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Laars Endurance 110 lukewarm DHW.

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tostroski
tostroski Member Posts: 4
in Domestic Hot Water

My DHW is suddenly no longer hot. It is not cold like it would be with a bad flow switch, it just does not get very hot. This happened suddenly, so I don't think it is scale build up in the heat exchanger, but I could be wrong there. Could this be a failing circulator pump? I can hear the pump kick on with a very quiet hum when there is a call for hot water. Could the pump just be starting to fail and not be as efficient? I am planning on replacing the boiler in the spring as it is 20+ years old. Before I sync money in this that I am never going to get back, I thought I would ask if you all think I am on the right track with the pump or am I barking up the wrong tree? My heating zones all have external pumps and work fine.

Comments

  • Larry Weingarten
    Larry Weingarten Member Posts: 4,042

    Hi, It may seem a little off topic, but it could be a plumbing problem… a crossover. This is easy to test for. Close the cold supply to your heater and open a hot tap. Does it stop running within seconds? If it does, good! If not, cold is getting into the hot side somewhere. That could be giving you cooler water. There are other possibilities, but I like to start with this one.

    Yours, Larry

    Alan (California Radiant) ForbesHomerJSmithtostroskibjohnhy
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,668

    Could be a cross-connection, could be a bad pump. Could also be a faulty DHW sensor.

    Does the sound of pump stop after awhile?

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    tostroski
  • tostroski
    tostroski Member Posts: 4

    Great suggestions guys, thank you. I should have mentioned that I tested tested for cross connection and that is good. Have replaced the DHW flow sensor a few times in the past and these symptoms seem to be different, the water never goes cold, just starts hot for a second and then fluctuates back and forth from lukewarm to warm. If it would not have started so sudden I would be thinking it was just scale on the heat exchanger, but I think I might have these symptoms if the circulation pump was not flowing enough water through the plates.?.?

    The pump seems to stay on the whole time a faucet is running. It does not seem to cycle on and off so I "think" I am in the clear with the flow switch.

  • hot_rod
    hot_rod Member Posts: 27,116

    Does yours have this thermostatic mix valve piped into the hot/ cold? The manual indicates it comes with the unit. Those 3 way valves can get limed up and present intermittent HW temperature output issues.

    Screenshot 2026-01-18 at 3.32.51 PM.png

    It and the HX need to be descaled occasionally, depending on how hard your water is.

    Ab tankless water heat cleaner kit may clean both if you have or can valves to circulate the cleaner.

    Screenshot 2026-01-18 at 3.36.05 PM.png Screenshot 2026-01-18 at 3.36.37 PM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • tostroski
    tostroski Member Posts: 4

    Hey Bob, mine is the older style. I do have a mixer valve on the hot output. I have worked the valve back and forth and it makes no difference whether I have it all the way "warmer" or "colder" other than the first second or so after the hot water gets to the faucet. I am not sure that I have the right tools to get the heat exchanger off it is really tight in there, but I am good with trying a descale if you think the pump is okay. I believe the installer is semi retired, and my go to HVAC guys avoid this boiler like the plague. :-) But I am sure they will give it a go if I can't since they are going to be installing the new one.

  • tostroski
    tostroski Member Posts: 4

    Thanks for all the suggestions. I think Bob nailed it, and my mixing valve is bad. I will update once I find parts.

    bjohnhy

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