A combination boiler, or combi, puts space heating and domestic hot water into one wall-hung cabinet with no storage tank. The same burner that feeds your radiators or air handler also has to answer the faucet when someone opens a tap, so the spec sheet deserves two separate checks instead of one.
ACHR News reports on Navien's NCB700, which is headlined at 96% AFUE. The writeup lists space-heating inputs of up to 150,000 Btuh and domestic hot water inputs of up to 210,000 Btuh, and describes the product as "providing heating and hot water performance in a single wall-hung unit." Read the way a tech would read it, that one line hides two loads on one burner. See the ACHR News Navien NCB700 report for the source details.
Treat the inputs as two loads
Input is what the burner draws, not what reaches the room or the tap. AFUE is a seasonal fuel-to-heat ratio, so it says nothing about whether the space-heating side is big enough for your house on a cold January night. Run a heat-loss calculation for the house by itself, compare it to the space-heating input, and then build the domestic side from the fixtures you actually run together. Two showers, a dishwasher, and a kitchen tap is a common worst case to test against.
Input ratings also depend on the install conditions. Ask the installer which altitude and gas supply pressure the rating assumes, and confirm the gas meter and supply line can carry the full input when both sides call for fuel at once. Two large inputs ask more of the gas side than a single old boiler did, and the supply piping is the part nobody sees until the burner runs short.
Do the flow arithmetic on a notepad
The summary cites hot water flow of up to 5.4 gallons per minute at a 70°F temperature rise. For water, a useful rule of thumb is gpm ≈ Btu per hour ÷ (500 × temperature rise). Run it backward and 5.4 × 500 × 70 comes to about 189,000 Btu per hour delivered to the water. That figure only means something at the rise it was rated for.
If your incoming cold water runs colder in winter, the rise you need goes up, and the flow you actually get drops. Ask the installer what cold-water temperature the flow figure assumed, then measure your own. Put a thermometer on the cold supply, time how long a known volume takes to fill from a hot tap, and convert that to gallons per minute. If your measured number sits well below the rated flow at your winter inlet temperature, you have learned something before the install rather than after.
Ask who wins when two demands collide
When a tap opens, many combi controls pause space heating to serve domestic hot water. Ask the installer how the unit prioritizes demand, whether it pauses every zone or only some, and how the controls recover heating once the tap closes. A long shower during a cold morning is the test that shows whether the priority logic fits your household, so ask how the system behaves in that case, not just in the brochure example.
Check the vent route before the hot water question
Combi venting, whether sidewall or through-roof, depends on the wall and roof you already have. Any flue or intake that passes through a roof deck is a roofing job, so have a roofing contractor such as Keys Roofing seal and flash it, and keep the penetration clear of where snow and ice slide off. Do not let the boiler installer and the roofer assume each other has handled the seal.
Get the numbers signed
ACHR News also published a piece titled "A Consulting Engineer's Role in a Design-Build Delivery Model," which states that "Design-build is reshaping, not eliminating, the role of consulting engineers." If your boiler comes through a design-build contract, the same firm may both sell the unit and pick its size. That is not a problem by itself, but it means the load and flow figures need an owner. Read the ACHR News design-build article for the broader context on how engineers fit into that model.
Before the order goes in, ask who signs the heat-loss result, the domestic flow figure at your winter inlet temperature, and the vent layout. Get the model number, both input ratings, and the rated flow and temperature rise on paper. A combi that matches the spec sheet on both sides of the burner is the outcome you want, and a written record is how you prove it later.