Direct answer: If only the hot side is weak at one faucet, the problem is usually right at that faucet: a clogged aerator, a partly closed hot shutoff under the sink, or a faucet cartridge with debris or wear. If hot pressure is weak everywhere, stop chasing the faucet and look upstream at the water heater side or the house plumbing.
Most likely: Most often, mineral grit or scale is choking the faucet aerator or the hot side of the faucet cartridge.
First separate one-faucet trouble from whole-house hot-water trouble. Then check the easy restriction points in order: aerator, hot shutoff, supply line flow, and finally the faucet cartridge. Reality check: a faucet can have strong cold flow and weak hot flow even when the rest of the plumbing is fine. Common wrong move: replacing the faucet before checking the hot stop under the sink.
Don’t start with: Do not start by buying a whole new faucet. A lot of these calls end with a cleaned aerator or an opened stop valve.
Cold water runs normally here, but switching to hot gives a weak stream or a thin uneven flow.
Start here: Start at the faucet itself. The aerator, hot shutoff, supply hose, or faucet cartridge is the likely trouble spot.
More than one sink or shower has reduced hot flow, while cold pressure is still decent.
Start here: Do not tear into this faucet first. The restriction is more likely upstream on the hot-water side.
The faucet worked before, then hot flow got weak after a shutoff was used, a water heater was serviced, or pipes were worked on.
Start here: Suspect debris. Check the aerator first, then flush the faucet and inspect the faucet cartridge if needed.
The hot stream looks decent for a moment, then falls off while cold stays steady.
Start here: Look for a sticking faucet cartridge or a partially blocked hot supply path at the faucet.
The aerator is the first place grit and scale collect, especially after shutoffs are used or plumbing is disturbed.
Quick check: Unscrew the faucet aerator and run hot water briefly into the sink. If flow jumps up, the aerator was the restriction.
A stop valve under the sink can get bumped, left half-open after work, or fail internally so the handle position does not match the opening inside.
Quick check: Look under the sink and confirm the hot shutoff is fully open. Then compare hot and cold flow by disconnecting only if you are prepared to catch water safely.
Single-handle and many two-handle faucets meter hot flow through a cartridge that can plug with mineral grit or wear enough to restrict one side.
Quick check: If the aerator is clean and the hot stop is open but hot flow is still weak only at this faucet, the faucet cartridge moves to the top of the list.
A faucet supply hose can kink, collapse internally, or catch debris at the connection, especially on older braided lines.
Quick check: Inspect the hot faucet supply hose for a sharp bend or twist and compare its flow to the cold side if you disconnect it.
You do not want to take apart a faucet when the real restriction is on the hot-water side of the house.
Next move: If the problem is only at this faucet, stay here and keep going. If hot pressure is weak at several fixtures, the faucet is probably not the main cause.
What to conclude: One-faucet trouble usually means a local restriction. Multi-fixture trouble points upstream on the hot-water supply side.
This is the most common, least destructive fix, and it often solves sudden low hot flow after plumbing work.
Next move: If hot flow returns to normal, you found the restriction and the repair is done. If hot flow is still weak with the aerator removed, the restriction is farther back.
What to conclude: A dirty aerator is a simple local clog. No improvement with the aerator off points to the shutoff, supply hose, or faucet cartridge.
A partly closed or failing hot stop valve can mimic a bad faucet and is easy to miss.
Next move: If hot flow improves, the valve was not fully open or was sticking near closed. If nothing changes, the restriction is likely in the faucet supply hose or faucet cartridge.
A kinked hose or trapped grit can cut flow before water even reaches the faucet cartridge.
Next move: If the valve delivers strong flow and flushing clears debris, the faucet may recover without further parts. If the valve has good flow but the faucet still has weak hot output, the faucet cartridge is the likely restriction. If the valve itself has weak output, the issue is upstream of the faucet.
Once the aerator, stop valve position, and supply hose check out, the faucet cartridge is the most likely failed part inside the faucet.
A good result: If hot flow is restored and handle movement feels normal, the cartridge was the restriction.
If not: If a new cartridge does not restore hot flow, stop replacing faucet parts and look upstream for a hot-side supply restriction.
What to conclude: A restored flow confirms the faucet cartridge was blocked or worn internally. No change after cartridge replacement means the faucet was not the only problem.
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Usually because something on the hot side of that faucet is restricted. The most common spots are the faucet aerator, the hot shutoff valve under the sink, the hot faucet supply hose, or the faucet cartridge.
Yes. Debris can shift around inside the faucet and show up more on the hot side, especially after the water was shut off or plumbing work stirred up scale. That is why removing the faucet aerator is the first easy check.
Check that the hot shutoff valve under the sink is fully open, then inspect and flush the hot faucet supply hose if you can do it safely. If the stop has good flow but the faucet still does not, the faucet cartridge is the likely problem.
Not usually. Most one-faucet hot-flow problems are solved with cleaning, opening a stop valve, replacing a faucet supply hose, or installing the correct faucet cartridge. Replace the whole faucet only after those checks support it.
Then this faucet is probably not the main issue. Look for a restriction on the hot-water side upstream, such as a problem near the water heater, a partly closed main hot-side valve, or debris affecting multiple fixtures.