What's normal boiler pressure?
Most residential hot-water boilers sit around 12–15 PSI cold and rise to roughly 20–25 PSI when hot. Above about 28 PSI, the 30 PSI relief valve gets close to opening.
Steam boilers are different. They run at a fraction of a PSI to about 2 PSI, and their gauge reads in a much lower range.
Pressure problems and causes
| Reading | Likely causes |
|---|---|
| Climbs well above 25 PSI when hot | Waterlogged or failed expansion tank; fill valve letting by |
| Relief valve drips | Pressure reaching 30 PSI, or a worn valve |
| Drops slowly over days | Small leak in piping, a radiator valve or the boiler |
| Drops to near 0 | Significant leak or a fill valve shut off |
| Swings widely each cycle | Expansion tank not absorbing expansion |
The expansion tank's job
Water expands as it heats. The expansion tank has an air cushion that absorbs that extra volume. When the tank's internal bladder fails or the air charge leaks out, the tank fills with water and pressure spikes every time the boiler fires.
A technician checks the tank's air charge (often 12 PSI to match the system) with the system drained down.
Fixing high pressure
- Confirm the gauge reading, cold and hot.
- Test the expansion tank's air charge, and replace or recharge it.
- Check whether the pressure-reducing fill valve is leaking through.
- Replace the relief valve if it has been discharging.
- Verify stable pressure through several heating cycles.
Fixing low pressure
- Look for drips at radiator valves, bleeders and joints
- Check the relief valve discharge pipe for water
- Look for damp spots under baseboards
- Check that the fill valve is open and set correctly
- Repressurize, then watch whether it drops again
Never plug a relief valve
The pressure relief valve protects the boiler from rupturing. If it's dripping, the system is telling you something. Capping or plugging the discharge is dangerous.
Pressure FAQs
Is it okay to top up boiler pressure myself?
On many systems with a manual fill, yes, following the manufacturer's steps. If you top up repeatedly, there's a leak that needs fixing.
Can high pressure damage a boiler?
Sustained high pressure stresses the heat exchanger, valves and fittings and keeps the relief valve discharging. Fix the cause.
Next steps
Ready to get it fixed? Call (888) 555-0144 to reach a licensed provider in your area. Related guides:
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