A dripping relief valve on a Bosch tankless water heater is a pressure or temperature safety response worth investigating promptly, not a part to simply plug or ignore.
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Quick answer
A relief valve on a Bosch tankless water heater drips or discharges when it is responding to excess pressure, excess temperature, or a worn valve seat, and it should never be plugged or capped. Common causes include high municipal supply pressure, thermal expansion in a closed plumbing system, a scale-fouled valve seat, or a valve nearing the end of its service life. A licensed technician tests actual system pressure and the valve itself to determine which applies before recommending a fix.
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The temperature and pressure relief valve, or a pressure relief valve on some tankless configurations, is a safety device designed to open and discharge water if pressure or temperature inside the system rises above a safe threshold. It is not optional equipment and it should never be capped, plugged, or have its discharge line blocked, because doing so removes a critical safety protection.
A small amount of occasional dripping can be a normal response to minor pressure fluctuation, but a valve that drips steadily, discharges repeatedly, or lets go with force is signalling that something upstream needs attention. Treating the valve as the problem and simply replacing it without finding the underlying cause often means the new valve starts dripping again shortly after.
Because a tankless unit heats water on demand rather than storing it, the pressure dynamics differ somewhat from a tank-style heater, but relief valve discharge is still most often tied to the plumbing system around the appliance rather than a defect in the tankless unit itself.
A proper diagnosis starts with measuring actual static water pressure at an outdoor hose bib or the main shutoff, since Metro Vancouver municipal pressure can run higher than ideal in some areas, particularly at lower elevations closer to pump stations. If pressure is elevated, a pressure-reducing valve on the main line is often the real fix, not a new relief valve.
The technician also checks whether the home has a check valve, backflow preventer, or water meter with a built-in check that creates a closed plumbing system, which requires a thermal expansion tank to safely absorb the volume increase as water heats. Without one, a relief valve on an otherwise healthy heater will keep discharging as a normal pressure-relief response to thermal expansion.
Finally, the valve itself is tested and inspected for scale, debris, or a failing seat, and the unit's set temperature is confirmed against manufacturer guidance and the home's fixture ratings.
Most relief valve discharge on a Bosch tankless system is resolved by addressing the underlying pressure issue: installing or servicing a pressure-reducing valve, adding a thermal expansion tank, or replacing a worn valve, rather than replacing the water heater itself. The tankless unit is typically unaffected mechanically by this issue as long as it is addressed promptly.
Replacement of the appliance becomes relevant only in the rare case where repeated high-pressure events have damaged internal components, which a technician can confirm during inspection of the heat exchanger and related parts.
| Pattern observed | Likely cause | Typical fix |
|---|---|---|
| Drips only after hot water use | Thermal expansion, no expansion tank installed | Install thermal expansion tank |
| Constant slow drip regardless of use | Elevated municipal pressure or worn valve seat | Pressure test main line, replace valve if needed |
| Forceful or repeated discharge | High pressure combined with a closed system | Pressure-reducing valve plus expansion tank |
| Drip stops after tapping/moving valve lever | Debris temporarily lodged in valve seat | Full valve replacement recommended, not a permanent fix |
Because the relief valve is a safety device, it should only be inspected, tested, or replaced by a Technical Safety BC licensed gas fitter, particularly given the gas and combustion components integrated into a tankless unit. CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas near the appliance, leave the home and call FortisBC at 1-800-663-9911.
Safety note
The temperature and pressure relief valve is a critical safety device and must never be capped, plugged, or have its discharge line blocked. Diagnosis and replacement of the valve, along with any gas or venting work on the appliance, must be performed by a Technical Safety BC licensed gas fitter. If you smell gas, leave the home and call FortisBC at 1-800-663-9911.
No. The relief valve is a safety device that prevents dangerous pressure or temperature buildup, and blocking it removes that protection entirely. A dripping valve should be diagnosed, not sealed.
This pattern typically points to thermal expansion in a closed plumbing system, where heated water has nowhere to expand except through the relief valve because a check valve or backflow preventer traps it upstream.
Yes. Municipal water pressure varies by elevation and area across Metro Vancouver, and pressure above recommended plumbing code limits is a common trigger for relief valve discharge.
If your home has a check valve, backflow preventer, or certain water meters that create a closed system, a thermal expansion tank is typically required to safely absorb pressure from heated water expansion.
Not usually. In most cases the tankless heater itself is functioning correctly and the issue lies in system pressure or a worn valve, both of which are addressed without replacing the appliance.
It should be addressed promptly. Occasional light dripping is a signal worth investigating, and any forceful or continuous discharge should be assessed by a licensed technician as soon as possible.
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