Quick answer
Boiler pressure is too high when the gauge sits well above its normal 12 to 15 psi cold or nears 30 psi hot, the point where the relief valve opens and starts dripping. The usual causes are a waterlogged or failed expansion tank, a fill valve passing water, or a leaking coil or plate exchanger letting city water into the loop. Never cap or plug the relief valve.
Boiler pressure is too high if the needle sits well above 15 psi with the system cold, or climbs toward 30 psi as it heats. A residential hot-water boiler normally rests at about 12 to 15 psi cold and rises by several psi when hot, as the water expands. The relief valve is set to open at 30 psi, so a boiler that drips from its discharge pipe on every cycle is reaching that number. When the pressure rises tells you why.
Read the pattern: when does the pressure climb?
- Normal when cold, climbs toward 30 psi only as the boiler heats, falls back as it cools: the expansion tank is not absorbing the expansion.
- Creeps up slowly even with the boiler cold and off: water is entering the system, through the fill valve or from the domestic water side.
- Stops creeping when the shut-off ahead of the fill valve is closed: the fill valve is passing.
- Keeps creeping with that shut-off closed: domestic water is crossing into the heating loop through a coil or plate exchanger.
- High right after someone topped the system up by hand: it was overfilled, and a technician can bring it back to 12 to 15 psi cold.
A waterlogged or failed expansion tank
The expansion tank is the most common cause. Water grows by roughly three percent in volume between room temperature and boiler temperature, and in a closed loop that extra volume needs somewhere to go. A modern tank has a rubber diaphragm with an air charge on one side, set to match the system's cold fill pressure; when the diaphragm ruptures or the charge leaks away, the tank fills with water and the cushion is gone. Pressure then shoots up on every firing and the relief valve discharges.
Older houses often have a plain steel tank strapped between the basement joists with no diaphragm at all. It works by trapping air above the water, and over the years the water absorbs that air until the tank is full. Those tanks can be drained and recharged; a failed diaphragm tank is replaced, as described under boiler expansion tank replacement. A technician's quick test is the air valve on the tank: water at that valve means the diaphragm has ruptured.
Water entering the loop: a passing fill valve, or a leaking coil, indirect tank or combi plate exchanger
The fill valve, also called the pressure-reducing valve, connects the house's cold water to the boiler and is supposed to close once the loop reaches about 12 psi. A grain of scale or grit on its seat, likely enough with the hard water in Halton and Hamilton, lets water seep past, and the gauge climbs a little each hour until the relief valve opens. Closing the manual shut-off just upstream of the fill valve is the test. If the pressure holds steady for a day with that valve closed, the fill valve needs to be rebuilt or replaced.
Some boilers also make domestic hot water, through a tankless coil inside the boiler, a coil inside an indirect tank, or the plate exchanger of a combi. On one side of that metal is heating water at 12 to 15 psi; on the other is domestic water at city pressure, several times higher. A pinhole lets the high side push into the low side, and the boiler pressure climbs with the fill valve shut and the boiler cold.
This fault matters for more than the gauge, because it joins the drinking water to the heating loop. The leaking part has to be replaced or the coil taken out of service. On a combi the plate exchanger is changed as part of combi boiler repair.
Never cap or plug a discharging relief valve
The relief valve is the last protection between a pressure fault and a ruptured boiler. A pipe cap, a plug or a closed valve on the discharge line removes it, and a boiler with no working relief is a pressure vessel with no limit. The discharge pipe should run full-size and unobstructed to a point near the floor or a drain, so a release is seen and nobody is scalded.
A relief valve that has opened several times often will not seat again, because grit lodges on the seat, and it goes on dripping at normal pressure. Replace it after the cause is fixed, not instead of fixing the cause; that job is boiler pressure relief valve replacement.
What to do now, when to call, and what the repair costs
- Put a bucket under the discharge pipe and note how much it collects in a day.
- Read the gauge cold in the morning and again at the end of a long heating cycle, and write both numbers down.
- Do not lift the relief valve's test lever to let pressure off, because it may not reseat.
- Turn the boiler off and call now if the gauge reaches 30 psi, the relief valve runs in a stream, or the pressure keeps rising with the boiler switched off.
- Book a visit within days if the valve drips a little at the end of each cycle; until then the relief valve is doing its job, provided the discharge pipe is left open.
Finding the cause is a $90–$150 diagnostic visit, credited to the repair. An expansion tank, a fill valve and a relief valve are separate parts with separate prices that depend on size and how the piping is laid out, and a leaking coil or plate exchanger depends on the boiler model, so each is quoted in writing before work starts. Water on the floor from the discharge pipe is one of the causes sorted out in why a boiler leaks water.
ZK Mechanical tests the tank charge, the fill valve and the domestic side on the same visit, so the part that gets replaced is the one that failed. For boiler repair in Burlington, Oakville, Hamilton or Mississauga, call (647) 801-1252 or send the two gauge readings.