Low hot water pressure: why it happens and how to fix it

Step 1: compare hot and cold

A. O. Smith's water heater manuals start low-pressure troubleshooting the same way: check both the cold and the hot water at a sink to see if the low pressure is only on the hot side. Then check a second and third fixture. The pattern tells you where the restriction is:

Only one faucet: aerator, cartridge or mixing valve

The aerator. Moen says aerators collect debris and mineral deposits over time, which can restrict water pressure or distort the spray. Unscrew the aerator from the spout tip (some are concealed and need the small tool that came with the faucet), rinse out the screens and reinstall. If the flow is still poor, replace it.

The cartridge. When only the hot (or only the cold) side of a faucet has less than normal flow, Moen's fix is to replace the cartridge, the valve inside the handle.

The shower valve. A. O. Smith notes that many shower controls have built-in mixing valves, and when they fail they can cut the hot water a shower delivers even with plenty of hot water in the tank. If a shower is the only weak fixture, see hot water in the sink but not the shower.

Also check the shutoff valves under the sink. Moen's first troubleshooting step for low flow is to make sure they haven't been turned partly off.

Every hot tap is weak: check the water heater

The supply valve. A. O. Smith lists a partially closed supply valve as one of the two main causes when low pressure is only on the hot side. The valve on the cold line into the top of the heater should be fully open. It's easy to leave it partly closed after a repair or a flush (see how to turn off a water heater).

Melted heat traps or dip tube. A. O. Smith says soldering copper pipes while they're connected to the heater can melt the heat traps inside the hot and cold connections or the plastic dip tube on the cold side, restricting hot water flow; the fix is to replace them. This tends to show up right after a new heater goes in (see water heater dip tube).

Tankless heaters: filter, scale and valves

Noritz lists four causes of low hot water pressure on tankless units: clogged inlet filters, mineral buildup, partially closed shutoff valves and plumbing restrictions. Its steps are to clean the inlet water filter, make sure all valves are fully open, schedule descaling if scale is suspected, and check the plumbing connections. Tankless units also need a minimum flow to fire: Rinnai says its gas models need about 0.4 to 0.6 gallons per minute to ignite. Descaling steps are in how to flush a tankless water heater, and other symptoms in tankless water heater not heating.

Is the flow actually low?

Newer fixtures are built to use less water, so some drop in flow is by design. Moen says that because of national conservation laws, it makes its kitchen faucets at 2.0 gallons per minute and its pull-out and pull-down faucets at about 1.5. EPA WaterSense says a standard showerhead uses 2.5 gallons per minute and a WaterSense model no more than 2.0. If hot and cold both feel modest but equal, that may just be the fixture.

House water pressure: the healthy range

A. O. Smith says household water pressure should be 50 to 60 psi and no higher than 80. If it's higher, it recommends a pressure reducing valve on the main cold line set to 50 to 60 psi; high pressure can make the heater's relief valve drip (see water heater relief valve leaking). A pressure gauge that threads onto an outdoor hose bib is a cheap way to check.

Hard water makes it worse

The minerals that clog aerators and scale up tankless units come from hard water. In Dublin and Hilliard, Ohio, Columbus says it softens its water to an average of about 120 ppm, roughly 7 grains per gallon, which it calls moderately hard, according to our town pages. If you're on hard water and aerators keep clogging, see water softeners and water heaters.

When to call a plumber

Call if hot water pressure dropped right after a heater replacement (heat traps or a dip tube may need replacing), if every hot tap is weak and the heater's valve is fully open, or if you find your house pressure is above 80 psi. If the hot water is weak and not getting hot, start with no hot water in the house.

Local pages mentioned

• Hilliard, OH
• Dublin, OH