The Hidden Danger of Check Valves in SF

In a previous post, we discussed the dangers of San Francisco’s incredibly high municipal water pressure. But what happens when the dangerous pressure isn’t coming from the city street, but is actually being generated inside your own home?

If your shower constantly fluctuates between scalding hot and freezing cold, or your faucets won’t stop dripping no matter how hard you turn the handle, you might be dealing with a locked plumbing system.

At HydroFlow, this is one of the most common—and most misunderstood—plumbing phenomena we encounter in the Bay Area. It all comes down to basic physics, thermal expansion, and a tiny device called a check valve.

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The Physics of Thermal Expansion

To understand the problem, you first need to understand how your water heater works.

When cold water enters your water heater, the burner fires up to heat that water to your desired temperature (usually around 120 degrees). As water heats up, it physically expands, taking up more volume inside the pipes. This is known as thermal expansion.

In a normal, open plumbing system, this expansion isn’t an issue. The expanding water simply pushes backward through your main water line and out into the city street. The massive municipal water grid easily absorbs this tiny bit of extra volume, and the pressure inside your home remains perfectly balanced and safe.

The Problem With Check Valves

The crisis begins when a contractor installs a check valve on your main water line.

A check valve is a one-way street for water. It allows municipal water to flow freely from the city into your home, but it firmly locks shut if the water tries to travel backward from your house out to the street.

Sometimes these valves are installed intentionally (like for irrigation systems or to comply with specific backflow prevention codes), and sometimes they are built invisibly into the design of a smart water shutoff device or a water meter.

When you have a check valve installed, your home’s plumbing goes from being an “open” system to a “closed” system.

Now, when your water heater fires up and the heating water expands, that expanding volume has nowhere to go. It hits the check valve and stops. Because water cannot be compressed, that trapped expansion immediately converts into extreme internal pressure.

The Damage of a Locked System

When expanding hot water is trapped inside a closed loop, the pressure can easily skyrocket to 100 or even 150 PSI inside your walls.

This extreme pressure desperately looks for a way out, violently attacking the weakest points in your home’s plumbing:

• Destroyed Gaskets: The rubber seals inside your faucets and toilets are crushed by the pressure, leading to constant, annoying drips.
• The “Ghost” Shower: This is the most common symptom. The extreme pressure destroys the delicate mixing cartridge inside your shower valve. Suddenly, the high-pressure cold water is able to force its way directly into the hot water line. You get into the shower expecting a warm start to your morning, but the temperature fluctuates wildly because the broken cartridge is randomly mixing the lines.
• Appliance Failure: Your washing machine and dishwasher hoses are pushed to the absolute brink, significantly increasing the risk of a catastrophic blowout.

The Solution: Thermal Expansion Tanks

Fortunately, the solution to a locked plumbing system is fast, affordable, and incredibly effective.

If your home requires a check valve (or has one built-in), you absolutely must install a Thermal Expansion Tank near your water heater.

An expansion tank is a small, domed tank that contains a rubber bladder filled with pressurized air. Because air can be compressed (unlike water), it acts as a shock absorber. When your water heater fires up and the water expands, the expansion tank safely absorbs that extra volume. The internal pressure of your home remains perfectly stable, your shower cartridges are saved, and your faucets stop dripping.

Don’t Wait for the Shower to Go Cold

If you live in San Francisco and suspect you might have a check valve wreaking havoc on your internal pressure, don’t wait for a gasket to blow out.

Contact HydroFlow today for a comprehensive plumbing inspection. We evaluate the pressure on every single service call, checking for locked systems and installing robust expansion tanks to protect your home’s most expensive fixtures.