I’ve had the Rinnai tankless for about 1 month. I did a lot of research before replacing the hot water tank (which was on borrowed time) with a tankless unit (RU160iP). Some comments were that it didn’t work with a well water due to fluctuating pressure. This was disputed by others, and the company that installed it stated many other customers had well water and there have been no issues. I have not had any issues with water pressure of varying water temperatures due to tankless and a well pump. Of course it is true that the water pressure in my house will fluctuate, it’s not really noticeable, since having the tankless installed no one has complained the water goes from too cold then too hot when taking a shower. Maybe Sense can give some insight to how Rinnai maintains the temperature?
My well is set pretty standard, cut in when the tank pressure reaches 40psi and shut off at 60psi.
The Rinnai unit knows the incoming water temperature which is between 48 and 50 degrees right now, the water flow into the unit and the required output temperature. It will burn the gas it needs to reach and maintain temperature. The speed of fan in the unit seems to relate to amount of BTUs it needs to heat the water, because of this Sense can get a picture of what’s happening.
Left side is the well pump cycling,on the right side Rinnai with someone is taking a shower from 12:13pm - 12:27pm. At 12:15pm the Rinnai energy use is going up, this is because the well pump kicked on and the water flow has increased so more heat is needed to maintain 125 degrees, when the pump kicks off at 60psi, the water flow starts decreasing so less heat is needed to maintain 125 degrees, then the cycle starts over about 5-6 minutes later. The big spike at the end is the water being shut off, the purge fan does max power then does a 3 minute post use purge before shutting down. When taking a shower you don’t notice any change in water flow or temperature since the Rinnai is constantly making adjustments to the flame and tempering servo position based on the incoming water flow and temperature to maintain output temperature. Who knew a water heater could be so interesting!
Regarding the tempering valve- In addition to a the amount of BTUs to heat the water, there is a tempering valve in the unit that mixes cold water into the heated water to maintain the temperature. Its a servo type valve that reacts quickly,in addition to increasing/decreasing flame, to meet needs for hot water such as someone turning on a tap while one is running - or the opposite 2-3 taps running and someone shutting off taps.