I have a 50 year old single level house (just moved in), that was added on to twice. I found supplemental water heaters in each of the three bathroom closets. The original water heater is a good ways away so I think this is what they did rather than increase the size of the heater. The more-or-less instant-on hot water is a nice touch. However, in one of the showers, the hot water will be comfortable for about a minute then go luke cold for about ten seconds, then back to original temp for about a minute, then the cycle repeats itself. You can almost set your watch to the timing of the cycle, which tells me this is a solvable mystery. Being a shade tree home improvement guy has brought no answers. I have replaced the cartridge to no avail. Any ideas?

Is this heater electric or gas? Have you flushed it and/or inspected the dip tube?

Electric

How handy are you willing to be? Ither a failed dip tube or one of 2 elements is not functioning. Start by flushing the heater into a drain, use a scrap of window screen or filter to catch any debris, tell me what you find.

Thanks! I will be trying your suggestion as soon as I have an extra minute, maybe this weekend. In the meantime, I have found a couple more details about the hot water "system." After calling a plumber about an unrelated problem, he discovered that the small water heater associated with this bathroom is "in line" with the main water heater, meaning as the small water heater supplies hot water, it's supply comes from the main heater, not a cold water line. We unplugged the small heater and the problem almost went away. Now the issue is this: The shower water takes over five minutes to warm up, and it is on a very gradual scale, not cold one minute, steamy hot the next. Additionally, there is still some temperature fluctuation, just not as major. Is the culprit still the main heater? When the house was inspected, the water heater was never looked at (had a insulation blanket), only tested via running the hot water at a sink. It was noted that the water heater was probably obsolete.
Thanks again! Look forward to any additional comments! Erin

Hey there ballengerb1, we let loose the drain valve and was pleasantly surprised at the lack of debris. A short stream of black gook that lasted less than a second, then clean hot water. We tried to check the install/manufacture date on this AO Smith 90 gallon heater, but couldn't find one on the heater itself. It looks brand new, but that could be because it has been under an insulation blanket its whole life. However, both the pvc relief valve tubing and the cold water supply pvc has manufacture dates of 7/23/07. This water heater may be only four years old.
So changed the temp settings (it has two thermostats) from 130 to 170. The affected shower still doesn't get hot after 10 minutes of running water. (The sink adjacent to it has smokin hot water.) A quick shower still has luke warm, fluctuating temps. The kitchen sink, closest to the main water heater has always had hot hot water (130 degrees). Now it has the smokin hot water!
So if it seems the water heater is relatively new and heating water properly, what gives with my shower? Not sure if I mentioned that we did replace the cartridge.. Are we to start from square one and replace the entire valve and ancillary plumbing at the shower? Just not sure where to turn next. Thanks in advance for your ideas! Erin