one or off position.

Thanks for the reply, jadnashua.


The pipe looks like a 1/2". This is the only bath room with this problem. The faucet in the same bathroom sink's water flow seems to be OK - the volume is OK and the pressure could be a little bit weak. My estimate for it is about 1-1.5 gallon per minutes.

With the cartridge back, for water flow from tub pipe, I don't see a difference between hot and cold water.

The water heater also serves hot water for another shower. The hot water supply for it is normal.

I would like to give an update on this in case somebody experiences the same problem.

The problem was fixed with a very low cost.

After some study, we think the water supply should give better water flow at shower head than we got. So we believe there is something wrong after the catridge. Since the problem is for both tub and shower, our guess is the mixed chamber before the pipes splits. We spent sometime to using any tools we can find to clean the mixing chamber but all failed. Eventually, we put the catridge back and left tub faucet and shower head unattached. Then we blocked the tub pipe and fill the pipe including mix chamber with white vineger via the shower arm. We left it soaked over night - 10 hours. Then next morning, we removed the tub pipe plug. The water came out was light green. We didn't see any pieces of debris. The water flow at the shoer head about doubled and is suitable for a shower now.