Thank you for responding Klem1. I do hope you can help me figure this out.

Sorry, I mis-spoke when I mentioned a "tank". I do not have a reserve tank of any kind. I meant to say that I thought the misbehaving shower and my kitchen sinks were on the same heater system. Turns out, they're not.

Tankless heater #1 serves:
Guest bathroom: shower and vanity sink (upstairs, gets a moderate amount of use)
Master bathroom: shower, separate bathtub, and vanity sinks (upstairs, and all get daily use)
Deck bathroom; vanity sink only (downstairs and very seldom used in the winter time)

Tankless heater #2 serves:
Bathroom #3: bathtub, vanity sink, shower (upstairs)
Laundry room: laundry sink and washing machine (upstairs)
TV Room: sink (upstairs)
Kitchen: Two sinks (downstairs)
Guest powderroom: vanity sink only (downstairs)

The problem seems to be limited to the guest shower and the guest vanity faucet. Interestingly, our master bathroom shower, tub, and sink faucets don't appear to have any issues even though they are on the same hot water circuit and are also upstairs. I took a quite long shower this morning in the master shower with no issues whatsoever. Had plenty of hot water, no variations in water pressure, and definitely no noise. But, turn on the guest room shower and, two minutes later, the water pressure goes "wonky" and the noise starts.

The tankless water heaters are Rannai, propane heaters. Model # R94-LSe.

I'm afraid I can't provide any information regarding the shower fixtures as my builder found that for me and never provided me with any model information. I do know that it is the exact same fixture as the one in my master bathroom which is still working fine.

If there is anything else that might help you to diagnose this for me, I'll be happy to supply the info. And again, thank you so much for responding!

This is veeryy important. #1.Can the problem be replicated at will? Stated differently,does the problem always occur when guest shower is used? #2. Have you tried turning master shower and vanity on while GUEST IS ACTING"WONKY"?

Right now it sounds suspeciously like an angle stop or other stop valve with a loose washer. Most sinks have one but showers don't. Is it possible that a remodel caused guest shower and sink to be served through a common shut off valve?

Well, the problem just started a couple of days ago but it definitely seems to be "reproducible at will." I've tried it at least half a dozen times today. It keeps recurring every single time. Once the problem starts, I have to let everything cool off for awhile before I start the shower back up again or it will go wonky immediately instead of behaving normally for the first couple of minutes.

I tried turning the master shower on first and then turning the guest shower on and everything ran normally for several minutes... longer than usual, I think. But, as soon as I turned the master shower off, the guest shower immediately started acting up. As in, it started going wonky the EXACT SECOND that I turned the master shower off. After that, even if I tried turning the master shower back on, the guest shower continued to be wonky. I'm thinking I should probably try the experiment again, maybe wait quite a bit longer to cut the master shower off to make sure that the "exact second" start of the problem wasn't merely coincidental? I'll rerun the experiment in half an hour or so, as soon as the water in the lines has had time to cool off and will let you know what happens.

I will also try the experiment the way you suggested. I.e., turn the guest shower on first and then turn the master shower on. Should I wait until the guest shower starts acting wonky to turn the master shower on to see if turning the master shower on seems to correct or at least minimize the problem? Or, should I turn the master shower on as quickly as I can manage to get from one room to the other to see if having the master shower on, keeps the guest shower from going wonky? Or, should I try both ways?

There has not been any remodeling done. This is the original plumbing that was put in when our new house was built four years ago. So if there is an error in the way the plumbing was run, it is a mistake that was made by the plumbers hired by our builder. But, given some of the other problems we've run into with this house, I certainly would not try to argue that "contractor error" can't be involved.

I'm afraid I have no idea where the shut-off valve for the shower would be located. There is no "access panel" for either shower. I do have open wall photos taken during the build so, if I knew what to look for, I might be able to figure out where the shut-off valve (assuming there is one) is located. Since there is no access panel, does that support the idea that the shut off valve for the vanity sink may also be serving as shut off for the shower?

Update - having the master shower turned on at the same time as the guest shower seems to postpone the problem but not eliminate it. It took about 4 minutes for the guest room shower to go wonky when the master shower was running at the same time. But, once the guest room shower's pressure drops down, turning the master shower on/off has no effect. And if the issue is in anyway related to the cold weather we've been experiencing, I'm not certain that running or not running the master shower has any effect at all. It warmed up quite a bit this afternoon so the attic area where at least some of the water lines run was probably in the 80s or 90s by the time I was testing things this afternoon.

So that you know why I had you try the master shower. To see if eventhough problem is triggered only by guest shower it does or does not drop pressure in the overall system connected to that heater. This is a common problem where a washer is loose in a facuet,hot or cold but I never heard of it effecting other facuets. Real common even right down to problem being helped when a secound facuet is opened. So to recap. Are you certain that when pressure dies in shower,it dies in vanity sink as well? Are you certain water lines run through the attic? Just so we know a Texan isn't trying to figure out a down under plumbing system,where do you live? Are you taking notes in case you decide to write a who done it dinner playhouse skit? My "Granny" would have said it sounds like you might have a "hant" in the houseLOL

I too am a Texas (just east of Austin) so your "Texas based" advice is EXACTLY what I need!

The pressure drop at the guest shower does not seem to affect the master shower at all. And, once the shower pressure cuts out at the shower, the guest vanity sink continues to work okay. HOWEVER, turning the guest sink and or the master shower on/off does have an effect on the problematic guest shower. That is, with the master shower and/or the guest sink turned on, it takes a lot longer for the guest shower to go wonky. In fact, this morning, with both showers and the guest room sink all running at the same time, the guest room shower never did cut out. But, as soon as I switched off the master shower, boom. I could immediately hear the "noise" that let me know the guest room shower had gone wonky. I quickly turned the master shower back on and the noise went away. Turned the master shower back off and the noise immediately came back again.

Yes, at least some of the water lines run thru the attic. About a year ago hubby and I added some blown-in insulation to the attic and pulled up and replaced much of the pink fiberglass roll insulation because, among other things, our builder shorted us on the amount of insulation he put up there and what he did put in he installed ACROSS the rafters instead of filling the spaces between the rafters. We had to avoid stepping on water lines while doing that. WHY any of the water lines run thru the attic, I can't tell you. It certainly would have been a shorter route to run them in the space between floor 1 and floor 2 but.... there ARE some up there. No question.

Is it possible that the loose washer is IN the shower fixture? Recall that this is one of those old fashioned kinds where I have two handles (one hot, one cold) that control the water.

What part of Texas are you located in? And, are you a licensed plumber or another self-taught individual. If you're anywhere close to the Austin area, at this point, I'd love to be able to hire you to come fix this thing for me. The old GW site used to have a way that posters could email one another off-line. I don't see a way to do that here on HOUZZ and I don't want to post my email address for spambots to find. But, if you think we might work something out, I'll figure out a way to let you get in contact with me.

This thing reminds me of a ground squirrel my buddy and I chased for a week after school. He was really fast and could turn on a dime. We collided with one another several times but never managed to even touch the squirrel before he disappeared into his den. One day Orvill told me to wait by the den while he chased the little devil. Sure enough here he came and I grabbed him. The story got really interesting after I captured him but we need to get back to the elusive facuet. There is something loose in the hot side of the facuet,99% chance it's the washer,.5% it's the seat and .5% it's one of a dozen other problems. The main thing to be learned from this is that you should be very clear while describing a problem,otherwise it drives the poor old worker man insane. Sorry but you wouldn't be able to afford the trip charge from N.E. of Dallas. Good luck and best wishes.

Now I've just GOT to find out what happened after you caught the ground squirrel. Do you still own all your fingers? LOL!

I do thank you for the help. At least I now know what to tell a plumber when I hire one...and he won't need to spend two hours experimenting with my faucets.

Do wish you were closer but you're right, no way I could afford the trip charge from N.E. of Dallas all the way to Austin. How did you enjoy the recent ice/snow up your way?

Thank you again!

What temp is the unit set for? Get a garden hose and connect it to the hot water service valve at the unit and check the flow. Is there any fault at the unit? At a 70* temp rise that unit should be good for 4.7 GPM or so. Has the inlet filter been cleaned? Is the unit throwing any error codes. Look in your manual. You can pull up to 9 faults. You can also read the flow rates. Look at the manual. How is your gas line sized? It is the most common error in tankless installations. I know these units very well. Check these things out and report back.

Hi Jackfre. Thanks for chiming in. Since none of the other fixtures that get hot water from this tankless heater are having any problems, is there a reason why you think the problem might be with the tankless heater rather than with the faucet itself? Specifically, the problem I'm having seems to be limited exclusively to my guest bathroom shower. My master shower is on the same tankless heater and it is still working just fine, as are the faucets on my master bathtub and master vanity sinks. As for flow rates, I can turn on the hot water at my master shower, master bathtub, and master vanity sinks all at the same time and get plenty of nice hot water at all of them. I'm guessing that between all of them, I'm pulling pretty close to 4.7 GPM if not a little more.

Some of the things you've asked about are going to take me awhile to figure out...especially as my builder did not leave the Owner's Manual for the tankless heaters with me. I'm going to have to go online and find it. I do know that the heater is set at 120 degrees and that, if there is an "inlet filter" that should be cleaned on a regular basis, it has NOT been cleaned in the four years since we moved into the house. (Once I find an owner's manual and figure out how to clean it, I will do it.) I suspect the owner's manual for the Rinnai's won't tell me how to check the gas line size. Can you tell me how to do that?

Again, thank you for chiming in. The more I know about what might be going on here, the more likely I feel that I will be able to get a plumber in to to fix the problem for me (if it turns out that I can't fix it myself) without charging me more than an arm and a leg.

If everything else is ok and you are getting good flow the filter is probably ok. I think you may need to get the shower going and read the flow on the touchpad at the unit to see how much hot is supplying the shower.for instance, my 2.5 gpm shower head takes 2.1 gal of hot (120*) and .4gpm of cold to give me my comfortable shower. I think from your description that the shower valve is bad or not set correctly.