ACHR News covered Navien's NCB700 combi-boiler announcement, which describes heating and hot water performance in "a single wall-hung unit." That layout suits a tight utility room, but it also moves a set of water problems onto the homeowner's side of the job. A condensing appliance makes water of its own, and a combi heats the water your house delivers. Before install day, walk both water paths from the wall to the street.
The condensate is part of the appliance
A condensing boiler pulls extra heat out of the exhaust, and that heat removal produces condensate. This liquid is acidic, usually somewhere between three and five on the pH scale, so it cannot go into just any pipe. Read the installation manual for the required drain slope and for whether a neutralizer is called out. Some manufacturers and local jurisdictions require one, and others leave the decision to the installer. Either way, ask the installer where the condensate will exit the house before the old boiler comes off the wall.
Common exits behave differently. A floor drain, a laundry standpipe, and a dedicated line each handle a clog, a backup, or a freeze in its own way. A standpipe that shares a trap with the washer can siphon or gurgle when the washer drains. A floor drain can hide a slow leak until you smell it. Pick the route on purpose and write it down.
Route the drain for the coldest week
An outdoor condensate run or a drain line through an unheated crawlspace will freeze during the first cold stretch that matters. Walk the route with a flashlight and a tape measure. Look for low sags where water can pool and freeze into a plug, and check that the line falls continuously toward the exit. Where the run must cross cold space, ask whether heat tape or insulated pipe is specified, who supplies it, and who checks it at the first cold snap. A frozen drain usually shows up as a shutdown on a cold morning, which is the worst possible time to find it.
Do not forget the pressure relief discharge. Its termination route is a separate question from the condensate, and it deserves the same attention. Ask where it will discharge, and make sure the path is visible and safe from the people who use the space.
Test the incoming water before the heat exchanger does
Domestic hot water in a combi is heated on demand, so the water quality feeding it shapes the life of the heat exchanger. Heavy hardness deposits scale where the water runs hottest. Before the install, log your incoming water with a basic test strip or the local utility's water report. Note the hardness and any visible sediment, and keep that record with the equipment file. If the numbers are high, the bid should say whether a softener, a scale inhibitor, or a maintenance interval is part of the plan.
Confirm the cold supply includes a filter or strainer where the manufacturer asks for one, and check that the strainer is reachable without cutting drywall. Ask whether the installer will flush the unit at commissioning and whether that flush is listed on the paperwork.
Check the electrical side with a meter
With the appliance off, use a multimeter to confirm the dedicated outlet reads close to its expected line voltage. Then ask the installer to verify voltage under load at startup, because a sagging circuit that looks fine at rest can behave differently when the unit fires. Note the breaker size and wire gauge on your own list, so you can compare them against the nameplate when the crew arrives. This is ordinary homeowner diligence, and it costs only a few minutes.
Put the water plan on the order
A product page tells you what the unit does. It does not tell you whether your drain runs downhill, whether your water is hard, or whether your pressure relief has a safe place to discharge. Those answers live in your house, and you are the one who has to collect them before the crew arrives. Ask for the condensate route, the neutralizer decision, the freeze protection, the water-quality plan, and the flush procedure in writing. Put them on the same sheet as the model number. If a bid cannot answer those questions, it is not finished yet.