Navien 240 newbie...advice on settings please

[dogbert]

This summer I bought a house that has a Navien NCB 240E installed. I've been reading the manuals and this forum trying to understand the system, and in particular the best way to operate the heating for the winter.

First, some details of the house:
- old house, completely renovated down to the studs 2 years ago. New everything (siding, roof, insulation, plumbing, electrical, drywall, baseboard heaters, windows, doors...all done prior to my purchase).
- house seems to be very tight thermally.
- basement unfinished but completely insulated.

Setup:
- 2 heat zones: main floor (900 sqf) and second floor (530 sqf). Basement unheated.
- Main floor (zone 1) has 54 feet of Haydon finned tube, 210 btu/ft/hr@120F.
- Second floor (zone 2) has 40 feet of Haydon finned tube, 210 btu/ft/hr@120F.
- External temp sensor installed, and seems to sense about 3F to 5F lower than actual (it has 20 feet of 18ga wire extension to get it to the boiler - necessary to keep it out of direct sun).

Settings:
- A: enabled
- B: 1
- rest are factory default.

The prior owner did not make use of the external temp sensor and had the boiler set at 140F, with both zones set to 70F.

According to the gas company, the gas usage during peak winter months for the first 2 years the Navien was installed was as follows:

2018/19: DEC, JAN, FEB, MAR
Therms: 54, 66, 78, 74
Avg temp: 34.4, 33.5, 23.1, 24.7

2019/20: DEC, JAN, FEB, MAR
Therms: 55, 70, 61, 59
Avg temp: 31.4, 30.8, 30.1, 32.0

I suspect the 240 is waaaay overkill for this house.
I would welcome any and all suggestions on the best settings to use to keep the boiler as efficient as possible.

Thanks,
Scott.

 

[Fitter30]

Fin tube really doesn't start to heat till 120* water. With outdoor sensor a heating reset can be programmed in. At 65* 90* water at 0* 140*-150*water. Just have to play with the ratios.

 

[Sponsor]

 

[dogbert]

Fin tube really doesn't start to heat till 120* water. With outdoor sensor a heating reset can be programmed in. At 65* 90* water at 0* 140*-150*water. Just have to play with the ratios.

Thanks for that suggestion...it looks very close to the "low mass radiant" reset curve, so I think I'll try that to start with.

Would it make sense to set item F. "space heating max heat capacity" to something lower, or am I not understanding that setting?

 

[Fitter30]

Start with 160-170* if you find at colder temps can always raise it to 180*