Navien Tankless Temperature swings (over/under shoot)

Discussion in 'Boiler Forum' started by doozer, Nov 18, 2020.

  1. doozer

    doozer New Member

    Nov 12, 2020
    I have installed a Navien NCB-180E in my pole barn apartment. I have the living area on one zone (20x40) and the garage on another (44x40). 1/2" PEX is run on 10" centers through 4" concrete. Slab is on top of 2" foam board. The walls are spray foamed with 3" of closed cell foam and blown in fiberglass on the ceilings.

    I did not put a temperature probe line in the concrete so I am relying on ambient air temp to run the system. I am seeing big temp swings. I have the thermostat set at 64 and I will see max temps up to 68 and min temps down to 60. Very uncomfortable swings. Water temp is set to heat to 111 degrees and by the end of the heating cycle, I am seeing return water from the floor at about 83 degrees.

    Any suggestions to get the temp to flatten out?

    Thanks, Doug
  2. fitter30

    fitter30 Well-Known Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    What brand and model of thermostat? What water temp are u running?
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    The problems is most likely to be the time lag between the thermostat sensing a drop or rise in air temp to when the surface temp of the concrete responds, all due to the significant thermal mass of a 4" slab.

    What are you using for a thermostat?

    Simple wall thermostats designed for low-mass radiation like fin-tube or even medium mass radiation like cast iron radiators won't compensate for this very well. Thermostats that have learning PID type algorithms can dampen out those over/undershoots pretty well, at least on most days. (Sudden extreme changes in outdoor temperature can sometimes screw those up too, but not as badly.) Tekmar's thermostats designed for radiant heat have pretty good performance with slabs, with or without a slab temp sensor, but there are others.

    The location of the thermostat can also make a difference.
    breplum likes this.
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