Boiler Pressure Too High or Too Low: What the Gauge Is Telling You
Last updated Reviewed by Waseem Shahzad, Lead Technician
Smell gas or hear a carbon monoxide alarm? Leave the house first, then call 911 or the gas utility from outside.
What is normal pressure for a hot-water boiler?
Most residential hot-water boilers run at roughly 12 to 15 psi when cold and rise to around 20 to 25 psi when hot. The relief valve is usually set at 30 psi. Your boiler's label and manual give the exact numbers, so check them before deciding a reading is wrong.
The gauge on the front of the boiler usually shows pressure and temperature together. Read it twice: once when the boiler has been off for a while and once after it has run. Steam boilers are different: they run at very low pressure, measured in ounces or a pound or two, and you watch the water level in the sight glass instead.
What causes boiler pressure to drop too low?
Low pressure usually means water is leaving the system: a leak at a pipe, valve or radiator, a dripping relief valve, or a lot of air bled out. A water feed valve that has failed shut can stop the system from refilling. Low pressure leaves upper floors cold and can make the boiler shut down.
- Look for water stains or drips at baseboard, radiator valves and around the boiler.
- Check the relief valve discharge pipe for drips or a wet floor.
- If someone recently bled air from the system, pressure can drop until the feed valve refills it.
What causes boiler pressure to rise too high?
High pressure usually points to a waterlogged expansion tank, a water feed valve letting city water keep pushing in, or, on boilers that make hot water, a leaking domestic hot water coil. When pressure reaches about 30 psi, the relief valve opens and drips or discharges.
A failed expansion tank is the most common cause. The tank holds a cushion of air; when its bladder fails, there is nowhere for heated water to expand, so pressure spikes every cycle. A leaking tankless coil lets house water pressure into the boiler, which we found and fixed in a Huntington boiler repair.
What can you safely check yourself?
You can read the gauge cold and hot, look for leaks, and check whether the relief valve pipe is dripping. Write down the numbers before you call. Do not open valves, drain the boiler or add water by hand unless a technician has shown you how on your system.
- Read the gauge cold: With the boiler off or idle for an hour, note the pressure.
- Read it hot: After the boiler has run for a while, note the pressure again.
- Check the relief valve: Look at the pipe below the relief valve for drips or water on the floor.
- Look for leaks: Check the boiler, pipes, baseboard and radiator valves for water or stains.
- Call with the numbers: Tell us both readings and what you saw. If the relief valve is discharging, turn the boiler off at its service switch first.
What should you never do with high boiler pressure?
Never cap, plug or tie off the relief valve, even if it drips. It is the last safety device between a pressure problem and a dangerous failure. Do not keep running a boiler with the gauge at or near 30 psi, and do not try to drain a hot boiler yourself.
If the relief valve is discharging, turn the boiler off at the service switch and call. If you also smell gas or a carbon monoxide alarm sounds, leave the house and call 911 or the gas utility from outside.
What does K2 do about boiler pressure problems?
We test the expansion tank, the feed valve and the relief valve, check the domestic coil if the boiler has one, look for leaks, and set the correct fill pressure. You get the cause and the price before the repair. Expansion tanks and relief valves are usually replaced on the same visit.
Recent examples: a leaking Dix Hills boiler with a new expansion tank and air bleeder, and a Deer Park boiler with a zone valve, tank and air vent replaced. For a longer explanation of air, pressure and zones, read our baseboard heat article.
Common questions
Is a dripping boiler relief valve dangerous?
The drip itself is the valve doing its job, but it means pressure is reaching its limit. Turn the boiler off and have the expansion tank, feed valve and coil checked. Never cap the valve.
Can I add water to my boiler myself?
Only if a technician has shown you how. If pressure keeps dropping, there is a leak to find.
Why does my boiler pressure go up when the heat runs?
Water expands as it heats, so a small rise is normal. A big jump, or one that reaches 30 psi, usually means the expansion tank has failed.
Need a technician?
Call or text us 24/7 from anywhere within 15 miles of Deer Park. You get a time window and the price before any repair.
Call 24/7: (631) 414-7141