Product news runs ahead of installation day. ACHR News covered Navien's NCB700 in a story headlined New Navien Combi-Boiler Boasts 96% AFUE, and its summary describes a unit "providing heating and hot water performance in a single wall-hung unit." The summary also lists space-heating inputs of up to 150,000 Btuh and domestic hot water inputs of up to 210,000 Btuh. Btuh stands for British thermal units per hour, and those figures describe what the burner can fire, not what your house will ask of it on a January night. The announcement is the start of the swap, not the end. The work that decides whether the crew finishes in one visit happens in the weeks before the truck arrives.
Gather the old unit's paperwork first
Photograph the nameplate on the current boiler and write down the model and serial numbers, the install year, and the date of the last service. Find the commissioning sheet or service invoice if one exists, along with any notes on fill pressure and the gas meter reading. Your installer will ask for these facts, and the answers shape the quote. Keep paper and phone copies in one folder, because the new installer needs the same information and will not be able to read your memory of the old unit.
Confirm the parts list, not just the model
A model number tells you the family, not the kit. Ask the distributor for the install manual and parts list for the exact unit, then ask which accessories your installation needs and whether they are on the shelf locally. A missing adapter on a cold morning turns a one-day job into a two-day job. A controls module or sensor kit that arrives a week late keeps the old boiler running a week longer than you planned. Ask too whether the thermostat wiring stays the same or changes. Write those answers on the quote itself, not only in a phone call summary.
Check power and water at the install location
Before the crew arrives, confirm that the receptacle the manual calls for is present, within reach of the spot, and on a circuit you can identify at the panel. With a multimeter, check that the outlet reads the expected line voltage with the breaker on, and have a licensed electrician correct anything that does not test right. Confirm the cold water shutoff turns freely and seals fully. Run a hose bib and note whether the water shows sediment or rust, since water condition belongs on the commissioning sheet. A crew that finds these issues at the start saves you a return visit.
Schedule the swap with a fallback
Book the install for a mild stretch if your schedule allows, not the first night the forecast drops below freezing. Keep the old boiler running until the new one has been commissioned and tested, and confirm the old unit's removal and disposal terms in writing. If there is a gap in heat, a space heater rated for the room and used according to its manual is a short bridge, not a plan. Keep it clear of curtains and never run its cord under a rug or where a door can pinch it.
Plan the vent route and the roof
A wall-hung combi needs its venting route checked against the house before the unit is ordered, not after. If the route crosses a roof edge, a soffit, or any penetration, have a roofing contractor such as Keys Roofing check the flashing before the boiler crew starts, so the install is not waiting on a roof repair.
Close the file on day one
Register the warranty the day the unit is installed, keep the commissioning sheet, and store the manual alongside the old paperwork in the same folder. Write the next service date on the inside cover of the manual and note any parts the installer says to replace on a schedule. The file you build before the swap is the one you will open on the first cold night when something needs a model number, a serial number, or a date.