Here's the setup: Bathroom sink faucet with two handles. All working great to start.

I had to shut off the hot water valve for a moment. I did that. When I turned it back on, the hot water came back on, but at about 50% of the previous flow. That's weird. Not sure what happened, and thought maybe the valve closure kicked loose something that got caught in the aerator. But cold water pressure was perfect going through the same aerator. (Checked the aerator anyway, and it was fine.)

Then I thought maybe the way I had turned the hot water back on had messed something up - maybe with the way the hot and cold joined together. So I tried turning the hot valve off/on with the hot water faucet opened up full. Now hot water is barely present: a trickle -- maybe 5% of original flow. Tried going off/on with the cold water flow opened up full. Same result. Cold water is 100%. Hot water is 5%.

Where should I look next? The hot water heater is fine (flow to all other sinks and showers is fine). It's definitely at the faucet level.

Air in the line? (I don't know how to figure that out or what to do).

The cartridge inside the hot water handle? And what would I do there? I've seen references to backflushing the faucet, but I'm not sure how to do that. But I'm game to learn.

Thanks in advance for the help.

  • In trying to back flow today, the valve itself failed. (Even when shut off, it was spraying all over the place.) As I understand it, this gives further evidence that the valve failed, sending debris up to the hot hose and hot cartridge. I'll figure out a time this week to shut off the water and replace the valve. (Separate question: Is there a way to avoid this kind of valve failure? It's only maybe 6-7 years old!)
    – TK in CA
    Jan 10, 2021 at 23:49

2 Answers 2


It sounds like some debris from the shutoff valve ended up in the faucet cartridge or the airator. The flow starting strong and quickly could even be a chunk of the shutoff valve seat that has crumbled getting stuck so the flow starts out good then the chunk blocks the flow to a trickle, disassemble and possibly flow water into a bucket will probably flush the debris out of the system.

  • I'm quite sure the aerator's fine (see above). When you say flow water into a bucket, I'm confused about exactly what you mean and how that will flush debris out of the system. I'm a newbie, but I see the water flowing from the hot water valve, to a hose, to the handle/cartridge, to another hose, to the T-section where it joins the cold water. (Beyond that should be fine, since the cold water flows normally.) Can you walk me through how I might flush out the likely spots in that pathway?
    – TK in CA
    Jan 9, 2021 at 21:31
  • You have a flow restriction prior the shutoff or cartridge open the lines and flow them on the floor or a bucket your choice. You have to check all the points of restriction on the hot water side that’s where stuff will lodge in the system. You can turn the hot water off at the heater open another tap on hot then disconnect the hot at this faucet shutoff and turn on both hot and cold again into a bucket back flowing may flush the valve and you may need to block the faucet with your hand to clear the debris. Right now the only thing that is known fine is the airator and supply prior to shutoff.
    – Ed Beal
    Jan 9, 2021 at 22:19
  • Thanks, Ed. Your expertise is much appreciated!
    – TK in CA
    Jan 9, 2021 at 22:24
  • Can I back flow using the functional cold water supply? In other words, disconnect the hose from the hot water valve and put that in a bucket. Then allow the cold to go from the wall-> hose -> cold cartridge -> cold hose -> T-joint under the sink... If I block the faucet, would it then back flow through the hot hose -> hot cartridge -> hose sitting in the bucket?
    – TK in CA
    Jan 9, 2021 at 22:31
  • Yes I would do that and it can even be done reverse into the supply valve with the hot connected and local valve open , water heater off and another tap open on hot but the plug is usually at the cartridge/valve. So back flowing into the bucket is almost always the fix in your case.
    – Ed Beal
    Jan 9, 2021 at 23:06

Try this before you start back flowing. Unscrew the hot supply line under the sink where it connects to the hot faucet. Turn the hot supply valve off first.

Aim the hose into a container and open the hot shut off valve all the way.

Note on the force of the water stream into the container. Does it spray with a powerful stream? If yes the problem lies between the supply line connection at the faucet and the faucets stem (where the washer is).

You'll need to disassemble the stem and look for obstructions.

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