Hot Water Capacity
0.6 to 5.3 GPM at 45°F Rise
“When I was using the hot water, the water got cold.
If you adjusted the flow from the tap to lessen it, you
may have gone below the minimum flow required. The
Rinnai water heater requires a minimum flow rate to
operate. (See the specification page for the flow rate
of your model.)â€
“Ensure you have at least the minimum flow rate required to fire unit.â€
“18. What is the minimum water flow required to operate a Rinnai tankless water heater?
Please keep in mind that the Rinnai tankless water heater needs to see over 0.6 gallons per minute of water flow through the hot side to ignite and to stay in operation. If the flow rate falls below this level, the unit will turn off because it senses that hot water is no longer needed. There are several possible solutions to minimum flow rate issues: lowering the temperature on the unit to a more comfortable temperature to avoid cutting in too much cold water, cleaning the inlet water filter on the cold water inlet of the unit (sediment build up in filter can decrease flow into the unit), cleaning or removing the aerators from showerheads or fixtures, or replacing low flow showerheads or fixtures.â€
Anyway you slice it, low flow through tankless water heaters is a problem. The more your pressure varies, the harder it is to maintain the minimum flow required to keep the burner on. The best I can tell it has a turbine type flow sensor. Flow sensors are notoriously unreliable and troublesome. If you gradually bring the flow up to .6 gpm, the turbine probably won’t start spinning. If you abruptly turn on more than .6 gpm, the turbine flow sensor will probably start spinning. That is until there is a little build up or wear on the flow sensor. And I have never seen a flow sensor that is accurate enough to work at exactly .6 gpm.
Even if everything is working perfectly, and you mix in some cold water to a low flow shower head, you may have .6 gpm flowing at high pressure, but not at the lower pressure. So the burner turns on and off during a shower, which is the complaint I always hear. The manufacturer even says…. “There are several possible solutions to minimum flow rate issuesâ€. So there are ISSUES with low flow.
Their solutions include cleaning the water filter, removing aerators, or replacing low flow showerheads. All of these solutions are ways to increase the flow. Maintaining constant pressure from the well pump will also increase flow, compared to letting the pressure decrease as it does on the low end of any pressure switch setting.