A loud bang near a pressure-reducing valve can be scary.
It can also be hard to track down.
In this case, the home has a tee off the main water service line.
That tee feeds the sprinkler system before water enters the house.
Once the line enters the home, it passes through a shut-off valve.
Then it goes through the pressure-reducing valve, also called a PRV.
After that, the water line branches through the house.
One branch feeds the water heater and expansion tank.
What Happens When The Sprinklers Start?
The loud noise happens when the sprinklers turn on.
This does not sound like normal water hammer.
Water hammer often happens when a valve closes quickly.
Here, the noise seems tied to pressure changes.
When the sprinkler zone opens, it may drop the incoming water pressure.
That drop can happen before the PRV.
At the same time, the house side may still hold higher pressure.
The water heater expansion tank may add to that pressure.
This can push pressure back toward the PRV.
Question And Answer
Question: Can an expansion tank make a PRV bang when sprinklers turn on?
Answer: Yes. If the sprinkler system drops inlet pressure, the house side can push back against the PRV. That may cause a bypass or check feature inside the PRV to chatter or bang.
Should You Add A Check Valve?
Adding a check valve after the PRV may seem like a simple fix.
It could stop pressure from pushing back into the PRV.
But it may also create a closed system inside the home.
That means heated water has fewer places to expand.
The expansion tank must be sized and charged correctly.
You can learn more about thermal expansion from
Watts.
Should You Remove The Expansion Tank?
Removing the expansion tank is usually not the best first move.
The PRV may include a bypass feature.
Still, that does not always protect every fixture.
There is about 25 feet of pipe before the tank.
Several fixtures may be tied into that section.
Those fixtures could see pressure changes without the tank.
Best Next Steps
First, check the expansion tank air charge.
It should match the home water pressure.
Second, test the pressure before and after the PRV.
Test it while the sprinklers turn on.
Third, inspect the PRV for wear or chatter.
A weak PRV can make loud sounds under changing flow.
For sprinkler system tips, visit the EPA WaterSense sprinkler guide.
A licensed plumber can also confirm code needs in your area.
The safest fix may involve PRV service, tank adjustment, or piping changes.
Do not remove safety parts without checking the full system first.
I’m Chris Mayer, writing for Plumbing 101.
