ACHR News has a write-up on the Navien NCB700, a combi-boiler with a 96% AFUE rating that provides heating and hot water in \"a single wall-hung unit\" (ACHR News, New Navien Combi-Boiler Boasts 96% AFUE). The efficiency number describes the burner and heat exchanger. It says nothing about the water side, where a closed heating loop depends on pressure you can check yourself.
Why the Loop Needs a Tank
Water expands as it heats. In a closed hydronic loop there is nowhere for that extra volume to go unless the system provides room for it. The expansion tank does that job. Most residential tanks are diaphragm types: a steel shell with a rubber bladder, water on one side of the bladder and air on the other. As the loop heats, water pushes into the tank and compresses the air. When the loop cools, the air pushes water back. The tank holds system pressure within a band instead of letting it climb until the relief valve opens.
Read the Gauge Cold
Start with the system cold. Find the fill pressure gauge, usually on the piping near the fill valve, and write down the reading. Compare it to the figure in the manufacturer's manual or on the nameplate. That cold reading is your baseline. Then call for heat and watch the gauge through a full cycle. A healthy loop rises a little as it heats and settles back as it cools. A gauge that climbs steadily with no call for heat, or that pushes toward the relief setting during a run, is telling you something important.
Test the Air Side of the Tank
A diaphragm tank only works if its air charge matches the system's design. Follow the manual's shutoff steps, isolate the tank, and drain the water side to zero pressure. Then press a tire gauge onto the Schrader valve on the tank's air fitting. Compare the reading to the manufacturer's precharge figure for that tank and the system height. If the air is low, add air with a pump. If the gauge swings more than it should and the tank feels heavy and full of water, the bladder has probably failed, and more air will not fix it. Replace the tank.
Know What the Relief Valve Is Telling You
Look at the relief valve and its discharge pipe. A weeping valve, a wet stain under the discharge, or mineral crust around the seat all point to pressure that got too high at some point. Do not cap the valve or block the discharge. Leave the discharge routed the way the installer set it up. If the valve drips during heating, record the time and the gauge reading, then have a licensed technician find the cause. A relief valve is the last safeguard when the tank cannot keep up, so plugging it hides the problem.
Watch the Fill Count
If you top up the loop with the fill valve every few weeks, you are not maintaining the system, you are chasing a leak. Makeup water brings fresh minerals and oxygen into the loop, which speeds corrosion. Track how often the fill valve runs and how far the pressure drops between checks. A small drop that levels off after a bleed may be trapped air. A steady loss that keeps coming back needs a leak search, starting at the lowest point in the piping and working through the fill valve, the pump seal, and the packing on radiator valves.
Make It a Seasonal Check
Pick one day before the first heat call of the year. Record the cold-fill reading, the tank precharge when you isolate it, and the condition of the relief valve and its discharge. Keep the numbers with the boiler's manual. Next year's readings will tell you more than any single check. A new combi can look clean on paper, but it is only as dependable as the water it heats, so give the water side the same attention you give the burner.