Usually not the pressure. A water heater passes along your home's water pressure and adds none. A new heater can still make showers better: steadier temperature, longer hot water, and no rotten egg smell. It fixes weak flow only when the old heater is the restriction, and an undersized tankless unit can make flow worse.
Does a water heater affect water pressure?
A water heater doesn't add pressure, but it can take some away. The tank sits on your cold water line and is fed at the same pressure as the rest of the house, so hot water leaves it at about the pressure that came in. Anything that restricts flow through the heater, its valves, or its connectors lowers pressure on the hot side only.
That gives you the first test. Run the shower on full cold, then on full hot, and compare the flow.
| What you see | Most likely cause | Will a new heater help? |
|---|---|---|
| Weak on hot and on cold | Showerhead, shower valve, or whole-house pressure | No |
| Weak on hot only, at every faucet | Something at or near the water heater | Maybe |
| Weak on hot only, at this shower | The shower valve's hot side | No |
| Strong flow, but temperature swings | Valve, heater, or both | Often |
| Hot water runs out too soon | Heater size, sediment, or a broken dip tube | Often |
If both sides are weak, our page on why shower pressure is low covers the showerhead and valve causes, and low water pressure in the whole house covers the supply side.
### What water pressure should a shower have?
A home's water pressure should stay at or under 80 psi. The plumbing code Phoenix uses says that where static pressure in a building "exceeds 80 psi," a pressure-reducing valve must bring it down to "not greater than 80 psi." A.O. Smith's maintenance guide adds that "Most experts recommend setting the Pressure Reducing Valve to 50-60 psi to protect appliances," and that "High water pressure can damage water heaters, toilet valves, ice makers, dishwashers and washing machines."
Showerheads also limit flow by design. The same code caps a new shower head at 2.0 gallons per minute at 80 psi. EPA WaterSense says "WaterSense labeled showerheads use no more than 2.0 gpm." A modern head feels softer than a 20-year-old one even at the same pressure. That's the head, not the water heater. For the pressure numbers in detail, see what home water pressure should be.
When can a new water heater fix weak hot water flow?
A new heater helps when the restriction is inside the old one. The classic sign is low flow on the hot side at every fixture in the house while the cold side is strong. Common reasons include:
- Sediment and scale. Minerals settle to the bottom of a tank over the years. A.O. Smith's troubleshooting table lists "Sediment or lime in tank" as a cause of insufficient hot water, and its manual recommends that the tank "be drained and flushed every 6 months to remove sediment which may build up during operation."
- A shutoff valve that isn't fully open. This one costs nothing to fix. Check the cold water valve above the heater before you replace anything.
Here is the catch: a flush can sometimes restore flow without a new heater. A tank that is otherwise sound may only need its sediment drained. If it's old, leaking, or rumbling, replacement starts to make sense. Our page on whether to repair or replace a water heater walks through that call.
Can a new water heater make showers better even if pressure doesn't change?
Yes, a new heater often brings steadier temperature, longer hot water, and no smell, even when pressure stays the same. Most complaints about "shower quality" are really about temperature and supply, not pressure.
### Hot water that runs out too soon
If showers start hot and turn lukewarm early, the heater may be too small, full of sediment, or have a broken dip tube. The dip tube is the pipe that carries incoming cold water to the bottom of the tank. When it cracks, cold water mixes in at the top, right where hot water leaves. A.O. Smith's troubleshooting table lists "Leaks or cracks in dip tube" as a cause of insufficient hot water, with the fix "Check dip tube. Replace if faulty." A new heater, sized with a higher first-hour rating, fixes the run-out problem for good. See why you run out of hot water fast in the shower for the causes in detail.
### Temperature that won't hold steady
Temperature swings usually come from the shower valve, not the heater. Under Section 412.3 of the plumbing code Phoenix uses, shower valves "shall be balanced-pressure, thermostatic or combination balanced-pressure/thermostatic valves," the kind built to hold your chosen temperature when someone flushes a toilet. If yours doesn't hold, the valve cartridge is the first suspect. See why shower pressure drops when a toilet flushes.
A new heater can still help here when the old one struggles to recover. A heater that can't keep up delivers hot, then warm, then hot again as its burner or elements cycle.
### A rotten egg smell in the hot water
A sulfur smell that only shows up in hot water often comes from the tank. A.O. Smith explains that the odor forms when sulfate in the water, low oxygen, harmless bacteria, and the tank's anode rod combine. Its manual says smelly water "may be eliminated or reduced in some water heater models by replacing the anode(s) with one of less active material, and then chlorinating the water heater tank and all water lines." So a new heater isn't always needed for the smell. Our page on smelly hot water covers the options.
Can a new tankless water heater lower shower flow?
It can, if it's too small. A tankless heater heats water as it flows, and it can only heat so many gallons per minute at once. Maricopa County's water heater guideline warns that when a tankless heater is undersized, "The temperature at a hot water fixture may remain the same, but the flow could slow to a trickle." The county also notes that "Typical tankless water heaters can provide hot water at a rate of 3 to 6 gallons per minute, with a 60°F water temperature rise."
So a switch to tankless can trade a tank that runs out for a shower that never runs out but flows weaker when two people shower at once. Size it for the flow your household actually uses at the same time. Our page on whether electric tankless water heaters work in Arizona has the flow math.
What should you check before replacing the water heater?
Check these in order, since the first few cost nothing:
- 1Compare hot and cold at the shower and at a sink. If both are weak, the heater is not the cause.
- 2Open the valves. Make sure the cold inlet valve on the heater and any shutoffs under the sink are fully open.
- 3Clean the showerhead. Scale from hard water is the most common cause of a weak shower. See how to clean a clogged showerhead.
- 4Flush the tank. If hot water is weak everywhere, draining sediment can restore flow. See how often to flush a water heater in Phoenix.
- 5Check the temperature. CPSC recommends setting water heaters to 120°F, and warns that "Burns will also occur with a six-second exposure to 140 degree water." If you turn the heater up to get more hot water, A.O. Smith says a mixing valve "should be installed at the water heater" or at the taps to bring the delivered temperature back down.
If those checks point to the heater, and it's old or failing, a new one with the right first-hour rating is the fix. Our page on what size water heater you need covers the sizing.
HQ Plumbing & Air diagnoses weak showers and replaces water heaters across the Phoenix area, with free estimates and upfront pricing. See our water heater service.
A new heater fixes hot water problems. For pressure, look at the showerhead and the valves first.