Boiler pressure too high or keeps dropping: what to do

Read the gauge cold, then hot. The two numbers tell the story.

The answer in 2 lines. This guide is for hot-water (hydronic) boilers that heat radiators, baseboards or radiant floors, not steam boilers. On many homes the gauge reads about 12 to 15 psi cold and a bit more when hot; your boiler's manual or installer has the exact figure. Pressure that climbs toward 30 psi often points to the fill valve letting water in or a waterlogged expansion tank; pressure that keeps dropping usually means a leak. Write the readings down and call a licensed heating contractor; don't keep draining or refilling.

The first 3 steps

Read the gauge (often a round dial showing psi and temperature) with the boiler cold, then an hour after the heat comes on. Write both numbers down.
Look for water: under the boiler, around radiator and baseboard valves, and at the end of the pipe coming down from the relief valve on the side or top of the boiler.
Check whether your system has a manual fill valve that was left partly open after someone added water.

Stop: danger

Smell of gas (rotten eggs): don't flip switches, light anything or start the car. Leave the house, leave the door open, and once outside call 911 or your gas utility's emergency line (propane: your propane supplier).
Headache, dizziness, nausea or confusion, especially in several people at once or easing when you go outside, or a CO alarm sounding: think carbon monoxide. Get everyone outside into fresh air, then call 911 from outside. Don't go back in until responders say it's safe.
Gauge at or near 30 psi, or the relief valve spraying or pouring hot water: turn the boiler off at its service switch, keep away from hot pipes and call a heating contractor. Never cap or plug the relief valve pipe.
Steam boiler (one or two big pipes to each radiator, a glass tube showing the water level): it runs at very low pressure and works differently. Don't follow this guide; call a steam heating pro.

What the gauge is telling you

About 12 to 15 psi cold: common for a one- or two-story house. Taller houses may need more; your installer sets it.
A few psi higher when hot: normal, as hot water expands into the expansion tank.
Climbing toward 30 psi (the red zone on many gauges): too high. The relief valve is there to open before damage happens.
Dropping, again and again: water is escaping somewhere, or the automatic fill valve is closed or blocked.

InspectAPedia explains how to read the combined pressure and temperature gauge: Hydronic boiler gauges.

Likely causes

"It climbs even with the boiler off": the fill (pressure-reducing) valve may be letting city water in, or, on boilers that also make your hot water, a leaking domestic hot water coil.
"It shoots up every time the heat runs, then the relief valve drips": often a waterlogged or undersized expansion tank. Many modern tanks have an air charge that can be lost over time.
"It dropped a little after I bled the radiators": normal; on systems with an automatic fill valve it refills itself.
"It drops every week": a small leak (valve, fitting, relief valve) or a fill valve that's shut.

What not to do

Adding water over and over without finding the leak: fresh water brings in oxygen and minerals that corrode the system.
Opening the boiler cabinet, adjusting the fill valve or recharging the expansion tank yourself.
Draining water at random to lower the pressure on a hot system: you can be scalded.

What you can do yourself, and where to stop

Pressure a bit low, no leak in sight: if your system has a manual fill valve and the manual explains it, with the boiler cold, open it slowly until the gauge reaches the cold pressure your installer set, then close it fully.
Pressure slightly high when cold: some guides suggest letting a little water out by bleeding a radiator with the heat off and the system cold (example for a Weil-McLain boiler). Follow your own manual, not a general guide.
Stop there: pressure that keeps climbing, a dripping relief valve, water under the boiler, or topping up more than once a month. That's for a licensed heating (HVAC or plumbing) contractor.

Who pays

Homeowners: the repair is yours; homeowners insurance usually covers sudden water damage, not wear and tear (NAIC).
Renters: tell the landlord in writing. Heating repairs are generally the landlord's job; rules vary by state and city.
Condo or apartment building with a central boiler: tell the building manager or super, and don't touch the boiler room.

Good to know

Tape a card next to the boiler with the date and the cold and hot readings: the technician will find the cause faster.
A yearly tune-up before winter is the usual advice for fuel-burning boilers (ENERGY STAR, CPSC).
No official national statistics on boiler pressure problems were found as of November 1, 2026.

In Hartford, Connecticut, Daniel notices his boiler gauge goes from 14 psi cold to almost 30 psi each time the heat runs, and a trickle comes out of the relief valve pipe. He shuts the boiler off, writes down the numbers and calls a heating contractor, who finds a waterlogged expansion tank and replaces it. The pressure stays steady after that.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on November 1, 2026, for the United States. Confidence: high for the safety steps; medium for the typical pressures (12 to 15 psi cold, relief at 30 psi), which are common for homes but must be confirmed in your boiler's manual. This guide describes common cases, not a diagnosis.