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10 years ago I had professionally installed a recirculating pump (Grundfos UP15-29SU) in our house. Essentially all hot supply lines were replaced by a single loop feeding all the faucets one after another, then returning to the main cold supply line inches from the heater; the pump was placed on that loop a couple of feet before the heater, always on (with a manual switch), and worked pretty flawlessly, with every faucet in the house getting hot water in mere seconds.

Recently, the kitchen faucet, which sits last on the loop, close to the heater & following maybe 100-120' of supply loop, started taking its sweet time to heat up--more than a minute. There is no noticeable difference at all the other supply points, incl. the farthest bathroom, that sits at around 75' from the heater, only the kitchen.

Time to hot water in the kitchen is similar to when the pump is off, but the water during that time is lukewarm with pump on, cold with pump off (after a few hours). The pump has gotten louder over time, but otherwise does not show any obvious signs of failure.

Occam's razor indicates it's the pump, so I may be overthinking this, but wouldn't it just stop running completely? And aren't they supposed to be quasi-perpetual? Also, wouldn't all the other supply points be noticeably slower to get hot water, with the farthest bathroom taking (proportionally to the distance) 60-75% of the same time it takes the kitchen?

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    You haven't ever shut the valves around the pump without shutting the pump off first, have you? That could lead to cavitation and impeller damage. Or drained the water system/turned off the main valve and dropped pressure with the pump still running...?
    – Ecnerwal
    Dec 4, 2023 at 2:17
  • I don't recall ever touching the valves--until now. But it's possible that I forgot to turn off the pump when fixing/replacing a faucet or two--turning the main valve off & some draining occuring. BTW, I looked again and only one valve is on the recirculating loop (right before the pump, like I said earlier), the second valve is on the cold supply to the heater right before the recirculating loop junction. I also have 2 thicker, brass-colored cylinders, maybe 1-1/2" in length, after the pump on the loop & before the valve on cold main--check valves?
    – user677660
    Dec 4, 2023 at 3:25
  • Here are some photos on the heater side & the pump side (which sits in crawl space), with A marking where the loop traverses the wall & B marking where cold water supply to the heater traverses the wall: photos.app.goo.gl/cDSAhgToEqETbJGL9.
    – user677660
    Dec 4, 2023 at 3:29

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Occam's razor likes the check valve just as well as the pump, particularly for the faucet last on the loop and thus closest to cool-water flow-back from the bottom of the water heater on the return line.

If the check valve is built into the pump you might think same as pump, but it isn't really. In most cases both valve and pump can be rebuilt (replace parts to restore function, rather than toss the whole thing.)

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  • I have not considered flow-back: are you saying that if the pump was broken (i.e., making noise but not moving any water), then the last faucet the closest to it could suck in some cold water from the main line and therefore perform much worse than the other faucets, appearing as if only the other faucets were getting help from the pump? Wouldn't that faucet always have an issue (i.e., never get hot water, no matter how long one waited)? Is there a way to test it? I have manual valves on the loop right before the pump & right after it (but before the junction with main supply from the city).
    – user677660
    Dec 4, 2023 at 0:09
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    Yes, backflow is possible if the pump is off, or pumping weakly. If hot water needs to travel 100+ feet the right way and 10-20 feet the wrong way, without a check valve or a happy pump it's going to take the path of least resistance. Which is from the cold return of the loop... Shut off the pump and shut the valves and see what happens. If it's pumping weakly, you may find that (after some time) other things on the loop take longer to get hot water with the valves shut, of course.
    – Ecnerwal
    Dec 4, 2023 at 0:45
  • Yes, all other faucets take noticeably longer to heat with the pump shut off, kitchen is about the same, but water is much colder until hot water arrives.
    – user677660
    Dec 4, 2023 at 22:37

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