Shower Running Boiling Hot or Freezing Cold - No In-Between...

Discussion in 'Tankless Water Heater Forum' started by FinchDaddy, Aug 16, 2019.

  1. FinchDaddy

    FinchDaddy New Member

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    Aug 16, 2019
    Location:
    South Carolina
    All -

    Fairly new homeowner here. I have a Rinnai Tankless Water Heater on the outside wall on my home (not my decision it was the builders) and when I start a shower in either bedroom the water temperature is either two temperatures - Boiling hot or (barely turning the shower faucet to cold) beyond freezing.

    This issue does not happen with any of my non-shower faucets in the bathrooms or kitchen sinks... To make things stranger this only appears to be an issue in the summer time. I live in South Carolina so it gets HOT in the summer.

    I did some research and I saw it could be a damaged flow sensor or a dirty water filter? I was also unaware I needed to have this checked annually so I am planning on that. Learning as I go...

    Any information would be helpful. Thank you!
     
  2. Dana

    Dana In the trades

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    These things have a minimum firing rate, which means at a given flow rate it has a minimum temperature rise. They also have a minimum flow rate to keep it firing. In summer when incoming water temps are warmer the flow rate through the tankless is lower during a shower, which means it may not be able to deliver the right temperature out of the shower head without either flaming out or delivering scorching hot water much higher than you want. To test if this is what's happening, open up another hot water tap and see if the shower temperatures become better regulated.

    If that's what's going on, one potential fix is to go with a higher flow shower head.

    With an actual model number it might be possible to figure out other potential causes/ fixes. (Rinnai has sold dozens of tankless water heater models with different specs over the past couple decades.)
     
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  4. FinchDaddy

    FinchDaddy New Member

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    Dana,

    Appreciate the response. I should have listed originally but the exact model is: Rinnai V53e Model #REU-VAM1620W-US
     
  5. Dana

    Dana In the trades

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    The V53e only modulates down to 19,000 BTU/hr-in (if natural gas) 20,200 BTU/hr if propane. At 82% efficiency the minimum output is then 15,580 BTU/hr (gas), or 16,564 BTU/hr (propane) for purposes of sketching out the math let's assume gas.

    15,580 BTU/hr is (/60 minutes =) 260 BTU/minute

    The minimum flow through the tankless it needs to stay lit is 0.6 gpm. At 8.34lbs/gallon that's 5lbs/minute.

    260 BTU going into 5 lbs of water raises it's temperature by 260/5= 52 F. Assuming your incoming water in summer can be as warm as 75F that would yield a scorching 127F out at 0.6gpm!

    But if the flow through the tankless is twice that (1.2gpm, or 10lbs/minute) the minimum temperature rise gets cut in half to 26F, so with 75F incoming water you should be able to get water as cool as 75F +26F = 101F.

    A typical low-flow shower is going to be 105F at the shower head, running 1.5-2 gpm so theoretically if it's all operating correctly to spec you should be able to get control of the water temp in the shower, if the temperature setting on the tankless is LOW enough. If your shower is actually running 1.2gpm and the tankless is set to 130F or higher it might not have sufficient flow through the tankless to stay lit, even if it's slightly out of spec.

    With many shower mixers with anti-scald valves the pressure difference between the cold side and hot side created by the pressure drop through the heat exchanger on the tankless can cause it to intermittently restrict the flow to the hot side enough to cause the tankless to flame out. If the heat exchanger on the tankless is starting to lime up that sort of problem will be worse, due to the greater pressure drop.

    Try setting the tankless to 110F-115F so that almost all the flow to the shower is going through the tankless, not the cold side of the shower mixer. It will likely either get better or worse, which may help point to the source of the problem. In general a tankless has better temperature control when the flow is well above it's specified minimum flow, and the lower the output temperature the less cold side water gets mixed in. If it gets a lot worse with the tankless set to a lower output temperature setting it may indicate an issue with the anti-scald function on the shower mixer.

    With a tankless that only modulates down to the 20K range it will have better temperature control if set to 115F or less to keep the flow through the tankless well above it's minimum. Most bath tub-fills only need 110F water. While 120F or hotter water is somewhat better for laundry and dish washing, most residential; applications don't need anything hotter than 110F.
     
  6. Reach4

    Reach4 Well-Known Member

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    Location:
    IL
    Could the two shower valves be installed wrong? If the water is hot, the valve tries to deliver more cold... but it is really hot. Just an idea. I am not a plumber.

    When you get the really hot or really cold, maybe run the lavatory and measure the water temperature there. If the problem is the WH, then the lavatory should have a similar symptom, I would think.
     
  7. Dana

    Dana In the trades

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    Given that "... this only appears to be an issue in the summer time. " I suspect it's not that the supplies to the mixer have been swapped.
     
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  8. FinchDaddy

    FinchDaddy New Member

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    Thanks, Dana. A lot of great information here. I will do this and report back my results.

    Would you also recommend cleaning the air intake filter and flushing the system with vinegar? I’m embarrassed to say I haven’t done it in 7 years...
     
  9. Dana

    Dana In the trades

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    Even though it's unlikely to be related to the symptoms, de-liming the heat exchanger and cleaning the air intake once every 7 years (whether it needs it or not ;) ) wouldn't hurt.
     
  10. FinchDaddy

    FinchDaddy New Member

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    Ah. Here is some great news. My Rinnai does not come with any other heat options besides 140 or 120 unless I buy the $100.00 controller and install it. I’ll update once it arrives and I install.
     
  11. Dana

    Dana In the trades

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    Can I assume you are already running it at 120F, and not 140F?
     
  12. FinchDaddy

    FinchDaddy New Member

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    Aug 16, 2019
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    Yes, I can confirm it is running at 120.
     
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