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We have a Rinnai tankless water heater in the basement. Our housemate has a bathroom in the basement. About once a week, the water turns cold during her showers or when she uses the sink. We have no problems at all upstairs, not even filling the bathtub. She recently told us the low water pressure light comes on on the water heater whenever this happens. I don't understand- if the flow is too slow then it should just not come on at all.

I just changed her shower head from 2.5gpm to 1.5gpm and tightened the valve for hot water to the sink a bit. I'm hoping this takes care of it but I'm also hoping to understand better.

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Need more information to answer the question. Need to know specific Model and serial number of unit. Also will need to know the exact error code the unit is showing. I believe i can direct you to the exact issue, or what might have caused it. I just need to know the specific error code. –  Mnc123 Jan 5 at 9:55
    
Also are you talking about low gas pressure or water pressure? –  Mnc123 Jan 5 at 19:18

2 Answers 2

Will revise answer once more information about the unit is given. This answer is assuming that the issue is low water pressure, as opposed to low gas pressure. IF the issue is indeed low gas pressure, i can edit my answer to accommodate the probable causes for this issue as well.

These are the possible causes for the issue from most common to least (1 and 2 are by far the most common causes for this issue):

  • Calcium build up of the heat exchanger (causing a restriction, will need to be flushed)
  • Blocked or dirty inlet filter
  • Faulty flow sensor (made of plastic, commonly will break or crack in older style Rinnai units)
  • Under sized inlet or outlet lines to and from the unit
  • Faulty control board/Short in the wiring harness somewhere
  • Plumbing issues within the structure (or inlet feed taken from improper location)
  • Issues with water pressure supplied by the city

If you have never had the unit serviced, i would highly recommend it. Flushing the unit with vinegar and cleaning inlet filter may fix the issue. As per manufacturer instructions, this is to be done at least once a year.

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I know this sounds like a long shot but I'd suggest looking into your PRV, it may be going out.

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Thanks! Any advice for how to check it? –  davidkovsky May 16 '12 at 2:54
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There are a few signs of a failing PRV. Noisy pipes, although this is a bit ambiguous as other things cause noisy pipes as well. Drop or spike in water pressure. Also when you put a water pressure gauge on a hose spigot, check if the needle steadily reaches a psi reading (healthy sign) or seems to wobble as it climes the readings (it may be going out). –  used2could May 16 '12 at 14:02
    
Thanks. I'll keep that in mind. The lower flow shower head seems to be working so far but it's too soon to say for sure. I haven't noticed any of the obvious signs you mentioned. –  davidkovsky May 17 '12 at 14:42
    
Low pressure will indicate low inlet water pressure. PRV will have no affect on this. ALso your PRV will be visually leaking, or have calcium build up on it if it has failed, or in the process of failing. –  Mnc123 Jan 5 at 9:56

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