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Takagi H3M-DV-N 100 Error 941

Andrew_DAndrew_D Member Posts: 6
I recently installed a Takagi H3M-DV as part of my shop in slab heating system, its a closed loop setup, 3 x 300ft 1/2" loops in concrete. 2" XPS foam under the concrete. The system has been in place about 6-7 months, and has worked well, no problem heating. I went out the other night and the system was not heating, the Takagi had an error 941 on it, which seems to indicate an abnormal exhaust temp. I opened the unit and the unit has some scorch marks under the burner and some chared paper in the case. I opened a case with Takagi (AO Smith) and they've basically said sorry, not for closed systems (the unit does say its supported for heating in open systems). I recognize its not a true boiler, but know many successfully use this heater for Radiant purposes. Any idea whats going on?

I've attached pics of the system.


  • DYIDYI Member Posts: 12
    I noticed that the vent piping does not conform to the installation manual. The manual states that for 2" vent installation 2" PVC is used only on the exhaust connection with a 3" PVC el connected to the intake from the room. Nowhere in the manual is 2" PVC shown for the intake connection.

    For 3" vent installation 3" PVC is used for both intake and exhaust.

  • Andrew_DAndrew_D Member Posts: 6
    edited April 2020
    I can definitely change over to 3" venting and intake, the unit is mounted on the back wall of the shop, and vents/intake right out the back wall, probably 1ft total length on each.

    I figure I'd also add the grundfos pump runs around 2-3 gpm, and the temp is set for 105 degrees.
  • DYIDYI Member Posts: 12
    I would suggest one change at a time. Make one change and see what difference it makes before making the next change.
  • silverlexsilverlex Member Posts: 5
    I wanted to follow up, takagi is going to reluctantly replace this unit. I personally think it has a defective systemboard or fan. I will replace the venting per the feedback here though.
  • Andrew_DAndrew_D Member Posts: 6
    edited January 12
    Well - Replacement H3M-DV-N installed and lasted not quite a year Now showing error 341 which is for a thermistor failure. I opened it up and same type of overheat issue (scorched wiring and glowing red hot when running). This unit claims to be used for commercial use, can heat water to 160 degrees (I had it set for 120). It sees relatively little use.. .what gives? I know takagi doesn't rate them for radiant use (although the manual provides specs on using glycol and a heating diagram so take that for what it is). Are these units just junk? I thought Takagi had a good reputation. System was running 40PSI and seeing roughly 2gpm, with a 50 degree delta on in/out temps. Any recommendations for a small tankless or boiler that won't burn up every 6 months? Located in Kansas, aprox 30k btu load for in slab radiant.

  • OliveoneOliveone Member Posts: 7
    wow that not a good sign that it gets that hot
  • kcoppkcopp Member Posts: 3,608
    edited January 13
    A tankless water heater is the wrong application for radiant floor heating.
    Its not designed to have water RE-circulating through it. 40F in 120F out...
    You really should have a boiler.
    A modulating condensing boiler would be the best option.
    I like the IBC HC unit. prob the 13-50 would work well for you.
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