ACHR News has run a product item on the Navien NCB700, and its report on the Navien NCB700 describes the unit as "providing heating and hot water performance in a single wall-hung unit." The headline leads with a 96% AFUE figure, and the summary adds space-heating and hot water input ratings along with a hot water flow number. Those are the figures a homeowner will see first on a quote, so they deserve the same reading a technician gives a spec sheet before it goes into the job file.
Treat AFUE as a Test Result
AFUE, or annual fuel utilization efficiency, is a rating from a standardized test that estimates how much of the fuel energy a boiler turns into delivered heat over a cycle. It is not a promise about your house. Piping layout, return water temperature, and how often the boiler cycles all shift what you actually get. For a combination unit, the number also depends on how the test treats the domestic hot water draw, so ask the installer which procedure produced the figure on the sheet. Keep the 96% in your file, but read it as a ceiling rather than a forecast.
Check the Flow Against the Input
The summary gives a space-heating input of up to 150,000 Btuh, a domestic hot water input of up to 210,000 Btuh, and a flow of up to 5.4 gallons per minute at a 70 degree Fahrenheit temperature rise. For water, the working relation is Btu per hour equals 500 times gallons per minute times the temperature rise. Put 210,000 Btuh and a 70 degree rise into that relation and you get 6.0 gpm, assuming every Btu of input reaches the water. The published 5.4 gpm sits below that ideal figure. The gap may reflect efficiency losses and test conditions, but do not guess at it. The sheet should state them, and you should ask which ones apply.
Count the Draws, Not the Fixtures
A flow rating is only useful when you match it to the taps you open at the same time. A modern showerhead commonly runs near 2 gpm, and a kitchen faucet often runs lower, so one shower plus one sink fits under 5.4 gpm on paper. A second shower in the house, or a utility tap opened during a shower, pushes the total past the rating. Write down your busiest morning: who showers, who runs the dishwasher, and whether anyone fills a tub. That list is your real flow requirement, and it is the number to compare with the sheet.
Keep the Burner Budget in View
The space-heating and hot water figures are separate maxima. They are not a shared budget that the burner splits evenly. When a tap opens during a heat call, the controls decide which demand gets priority, and that choice changes how the house feels on a cold morning. Ask the installer to explain that priority in plain terms and to show you where it is set in the control menu, so you can see it working rather than trusting a description.
Verify the Model Before the Crew Arrives
Paper claims are easy to copy and harder to audit. ACHR News also reports on HARDI's warning about non-compliant equipment, covering potentially illegal HVAC equipment and the red flags contractors should watch for before a unit goes in. Apply the same habit at home. Match the model number on the unit to the manufacturer's current documentation, confirm the rated values against the spec sheet, and ask where the paperwork will be kept once the job closes.
What to Ask Before You Sign
Put these questions in writing before the purchase order is signed. Which test procedure produced the AFUE figure, and does it include domestic hot water? Which gas type and altitude does the input rating assume, and does your site match? Which flow figure is the manual's basis, and at what temperature rise? Where do the venting and condensate requirements appear in the install plan? Who owns the model-number check before the first firing? A written answer to each one tells you more than the headline ever will.