Quick Answer:
If every cold tap is dead but hot works (or vice versa), it usually means a single shutoff or mixing point is closed or cross-connected. Start by testing several fixtures to confirm which side is affected, then check the water heater cold feed, any whole-house tempering or mixing valve, and under-sink shutoffs on single-handle faucets. If you need context about neighborhood vs. house supply, see How to tell if outage is city or house and for intermittent whole-house problems check Whole house water intermittent.
Why This Happens
- One closed shutoff (at the meter, water heater, or a branch valve) can cut one side of supply to the whole house.
- A stuck tempering or mixing valve at the heater can block cold or hot flow to downstream plumbing.
- Single-handle faucets or a failed cartridge can allow cross-connection, making it seem like one side is lost when only a few fixtures are affected.
- Work or valve changes by a previous homeowner or contractor left a valve partly closed or in the wrong position.
Step-by-Step What to Do
1. Confirm which side is dead by testing multiple fixtures
- Test cold and hot at a sink, tub, and an outdoor spigot if you have one. Note which rooms show the same issue.
- If all cold taps across the house are dead but hot works everywhere, that points to a single cold isolation point. If only one bathroom or sink is affected, the problem is local.
2. Check the main shutoff and the water meter area
- Locate the house main shutoff and confirm it is fully open. Some valve types look half-open when they’re not.
- If your supply has a separate cold and hot feed shutoff near the meter or entry, confirm both are open.
3. Inspect the water heater cold feed and its shutoff
- Find the cold inlet shutoff feeding the water heater and make sure it is fully open. A closed or partially closed valve can stop cold to the rest of the house.
- If the water heater has a mixing or tempering valve attached, inspect that next (see next step).
4. Look for a tempering/mixing valve stuck or set wrong
- Many systems use a tempering valve at the heater to limit outlet temperature. If it fails or is set wrongly it may block one side.
- Visually inspect for leaks, corrosion, or a stuck handle. If adjustable, note current setting before making small, careful changes and re-test fixtures.
5. Isolate single-handle faucets and cartridges that can cross-connect
- Under-sink shutoffs allow you to test a single fixture. Close the cold shutoff under one sink and check house cold flow; repeat with hot shutoff. This isolates whether that faucet is backfeeding.
- Single-handle mixers with damaged cartridges can let hot and cold cross. If isolating the fixture fixes the house problem, the cartridge likely needs replacement.
6. Restore correct valve positions and re-test
- Once you find the closed or mispositioned valve, open it fully and test hot and cold at multiple fixtures again.
- After any adjustment, check for leaks around the valve and the water heater fittings. Make sure all valves are in the intended “open” position.
What Not to Do
- Don’t drain the water heater unless you’ve confirmed cold supply is actually available to refill it.
- Don’t force valves that are clearly corroded — forcing can break the valve and make repair harder.
- Don’t remove mixing valve parts without turning off power or gas to the water heater and closing the proper shutoffs first.
When to Call a Professional
- If you find a faulty tempering or mixing valve and you’re not comfortable replacing it.
- If the cold feed at the water heater is blocked but you cannot locate or operate the shutoff valve safely.
- If you suspect a municipal-side issue after confirming all house valves and the meter connection — your plumber can coordinate with the utility.
Safety Notes
- Turn off electrical power or gas to the water heater before doing any work that could involve the water heater’s components.
- Wear eye protection when working under sinks and around valves to avoid splashes or debris.
- If you shut off the main water, have a plan for refilling and bleeding air from the system before restoring water heater operation.
Common Homeowner Questions
- Why is only cold water gone but hot still flows?
Usually a closed cold shutoff at the heater or meter, or a mixing valve failure blocking the cold line. - Can a single faucet make the whole house lose one side?
Yes — a faulty single-handle faucet cartridge can cross-connect and affect perceived supply until isolated. - Will turning valves open and closed fix it?
Often yes if a valve was partially closed; if a valve is broken or corroded you’ll need repair or replacement.