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I appear to have a strange problem. For the last 6 months, at least, in my master bath the sink faucet pours slowly when the hot water reaches the faucet itself.

That is,

  • If the pipes have cooled off, it runs as fast as the cold side.
  • As the pipes warm up, and warm water starts flowing, it slows down.
  • As long as the water is warm/hot, it flows slowly.
  • But it clears up and resumes full flow

The guest bath has a sink attached to the same pipe head, no problems. Same for the master and guest showers.

The faucets, shower heads and hoses are about 5 years old. I replaced them all when we moved in.

I suppose it could be debris coming loose when the pipes or hoses expand from the hot water, but it's very consistent. The flow is always the same speed. If you run the shower to get the water hot, then the sink faucet, it will run at normal flow for a few seconds until the hot water reaches it, then the flow decreases sharply as the water heats up sharply. Otherwise, the flow slows down... slowly.

What could be the problem? I'm tempted to replace the faucet and hoses again, but it's not as simple as the guest bath to get under there. It also drives me insane because it's something I forget as soon as I leave the bathroom but every morning when I try to shave I'm reminded. I imagine it's saving me a bit on energy, though, on the bright side.

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It sounds to me like an overactive "scald prevention valve" - pretty much all USA faucets (and likely some other areas around the world) have been required for some time (certainly much more than 5 years) to have a mechanism that is designed to prevent scalding, so there's a thermally-active element in the faucet that is supposed to reduce flow when it's too hot.

I would guess that yours is mal-adjusted (if adjustable) or simply failing in some way that makes it over-react long before it should. If you can find literature on your faucet (whether you actually saved it, or just can sort out the model and look it up with the manufacturer) you might discover if there is supposed to be some sort of adjustment. Depending on manufacturer there might also be a help or support number you can call (particularly among the "guaranteed for life" manufacturers.)

  • This, or there is an issue with the faucet valve if it is single handle. I'd find the manual like Ecnerwal suggests and if still not solved, and least pull the cartridge and clean it and the seat area. – noybman Mar 6 '18 at 5:07
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What type of valve is supplying this shower? Guessing a cartridge type since it's newer. How much use does this shower get?

To me, it sounds like some sort of heat expansion issue since the failure is consistent from cold. I'm thinking of a couple of potential situations:

  1. Valve has an "anti-scald" capability that could be failing either from non-use or sediment issue?
  2. O-ring or something in the valve cartridge is expanding when heat rises?

Either way, I'd suggest turning off the supply to that shower and pull the cartridge. You can inspect it and try cleaning it in some CLR or something along those lines if it seems to be hard water... If replacing it, I would recommend against getting a knock-off replacement cartridge and go with the original manufacturer. I've fallen for this in the past and its lead to a leaking faucet.

If these valves aren't used regularly, they tend to not perform well. I'd suggest exercising them regularly... Maybe monthly.

  • The questioner has a problem with the "master bath sink" - not the guest sink - that would be a sink that gets used several times every single day in most houses, so your "disuse/monthly exercise" bit makes no sense given the question. – Ecnerwal Mar 6 '18 at 3:12
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Well, there was no anti-scald device. I replaced the entire faucet and the problem vanished. So something in the faucet was probably the culprit, or less likely something in the lines that came out as I flushed them during the install of the new faucet.

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