I moved into a home with two brand new rheem instant hot water heaters. They both act this way:

Randomly, turning on a faucet won't activate the hot water heater, even at 1GPM. You turn on the shower or a sink faucet, walk over to the hot water heater, and it never kicks on. And the water is obviously cold. Even running it for 10+ minutes never kicks it on.

IF you turn on another sink or shower at the same time, the heater will kick on.

I measured the GPM of 1 faucet, with only hot water on, it's 1GPM.

The minimum flowrate on the hot water heater is 0.26 GPM, and an activation flowrate of .4 gpm

Rheem Model:RTG-95DVLN-1

The plumber has changed the igniters, cleaned the flame rods, checked the filter for debris.

What else could it be?

  • Has this been a consistent problem since installation? Did the installer perform any flow and temperature checks after installation? Feb 20, 2022 at 3:52
  • One other question - what is the altitude of the property in question? Feb 20, 2022 at 3:52
  • @Telescope2334 Only been in the home 3 weeks, been happening since the beginning. They're plumbed in parallel. Altitude is 430′. They did flow and temperature checks. The problem seems to be that the hot water heater's minimum flow to activate is not happening correctly. The the sensor is bad on both of them. The flow rate doesn't change and I measured it with a 32oz cup that took 15 seconds to fill up.
    – shane
    Feb 20, 2022 at 4:26
  • what did Rheem technical support say when you called them?
    – Llaves
    Feb 20, 2022 at 4:48
  • 1
    @Telescope2334 - I have Bosch Hydrostar water heaters and the low water pressure issue is quite apparent. (I'm on a well with a pressure tank in the house). Like the OP, there are times when the burners don't light from a single faucet, but turn on a second one and you get the satisfying "whoosh" of the burners lighting. There's never an issue with the tub spout, which has very high flow.
    – Llaves
    Feb 20, 2022 at 16:23

1 Answer 1


Looking at the service manual for this system, there are so many potential problems (with the gas, plumbing, electrical connections, computer control board, mixing valves, etc) that it's practically impossible to diagnose from a distance. What I can give as an answer is the list of issues that the service manual says to consider when the water is completely cold, or not hot enough:

  • Water shutoff valve not fully opened
  • Hot water faucet is not fully opened
  • Water pipes are frozen
  • Electrical or water supply cut off
  • Unit not switched on
  • Temperature set point is too low
  • Mixing valve malfunction
  • Error code displayed on control unit
  • Not enough water demand
  • Water inlet filter clogged
  • Fixture aerator is clogged
  • Scale buildup in the heat exchanger
  • Hot and cold water lines reversed
  • Gas valve is not fully opened
  • Gas supply pressure too low
  • Bleed over in one of the hot water fixtures

As you can see, this is a fairly long list to work through (although some of the items can be ruled out quite quickly).

In addition I note that the system has to be calibrated for the right operating altitude, although as you are fairly low this is unlikely to be the issue.

In addition, as I noted in my earlier comment, I see that the system requires a minimum water pressure in addition to a minimum flow rate. When there are multiple units in parallel, each individual unit needs to see the minimum pressure (suggested to be 40 psi). The manual specifically warns against use in low pressure systems. I recommend you investigate, to ensure that each unit is seeing at least the minimum pressure and flow.

P.S. If and when you have a final diagnosis from your technician, can you add a self-answer to share the solution with us?

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