Boiler Pressure Keeps Dropping? Here Is What to Check
Low boiler pressure is one of the most common heating problems in UK homes. If your pressure gauge keeps creeping below 1 bar, your boiler will eventually lock out and refuse to fire. The good news is that the cause is usually straightforward to find.
What is normal boiler pressure?
Your boiler pressure gauge should read between 1.0 and 1.5 bar when the heating is off. When the heating is running, it can rise to around 2 bar, which is perfectly normal. Below 0.5 bar, most boilers will lock out as a safety measure.
Why does boiler pressure drop?
Pressure drops because water is leaving the sealed system somewhere. The four most common causes are:
1. A leak in the system
This is the most common cause. Even a tiny drip from a radiator valve, a pipe joint, or a fitting under the floorboards can cause a slow pressure loss over weeks. Check around every radiator valve, under the boiler itself, and look for any damp patches on ceilings or walls near pipes.
2. Bleeding radiators
When you bleed air out of a radiator, a small amount of water also escapes, which drops the system pressure. This is normal. Just repressurise after bleeding using the filling loop.
3. Faulty pressure relief valve
The pressure relief valve (PRV) is a safety device that releases water if pressure gets too high. If it is faulty or slightly open, it will drip constantly. You will usually see water coming from a copper pipe on the outside wall of your house. This pipe runs from the PRV to the outside, so if it is wet or dripping, the valve needs attention.
4. Expansion vessel failure
The expansion vessel is a small tank inside your boiler that absorbs pressure changes as water heats and cools. Over time, the diaphragm inside can fail or lose its air charge. When this happens, pressure fluctuates wildly, often climbing high when the heating is on and dropping when it cools down.
How to repressurise your boiler
- Find the filling loop. It is usually a braided silver hose underneath the boiler with one or two small valves
- Open the valves slowly. You will hear water flowing into the system
- Watch the pressure gauge and stop when it reaches 1.2 bar
- Close both valves tightly
If you are having to top up pressure more than once a month, there is definitely a leak or a component fault that needs finding.
Need help with your boiler? Our Gas Safe registered engineers cover the North East. Get in touch today or call 07579 760536.