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Plumbing Pro
Plumbing Pro, Plumber
Category: Plumbing
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Experience:  Licensed plumbing contractor state of Il, & Chicago
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We have a Rinnai R85i-1 tankless waterheater that is 3-4 years

Resolved Question:

We have a Rinnai R85i-1 tankless waterheater that is 3-4 years old. We're on a well, and when the water heater was installed, we had the plumber install turbidity and carbon whole-house filters on the cold water inlet to the house (fortunately with bypass valves.) Because of the initial carbon dust in the water, we switched the valves to bypass the filters and never got around to using them until yesterday. When the first filter went into its backwash cycle, the water heater started leaking. Taking the cover off the Rannai, the water is coming out of the exhaust fan, so it seems to be leaking in the heat exchanger above the exhaust fan. When the backflushing stops, so does the leak, and if I bypass the filters again, the water heater is not leaking.

When in backflush mode, the filters should be drawing water out of the system and discharging it out the backflush valve (since there is an airgap and the drain is by the filters, I can confirm it appears to be backflushing properly)...if anything, there should be a pressure drop, not an increase (I have a hose bib right before the inlet into the house and the water heater, and if I open that valve to cause a pressure drop, the Rinnai does not leak.) If I recall correctly, our water pressure is only about 50 psi, and the filter does not have a pump, so even if the backwash valve was not operating correctly and was somehow flushing back into the supply line, the pressure shouldn't be high enough to spring a leak. If there is a seal, joint or pipe leak in the heat exchanger, once it started, I would think it would leak regardless of whether or not the filters are in backflush mode.

Any thoughts on what I should do now. I'd rather be able to use the filters, but I can leave them bypassed, if the water heater is otherwsie going to be ok. I can use the flush valves to isolate the water heater and manually backwash the filters (a PIA), or...?
Submitted: 6 years ago.
Category: Plumbing
Expert:  Plumbing Pro replied 6 years ago.
Welcome to Just Answer, I will do my best to Answer your question to the best of my professional ability. things happen on coincidence, when ,or if you had a leak it would be all the time and not come and go . Water is under pressure so if you had a leak it would be all the time and not stop it self . If you had a leak in the heat exchanger then the heater would be shot and need replacement there is no fixing it, but it would leak all the time . If you are having water come out the pressure relief valve then , the valve might be bad and need replacement . The other thing is if yo u have filters that need back wash then that discharge hose needs to drain somewhere and you could just be seeing the water from the back wash , that would be normal , expect the hose should be placed into a drain so it doesn't leak all over , but a filter that does its own back wash means that it needs a drain to drain the contaminates. The other thing would be if you see water coming from the flue pipe then you might just be seeing water coming back in from the flue , as hot air rises it cools off and could fall back down the flue pipe and look like its dripping.
Customer: replied 6 years ago.
I understand water is under pressure and it should leak all the time, but it is not - it is only leaking in backflush mode. There is a backwash discharge hose from the tank valve on the filters that goes through an airgap to a nearby drain. The leak is clearly coming from the water heater, and it is more water than a bit of condensation from the flue pipe. Is there a pressure relief valve above the exhaust fan housing in the heat exchanger? If so, how do I get to it to replace it?
Expert:  Plumbing Pro replied 6 years ago.
the panel removes by unscrewing the little screws that hold the face panel on, there is a pressure relief valve , and they do go bad over time, and could need replacement. Other then that if your heat exchanger is leaking in any way then the heater is shot and will not be fixable. These thankless heaters do not recommend home owns to do there own repairs or service and strongly recommend calling a certified professional, or you will void any warranty that you might have, the heater should have a 10 years parts warranty and a 3 year warranty on labor if you contact the manufacture you can find out if your still covered.
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