A hot water heating system is a closed loop of water under modest pressure. The gauge on the boiler is the one instrument that tells you, at a glance, whether the system is healthy. Learning to read it takes a minute, and it avoids a great many unnecessary calls, and a few necessary ones that get made too late.
This guide is for hot water boilers. A steam boiler runs at a much lower pressure, a pound or two, and has a water level glass instead; see our guide to steam vents.
What it should read
Cold, with the heating off, most residential systems sit around 12 to 15 psi. Running hot, it rises a few psi, perhaps to 18 or 20. Many gauges have a green band or a red needle set at the limit. The relief valve on the boiler opens at 30 psi, and that is the number to stay well away from.
Too low
Below about 10 psi cold, water may not reach the top floor, and radiators upstairs go cold or need bleeding constantly. Causes:
- Bleeding radiators, which lets water out. Normal, and the system needs topping up.
- A small leak somewhere: a radiator valve, a joint, the pump seal. Look for wet patches and stains.
- The automatic fill valve has failed or been closed. It is on the cold water pipe feeding the boiler, often with a lever on top.
Topping up: if the boiler has an automatic fill valve, opening the lever on it briefly, or opening the manual valve on the fill line, raises the pressure. Watch the gauge and close it at 15 psi. If you have never done this, ask the plumber to show you at the next service. If the pressure falls again within days, there is a leak, and that needs finding.
Too high
Above 25 psi cold, and certainly if the relief valve is dripping, the system is overfilled or something is pushing the pressure up:
- The fill valve is letting water through continuously. Close the manual valve on the fill line and watch whether the pressure stops climbing.
- The expansion tank has failed. Hot water boilers have an expansion tank, usually a small cylinder hanging from a pipe near the boiler. If its diaphragm has failed, the tank fills with water and there is nowhere for expansion to go, so the pressure climbs every time the boiler fires. Tap it: it should sound hollow at one end. This is a plumber's job.
- A leaking coil, on boilers that make domestic hot water through a coil inside, lets the higher house pressure into the heating loop. Also a plumber's job.
Reducing the pressure: draining a little water from a boiler drain valve or a radiator bleed valve into a bucket brings it down. If it goes straight back up, the cause is one of the three above.
The relief valve
If it drips, the pressure has been too high. Do not cap it, replace it with a higher-rated valve, or ignore it. Fix the cause, then replace the valve if it continues to weep.
Other things the gauge tells you
- Pressure that swings widely between hot and cold: expansion tank.
- Temperature reading far above the setting on a combined gauge: the aquastat may have failed. Turn the boiler off and call a plumber.
- A gauge that reads zero with the system obviously full: the gauge itself has failed, and should be replaced so that you can see what is happening.
When to call a plumber
- Pressure that keeps falling after topping up
- Pressure that climbs on its own
- A dripping relief valve
- A failed expansion tank or fill valve
- Any doubt about the fill procedure
Find a licensed plumber near your town.