Question about Rinnai Water Heaters

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I have had my Rinnai tankless for about 8 years without problems. Recently I am noticing that the water temperature is fluctuating from the set temperature to a cooler temp and then back up to the set temp. It does that regardless whether it is at a sink tap or a shower head. We have the board that allows us to set the temp at any desired temp. What is causing the water temp to fluctuate?

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The Rinnai will shut off it can't maintain the temperature you have set to within +/- 3 degs so I'd guess that might be the problem. The cause can be a few options. Check water flow first; any filters you have in tapware, main water flow etc. get cleaned. The Rinnai has a water line filter in the incoming supply, middle pipe under the heater just after the isolating valve. Gas supply could be the other option mainly being a possible intermittent fault in the gas regulator. Hard to check this one short of just changing it. Final option is some sort of electronic malfunction, pretty rare. I'd start with the water flow issue as you can do this yourself. Checking the gas will need a good technician with digital gas pressure guage to check incoming and working gas pressures; it might just need to be recalibrated. Just hasd a thought about the heat exchanger being scaled up if you are in an arera where water scale is a problem but this would normally take far less than 8 years to be a problem. Sorry can't be of more help; hard when it might be a few different causes.

Posted on Jun 06, 2011

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Posted on Jan 02, 2017

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I am trying to increase the temperature on the controller and it wont change. I have installed a new one and it still will not increase


[PDF]Operation / Installation Manual - Rinnai

https://www.rinnai.us/.../V_Series_Indoor_25xx_U...

Rinnai America Corp.temperature (shown on the digital remote control) the gas burner will ... Without the connection of a remote controller(s), the Rinnai Water Heater is preset to.

PDF]Tankless Water Heater Service Manual - Rinnai

https://www.rinnai.us/.../100000098_Rev_B.pdf

Rinnai America Corp.Indicates condensing water heater models. ... Rinnai Water Heater Service Manual. 2 ..... Digital manometer or U tube type manometer with 14 inch water column (W.C.) scale, a hose and two .... will flash on the display of the remote controller.

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Water temperature fluctuates with tankless


Have you ever tried to pump water for a fountain... It is so tough and you need larger and larger pumps to get it to go 7 feet high to let's say 10 feet high. Having the on demand water heater in the attic dramatically drops it's pressure. It's best to feed it with a 1 inch or at least 3/4 inch pipe downstairs before it splits the hot and the cold to get the best pressure everywhere then the least pressure will be when it reaches the 2nd floor. but the heater won't cut out because the pressure it even greater downstairs.

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Our rheem pronto tankless rutg-74xp red light will come home indicating hot water and stay on about 2 minutes and then turns off we have turned off and back on and let water run for 5 minutes and will come...


Before calling service technician, open following link for Rheem troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
Tankless fuses and control board.
Water flow rate too low. Minimum .8 gallon required (depending on model)
Tankless inlet filter clogged.
Household aerators clogged at faucets.
Crossover at single-handled faucet
If installed, there can be crossover at mixing valve.
Faulty household check valve, including check valve on re-circulation system if that system is installed.
Restriction in household water line, such as clog at elbow.
Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow and tankless will not turn on.
Loose wire connection inside tankless.
Unit temperature set too high.
Unit temperature set too low.
Water temperature is very hot coming out of tap, so more cold is added, but that reduces amount of hot water flow, so tankless turns off because flow rate is too low.
On some tankless, if water is preheated by solar or other indirect source, the burner will not activate.
High mineral content in water > delime tankless yearly
Misaligned temperature sensor
Low gas pressure. Wrong type of gas. Undersized gas line.
Too many faucets on at same time.
Faulty tankless water valve. Water valve stuck.
Fluctuating household water pressure, for example MUD and PUD district water pressure problems or water well pressure.
Faulty household pressure reducing/regulating valve if installed.
Faulty household pressure tank, if installed -or- pressure tank needs to be installed.

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1 Answer

How do I turn it on? I press the "on" button and


The manual says:
http://www.coinwash.com/temp/rinnai.pdf

"For the name of the Authorized Distributor or Installer nearest you, please contact your local HVAC dealer or gas service technician, visit the Rinnai website (www.rinnai.us), call Rinnai at1-800-621-9419 or write to Rinnai, 103 International Drive, Peachtree City, Georgia 30269"

Manual says that the remote is for setting temperature.
When you turn on hot water spigot, tankless water heater will heat the water to the set point temperature.
If water heater is not set up to use remote, then tankless unit will heat to 140 degrees F.
"With the MCC-45-4US remote control installed the water temperature is adjustable from 96
to 180°F. The water temperature cannot be set to a temperature other than 140°F
without the use of the MCC-45-4US remote control unit. If you require a temperature
other than 140°F you must install the MCC-45-4US remote control unit."

"Error messages are displayed on the remote control units, simplifying service calls."

Page 9 of manual shows hot to set temperature using remote.
Water taps must be shut off to set temperature.
Circulating pump must be turned off.
"Simply press the H or C button until desired temperature is displayed"
"green in-use indicator will display"

If your unit is not working, the manual says to contact local plumber.
Fixya has specialists who will talk to you by phone about your problem for a fee.

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1 Answer

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this is usually a problem with all tankless water heaters.
the solution is to install a small buffer tank (5 or 10 gallon electric water heater) after the Rinnai. this will give the water something to mix in. it will solve your problem.

did you make sure to use a 3/4" gas line to the unit. not just 3/4" at the unit itself. sometimes when there is not enough gas, the unit will act funny and cycle from hot to warm or even cold. Make sure you check the gas line size, its very important.

Aug 10, 2010 | Rinnai Water Heaters

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1 Answer

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Sounds like there is not enough flow in order to keep the burner in the heater firing continuously

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I think the dip tube in your water heater has failed, this is a plastic tube that goes in the cold side of your tank and forces incoming cold water to the bottom of the tank allowing hot water out at the top. When this tube deteiorates the incoming cold water mixes at the top of the tank causing the problem you have. the dip tube runs about 5 bucks but takes someone with pluming skills to install.

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