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Rinnai Tankless Heater Short Cycling

Hello! New on this site, found it through a search and seems super helpful.

I'm hoping to get some information on an issue I'm having with my Rinnai Tankless heater. Model number is R75-LSi (VA version). I have tried to troubleshoot the issue several different ways now, including replacement of parts, but the issue persists.

Issue: The unit works properly when turning on faucets and heat is called for. We have a hydronic heating system with a pump to circulate hot water through the tank continuously to feed a heat exchanger for the radiant heating loop. When the hydronic pump runs the Rinnai heater correctly turns on and heats the loop up to temperature (140F). After a few minutes (3-5 min) the heater will start 'short cycling' continuously. The unit flame turns off, the fan kicks high (to vent the burner) for about 5 seconds, then the unit will re-ignite the flame. Sometimes the unit will stay off for 1-5 seconds before re-ignition. This continues on indefinitely every 30sec to 3 min while the hydronic loop pump is running.

Troubleshooting: I have tried to replace the flame detection rod, ignition rod, and the flow sensor assembly. I can measure with a DMM on the flow sensor output and I'm seeing about 170Hz signal, plenty of flow to engage the heater consistently. Thermistors seem to be reading in the correct range based on their temperature. I have not checked any of the gas control components.

My leading theories are that there is something overheating or causing the flame to extinguish. I can see through the viewing window there is flame at the back of the burner. I did blow compressed air inside and vacuumed any debris I saw in the burner chamber.

Error codes have not changed since this issue started. They are reporting as follows: 1 12 2 10 3 10 4 12 5 - 6 - 7 - 8 - 9 -

A little background: The unit has been installed here at my home for years now without any issues. I perform regular maintenance such as vinegar flushes, cleaning filter, clearing out the fan assembly, etc. I have not noticed this short cycling behavior until recently. I'm afraid that parts may wear out prematurely if it continues to cycle.

Any other leads or parts that I should check would be super helpful. I have a background in electronics and can measure any voltage/current on the unit as needed.

Thank you!


  • nibs
    nibs Member Posts: 515
    Our Rinnai combi is very programmable, if yours is and if you are reasonably sure that none of the piping and parts have failed, you might take a hard look at the program for the boiler.
    We have a very different set up, we use the boiler circ pump to serve the hydronic floor set with a max temp of 110F and I do not remember the ramp up setting but it is very slow less than 1 degree / minute.
    Hope that helps.
  • e12voltsdac
    e12voltsdac Member Posts: 4
    On the above comment, I wasn't able to find very many settings for the Rinnai. I did verify the settings on my thermostat are correct. The interesting part is the system has always worked properly until recently, so leaning less towards needing to adjust any settings.

    Quick update on more findings:

    After replacing a bunch of parts on the Rinnai I decided that the issue must be related to the speed of the circulation pumps and/or efficiency losses transferring heat to the closed loop. The Rinnai keeps the output temperature at a constant 140F so it's definitely working as expected.
    The closed loop temperature is at about 130F which seems to be correct for this type of hydronic system.
    The temperature of the return line to the heater is running at 135F, this also seems consistent even when the system is cycling (at start of cycle and at end of cycle when heater turns off).

    So the open/closed loop pumps appear to be functional, I'm getting good heat transfer to the closed loop. Now I'm back to thinking it's a Rinnai issue... perhaps I will replace all the thermistors in case any are starting to get out of tolerance.

    Any other ideas would be appreciated. Thanks!
  • hot_rod
    hot_rod Member Posts: 22,256
    This is not a combi? Sounds like a flow restriction. All the strainers clean?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GGross
    GGross Member Posts: 1,069

    When running only hot water to a fixture does the unit short cycle?

    Error codes seem to be the following, these could maybe be saved from a previous problem? I am not sure how long it saves them for

    12- no flame

    10- air supply blockage

    You need to check the incoming air supply, and the gas supply

    Beyond that, this could potentially be an application issue. These heaters generally work on a wide Delta T, 50 degree water in, 120 out. It would stand to reason that with the 100+ entering water during space heating, the unit is just short cycling due to application. I don't want to beat this topic to death, but tankless water heaters sometimes don't fit well in a space heating application, it is not what they are designed to do, but in your case it seemed to function correctly for several years.

    You mention never noticing this before. Did something break on this unit that has caused you to pay more attention to how it operates? Could this be confirmation bias kicking in and the unit always short cycled?

    That being said, the short cycling could absolutely lead to premature component failure.

  • e12voltsdac
    e12voltsdac Member Posts: 4
    edited December 2022
    Thank you for the responses!

    All filters have been checked to be clear. I recently also did a flush on the heater to ensure there is no blockage.

    One interesting thing is that the heater does NOT cycle when the hot water is called from a fixture. I can actually leave one faucet on just slightly (not enough to trigger the heater) and it will prevent the heater from cycling when the circ pumps are on. This is why I am thinking something is marginal and leaving a faucet on is enough to overcome the cycling.

    Yes, I am now finding out that these heaters were not designed for this radiant heating application. Unfortunately, all of the units in our townhouse community have been designed and built the same way. In the 3 years that I've owned my unit I have not noticed the short cycling during winter months (this is the 4th winter). I would definitely have been aware as the heater is in my bedroom closet and it would be apparent because of the loud fan noise during a short cycle.

    I'm wondering if the error codes are simply historical codes. I haven't seen any blockage on the air intake, it's a short pipe just to the outside. I will need to call someone to check the gas supply if I exhaust all other options, it may be that the system needs recalibration with the gas/water pressure.

    I'll continue to do some research and possibly replace other thermistors since they are low cost. Will report back on any additional findings.
  • e12voltsdac
    e12voltsdac Member Posts: 4
    Final update!

    I was able to fix the short cycling issue. It turns out that one of my hydronic loops was partially closed. This was my mistake as I had tried to adjust the temperature down on this loop prior. Also, I have a wall fan system with its own loop in the basement and it was set to half speed.

    If I open up all the loops fully and turn the fan on full speed, there is sufficient temperature drop from the closed loop to prevent the Rinnai from short cycling!

    I did end up finally narrowing it down to a temperature issue based on the water being fed back to the Rinnai on the open loop. I setup my DMM to monitor the input water thermistor and saw that as the voltage drops (temp increases) there was a trigger point that would eventually kick off the heater. And sure enough after a few seconds the temperature drops back down enough to kick the heat back on. 
    So the Rinnai has a feedback mechanism to turn off when the input water is already within a certain threshold of set temperature.

    Thank you all for the suggestions! My system is back up and running like normal and with a few new parts now :smile: