Navien NFB-175H

FB

A Contractor is recommending a Navien NFB-175H with a turn down ratio of 13.2:1. We've had plenty of heat for DHW (using a SuperStor indirect storage tank), radiators and baseboards from an aging Buderus mod-con with an output of less than 80K. Will the Navien's generous TDR throttle it back enough to avoid short cycling? Seems like overkill. Our house is a small, well insulated single story in the Denver area. Would a Navien NHB110 be a better choice to promote longer cycling, condensing and lower natural gas bills?

dko

It's possible tech is avoiding water tube Naviens. Navien fire tubes are top notch, but unfortunately start at 175.

While it does have a great TDR, I would say it's still grossly oversized and you would be paying quite a premium for no reason.

Hot_water_fan

I’d get a similarly sized 80kbtu replacement, no sense oversizing.

Teemok

Unused capacity is not necessarily over sized with modcon's. When the boilers minimum firing rate is much larger than the minimum load, that is one way to over size. I like the NFB. The larger heat exchanger running at low firing rate in condensing mode is a good thing. 80K @ 5:1 turn down is a 16K min. 175k @ 13.5:1 turn down is a 12.9K min. The NHB is not better. It's apples to oranges different. On cold starts that NFB will quickly put BTU's into cold mass like a beast and then genitally back off as the return water temp rises. What's bad about that? I have one in a large 1905 radiator craftsman un-insulated house. It works great for the client who valves off 80% of the emitters 80% of the time. When the house is full and the whole system is brought on line it heats up fast. A stable burning large turn down ratio is an attribute of a great modern boiler. It means max. firing rates are not as important as they used to be, given it meets max load.