A boiler losing pressure is one of the most common service calls in the hydronic heating industry. The fix is usually simple. Finding the root cause takes a methodical approach.
This guide covers every hydronic boiler regardless of brand. Weil-McLain and Burnham owners will find specific callouts for their equipment.
Normal Boiler Pressure Range
Your boiler operates within a specific pressure window. Know these three numbers:
12-15 PSI cold. This is the sweet spot. When the system is fully cold and the gauge settles, you want to see 12 to 15 pounds per square inch. This gives the circulator enough pressure to push water through the highest radiators without forcing the relief valve open.
Up to 25 PSI hot. As water heats, it expands. Pressure rises with temperature. A reading between 18 and 25 PSI on a fully hot system is normal.
30 PSI means the relief valve pops. The safety relief valve opens automatically at 30 PSI to prevent a rupture. If you see water dripping from the relief valve discharge pipe, the pressure hit the ceiling. Fix this immediately. A stuck-open relief valve after the event means replacement is required.
Step-by-Step Diagnosis
Follow these six steps in order. Each step rules out one cause before moving to the next.
Step 1: Check the Pressure Gauge
Start at the gauge. A cold system should read 12-15 PSI. Below 10 PSI means water volume is too low. At 0 PSI the boiler will not fire. Many modern gas valves lock out below a minimum pressure threshold.
If the gauge reads 0 PSI, the boiler is likely in a safety lockout. Reset the boiler after adding water. If it reads 50 PSI, the gauge itself is probably broken.
Step 2: Add Water Through the Auto-Fill Valve
The auto-fill valve (also called a pressure reducing valve or PRV) connects your boiler to the domestic cold water supply. It looks like a brass fitting with a lever or handle on top.
Turn the handle counterclockwise or lift the lever to open. Let water flow until the gauge hits 12 PSI. Listen for water moving through the pipes. Close the valve fully when done. A valve that does not close completely will raise pressure gradually overnight.
Some systems use a manual fill valve instead of an auto-fill. Same procedure. Open, fill to 12 PSI, close.
Step 3: Bleed Air from Radiators
Air trapped in the system causes pressure fluctuations and cold spots. Start with radiators on the lowest floor. Work your way up.
Use a radiator key on the bleed valve at the top of each radiator. Place a cup or rag under the valve. Turn the key counterclockwise. You will hear a hiss. When a steady stream of water comes out, close the valve. Repeat for every radiator in the house.
After bleeding, check the pressure gauge again. It will have dropped. Add water to bring it back to 12 PSI.
Step 4: Check for System Leaks
Walk every inch of exposed piping. Look for wet spots on floors, walls, and ceilings. Check radiator supply valves and return valves at both ends of each unit. Inspect the circulator pump flange seals for drips. Examine the pressure relief valve discharge pipe for signs of discharge.
A pinhole leak in a copper pipe can lose a gallon a day. That adds up to a full system pressure drop every 48 hours. Even a slow drip at a radiator valve will cause gradual pressure loss over a week.
Step 5: Test the Expansion Tank
The expansion tank absorbs the extra volume created when water heats up. A waterlogged tank cannot do this job. The result is pressure spikes every time the burner fires.
Tap the tank with your knuckle or a small wrench at different heights. A hollow, ringing sound near the top means the air charge is intact. A dull thud at the bottom means water has filled that portion. This is normal and expected. A full dull thud from top to bottom means the bladder inside has ruptured. The tank is waterlogged. It needs replacement.
A properly charged expansion tank should read 12 PSI at the air valve on top when the system is cold and depressurized. Check this with a tire pressure gauge.
Step 6: Verify the Pressure Gauge
Gauges fail. It happens. If your gauge reads 0 PSI but the system clearly has water and heat, the gauge is dead. If it reads pinned at 50 PSI and the relief valve is not leaking, the gauge is stuck.
Replace a bad gauge. The port is typically 1/4” NPT. Turn off the boiler and drain the system below the gauge port before removing the old gauge. Use pipe thread sealant on the new gauge threads. Do not use Teflon tape on NPT fittings for boiler gauges.
Diagnostic Reference Table
| Pressure Reading | What It Means | Action Needed |
|---|---|---|
| 0-10 PSI cold | Low system pressure. Water loss or air needs bleeding. | Add water through auto-fill. Check for leaks. Bleed radiators. |
| 12-15 PSI cold | Normal operating range. | No action needed. |
| 20-25 PSI cold | High static pressure. Expansion tank is waterlogged or fill valve is stuck open. | Drain water to 12 PSI. Test expansion tank. Replace if waterlogged. |
| 30+ PSI cold | Relief valve will open. | Immediate fix. Waterlogged expansion tank or failed PRV. |
| 15-20 PSI hot | Normal for a warm system. | No action needed. |
| 25+ PSI hot | Pressure rising too high with heat. | Waterlogged expansion tank is the most likely cause. Replace the tank. |
| Pressure drops every few days | Small leak or failing auto-fill valve. | Inspect all visible piping, radiator valves, and pump seals. |
Brand-Specific Notes
Both Weil-McLain and Burnham use standard hydronic components. The layout varies slightly.
Weil-McLain. The auto-fill valve is a 1/2” brass fitting with a handle on top. It sits on the supply line near the boiler. The pressure gauge mounts on the front panel above the burner access door. The expansion tank sits near the boiler top, often on a bracket or directly on the outlet pipe. Weil-McLain CGi and Gold series share this configuration.
Burnham. The auto-fill setup is similar. Some Series 2 models include a manual fill lever in addition to the auto-fill valve. The gauge is on the front jacket panel. Burnham ES2 and SCG models use standard 12 PSI cold settings. The expansion tank on a Burnham boiler is typically mounted on a wall bracket above the boiler, not directly on the boiler top. This makes tapping the tank easier.
When to Call a Professional
Some problems need a licensed heating contractor.
Call a pro for persistent pressure loss you cannot trace. A leak inside a finished wall or under a concrete slab requires specialized detection equipment. Do not cut into walls guessing.
Call a pro for expansion tank replacement. The system must be drained, the old tank unbolted, and a new one sized and installed. A propane torch or pipe wrenches on old galvanized fittings are everyday tools for a pro. Not for a homeowner.
Call a pro for relief valve replacement. Relief valves are safety devices. A misinstalled valve can fail to open when needed. Correct installation requires matching the pressure rating and temperature rating of the existing valve. Use the same specs the manufacturer installed.
Call a pro if the boiler goes into lockout and will not reset after you add water. There may be a blocked pressure sensor tube or a failed low-water cutoff switch.
Low boiler pressure is almost never a mystery. Start at the gauge. Work through each step. Nine times out of ten the fix is water, air, or the expansion tank. The tenth time is a call to a pro. That is the right order.