The first portion of the tankless water heater inspection is performed in the bathroom. Turn on the hot water at the sink and the tub simultaneously and look at the flow. If the flow appears to be low, mineral deposits might have built up in the tankless coil, and maintenance might be needed. On the other hand, the low flow might reflect a constriction in the distribution piping. Record the fact that there is a low flow on your worksheet as a reminder that the condition must be corrected regardless of the cause.
At the boiler, look at the joints around the tankless heater mounting plate. This is a vulnerable area for leakage. If there are water stains and deposits around the joint, even if you do not see water dripping, maintenance is needed. The deposits tend to self-seal the leak, but this is not a permanent fix. Any movement or vibration of the boiler can cause the deposits to loosen, reactivating the leak. Often, all that is needed is tightening of the mounting bolts around the joint or replacing the gasket around the joint. Look at the inlet and outlet pipes to the heater and the mixing valve (if there is one) for signs of past or current leakage.
The temperature of the boiler water that heats the domestic hot water is controlled by an aquastat. You can check the operation of the aquastat by opening one of the hot-water faucets. After a while, the temperature of the boiler water will drop to a point where the aquastat will cause the oil or gas burner to fire. Tankless coils are designed to operate with a boiler-water temperature of about 200° F. Look at the boiler-mounted thermometer when the burner fires. If the boiler-water temperature is less than 180° F, the aquastat may be in need of adjustment. In some homes, the aquastat is intentionally set lower. This, of course, will result in a lower hot-water temperature.
Look for a temperature-pressure relief valve on the hot-water outlet line of the tank-less coil. If there is a storage tank associated with the tankless coil, the relief valve might be located on the tank. A temperature-sensitive relief valve is a necessary safety control and is normally factory-set to discharge when the domestic hot water reaches a scalding temperature. If you do not find a relief valve, record the fact on your worksheet as a reminder to have one installed after you move in.