Navien’s model numbers look cryptic until you know the pattern, and then sizing a unit takes about five minutes. This guide explains what NPE, NCB and NFB mean, and how to work out which one your house actually needs.
Decoding the model names
- NPE – a tankless water heater. Hot water only. This is what most homes replacing a tank want.
- NCB – a combi-boiler. Hot water and space heating from one wall-mounted unit. Two BTU figures are quoted: domestic hot water, then heating.
- NFB – a fire tube boiler. Heating only, for hydronic systems. Not a water heater.
The S and A suffix on NPE models is the one people miss. S is the standard unit. A adds a built-in recirculation pump and buffer tank, which cuts the wait for hot water at distant taps. If you have ever stood at a far bathroom counting to twenty, you want an A.
Sizing by flow, not by house size
Tankless units are rated by how much hot water they can raise to temperature at once. Two numbers decide it: how many fixtures run simultaneously, and your incoming groundwater temperature.
Rough fixture demand in gallons per minute:
- Shower – 1.5 to 2.5 GPM
- Kitchen sink – 1.5 GPM
- Dishwasher – 1.0 GPM
- Washing machine – 2.0 GPM
Add up what realistically runs together. Two showers and a kitchen sink is about 6 GPM.
Why your climate changes the answer
This is the step most people skip. A tankless heater raises incoming water to about 120°F. In southern states groundwater arrives around 70°F, so it only has to add 50 degrees. In the northern US it can arrive near 40°F, needing an 80-degree rise – and the same unit delivers substantially fewer gallons per minute.
A heater that comfortably runs three showers in Texas may manage two in Minnesota. Size for your winter, not the brochure.
Matching a model
- NPE-150S2 – 120,000 BTU. Apartments, small homes, one to two bathrooms in a warm climate.
- NPE-180S2 / 180A2 – 150,000 BTU. The typical two to three bathroom family home.
- NPE-210S2 / 210A2 – 180,000 BTU. Three-plus bathrooms, or a cold-climate home that would otherwise pick the 180.
- NPE-240S2 / 240A2 – 199,000 BTU. Large homes, heavy simultaneous demand, or northern winters.
When you fall between two sizes, go up. Oversizing costs a little more to buy and nothing to run – these units modulate down. Undersizing means cold showers every January.
When you want a combi-boiler instead
If you heat with hot water – radiators or in-floor – an NCB combi replaces both your boiler and your water heater with one wall unit, freeing a remarkable amount of basement. The two BTU figures quoted are hot water first, heating second, so an NCB-190/080H gives 160,000 BTU of hot water and 80,000 BTU of heat.
Size the heating figure to your home’s heat loss, not its square footage. That calculation is worth paying a contractor for.
Installation realities
Be honest about these before you buy:
- Gas line – a 199,000 BTU unit often needs a larger supply line than a tank heater used.
- Venting – condensing units vent through PVC, which is cheap, but the run has to be planned.
- Condensate drain – condensing units produce acidic water that needs a drain and usually a neutraliser.
- Hard water – descaling annually is not optional above roughly 7 grains of hardness.
This is a licensed-plumber job in most jurisdictions, and permits usually apply.
Browse the range
See all 15 models in Navien – tankless heaters, combi-boilers and fire tube boilers, shipped free across the lower 48.
Tell us your bathroom count and your state and we will tell you which model fits – and when a smaller one is genuinely enough.