Boiler Pressure Too High or Too Low? What the Gauge Means and What to Do

On a home hot-water (hydronic) boiler, the gauge should read about 12 psi when the system is cold and rise a little as the water heats, staying well under the relief valve's setting, which is 30 psi on many residential boilers. Weil-McLain's manual gives 12 psig as the normal cold fill pressure for residential systems, and its user's manuals say the reading should not exceed 24 psig; Slant/Fin expects about 12 to 15 psi at 70 to 100°F and 15 to 25 psi as the water heats toward 240°F. A reading well below 12 psi cold usually means the system has lost water, from a leak or from bleeding radiators. A reading that climbs toward 30 psi usually points to the expansion tank or a fill valve, and that's a call to a technician.

The exact number for your house is in your boiler's manual; Weil-McLain notes that the correct pressure varies with each installation.

Reading the Boiler Gauge

Most hot-water boilers have a combination gauge, often called a tridicator, on or near the boiler. Boshart, a gauge maker, describes it as one dial that shows pressure, temperature, and altitude, or pressure in psi and in feet of water.

• Use the psi scale, not feet of water or kPa.
• Read it cold, then hot: first thing in the morning, then after the heat has run a while. A rise of a few psi is normal (Slant/Fin).
• Write the numbers down. A cold reading that falls over days, or a hot one that creeps up, tells the technician more than one reading does.

Boiler Pressure Too Low

Low pressure often shows up as radiators cold at the top, gurgling pipes, or upstairs rooms that don't heat; Weil-McLain lists low system pressure as one reason some radiators don't heat.

Common causes

• You just bled the radiators. Each one lets out a little water with the air (see below).
• A leak: a fitting, a valve stem, an air vent, or a dripping relief valve. Weil-McLain says a system that often needs makeup water has leaks a technician should repair at once.
• The fill valve isn't adding water. Many systems have an automatic fill valve (a pressure reducing or feed valve) that tops the system up from the house's water line; Bell & Gossett's dual unit valve comes set at 12 psi. Caleffi has technicians check that its screen isn't clogged and the valves feeding it are open.
• A failed expansion tank. Caleffi says a waterlogged expansion tank can let the system fall into a vacuum (negative pressure) as the water cools.

What you can safely check

1. Read the gauge cold and write it down.
2. Look for water. Check under the boiler, along exposed pipes, at radiator valves, and at the end of the relief valve's discharge pipe. Caleffi says a bucket under that pipe that holds water, or a mineral trail on the floor below it, is a sign the valve has been leaking or discharging.
3. Follow your boiler's manual for topping up. Weil-McLain's user's manual for the GV90+ says to fill to the correct pressure and check for leaks; Slant/Fin's says to call your service organization if pressures are higher or lower than normal. If your manual doesn't explain how to add water on your system, or you can't find the fill valve, leave it to the technician.
4. Never add cold water to a hot boiler. Weil-McLain warns the thermal shock can crack the heat exchanger. Let it cool first.
5. Don't overfill or pour in stop-leak. Extra water pushes the hot pressure toward the relief valve, and Weil-McLain says not to use petroleum-based stop-leak compounds.

Pressure that keeps dropping means a leak, and Weil-McLain says continual fresh water causes mineral buildup and corrosion. Peerless adds that an automatic water feeder can mask the problem, so a normal reading doesn't rule out a leak.

Boiler Pressure Too High

Water expands as it heats, and the expansion tank absorbs it. When the reading rises past 24 psi (Weil-McLain) or the relief valve drips, the tank isn't doing that job, or extra water is getting in.

Common causes

• A waterlogged or failed expansion tank. Weil-McLain's troubleshooting charts list an expansion tank that's too small, waterlogged, or flooded as the cause of frequent water release through the relief valve. Caleffi adds a ruptured diaphragm, and a tank whose air charge doesn't match the system's cold fill pressure. Bell & Gossett says a diaphragm tank slowly loses its air charge, and if it gets low enough the system takes on water. (This is the tank on the heating system. The thermal expansion tank on a water heater is a different part; see missing expansion tank.)
• A fill valve that won't shut. Bell & Gossett lists a pressure reducing valve that fails open as a top reason relief valves discharge: scale and mineral buildup on its seat can keep it from closing, so house water pressure keeps feeding the boiler. Caleffi lists debris in that valve too.
• A leaking domestic hot water coil. House water pressure is higher than boiler pressure. If your boiler makes hot water through a coil inside it, Bell & Gossett says corrosion can let the cold water line's pressure into the boiler, which keeps the relief valve open and discharging.
• A faulty gauge. Weil-McLain says a high reading can also mean a problem with the gauge itself, which a technician can check.

The relief valve dripping or discharging

The relief valve protects the boiler from overpressure; Weil-McLain's WGO manual says it discharges if boiler pressure exceeds 30 psig. A drip means the pressure is reaching its setting or the seat is dirty (Caleffi).

• If it weeps often: Weil-McLain says the expansion tank may not be working properly and to contact your qualified service technician immediately.
• If it discharges water: Peerless says to turn off the gas to the boiler following the lighting instructions on the boiler and contact a qualified service agency.
• Stay clear of the discharge pipe; the water can scald.

Pressure Drop After Bleeding Radiators

Every radiator you bleed lets out a little water with the air, so the gauge can dip a few psi. That's expected; our radiator-bleeding guide covers the steps.

• With an automatic fill valve, the pressure usually comes back up on its own. Weil-McLain's start-up steps, after venting each radiator, are to refill to the correct pressure.
• Without one, or if the reading stays low, follow your owner's manual for adding water or have a technician do it. Let the system cool first.
• Vent again once it has run. Weil-McLain's manual calls for venting air again after the boiler is placed in operation.
• If you need to bleed often and the pressure keeps falling, that's a leak, not an air problem.

If a radiator stays cold with the pressure right, see radiator not heating; for knocking pipes, see banging pipes.

What a Heating Technician Does

• Checks the expansion tank: that it's sized for the system and its air charge matches the system's cold pressure (Caleffi), then recharges or replaces it.
• Tests the fill valve, which can stick open from scale on its seat (Bell & Gossett).
• Checks for a leaking domestic hot water coil, one of the causes Bell & Gossett lists for a relief valve that keeps discharging.
• Repairs leaks in the boiler and piping, and checks the relief valve and gauge (Peerless, Weil-McLain).

Weil-McLain says to have the boiler started up and serviced at least annually by a qualified technician; our winter guide covers the rest of the season. If you'll be away while the heat is out, keep the house no lower than 55°F (American Red Cross).

Frequently Asked Questions

What should my boiler pressure be?

On a residential hot-water boiler, about 12 psi cold, rising as it heats but not over 24 psi (Weil-McLain). Your boiler's manual has the exact number. Steam boilers read zero cold and usually 2 to 3 psi running (Hoffman).

Why does my boiler pressure keep dropping?

The system is losing water through a leak, and Weil-McLain says a technician should repair it at once. An automatic fill valve can mask the loss (Peerless), so watch for water stains and a wet relief valve pipe.

Why does my boiler pressure rise when the heat is on?

A small rise is normal, since water expands as it heats. A big rise, past 24 psi (Weil-McLain), usually means the expansion tank is waterlogged, undersized, or has lost its air charge. If the pressure also climbs while the boiler is off, more water is getting in, from a fill valve that won't shut or a leaking domestic hot water coil (Bell & Gossett).

Is high boiler pressure dangerous?

It can be, which is why boilers have a relief valve, set at 30 psi on many residential boilers. If the pressure nears that or the valve discharges, turn the boiler off per its lighting instructions and call a technician (Peerless). Never cap or plug the relief valve.

Can I bleed my boiler to lower the pressure?

Not to fix high pressure. The cause is usually the expansion tank or fill valve, so the pressure comes back, and draining a boiler means handling hot, pressurized water. Leave it to a technician.

The Bottom Line

On a hot-water boiler, about 12 psi cold and no more than about 24 psi hot is normal for most houses. Low pressure usually means lost water: top up per your manual with the boiler cool, and call if it keeps dropping. High pressure usually means the expansion tank or fill valve, and a dripping relief valve is a warning, not a nuisance. If the gauge nears 30 psi or the relief valve discharges, turn the boiler off and call. Never cap the relief valve, and if you smell gas, get out and call from outside.

Sources