Rinnai Cycling on and off
I have a Rinnai RUC98i and it has been working well for 7 years. Recently it started going cold when taking showers (cycling hot and cold). The only code history it has was 1-11 and 2-11 the rest are not showing codes. I have always religiously done annual vinegar flushes. I have replaced the water flow servo and flow sensor assembly because I was concerned that the unit may have not been detecting proper water flow but this did not fix the issue. I read that it may be related to the burner assembly being dirty or clogged because of the code 11 indication. Since I was going through the trouble of removing the whole heat exchanger and burner/condenser I decided to just purchase a brand new burner. I just disassembled the burner/heat exchanger last weekend and replaced the burner (4 hours work) and managed to get it all back together and it still does the same thing!
The unit starts normally when the shower is turned on, then sometimes the fan speed goes to high and then the unit turns off the burner for maybe 30 seconds and then it will ignite again and maybe work for a minute or two sometimes with the burner on high and then it eventually turns off the burner. Not sure what else to do with this thing at this point. I may replace the two thermistors next because at least it doesn't involve gutting the innards of the unit! This thing is very frustrating at this point…. Any ideas?
Comments
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cycling hot and cold with no error is generally a scaled up heat exchanger. What is your water hardness? when you flush with vinegar how long do you pump the fluid through the unit, and are you using a powerful pump? Flush the heat exchanger while circulating the cleaning fluid for at least an hour and then try again. Sometimes with very hard water you need to flush more than once a year.
For ignition issues you should always start with the basics. Gas pressure, venting, flame sensor, igniter cleaned. If you know the ignition issues are current you should start by checking gas pressure. firing the parts cannon is expensive and not wise to do prior to checking site conditions. You will need to get the unit to fire before checking these values.
static pressure when unit is off
record the value on ignition
run to high fire record the value'
shut unit off and record the value
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Do you have a mixing valve after the unit? I've had a similar issue where a bad valve would cause the tankless to cycle.
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I annually descale the unit with vinegar using a 1/5 HP de-watering pump (a very strong flow) with at least 3 gallons of vinegar and I let it run for 3 hours. The pump even warms up the vinegar so I'm pretty certain there is no scale build-up and I most recently flushed it just after I first started getting these symptoms. The heater only sees soft water otherwise.
There is no ignition issue because it fires up just fine initially it just stops after a few minutes or sometimes after 30 seconds. It has a new ignitor, flame-rod and thermocouple because they came as part of the burner assembly. The complete burner assembly for this unit is $57 from SupplyHouse.com so I'm not exactly throwing money at it since I am doing all the work myself!
While I don't have the means to immediately test gas pressure while running, nothing has changed regarding the regulator and the meter was upgraded several years back in order to support my pool heater which is 350,000 BTU. The unit runs and heats just fine but just shuts-down periodically so I was thinking it may be related to some closed-loop operation involving thermocouples and the controller.
I had a "professional" HVAC company come out and look at it and they ultimately were a bit dumbfounded and wound up calling Rinnai support who spent an hour with them on the phone and Rinnai told them it was most likely the burner assembly which is why I went and replaced it myself (vs. the $1000 quote I got from the plumbing folks).
No mixing valve is used after the heater and it is being used for standard plumbing fixtures. No hydronics etc…
Has anyone else seen anything like this?
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Hi, It may sound silly, but next time you shower, also open a hot tap to get the hot water flow up. See if that makes any difference. I've found some tankless heaters don't appreciate low flow showerheads.
Yours, Larry
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Thanks Larry. I have already experimented with increasing demand by running faucets in parallel and even went as far as running the tub and faucets and shower all at the same time just to see what the effect is. It does force the burner into "high" but it still has the same instability where the blower will vary its speed and then shutdown and restart etc… I have previously removed the restrictor from the shower head so it typically caused the unit to run on high when things were working normally.
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even know it has been flushed for 3 hours, is it possible that it is overheating? Still yet, go 8th performance data and watch #11. That is the hex temp.. then compare to #2 which is the outlet temp. Should be about 20 degrees difference. If #2 isnt getting to the set temp but overheating on #11 then replace the bypass valve. You can learn to get into performance data by the tech sheet in the pouch on the backside of the cover
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I think Hilltown is on the right path. I would guess issue with either the flow sensor or the bypass valve.
Flow sensor can also be checked by monitoring flow rate in the performance data menu.
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Thank you for the comments regarding performance data. I will look at that more closely since the bypass valve is pretty much the last thing on my list of possibilities. In general I can feel the cold water from the bypass getting to the "T" where it mixes with the hot water coming from the heat exchanger and it does seem to be bypassing otherwise I would think everything would get warm but I will dig into it more to try to get some numbers.
I initially thought the whole problem was the flow sensor causing things to drop out because it thought the flow-rate was artificially too low. I wound up replacing the flow sensor and nothing changed as a result…
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OK maybe we are getting somewhere @hilltown I suspect you are on to something. I got it into maintenance mode and took a look at the parameters you pointed out (at least I think I did!)- I have the heater set to 125F by the way…
Looking at #2 it does indeed go up to 125F pretty precisely (when it is running and not cycling). However #11 will ramp all the way up to around 210F at which point it shuts off the burner and then of course the outlet temp starts dropping (cold shower) and the heat exchanger temp drops quickly until it gets to maybe 75F then the burner kicks back on and the cycle repeats.
You are onto something here… Does this mean the bypass valve is not acting correctly?
I really appreciate your expertise here!
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the hex is overheating, but we are making set temp, I heard you say you flushed for hours, but I believe it needs more, 4 gals vinegar in a 5 gal pail, , unit off. Flush away, I dont believe a bypass will change anything, but I could be wrong. Let us know
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If the HX temp is increasing and the outlet temp is stable, your bypass valve is working properly.
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Thanks. I thought you said previously that if it is overheating on #11 then replacing the bypass valve should do the trick. I am going to have to try that because I am certain the HEX is not plugged because if anything, I have overflushed it and it has already been done again just a couple weeks ago. It makes set temperature immediately without any issue while flowing more than 5 GPM so that doesn't sound like a scale build-up issue to me but what do I know?
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Make sure that when your descaling the unit your in descaling mode. I'm not familiar with your model but some need to be unplugged, then plugged back in until you hear all the valves stop, and then unplug it again. This will get the mixing valve closed and everything will go thru the HX. Others need to be set in the software to descale
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Cycling issues are sometimes caused by sticky bypass valve. Since the temp is stable until the HX overheats, the issues isn't the bypass.
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Well then I have no idea where to go next with this thing. I don't see any reference to setting it in descaling mode and the unit is generally unplugged when I do the descaling. After 3 hours the vinegar is quite warm because of the pump heating it and the vinegar is quite green which I always thought was due to copper corrosion/byproducts as expected running through all that copper tubing.
I am pretty much at my wits end because the only thing I haven't replaced is the bypass valve (and of course the heat exchanger). If I do another flush on the Heat exchanger tomorrow it will be 6+ hours of vinegar flush within the last 2 weeks!
Also, how can the outlet jump right up to the set temperature (125F) in only a few seconds if the heat exchanger is supposedly this heavily scaled. I would think that would impact heat transfer in a significant way and flow would be impacted. It gets to temperature virtually instantaneously and everything is flowing normally…
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give the bypass a whirl it’s only a few bucks
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Yes, I will let you know. Many thanks for the support from everyone on this forum
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When you run the unit, how does the flame look in the window? If you run at lower flow, say a sink faucet, does it still overheat or only at higher flow rates?
Is this propane or natural gas? Can you measure the supply pressure when the unit is running.
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If #11, the HEX temp is going up over 200 degrees, you have a scaled up HEX. You can try using some powdered citric acid and dilute with water for a 10% solution and flush for a while and flush with water afterward for a while too. Also check to see if the HEX thermistor is measuring correctly. A good way to do that is while the unit is running, turn off power at the controller. This will turn off the burners and then go into the maintenance mode and check 08 and #11 should be close to the same temp. You're basically reading the incoming water temp and both thermistor's should be about the same once the HEX cools down. I'm attaching the maintenance codes to help you. With this, you can also check the bypass position.
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Try i believe it s whitlams flow aid doing a cleaning w vinegar is not for heavy build up . I ve used this product on some indirects and have had good luck with it . Hilltown is correct that the heat exchanger over heating . Has the boiler side of the heat exchanger ever been flushed . When operating in heating mode are there any over heating or short cycling on temp issues that occur ,they may not show a code or generate a fault but in observing a call for heat may give ya a clue if the over heating occurs then its surely the boiler side exchanger which may be the root of the issue .
peace and good luck clammy
R.A. Calmbacher L.L.C. HVAC
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating0 -
Flame looks blue when running the burner with one faucet running but the flame is clearly reduced when viewing in the window. The flame is bright blue and lights up the entire window when demand is high. I don't have the means to test pressure when operating however I am not suspecting a gas problem since the same gas line, downstream from the heater, is running a 120,000 BTU furnace and an 80,000 BTU garage furnace. Those gas connections are after the water heater and I have run the water heater with both furnaces turned off just to make sure. There is plenty of gas capacity I think…. Gas meter was uprated to support my 350,000 BTU pool heater which has no issues with the supply.
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Thanks @Slimpickins , I will do another descale today and see where that takes me…
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Clock the gas meter to verify BTU does not exceed maximum or minimum settings.
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If it were shutting down because of a gas supply issue surely you would get an error code. Anyway if you were checking the gas supply I wouldn't bother clocking the meter, that won't tell you what you need to know. I'd check incoming gas pressure, static, dynamic and high lockup. It's the only way to check its within spec. The symptom is still the symptom of a scaled heat exchanger, or running close to minimum flowrate (unit turning on/off with no error code, water not consistent temp hot-cold-hot-cold) I wouldn't spend more energy investigating the gas supply unless you get flame sensor errors or other ignition errors.
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