Cold works, hot does not
The cold side runs normally, but the hot side gives nothing or just a weak dribble.
Start here: Confirm the problem happens at more than one fixture or only one fixture before you assume the water heater failed.
Plumbing
Frozen Hot Water Line
Direct answer: If cold water still runs but the hot side slows to a trickle or stops during freezing weather, the hot water line is often frozen somewhere between the water heater and the fixture. Start by confirming it is only the hot side, then warm the most exposed section gradually and watch closely for leaks as it thaws.
Most likely: The usual culprit is a hot water branch line running through an exterior wall, crawl space, attic edge, garage wall, or uninsulated cabinet area.
A frozen hot line can look like a bad faucet, a failed water heater, or low house pressure, but the pattern matters. If the cold side works normally and the problem showed up right after a hard freeze, treat it like a frozen branch until proven otherwise. Reality check: the ice plug is often a few feet away from the dead fixture, not right at the handle. Common wrong move: heating the faucet body while the frozen section is still sitting in a wall or crawl space.
Don’t start with: Do not start with a torch, open flame, high-heat gun jammed against the pipe, or by cranking up pressure and waiting. That is how a hidden split turns into a flood.
If only one faucet has no hot waterFocus on that branch line first, especially any run in an outside wall or under a sink on an exterior wall.
If several fixtures lost hot water at onceCheck the hot water line leaving the water heater and the first cold draft area it passes through.
Last reviewed: 2026-04-05
What a frozen hot water line usually looks like
Only one fixture lost hot water
A kitchen sink, bathroom sink, or shower on an outside wall has little or no hot flow, while other hot fixtures still work.
Start here: Look for the most exposed section of that single branch, usually under the cabinet, in the wall behind it, or below the floor.
Several fixtures lost hot water
Multiple faucets have weak or no hot flow, but cold water pressure is still decent around the house.
Start here: Check the hot water piping near the water heater and any nearby unheated space before chasing individual faucets.
Hot water came back after warming, but now you see drips
Flow returns as the pipe thaws, then a damp spot, drip, or spray shows up.
Start here: Shut off water to that branch or the house and treat it as a split pipe, not a thawing problem anymore.
Most likely causes
1. Hot water branch line frozen in an exterior wall or unheated cavity
This is the most common pattern when one sink or one bathroom loses only hot water after a cold night.
Quick check: Open the hot side slightly and feel for the coldest section under the sink, in the basement below, or where the line passes near outside air.
2. Main hot water line frozen near the water heater or in a crawl space, garage, or attic edge
If several fixtures lose hot flow at once, the freeze is usually on a shared hot line before it splits off.
Quick check: Trace the hot line leaving the water heater and look for the first exposed stretch in a cold area.
3. Localized freeze at a shutoff valve, faucet supply tube, or pipe section inside a vanity or kitchen cabinet
Cabinets against outside walls can trap very cold air, especially if doors stay closed and insulation is missing behind the pipe.
Quick check: Open the cabinet, remove stored items, and compare pipe temperature at the shutoff, supply tube, and wall area.
4. Not actually frozen: water heater issue, closed valve, or faucet problem
If the weather is mild, the hot side is weak everywhere all the time, or only one faucet acts up without any cold-weather pattern, the cause may be elsewhere.
Quick check: See whether other fixtures have normal hot flow and verify any local hot shutoff valves are fully open.
Step-by-step fix
Step 1: Confirm it is the hot line and not a bigger water problem
You want to separate a frozen hot branch from a water heater problem, a whole-house pressure issue, or one bad faucet before you start opening walls or heating pipe.
- Test both hot and cold at the affected fixture.
- Test at least one other nearby fixture and one fixture farther away.
- If several fixtures have no hot flow, check whether the cold side still runs normally throughout the house.
- Look at the water heater area for obvious signs of trouble like no hot water anywhere, a closed valve on the hot outlet, or active leaking.
Next move: If the pattern is cold works normally but hot is weak or dead, especially during freezing weather, keep treating it as a frozen hot line. If both hot and cold are weak, or no fixture has usable hot water even in warm conditions, stop chasing a frozen line and inspect the water heater or house supply instead.
What to conclude: A true frozen hot line usually shows up as a hot-side-only problem tied to cold weather and a specific pipe run.
Stop if:
- You find active leaking around the water heater or piping.
- You cannot tell whether the issue is isolated to one branch or affects the whole house.
- The fixture is in a wall or ceiling area already showing water stains.
Step 2: Find the most likely freeze point before applying heat
The frozen section is usually where the pipe is closest to outside air, not where the water stops at the faucet.
- Trace the hot water path from the water heater toward the dead fixture as far as you can safely access it.
- Check exterior walls, crawl spaces, attic edges, garage walls, rim joist areas, and pipes under sinks on outside walls.
- Open cabinet doors and remove items so room heat can reach the piping.
- Touch accessible sections carefully and look for the coldest spot, frost, bulging insulation, or a line that feels much colder than nearby pipe.
Next move: If you find one exposed cold section that matches the dead fixture or dead group of fixtures, start thawing there first. If you cannot locate any accessible cold section, keep the hot side slightly open and warm the room or adjacent accessible spaces while you monitor for returning flow.
What to conclude: A clear cold spot usually points to the ice plug or the section just upstream of it.
Step 3: Thaw the line slowly and give the water somewhere to go
Gentle, steady heat is safer and works better than blasting one point. A slightly open faucet lets melting water move and relieves pressure.
- Open the hot side at the affected fixture just enough to notice when flow starts to return.
- Use safe ambient heat first: raise room temperature, open cabinet doors, and direct warm household air toward the suspected area.
- If the pipe is fully exposed and dry, you can warm it gradually with a low-setting hair dryer, moving constantly from the faucet side back toward the colder section.
- Keep warming the pipe and nearby air space, not one tiny spot, until flow improves.
- Check nearby accessible sections every few minutes for drips as the ice plug releases.
Next move: If flow starts returning, keep warming gently until the hot side runs steadily, then move to leak checks right away. If there is still no change after a reasonable period, the frozen section may be farther back, hidden in a wall, or already damaged.
Step 4: Check for a split pipe before you put everything back to normal
Many frozen lines do not leak until the ice melts. This is the point where a small hidden split shows itself.
- With water flowing again, inspect every accessible section of the thawed hot line from the fixture back toward the water heater.
- Look for pinhole sprays, seam splits, damp insulation, cabinet floor moisture, and fresh staining below the pipe run.
- Run the hot side for a minute, then shut it off and watch for slow drips that only appear under pressure.
- If the line serves an upper floor or hidden wall, check ceilings, baseboards, and the floor below for new moisture.
Next move: If the line stays dry and hot flow is normal, move on to insulation and freeze prevention. If you find any leak, shut off the nearest local valve or the main water supply and plan for pipe repair before using that line again.
Step 5: Protect the line so it does not freeze again tonight
Once a hot line has frozen, it will usually freeze again in the same weak spot unless you change the conditions.
- Add frozen pipe insulation to exposed accessible hot water piping in basements, crawl spaces, garages, and under sinks where space allows.
- Seal obvious cold-air paths around the pipe area with an appropriate draft-blocking repair for the surrounding surface.
- Keep cabinet doors open during hard freezes for sinks on exterior walls.
- If the same exposed section freezes repeatedly and the product is rated for plumbing freeze protection, install frozen pipe heat tape exactly as labeled and only on suitable pipe materials and locations.
- During severe cold, let the affected fixture's hot side trickle slightly if that has been your recurring freeze point and you cannot correct the exposure right away.
A good result: If the pipe stays warm enough to maintain normal flow through the next cold snap, your prevention work is doing its job.
If not: If the line keeps freezing despite insulation and airflow changes, the pipe route or insulation gap may need a more permanent correction by a plumber.
What to conclude: Repeat freezing usually points to a pipe run in the wrong place, missing insulation, or a strong cold-air leak that simple warming cannot overcome.
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FAQ
How do I know if only the hot water line is frozen?
If the cold side runs normally but the hot side is weak or dead, especially right after freezing weather, that points strongly to a frozen hot line. If no fixture has hot water at all in any weather, look at the water heater too.
Can I pour hot water on the pipe to thaw it?
Only if the pipe is fully exposed, dry around it, and you can control the water safely, but it is usually messy and less controlled than warm room air or a hair dryer. Never soak hidden walls, insulation, or electrical areas.
Why would only one sink lose hot water?
That usually means the branch serving that sink runs through an exterior wall, cold cabinet, or other exposed spot. The freeze is often in the wall or below the cabinet, not at the faucet handle itself.
Should I leave the hot faucet dripping during a freeze?
A slight trickle can help in a known problem area during severe cold, but it is a short-term tactic, not the real fix. Insulation, draft control, and correcting the exposed pipe run matter more.
What if the hot water comes back and then pressure seems odd?
Check carefully for a partial blockage from remaining ice or a small split that only leaks under pressure. If flow is still weak or you see any moisture, shut the line down and inspect further before normal use.
Is a frozen hot water line an emergency?
It becomes one if the pipe has split, water is leaking into walls or ceilings, or the frozen section is hidden and you cannot thaw it safely. A frozen line without a leak is urgent, but a thawing split pipe is a water-damage emergency.