If the needle on your boiler pressure gauge keeps creeping down, or the boiler locks out with a low pressure fault, something in the system is letting water or pressure escape. Topping it up gets the heating back on, but it does not fix the cause.
Here is what normal pressure looks like, the usual reasons it drops, what you can safely check yourself and when it is time to call an engineer.
What pressure should a boiler be at?
Combi and system boilers run on a sealed heating system that holds water at a set pressure, shown on a gauge on the front of the boiler or on the pipework underneath it. Ideal Heating gives a normal cold operating pressure of between 1.0 and 1.5 bar. Which? says most combi boilers sit somewhere between 1 and 2 bar, rising a little when the heating is running and the water is hot.
The fairest time to check is when the system is cold, first thing in the morning before the heating comes on. Check your boiler manual too, because the exact figures vary between makes and models.
Common reasons a boiler loses pressure
You have recently bled the radiators
Bleeding radiators lets trapped air out, and that lowers the pressure. This is normal. It is one of the few causes where a single top-up should sort it for good.
A small leak somewhere in the system
A slow drip from a radiator valve, a joint under the floor or a fitting inside the boiler can lose enough water over a few days to bring the pressure down. Look for damp patches, staining or green and white crusting around radiator valves, exposed pipework and the bottom of the boiler. Plenty of leaks are hidden under floors or behind kitchen units, though, so not seeing water does not rule one out.
The pressure relief valve is letting water by
Every sealed system has a pressure relief valve that releases water if the pressure climbs too high. It usually discharges through a copper pipe that runs to the outside of the house. If that pipe drips, or there is a wet mark on the wall below it, the valve may be passing water. That can happen when the valve is worn, or when the system has been overfilled.
The expansion vessel has lost its charge
Water expands as it heats up, and the expansion vessel gives that extra volume somewhere to go. When the vessel loses its air charge, the pressure shoots up as the heating warms, the relief valve opens to protect the system, and the pressure then drops right back down when everything cools. If you see the gauge swing high when the heating is on and low when it is off, this is a likely culprit.
What you can safely do yourself
Topping up the pressure is something most householders can do, as long as you follow the manufacturer's instructions for your boiler. Most combi boilers have a filling loop, either a silver braided hose under the boiler or a built-in key or lever, that lets mains water into the heating system.
- Switch the boiler off and let the system cool
- Open the filling valve slowly while watching the gauge
- Close it fully once the pressure is back in the normal range
- Switch the boiler back on and reset it if it shows a fault
Ideal warns not to operate valves under the boiler that you cannot identify, and Which? advises against taking the boiler casing off. Anything inside the boiler is a job for a Gas Safe registered engineer.
When a top-up is not enough
Ideal advises getting the system checked if the pressure keeps dropping over days or weeks, falls to zero, keeps rising after the filling valves are closed, or needs topping up more than once or twice a year. Call sooner if you can see water coming from the boiler or the pipework.
Ignoring it tends to make things worse. Every top-up adds fresh water to the system, which can dilute any corrosion inhibitor in it. And a boiler that keeps locking out on low pressure has a habit of doing it on the coldest night of the year.
What Gary checks
When Gary looks at a boiler that keeps losing pressure, the aim is to find the cause rather than top it up and leave. That usually means:
- Checking the expansion vessel charge, and recharging or replacing it if needed
- Checking the pressure relief valve and where its discharge pipe ends up
- Looking for leaks at the boiler, radiator valves and visible pipework
- Making sure the filling loop is not letting water through when closed
- Explaining what he found and agreeing a price before any work starts
Need your boiler looked at?
If your pressure keeps dropping, see heating fault finding and repair, or boiler breakdowns and emergency repairs if the boiler has stopped working altogether. A yearly service also picks up a tired expansion vessel early, which our guide on how often a boiler should be serviced covers in more detail.
Gary covers Gloucester, Quedgeley and the surrounding area. Quotes are free and there is no call-out charge. Call 07454 117678 or send Gary a message.