Does this sound like a plumbing issue or a tankless-water-heater issue?
August 15, 2018 10:01 AM   Subscribe

Cold showers and my first tankless-water heater experiences - does this sound like a plumbing issue or a tankless-water-heater issue? So much more inside...

We have a tankless-water heater in the 1926 house in southern California we just purchased. This is our first experience living with a tankless-water heater so we are both a bit unsure of how it works. As you'd expect, I can run cold, warm, or hot water all day long from the kitchen sink. As far as I can tell, the same is true for the washing machine in a room off the kitchen adjacent to the water heater. The bathroom, approximately 10 feet away from the water heater, is where we have the problems.

Toilet: Upon flushing, the toilet (hooked up to the cold-water line) appears to fill at a normal rate. No problems here.

Sink: There is only one handle that you push towards the wall to turn on the water and turn a little to one side for hot, a little to the other side for cold, and straight back for a mixture. The sink is hooked up to both the cold-water line and the hot-water line (as you'd expect) but we can only get a trickle of cold water out of the faucet. I can turn off the cold-water line to stop the trickle, and I can open up the cold-water line all the way, but will still only get a trickle. This makes me think there is (in addition to anything else) an issue with either 1) the cold-water line in the wall near the sink but not affecting the toilet, -OR- 2) the facet itself. I can get hot water all day long from the sink.

Shower: Like the sink, the shower only has one handle but it's a "dial" that you turn counter clockwise from pointing towards the floor (6 o'clock) to pointing towards the ceiling (12 o'clock). As you turn it from 6-12 you get pretty much the same pressure of water but it goes from exclusively cold (from 6-3) to a mixture (from 3-2) to exclusively hot (2-12). Counter to the sink situation, there appears to be plenty of cold water pressure. But, also counter to the sink situation, if I am in the shower with either a mixture of warm water or exclusively hot water, I will run out of hot water after ten minutes or so and will have cold water no matter what I dial the handle to... This isn't the case in the kitchen or the bathroom sink (or to my knowledge in the washing machine) and still happens if I turn-down or turn-up the temperature of the water coming from the tankless-water heater. I can get the water in the shower to return to the proper temperature if I turn off the water in the shower and wait a minute or two before turning it back on.

In addition to options 1 or 2 (above) what I think might be happening but would love to hear your thoughts about is:
3) There is nothing wrong with the pipes or the shower and something is wrong with the water heater causing it to shut off or overheat or something.
4) Nothing is wrong with the water heater and it is shutting off as it should do to protect itself from overheating, but similar to the bathroom sink, there is something wrong with the pipes or the shower handle causing such little cold water to mix with with the hot (once the handle is turned past 4 and on to 3 or so...) that either the water heater or the faucet senses "too much" hot water and some anti-scalding feature turns on to cut the hot water.

Of course, none of these four options are mutually exclusive and obviously either 1 or 2 are definitely a problem unrelated to the water heater. Ugh. Does the shower issue sound like a plumbing issue (like the sink) or a tankless-water-heater issue? Who do I call?

The reason why I'm asking instead of just calling a plumber is that the house we purchased came with a 1-year home appliance insurance policy that will cover issues with the water heater but will not cover issues with the plumbing. Calling the insurer for a plumber to visit us about the water heater will incur a flat $75 fee (and will only make sense if the problem is due to the water heater) but calling a plumber on our own will incur an unknown fee (and will only makes sense if the problem is due to piping issues not associated with the water heater.)
posted by pwb503 to Home & Garden (7 answers total)
I cannot answer your sink question, but i might be able to shed some light on the water heater.

SHORT VERSION: Make absolutely sure all the air intake passages are squeaky clean.

STORY TIME: My WH began doing the same thing a few months ago - it would produce hot water for a while using the shower, but then shut off. Stopping and restarting the shower would clear it. First I tried turning down the temp from 140F to 120F. This fixed it for a couple weeks.

It started exhibiting problems again and they got much worse - it wouldn't stay on for more than 1 minute - 2 at the most. So after googling around, I decided it must have scale buildup. It's been in use for about a decade and i haven't flushed it. I took the lid off to see if i could hear strangled water sounds, or gurgling, or whatever, or see any status lights it might have. During testing with the lid off, it ran perfectly, for as long as I cared to leave it on.

In mine, there's a couple of elongated holes in the cover, and they have a metal channel behind them on the inside. This is the air intake. At the top of the air intake, on the inside of the lid, there's a screen. It doesn't cover the whole intake, so the air could flow around it, but this tiny screen was covered in dust entirely, and the intake chute had a layer of dust on it. I cleaned these off and it has worked flawlessly ever since. I did end up flushing it out also, but there wasn't any appreciable buildup in it.

It is my guess that (in my case) the air/fuel mix sensors are so sensitive, that even a 5-10% reduction in airflow (guessing, based on area of the entire chute and area of the screen) causes error conditions.
posted by mu at 10:33 AM on August 15, 2018

For the sink, shut off the cold or hot water under the sink, and then turn it on to hot and cold. If you shut off the hot water under the sink and no cold water comes out, then you have a problem with your faucet probably. It's possible that they can break.

If you are brave and have towels and a bucket, you should also be able to disconnect the short line from the faucet to the cutoff and see the full power of your water line, uneffected by the sink faucet itself.
posted by The_Vegetables at 11:05 AM on August 15, 2018 [1 favorite]

I'd say you have two unrelated problems. There's either bad supply to under the sink on the cold line, or something in the faucet is mucked up. Run cold water from the wall into a bucket as suggested. If that has good pressure, take the faucet apart. Clean and reassemble. You can also get a rebuild kit.

For the water heater, I would be helpful if you list the make and model so that we know exactly what we're working with.

I suspect it is auto-shutting down for some reason. It's also possible that the sensor that detects flowing water "gets tired" after an extended shower run. The first thing you should do is let the kitchen sink to run hot water for as long as it takes the shower to go down to see if you can replicate the failure.

Servicing the water heater should best be done by a pro. I doubt that theres some plumbing issue causing the hot water in the shower to go down. Much more likely that the water heater needs servicing.
posted by humboldt32 at 11:50 AM on August 15, 2018

Agreed with above, the next test to run on the sink is to close the valve, disconnect the hose, and turn the valve back on running into a bucket. If you get good pressure from the valve, it's either the faucet of the hose.
Don't neglect the hose in your troubleshooting, though. My parents had a similar (though intermittent) issue, and it turned out to be a anti-flood hose that had gotten fouled by sediment in the line, and was now hyper-sensitive, detecting normal flow as a burst hose and shutting off the flow. After replacing the hose, the faucet worked fine.
posted by yuwtze at 12:52 PM on August 15, 2018

I agree with the others that these seem likely to be two seperate issues. I also agree that the most likely culprit for the sink is either a bad supply hose (from the shutoff to the faucet) or within the faucet itself. If I had to put some money down, I think I'd bet on the faucet rather than the hose, but the test of disconnecting the hose from the faucet and turning it on while pointing it into a bucket is a good one. Also look at the inside of the hose and see if you can notice any "cholesterol" buildup.

I also think the water heater is likely the culprit for your shower - I'm not sure the location of your shower is relevant, but rather that this is just the highest volume use of your water heater. The only other outside possibility I can think of is that it's possible that there's a wonky mixing valve in your shower valve. There's anti-scald protection built into most of those "dial" shower valves that don't let you run your showers above a certain temperature. Maybe something's up with the thermostat?

But I think it's likely it's the water heater. Is there any way you can test the hot water flow in a high-volume way for the amount of time you typically shower? Is there a place to hook up a hose at your water heater? Could you disconnect your washer line (maybe not a good idea)?. That would be one way to test whether the water heater could keep up.

Please do post here if/when you figure it out. Mysteries like this are oddly compelling.
posted by Betelgeuse at 1:33 PM on August 15, 2018

Here is a trick that sometimes will work to clean out a clogged mixing faucet without disassembling it. Disconnect the cold supply line from the shut off valve. Put a bucket under it. Now adjust the control valve to the hot and cold position and turn it on. Hot water should come out of the faucet. Plug the faucet tap with your hand and you will force the hot water to go through the cold valve and into the bucket, backwards. You must have the faucet controls open for both hot and cold so that hot water comes in, but goes out the cold in the reverse direction. This may clear the obstruction, which is usually a piece of water scale or rust. Look in the bucket and you might see it.
posted by JackFlash at 1:50 PM on August 15, 2018 [1 favorite]

If it was my place I'd just replace the cartridge in the sink faucet. They are pretty cheap (Moen's are free because they have a lifetime warranty) and replacement is usually pretty straight forward (I can do the sinks at the hotel I work at in less than five minutes if the shut offs under the sink aren't seized).
posted by Mitheral at 10:23 PM on August 15, 2018

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