How to top up your boiler pressure: the three UK filling methods, and the German rule

Why the pressure drops

Every time you bleed a radiator, a little water leaves the heating system, so the pressure falls (Viessmann). Worcester Bosch says to top it up after bleeding. If it keeps dropping when you haven't bled anything, Viessmann names a leak or a faulty expansion vessel as the most common causes. That is a job for a heating engineer, not a top-up.

UK and Germany side by side

Step 1: read the gauge with the heating cool

Look at the round gauge on the boiler (Viessmann notes that some boilers show it on the control panel instead). Worcester Bosch says the reading rises while the heating runs and falls back when it cools, so check it cold. On a Worcester Bosch boiler, a reading below 1 bar is under the range it gives.

Step 2: find out which filling method your boiler has

Worcester Bosch describes three. Look under the boiler, and behind the bottom cover if it comes off.

Blue lever (keyless filling link)

Remove the bottom cover and find the black manifold with the blue lever, normally on the right. Pull the lever down: you'll hear water running, which is normal. Watch the gauge and let go of the lever between 1 and 1.5 bar. Put the cover back.

External filling hose

The hose may be under the kitchen or utility sink. Take the cap off the valve on the copper pipe and attach the hose. Open one lever, then open the second one slowly. When the gauge reads 1 to 1.5 bar, turn both levers back so they sit across the pipe. Unscrew the hose with a rag ready for drips, then refit the cap.

Internal filling key

The key is usually kept inside the bottom cover. Find the manifold with the nut, put the key in at the padlock symbol, push it in firmly and turn it 45 degrees to the left until it locks. Turn the nut to the left to let water in. Close the nut when the gauge reaches 1 to 1.5 bar, turn the key back 45 degrees and pull it out. A few drips are normal. If it keeps dripping, tighten the nut a little more.

Step 3: stop between 1 and 1.5 bar

If you go past it, Worcester Bosch says to bleed a radiator to bring the pressure back down. Other makes may use a different range, so check your boiler's manual.

In Germany

Viessmann recommends leaving the refill to a heating professional. The water must be clean and meet the VDI 2035 guideline, and heating water must never get into the drinking water. For owners who have done it before, its steps are: switch off the circulation pump, open all thermostats fully, and bleed the radiators, starting on the lowest floor. Then fill the hose with water until it overflows, connect it with a proper filling fitting, open both valves and fill to the right pressure. Close the valves, turn off the tap, and bleed the radiators again. Viessmann says the pressure should stay steady afterwards and not drop below 1.3 bar.

If this doesn't work

• The pressure keeps falling: a leak or the expansion vessel is likely (Viessmann). Call a heating engineer or the boiler maker.
• You can't find a filling link: check your boiler's manual. Worcester Bosch has a video for each method, and its helpline is in the guide.
• You rent: tell your landlord before you touch the boiler.