Quick Answer:
When two showers run at once the available cold-water pressure can drop enough to reduce flow or make the mixer valve pull more hot water. Start by isolating fixtures to test pressure, check the main pressure-reducing valve (PRV) setting, and watch your water meter for flow patterns before assuming a broken shower valve.
Why This Happens
Homes have a finite supply of cold-water flow. When two showers demand water at the same time, the cold side can be starved if the incoming pressure or pipe size can’t keep up. That causes shower mixers to shift the balance toward hot water, or the cold side flow to slow so much the shower feels warm and weak. Age, a low PRV setting, undersized plumbing, or other appliances running at the same time make the effect worse.
It’s common to see similar symptoms in other situations — for example, appliances can create brief drops in available pressure. See Pressure drops when dishwasher fills for a related example of demand-related pressure loss.
Step-by-Step What to Do
1. Isolate and test a single fixture
- Shut off every fixture and appliance that uses water (toilets, taps, washer, dishwasher, irrigation).
- Open the cold side of one shower fully and note the flow. If you have a simple pressure gauge, attach it to a hose bib and read static and running pressure.
- If pressure is normal with others off, the issue is demand-related — multiple outlets are competing for cold water.
2. Observe the meter flow patterns
- Go to your water meter. With both showers off, note the dial or digital flow indicator.
- Start one shower, then the second. Watch for large spikes or a steady high flow. A steady high reading while fixtures are closed suggests a leak; spikes that match shower use indicate demand peaks.
- Make a simple note: steady continuous flow vs short bursts tied to use. That pattern tells you whether supply or a leak is the real problem.
3. Confirm PRV setting
- Locate the pressure-reducing valve near the main shutoff or meter. Many are adjustable; typical domestic settings are 50–60 psi.
- Check the PRV’s gauge reading. If it’s set very low, raising it slightly can improve cold-water availability. If you’re not comfortable adjusting it, stop and call a pro.
- If the PRV is working but pressure is still inadequate when two showers run, the system may be undersized for demand or have a partial blockage.
4. Check for simultaneous demands elsewhere
- Confirm other devices aren’t running (dishwasher, laundry, irrigation). Even short-term draws can cause the problem.
- When you see demand-related dips, consider staggering large uses or limiting simultaneous showering times as a practical short-term fix.
5. Record what you find and decide next steps
- If isolation, meter observation, and PRV check point to demand rather than a single failed fitting, plan upgrades (bigger supply line, pressure adjustment) or call a plumber for options.
- If meter patterns show continuous flow with everything off, investigate leaks or faulty valves right away.
What Not to Do
- Do not assume fixture failure when pattern is demand-related — replacing a shower cartridge or valve won’t fix a supply shortage.
- Do not dramatically increase PRV pressure without confirming the rest of your plumbing can handle it; that can damage fixtures and appliances.
- Do not work on the main without shutting the water off properly or without basic knowledge; call a pro if unsure.
When to Call a Professional
- If your PRV is faulty or you can’t access or interpret its settings.
- If meter observation suggests a hidden leak (continuous flow when all fixtures are off).
- If supply piping is undersized and you want a reliable long-term solution (pipe upsizing, system redesign, or booster pump assessment).
Safety Notes
- Shut off water at the main before doing any work on valves or the PRV. Sudden opening can cause surge or injury.
- Be cautious around hot water when isolating or testing a shower — run cold side only when measuring cold supply.
- If you’re uncomfortable adjusting valves or reading the meter, hire a licensed plumber. Avoid DIY work that could cause flooding or contaminate potable water.
Common Homeowner Questions
- Why does the problem only happen when two showers are on?
Because two fixtures together can exceed the available cold-water flow or pressure, causing a noticeable drop. - Will a new shower valve fix this?
If the root cause is supply or PRV-related, replacing the valve won’t solve the low cold-water flow. - Could the water heater be at fault?
Often the heater isn’t the issue; pressure or cold-water supply problems are more likely. See also Water temperature changes with pressure drop.
For more related articles, see the Pressure Drops When Multiple Fixtures Run hub.