5 minute read
How to repressurise a boiler, and when it is telling you something
Topping the pressure up takes five minutes and clears most low-pressure fault codes. Doing it once or twice a year is normal. If you have had to look this up more than once this month, read the end of the page as well as the steps.
Every boiler is slightly different. Your manual is the final word, especially on combi boilers with a built-in filling key or lever instead of a hose. These steps follow the general method ADI gives for Ideal, Baxi and Potterton boilers.
What you are aiming for
The gauge is on the front or underneath the boiler: a dial, or a number on the display. With the heating cold, most boilers want between 1 and 1.5 bar; your manual gives the exact range. When the pressure drops too low, the boiler stops and shows a low-pressure fault code. Most domestic pressure relief valves open at 3 bar to protect the boiler.
Topping it up, step by step
- Turn the boiler off and let it cool. Switch the heating off and give the system time to go cold; ADI's F1 guide allows four to six hours. The target pressure is a cold reading, so topping up a hot system leaves it under-filled once it cools.
- Find the filling loop. On most boilers it is under the casing: a braided metal hose with a valve at each end, connecting the cold mains to the heating. If there is no hose, look in the manual for a built-in lever or key.
- Connect it if needed. Some loops are left connected, others are stored separately. If yours is detached, take the end caps off and screw it on hand tight.
- Open the valves slowly. Turn the handles, or use a flat screwdriver on the slots, and you will hear water running in. Keep your eyes on the gauge the whole time.
- Close both valves at 1 to 1.5 bar. Close them fully. A valve left slightly open keeps pushing water in, and the pressure creeps up until the relief valve lets it out again.
- Reset the boiler and put the heating back on. Clear the fault code with the reset button or knob, then turn the heating or hot water back on.
- Bleed the radiators if they need it, then check again. Bleeding lets air out and lowers the pressure, so read the gauge afterwards and top up once more if it has dropped.
If your loop is the detachable kind, disconnect it and put the caps back on when you have finished. Water regulations expect a temporary filling loop to be disconnected after use, so mains water cannot be drawn back from the heating.
If the fault code will not clear
If the pressure is back at 1 to 1.5 bar and the boiler still shows a fault, the problem may be a faulty pressure sensor, a pump, or something else inside the boiler rather than the water level. That needs a Gas Safe registered engineer. What the common low-pressure codes mean.
How often is too often
This is the part most guides skip. A sealed heating system does not use water. Topping up once or twice a year covers the small losses from bleeding and servicing. Anything more frequent means water is leaving the system:
- Every few weeks: possibly the pressure relief valve, the expansion vessel or a weeping radiator valve. Check the relief valve pipe outside for drips and every radiator valve for rust or moisture.
- Every few days: water is going somewhere, and if you cannot see it, it may well be going into a floor, a wall or a ceiling.
- Every day, or it goes to zero: turn the boiler off rather than keep refilling it, and call.
Every top-up also brings in fresh water and the oxygen dissolved in it, which is one reason a leaking system tends to get dirtier over time. In Market Deeping, the engineer who found and repaired a hidden leak also recommended flushing the system because the water was so dirty. Read that case.
Topping up more than you should?
Tell the office how often, how low the gauge gets, and what you have already checked. They will tell you whether it sounds like a leak and what a survey would involve.
Quick answers
What pressure should I top my boiler up to?
Between 1 and 1.5 bar with the heating cold, unless your boiler manual says otherwise. It will rise a little when the heating is on. Filling much higher than that can make the pressure relief valve open when the water heats up.
Do I need a Gas Safe engineer to top up the pressure?
Usually not. Most boiler user manuals show you how to top up through the filling loop yourself. If yours tells you to call an engineer, or the filling point is inside the casing, follow the manual. Anything inside the casing of a gas boiler needs a Gas Safe registered engineer.
How often is it normal to repressurise a boiler?
Once or twice a year. If you are doing it every few weeks, something is wrong, and if you are doing it every few days, the system is losing water somewhere.