You can fix this
Likely cause: a restriction on the hot supply
Weak flow on hot only means something on the hot side is restricted. Check the hot stop and supply line first, then whether other hot taps are weak too.
Why this happens
When cold flows well and hot does not, the restriction is somewhere on the hot supply rather than in the spout. Mr. Rooter lists partly closed shutoff valves among the causes of low pressure, and Moen says to check the supply lines from the shutoffs. If every hot tap in the house is weak, the problem is upstream at or near the water heater.
How to fix it
- Check that the hot stop under the sink is fully open. Do not force a stuck one.
- Look at the hot supply line for kinks.
- Run hot at another sink or the tub. If those are strong, the problem is at this faucet; clean the aerator and see the cartridge result.
- If every hot tap is weak, check that any shutoff on the water heater's lines is fully open, then call a plumber.
If this does not fix it, tell the plumber
- Flow is weak on hot only.
- Whether it is one faucet or every hot tap in the house.
- The water heater's fuel type and age.
Build the full list: what to tell the plumber.
Start this troubleshooter over
References (every fact above comes from one of these pages)
Vent, Valve & Volt is published by the team behind FourHome. Browse all FourHome replacement faucet and shower parts on the FourHome Amazon store.