A bathtub can fill slowly while the shower seems fine because showerheads are capped at a low flow, which hides a restriction the open tub spout shows. Common causes are a supply not fully open, a worn or clogged valve cartridge, scale in the spout, or a spout piped with the wrong pipe or too many fittings.
Why can the shower seem fine when the tub is slow?
Because the showerhead limits flow, and the tub spout doesn't. Phoenix's adopted plumbing code caps a shower head at 2.0 gallons per minute at 80 psi. A tub spout has no such cap in that table. It's meant to move a lot of water. The same code's table of minimum flow at fixtures lists a bathtub with a balanced-pressure, thermostatic or combination valve at 4 gallons per minute, at 20 psi.
So a valve that can only push, say, a bit over 2 gallons a minute will still feel normal through a showerhead that's limited to 2. Send that same water to the tub spout, and the tub fills at half the rate it should. The shower wasn't fine. It was just hiding the problem.
That tells you where to look. The restriction is usually somewhere the tub and shower share, like the valve or the supply to it, or in the path to the tub spout itself.
What are the usual causes?
Here's where to look, roughly from easiest to hardest to check.
### A supply that isn't all the way open
Start with the simplest cause. Moen's low-flow troubleshooting for its PosiTemp valve says that when there's no flow from the tub spout and shower, "Confirm both hot and cold supply lines are both turned all the way on." A valve left partly closed after a repair limits every outlet it feeds.
If the slow fill started right after someone worked on the shower or the plumbing, check this first.
### A worn or clogged cartridge
The cartridge inside the valve controls flow and temperature. Moen's guide for its PosiTemp valve says that when low flow of hot and cold shows at both the tub spout and showerhead, you replace the cartridge. When only one side, hot or cold, is low, it also points to the cartridge.
On Moen's push-pull Standard valve, its guide adds a simple check: "Confirm the handle is being pulled out all the way to turn the water on to full capacity." For how the replacement goes, see how to replace a shower cartridge.
### Debris or scale in the tub spout
The spout itself can be the choke point. Moen's troubleshooting for a tub that also sprays from the showerhead says the problem is "caused by either a restriction in the system or an installation issue," and its first step is to remove the tub spout and "inspect for any obstructions that may restrict the water from the spout."
Phoenix water is hard, which adds to buildup over time. The city's 2025 water quality report lists hardness of 10 to 17.6 grains per gallon, and says hard water "can cause some types of scaling in pipes and water heaters, and on plumbing fixtures such as faucets and showerheads."
### A spout piped the wrong way
This one is common after a remodel. Moen's same troubleshooting guide lists several installation problems that restrict the tub spout:
- The wrong pipe. "From the tub port of the valve, only ½" Copper or ½" Galvanized Iron pipe (IPS) can be used." If PEX or CPVC was used there, Moen says it will need to be replaced.
- The wrong distance. "The tub spout can be no less than six inches and no greater than eleven inches below the valve."
- Too many bends. "There should only be one elbow from the tub port of the valve to the tub spout."
- Restrictive fittings. Transitional fittings that might restrict the water flow need to be removed.
Those rules are for Moen valves. Other brands publish their own, so check the instructions for yours.
A telltale sign of a restricted spout on a diverter setup is water coming out of the showerhead while the tub fills. That's the exact symptom Moen's guide was written for.
### Low pressure to the whole house
If every fixture is weak, the tub is just showing it most. The City of Phoenix lists 40 to 100 psi as its acceptable city-service range; that is separate from the 80 psi maximum for building distribution under IPC 604.8. Higher static pressure inside the building requires the specified pressure reduction. The City also lists "A partially closed house valve" among the potential causes of reduced water pressure. See low water pressure in the whole house for the other causes.
While the spout is off, check the diverter
If your tub spout has a lift rod, it's a diverter spout. Moen explains that a diverter tub spout "will have a lift rod that you pull up and push down on to divert the water between the showerhead and tub spout." A non-diverter spout has no lift rod.
Diverters are a common weak spot in the other direction too. A study published by NYSERDA, New York's energy research agency, surveyed about 130 apartments and houses with 120 combination tub and shower units. It found that 34 percent of the diverters leaked more than 0.1 gallons per minute in shower mode, with the largest leak at 3.0 gallons per minute. That water runs out the tub spout and down the drain while you shower, wasting both the water and the energy used to heat it.
So if water dribbles from the spout while the shower runs, the diverter is worn. If the tub fills slowly and some water sprays from the showerhead at the same time, the restriction is more likely in the spout or its piping. Either way, the spout is off, so it's a good time to look at both.
How do I narrow it down?
Work through it in this order:
- 1Compare hot and cold. Run the tub on full hot, then full cold. If only one side is weak, the problem is likely that side's stop or the cartridge.
- 2Check the supplies. Make sure the valves feeding the shower, and the house valve, are fully open.
- 3Pull the spout. Moen notes there's a small hole on the bottom of the spout. If you see a screw, it's a slip-fit spout. If not, it's threaded onto the pipe. Look inside for debris or scale.
- 4Look at the piping. With the spout off, note whether the pipe coming out of the wall is copper, galvanized, PEX or CPVC.
- 5Check other fixtures. If the kitchen and bathroom sinks are weak too, look at whole-house pressure instead.
### When is it time to call a plumber?
When the fix is inside the wall. Replacing the pipe to the spout, correcting the spout distance, removing extra elbows or replacing a stuck cartridge can all mean opening the wall or working on the valve body. If the slow fill started after a remodel, the spout piping is the first thing we'd check.
HQ is licensed and insured, and we give free estimates with upfront pricing. We can find the restriction and fix it without guesswork.