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Published by Mo's Plumbing and Heating
How to Repressurise a Boiler
The gauge is low, the heating has stopped, and you want it back on before you do anything else. This is a genuine two minute job on most boilers once you know which type of filling loop you are looking at, and knowing that is most of the difficulty.
The Short Answer
Turn the boiler off and let it cool. Find the filling loop underneath it, open both ends slowly, and watch the gauge until it reaches about 1.2 bar. Close both ends firmly, in the reverse order you opened them, then reset the boiler if it needs it. The whole thing takes two minutes. The part that catches people is that there are four different filling loop designs and they do not look alike.
Before You Touch Anything
Three things, and they take thirty seconds between them. Turn the boiler off at the front panel and, ideally, let the system cool for an hour or so. Filling a hot system means the pressure climbs again as it cools and you end up undershooting, and it also means handling pipework that is hot enough to hurt.
Have a towel and a small bowl to hand. Filling loops drip, particularly the flexible hose kind, and a cupboard floor is easier to protect than to dry out. And note what the gauge says before you start, because that figure is the useful one later: a system found at 0.9 behaves differently from one found at zero, and zero often means something rather more than evaporation.
If the gauge reads zero and the system has been fine for years, stop and look for a leak before filling it. Water has gone somewhere. Filling a system that is emptying itself into a floor void is how a small problem becomes a ceiling.
Find Out Which Filling Loop You Have
This is the step every generic guide skips, and it is the one that leaves people staring into a cupboard. There are four common arrangements and the procedure differs slightly for each. Look underneath the boiler, at the pipes coming out of the bottom.
- A braided flexible hose, silver and about a foot long, connecting two pipes with a small lever valve at each end. The classic external loop. Open both levers so they sit in line with the hose, watch the gauge, close both, and the loop is then supposed to be disconnected at one end afterwards.
- A keyed internal filling link, built into the underside of the boiler, with a small plastic key that slots in and turns. Common on Worcester and on some Ideal models. The key is usually clipped to the boiler somewhere or sitting in the manual. No key is the single commonest reason people cannot top up their own boiler.
- A pair of small lever or screwdriver-slot valves with no visible hose, joined by rigid pipe. Turn one a quarter turn, then the other, listen for the flow, and close them in reverse. These move a lot of water fast, so open the second one slightly rather than fully.
- A single push-fit or cartridge type filler on the boiler underside, worked by pushing up and turning. Read the manual for this one, because the return-to-closed position is not always obvious and leaving it part open is exactly how a system ends up overpressurised.
Doing It
With the boiler off and cool, open the first side of the loop. You will usually hear nothing. Then open the second side slowly, and you will hear water moving, a rushing sound like a tap filling a bath in the next room. That noise is your real feedback, more than the gauge, which lags slightly.
Watch the needle or the display. It will move slowly at first and then quicker. Aim for 1.2 bar, or nearer 1.5 if your radiators sit two floors above the boiler. Close the second valve as you approach the number rather than when you hit it, because there is a little in the pipe still to arrive.
Close both ends firmly and check the gauge has settled. If you have gone over, that is easily corrected by bleeding a little out of a radiator, which is covered in the guide on releasing pressure. Overshooting is common and it is not a disaster.
Then turn the boiler back on. Many boilers clear a low pressure lockout by themselves once the pressure is back; others want a reset button held for a few seconds. If it fires and runs, you are done. Check the gauge again the following morning when everything is cold, because that reading is the one that tells you whether it held.
The Part That Actually Matters
Topping up is easy. Knowing whether you should have had to is the useful skill, and it is the difference between a two minute job and a repeat every fortnight.
Write the date on something. A sealed system that wants topping up once or twice a year is behaving normally, and the small losses come from valve glands, from bleeding radiators, and from the ordinary business of a system that has water in it under pressure. Once a month is not normal. Once a week is a fault that is getting worse while you keep hiding it.
There are only really three destinations for the water. It is leaking somewhere you can see, which is the easy case. It is leaking somewhere you cannot, usually a joint under a floor or a pinhole in a pipe run inside a wall. Or the expansion vessel has lost its air charge, which produces no leak at all but pushes water out through the safety valve every heating cycle, so it disappears down the discharge pipe outside without ever wetting anything indoors.
That third one is the sneaky case and it is common. If you are topping up regularly and can find no damp anywhere, go outside and look at the small pipe that comes through the wall near the boiler. If it is dripping, wet, or stained, you have found where your water is going and no amount of filling will fix it.
When Topping Up Does Not Work
Sometimes the gauge will not move, or moves and immediately falls back. If nothing happens when both valves are open, the mains supply to the loop may be isolated, or on a keyed link the key may not be fully engaged, which is easy to get wrong because it turns partway without doing anything.
If the pressure climbs and then drops the moment you close the loop, that is a genuine leak of some size and it needs finding rather than feeding. If it climbs, holds, and then the boiler still will not fire, the fault is not pressure at all and you have simply eliminated one cause, which is worth knowing.
And if the pressure will not stay up but you have just had work done anywhere in the house, mention that when you call. A nail or a screw through a heating pipe under a floorboard is not rare after flooring goes down, and it can weep slowly for weeks before it shows.
Where the Filling Loop Hides in a Glasgow Flat
The physical hunt for the loop is harder here than in a modern house, and it is the reason a lot of these calls start with somebody unable to find anything to turn. Flat conversions put boilers wherever the flue could legally go, which in practice means a kitchen wall cupboard, a hall press, occasionally a bathroom, and the pipework then runs into a boxed-in section that somebody tiled over in a refit.
The boxing-in is the recurring problem. A boiler fitted in 2009 with a tidy external loop underneath it gets a new kitchen in 2016, and the loop ends up behind a panel with no access hatch. It is still there and it still works, but you are not reaching it without taking a plinth or a cupboard base off, which is not a job to attempt at ten at night with the heating off.
The other local pattern is the missing key. Keyed internal filling links come with a small plastic key that is meant to live clipped to the boiler, and in rented flats and in any property that has changed hands a few times that key is long gone. They are inexpensive and manufacturer specific, so it is worth finding the model number and buying one before you need it rather than during a cold snap. Mo has lost count of the callouts that were just a missing key.
And in older converted property, check whether the loop was ever fitted permanently at all. Some installers leave a loop connected, some fit one only when needed and take it away, which is the more correct approach and much less convenient. If there are two capped connection points under the boiler and nothing joining them, that is what has happened, and topping up needs a loop rather than a technique.
When to Call Someone Out
If you cannot find the loop, cannot free a seized valve, or the system will not hold what you put in it, that is the point to stop. Repeated topping up is the thing worth calling about, because it is a leak or an expansion vessel and both get worse and more expensive while being managed with a filling loop. Mo or Muhammad will find where the water is actually going rather than filling it and leaving, and finding it properly is usually a matter of pressure testing rather than lifting floors on a hunch.
Boiler Repair in GlasgowCommon Questions
It is the same job as repressurising, described above, and the two phrases mean one thing. Open both ends of the filling loop with the boiler off and cool, let mains water into the sealed heating circuit until the gauge reads about 1.2 bar, then close both ends firmly. If there is no obvious loop, identify which of the four common types you have before going further.
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Read Next
Full guide
What Should the Pressure in a Boiler Be?
On a sealed system, which is nearly every combi and most modern system boilers, aim for 1.0 to 1.5 bar when the heating has been off for a few hours and the system is cold.
Why Does My Boiler Keep Losing Pressure?
The water is leaving through a visible leak, through a leak you cannot see, or through the safety valve to the outside because the expansion vessel has failed.
How to Release Pressure From a Boiler
Turn the heating off and let the system cool.
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