Jac04
Member
I recently had a Navien NCB-180E (14ooo Btu/hr min fire) installed to replace a Navien CH-180 (16000 Btu/hr min fire) that had a heat exchanger failure.
The old CH ran well, and heated the house well. My only issue was that I had to manually adjust the supply temperature since none of the ODR curves seemed to work for me.
Last night was the first time that we ran the new NCB unit for heat, and it’s acting a little different than the old CH. The old CH would always run continuously during a call for heat, but the new NCB will cycle on a call for heat.
Example, With a supply set point of 115F, and outdoor temp about 35F:
The old CH would fire and remain firing 100% of the time on a call for heat. Burn times were typically about 10-15 minutes, and supply temperature would typically creep up above the 115F set point to about 125-127F before the t-stat would be satisfied and end the call for heat. From my observations, the CH had a non-adjustable hysteresis of +/- 15F (i.e.: for setpoint = 115F, the burner “on” temp would be 100F and the burner “off” temp would be 130F).
Now for the new NCB unit. My custom ODR curve has the supply setpoint at 114F for 35F outside temp. The call for heat lasts indefinitely, but the burner keeps cycling. It cuts off at 118F and it doesn’t re-fire until it gets down to 109F. It just keeps doing this and the t-stat is never satisfied.
According to the Navien manual, the default burner “off” setting is 4F above the setpoint, and the default burner “on” setting is 5F below the setpoint. This is why I am seeing the 109F to 118F swing with a setpoint of 114F.
Now for the sanity check.
My plan was simply to bump up the burner “off” setting to 10 – 15F. At 114F supply, I know that the unit is putting out more heat at minimum fire than I am putting into the house, so the supply temp is going to continue to rise. My thought is to let the supply temp slowly rise 10-15F above the set point which will 1) keep the burner firing, and 2) satisfy the thermostat.
Does this make sense?
The old CH ran well, and heated the house well. My only issue was that I had to manually adjust the supply temperature since none of the ODR curves seemed to work for me.
Last night was the first time that we ran the new NCB unit for heat, and it’s acting a little different than the old CH. The old CH would always run continuously during a call for heat, but the new NCB will cycle on a call for heat.
Example, With a supply set point of 115F, and outdoor temp about 35F:
The old CH would fire and remain firing 100% of the time on a call for heat. Burn times were typically about 10-15 minutes, and supply temperature would typically creep up above the 115F set point to about 125-127F before the t-stat would be satisfied and end the call for heat. From my observations, the CH had a non-adjustable hysteresis of +/- 15F (i.e.: for setpoint = 115F, the burner “on” temp would be 100F and the burner “off” temp would be 130F).
Now for the new NCB unit. My custom ODR curve has the supply setpoint at 114F for 35F outside temp. The call for heat lasts indefinitely, but the burner keeps cycling. It cuts off at 118F and it doesn’t re-fire until it gets down to 109F. It just keeps doing this and the t-stat is never satisfied.
According to the Navien manual, the default burner “off” setting is 4F above the setpoint, and the default burner “on” setting is 5F below the setpoint. This is why I am seeing the 109F to 118F swing with a setpoint of 114F.
Now for the sanity check.
My plan was simply to bump up the burner “off” setting to 10 – 15F. At 114F supply, I know that the unit is putting out more heat at minimum fire than I am putting into the house, so the supply temp is going to continue to rise. My thought is to let the supply temp slowly rise 10-15F above the set point which will 1) keep the burner firing, and 2) satisfy the thermostat.
Does this make sense?
