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beginner plummer. New home with Lochinvar Noble temperature fluctuations and OUTLET MRHL error

meister19meister19 Posts: 2Member
hey guys so i recently moved to a new home and was trying to trouble shoot an issue with our tankless water heater. The mode lis Lochinvar Noble. I tried to solve this problem before I tried to hire a professional to deal with it. I would like to understand the basics at least so I don't get ripped off.

The problem I have occurs since I moved in. The shower temperatures would fluctuate hot and cold during usage. Faucet seems okay and doesn't seem to have the same issue. Recently there was errors from the unit which are OUTLET ARHL and MRHL. I resetted it and it seems to do the same thing. I did the full reset with the easy setup with factory recommended settings and it didn't seemed to help.

I was told to check a few things, which includes to purge the air inside the system, and to check if i'm losing pressure on the valve. However I would like to make sure if there are settings are correct before continuing.

I've searched everywhere on the internet and haven't found an answer at all but if I can get any help then that would be amazing.


  • nicholas bonham-carternicholas bonham-carter Posts: 7,841Member
    How old is the water heater? The settings must be right, if you have followed instructions from the manual.
    If you have a constant temperature from the faucet, but not the shower, then it would seem the problem is with the shower valve. The faucet may be passing enough water to keep the heater flame on, but the shower valve may be too restrictive.
    Try running a bit of hot water from the sink, while you are having a shower.
    If the shower stays consistently hot, then the problem is in the water heater part that reacts to the flow of water through the water heater. Some shower heads may not allow enough water to flow, for constant hot water even from a new heater.
    The shower valve itself may have an anti-scald feature, which shuts down the hot water, when the temperature exceeds a set point.—NBC
  • IronmanIronman Posts: 5,082Member
    It's a combi BOILER, not a water heater.

    The faults are Auto Rest High Limit and Manual Reset High Limit. They both indicate that there's not proper flow through the heat exchanger of the boiler.

    This could be caused by:
    1. Air in the system.
    2. A bad internal circulator.
    3. A bad internal diverter valve.
    4. The plate heat exchanger is fouled on the domestic side.

    The first three refer to the hydronic side.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rodhot_rod Posts: 11,608Member
    It is a bit tricky to purge those boilers for initial start. I've put a bucket under the relief valve and let them flow full bore for a bucket tor two.
    Assuming you have a fill valve that can keep up.

    There is a small manual purge valve on the boiler HX, did you run that clean?

    Does the boiler have a good air purger? if so running heat mode can help get the small air out.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • IronmanIronman Posts: 5,082Member
    edited August 12
    Another question would be: is the space heating side piped in with the necessary components?
    You can't run the unit without the basic components: expansion tank, relief valve, fill valve/backflow preventer, air separator (MBR).
    All of these are on the hydronic side, but are necessary for the boiler to operate in either domestic or space heating. If someone installed it thinking it was an on-demand water heater and didn't do these components, then there's your problem.
    Some pics would help.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • meister19meister19 Posts: 2Member
    Hey guys,

    Thank you so much for the help. I am going to start to get the air out of the system first and then go from there.

    I had a difficult time finding info online but i ran across the Lochinvar University videos on the unit. Hopefully that helps others when you're looking to DIY.
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