Many UK combination boilers show system pressure on a small dial or a digital bar reading. When that figure drops, the boiler may lock out, radiators stay lukewarm, or hot water arrives in fits. The useful question is not “what does a plumber charge”. It is whether this is a five-minute top-up you can do from the cupboard, or a leak and a gas appliance that should be left alone.
This piece is a check, not a repair manual and not a quote. Gas work belongs with someone whose profile shows Gas Safe. If you already know you need a number, open plumbing and heating on the directory.
Read the gauge before you touch a tap
Find the pressure display on the front of the boiler or behind the drop-down flap. The handbook that came with the unit is the authority for that model. A common cold-system band on domestic boilers is about 1 to 1.5 bar. Some makers print a green arc on the dial. If the needle sits in that arc and the house is still cold, pressure is probably not the whole story.
Write down the reading, whether the heating has been on, and whether any radiator has been bled in the last day. Bleeding a radiator lets air out and water volume can drop. That is a different sequence from a hidden leak under a floor.
If you cannot see a gauge, do not dismantle the case. A sealed unit is not a DIY panel. Call someone who can open it as part of a service.
When a filling loop is a homeowner job
Some boilers have a silver braided filling loop with two small valves, or a key that the maker supplied. The handbook will say whether you are expected to use it. If it does, the usual sequence is: heating off, both valves opened slowly until the needle reaches the band the maker printed, valves closed firmly, loop left as the maker designed it — some loops stay fitted, some are removed after filling.
- Turn the boiler and the heating controls off and let the pump stop.
- Open the filling valves slowly. You should hear water enter. Watch the gauge, not the clock.
- Stop in the band the handbook names. Do not keep going toward the red high-pressure mark.
- Close both valves. Check for drips at the joints.
- Restart and watch the gauge after twenty minutes. If it has fallen back, stop topping up.
Repeated topping-up is not maintenance. If you need the loop every few days, water is leaving the system. That is a leak-finding job, not another twist of the valve.
When to put the loop down and call
Stop the homeowner sequence in any of these cases. They are not “try once more” problems.
- You smell gas, hear a hiss you cannot place, or the carbon monoxide alarm sounds. Get out, call the Gas Emergency Service on 0800 111 999, and do not use electrical switches as you leave.
- Water is pooling under the boiler, along a pipe, or through a ceiling.
- The pressure rises into the high red as soon as the burner lights, then dumps from a discharge pipe.
- There is no filling loop, or the valves will not move without force.
- You have filled once already today and the needle is low again.
A locked-out boiler that shows a fault code is also a reason to stop guessing. Photograph the code, the gauge, and the boiler model badge. That is what you read out on the phone. It is not a diagnosis this site can make.
Who the job actually is
Gas, flues and the sealed heating circuit are Gas Safe territory when the work goes beyond a filling loop the maker documented. Ten businesses on this directory show Gas Safe on the profile. A mark appears only when it is there. It is not a grant badge and it is not a promise from this site.
If the house is cold because a radiator valve is shut or a programmer is on a holiday setting, that is not a boiler failure. Check the room thermostat and the programmer before you describe an emergency. If the boiler fires and the flue is clear but one room stays cold, the next question is often a stuck valve or sludge — still a plumber, not a new boiler by default.
For a replacement rather than a top-up, read the grant notes before you treat a Boiler Upgrade Scheme headline as the whole bill. That scheme is for eligible heat pumps, not for a filling-loop visit. For a sense of how listings talk about money, use the cost notes.
What to have on the call
Have the boiler make and model, the gauge reading, whether you used the filling loop, and whether anyone has bled radiators. Say if you are a tenant: a landlord gas certificate is a separate paper trail from a same-day repair. Ask whether the person coming is Gas Safe for the work you described, and whether the first figure is plus VAT.
This site does not email the business. Open the directory — for example plumbing in London or plumbing in Manchester — and use the number on the card.
Need a number after the check?
Open plumbing and heating, pick the city, and contact the business yourself.