How often should you check boiler pressure?
Updated July 2026
Glance at the boiler pressure gauge once a month during heating season, and it should read about 1.0 to 1.5 bar (roughly 12 to 18 psi) when the system is cold. The needle sits in a green band on most gauges; it rises a little as the water heats and settles back when it cools. A reading that drifts below 1 bar means the system is losing water and will eventually lock out, while a cold reading pushing past 2.5 bar means too much water or a fault, and the relief valve will start dumping.
What changes the interval
| Your situation | Interval | Why |
|---|---|---|
| Reads 1.0-1.5 bar cold | Normal, just log it | This is the healthy resting range for most systems |
| Below 1 bar (in the red) | Top up via the filling loop | Low pressure trips a no-heat lockout by design |
| Climbs above 2.5-3 bar when hot | Have it checked | Points to a waterlogged expansion vessel or a stuck valve |
| Pressure keeps dropping weekly | Find the leak | A steady fall means water is escaping somewhere |
| Relief valve dripping outside | Call a heating engineer | Chronic overpressure is wearing out the safety valve |
| Right after bleeding radiators | Recheck and top up | Releasing air also lowers the pressure |
The cost of skipping it
What ignoring it costs: low pressure is the single most common reason a boiler locks itself out and leaves you cold, usually on the first hard freeze when you least want a service call at $100-200. Run it low long enough and the pump can cavitate and wear early, a $300-600 part. Chronic high pressure is the flip side: it fatigues the expansion vessel and forces the relief valve to weep, and a failed expansion vessel replacement runs $200-500. A 30-second monthly glance and a filling-loop top-up head all of it off; anything stubborn belongs in the annual boiler service.
How to do it (30 seconds to check · DIY top-up in 5 minutes)
- Find the pressure gauge on the boiler front or on the pipework nearby, and note the colored normal band.
- Read it when the system is cold; expect roughly 1.0-1.5 bar and a small rise when it heats.
- If it reads below 1 bar, locate the filling loop, the silver braided hose with two valves under the boiler.
- Open both filling-loop valves slowly and watch the gauge climb to about 1.2-1.5 bar, then close them firmly.
- If the pressure keeps falling week after week, stop topping up and have the leak traced by a pro.
Knowing the interval is the easy half. OnOtto reminds you at the right interval, and it won't let you snooze it into next year. Tasks persist until they're done, and completing boiler pressure checks schedules the next one automatically.
Put this on autopilot, free for 30 daysFrequently asked questions
What should boiler pressure be when the heating is on?
Cold, a healthy system sits around 1.0-1.5 bar; once the heating is running and the water expands, it is normal for the gauge to climb to roughly 1.5-2.0 bar. It should fall back as the system cools. A jump well above 2.5-3 bar when hot suggests a waterlogged expansion vessel or a stuck filling loop, and the relief valve may open to protect the system. Judge the resting cold reading first, then watch how far it swings when hot.
Why does my boiler keep losing pressure?
A slow, steady drop usually means water is escaping: a weeping radiator valve, a pinhole in a pipe, a leaking pump seal, or a relief valve dumping outside because of a separate fault. Bleeding radiators also lowers pressure, but only once, not continuously. If you are topping up more than every few weeks, stop chasing it with the filling loop and have the leak traced. Repeated top-ups add fresh oxygenated water that speeds internal corrosion.
How do I top up boiler pressure?
Find the filling loop, a short braided silver hose with a valve at each end, usually beneath the boiler. With the boiler off and cool, open both valves slowly and watch the gauge rise to about 1.2-1.5 bar, then close them firmly and check they are not left dripping. Some modern boilers have a built-in keyed or lever filler instead. If you cannot find the loop or the pressure will not hold, that is a job for the annual service.
Can low boiler pressure be dangerous?
Low pressure itself is not dangerous; it is a nuisance that trips the boiler into a protective lockout so you lose heat and hot water. The safety concern runs the other way: very high pressure is what the relief valve and expansion vessel exist to manage. The real hazard of a no-heat lockout in deep winter is indirect, an unheated house risks frozen and burst pipes. Restore pressure, get the heat back on, and trace any persistent leak.