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The Pressure Gauge

A hydronic system is a closed loop under pressure. The gauge is the only window into it, and it will tell you more about the system than anything else in the room.

In brief

The pressure gauge is the most useful number on a boiler and the one nobody reads. The expansion tank absorbs the rise as water heats and the relief valve opens if it cannot. Pressure that climbs or falls between readings is diagnostic in itself.

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Why a boiler has pressure at all

Water expands when it is heated. In a sealed loop that expansion has to go somewhere, so the system carries a designed working pressure and an expansion tank to absorb the change. The gauge shows the result.

That makes a boiler unlike almost every other appliance in the house: it is a pressure vessel with a safety device on it, not just a heater.

The relief valve, which is the safety device

If pressure rises beyond the safe limit, a relief valve opens and lets water out. A valve that has discharged has done its job — but it has also told you the pressure got somewhere it should not have, and that cause is the actual fault.

Water on the floor near a boiler is frequently this, and treating it as a leak to be mopped misses the point entirely.

What not to do with it

Nothing on this site tells you to lift, cap, plug or adjust a relief valve. It is the device that stops a pressure vessel becoming a hazard, and a valve that keeps discharging is a reason to call somebody rather than to silence it.

Pressure that keeps falling

A closed loop should hold its pressure. One that needs topping up repeatedly is losing water somewhere, and the loss is the fault. Topping it up is not a repair, it is a way of not noticing.

Each refill also brings fresh oxygenated water into a system designed to hold the same water for years, which is its own slow problem.

Pressure that climbs

Rising pressure on a system that has not been touched usually points at the expansion tank having lost its charge, or at water getting in from the fill. Both are diagnosable and neither is a homeowner adjustment.

What to tell whoever comes

The reading when the system is cold, the reading when it is hot, and whether the relief valve has discharged. Those three facts narrow the problem more than any description of the noise it is making.

A boiler is not a furnace, and the person you want knows the difference.Tell us it is a boiler, tell us what the gauge reads if you can see it, and we will route you to somebody in your area who works on them. You are never charged. Boilers give this site its name and not its limits.

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