This year Lily started to complain that it took long time for the shower water to turn warm, and after a while (5+ minutes), it would turn cold, and the water flow rate was low - shower was "weak".
The time for the shower to turn warm was long per my experience and observation as well. It will take a while for warm/hot water at heater to reach this bathroom because the bathroom is furthermost from the tankless water heater. The cold water has to be flushed out before the warm water can come.
I had not experienced the the cycle of water going from warm to cold and warm again while in shower when she first complained. I attributed the cycle to the fact that she turned the hot water volume down too much while in shower. But I experienced the same thing last few weeks.
Thinking about it, I determined that there are two factors related to the phenomenon. First the water pipe and shower head clogging, which reduce the water flow rate in hot water pipe, which could shutdown the tankless heater; second tankless water heater does not work properly. I checked tankless water heater quickly and superficially - it seemed to work properly, and temperature setting was correct. I also observed that other hot water outlets worked just fine, even the hot water faucets in the same bathroom. The potential heater issue is eliminated.
Tankless water heater turn-on/turn-off depends on both water flow rate and temperature inside the heating unit per this reference. The hypothesis was that the issue was with the shower head and a secondary factor might be the way we adjust the volume of hot water. A clogged shower head or hot water pipe could make the water flow rate low despite the fact that we turned the hot water volume at shower stall to maximum. In this case, the tankless water heater control is dominated by temperature for turn-on and by flow rate for turn-off. This could explain the cycle we observed - initially when we flush water long enough, the water inside the heating unit will be low enough to turn on the heater; after temperature reached set temperature, the control unit is looking for signal to turn-off, it sensed the low flow rate and turn off the heater. But the water is still flowing, eventually the hot water pipe inside the heating unit will low enough to turn on the heater, the cycle repeats.
The next thing I did was to check the shower head: some mineral deposits were observed in the orifices of the shower head and were cleared. Minor improvement achieved and the issue persists.
One night last week, while showering the water never turned hot but lukewarm. I was about to finish.. water turned cold ! .. I removed shower head ... water turned warm and then hot in few seconds! just like the nearby faucets.
It is the shower head that caused the troubles, root cause is confirmed!
Removing shower head can eliminate the water temperature cycle issue but showering without shower head is not comfortable, and functionally not acceptable.
What can I do?
I checked the shower head again, no mineral deposit on the outer orifices, I further dissembled the inflow control unit at the inlet of the shower head, there is a rubber weep on peek base, and a mesh screen.
The mesh is fine. The rubber weep on the peek base swelled, and released black residue in water when flushed. The real root cause of the shower head problem was that the rubber weep swelled to block water flow too much. (Note: The swell itself is due to rubber property deterioration after about a year in usage)
The solution is really simple and without cost, after the root cause is found, I simply removed the inflow control unit. The shower head performs consistently, water is hot, the flow is strong, giving us great satisfaction.
Note: without the inflow control unit, the water usage per shower can increase. we control the flow rate at the faucet to the shower head, only 1/4 to 1/3 turn is needed to get hot and strong enough flow.
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