Rinnai Tankless Water Heater feeding about 6 fixtures. The two shower valves are thermostatic mixing valves. ALMOST reliably the water will slowly warm up and deliver hot water for about 3-4 minutes then turn cold for about 30-40 seconds before heating back up again. This happens with shower and faucets. It happens if only one fixture is running or multiple, and is flow rate independent (as long as its fast enough to trigger the unit to fire) NOT all the time though, but almost all the time. Thoughts?
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2 Answers
This might be a symptom of low water-flow causing the unit to cycle off and on. From Rinnai's web site: With low-flow faucets and showers, "If you find you’re not getting the hot water you need, start by lowering the temperature setting... It’s counterintuitive, but it affects the hot/cold mix ratio and will activate the tankless water heater."
"Rinnai tankless gas water heaters need a demand of at least 0.4 to 0.6 gallons per minute [1.6 - 2.5 L/min, ed.] of water flow through the hot side to ignite and 0.26 gallons [1 L/min, ed.] per minute to stay in operation. If the flow rate falls below these levels, the energy-efficient Rinnai unit will turn off because it senses that hot water isn’t needed."
It might be due to low air flow, such as caused by a blocked vent or by dirt in the squirrel-cage blower. B. Smith provides a video for cleaning the fan.
It might be due to a clogged condensate line: "I have a small tube that runs directly from the unit into the water pump... to expel excess... condensation. The issue was that the tube had slipped too deep into the water pump reservoir... Because my condensate line was sitting in the water, it wasn't able to drain properly which I believe caused my water heater to shut down as a safety precaution (similar to if the condensate line/trap was completely clogged)."
Finally, it might be a flow-sensor issue. If water-pressure is too low, or the sensor is faulty, it can cause cycling on-and-off.
I'd trouble-shoot low-hanging fruit first.
- Try reducing temperature a few degrees.
- If there is still an issue, check condensate line and vent.
- If the issue persists, flush and clean as recommended in the owner manual.
- Last, getting at the blower is a nuisance, but might help.
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This is helpful will try all this over the weekend. It's certainly not because the shower head is low flow. I made sure of that when I bought the head... Commented Jul 20 at 14:34
So turns out the base problem was the two shower safety hot water stops were turned WAY too low. So I had cranked up the heat on the hot water unit to 135 and I think that created a low flow hot water state. I adjusted the hot water stops and was able to turn tankless heater to 125. Problem seems to be solved.