Pressure falling means water is leaving – a leak, or a relief valve discharging because the expansion vessel has failed. Pressure swinging widely between cold and hot means the expansion vessel has lost its charge. Repeated topping up damages the system.
What the gauge should show
A sealed heating system is filled to a set pressure when cold, typically in the region of one to one and a half bar, and it should stay there indefinitely. The gauge will rise modestly as the system heats and fall back as it cools – that is normal and it is what the expansion vessel is managing.
Most boilers have a minimum below which they will not fire at all, which is a protection against running a system that has lost its water. That is why a boiler that has stopped is very often simply low on pressure.
| What the gauge does | What it means |
|---|---|
| Steady at the fill pressure, small rise when hot | Normal |
| Gradually falling over weeks or months | Water is leaving. A leak, or relief valve discharge |
| Falling quickly, over days | A more significant leak |
| Very low when cold, very high when hot | Expansion vessel has lost its charge |
| Reaching the red zone when hot | Over-filled, or expansion vessel failed |
| Zero | System has drained, or the gauge has failed |
| Rising slowly even when cold | A filling loop left open, or passing |
Why it drops
- A leak somewhere in the system. Radiator valve glands weeping too slowly to make a puddle, a corroded radiator, a joint under a floor, or a pinhole in buried pipework. Slow leaks can be genuinely hard to find – see leak detection.
- The pressure relief valve discharging. This is the one people miss. Go outside and look at the small copper pipe that comes through the wall near the boiler. If it is wet, dripping, or has a stain or green algae below it, the valve has been discharging – and the usual reason is a failed expansion vessel.
- Bleeding radiators. Each bleed releases a little water. Bleeding several radiators will drop the pressure noticeably and it is normal.
- Recent work that drained part of the system.
- A passing or failed relief valve that no longer reseats after having discharged.
- Air escaping through automatic air vents, which is normal for a while after filling.
It is free, it takes thirty seconds, and it explains a large share of mystery pressure loss. A wet or stained discharge pipe means the relief valve is opening, which almost always means the expansion vessel has lost its charge – a modest repair, not a hunt for a leak.
Why it climbs
Pressure that rises far more than it should when the system heats means the expansion is not being absorbed.
- Failed expansion vessel – the overwhelming cause. The air charge has gone, so there is no cushion and pressure tracks temperature directly. See expansion vessel failure.
- Over-filled system. Filled to too high a cold pressure, so there is no room for expansion. Drain a little from a radiator bleed valve or a drain point.
- Filling loop left open or passing, continuously adding mains water.
- A failed plate heat exchanger on a combi, letting mains pressure into the heating circuit. This produces pressure that climbs even when the heating is off and hot water is used – a distinctive symptom.
That last one is worth knowing. If the pressure rises when you run a hot tap, rather than when the heating runs, suspect the plate heat exchanger.
Repressurising safely
- Find the filling loopUsually a braided flexible hose with a valve at each end, connecting the mains to the heating circuit beneath or near the boiler. Some boilers have a built-in filling key or lever instead.
- Note the current pressureCold, before you start.
- Open one valve, then the other, slowlyYou will hear water entering. Watch the gauge.
- Stop at the correct cold pressureUsually marked on the gauge or in the manual – commonly around one to one and a half bar. Do not overfill.
- Close both valves fullyAnd check the gauge holds.
- Disconnect the loop if it is a removable typeA filling loop left connected is a permanent cross-connection between mains and system water, which is a backflow concern. Many are designed to be disconnected.
- Bleed any air introducedAnd re-check the pressure afterwards, because bleeding drops it again.
If you are repressurising more than very occasionally, the system is losing water and that needs finding. Every top-up also introduces fresh oxygenated mains water into a sealed system, which is precisely what drives corrosion and magnetite formation. A system that is topped up monthly is a system being slowly destroyed – see radiators cold at the bottom.
What repeated topping up actually costs
- Corrosion. Fresh water carries dissolved oxygen. Oxygen plus steel radiators produces magnetite, which blocks heat exchangers, settles in radiators and destroys pump bearings.
- Dilution of inhibitor. Every top-up dilutes the corrosion inhibitor, and few people re-dose.
- Scale, in hard water. Fresh hard water brought into the heating circuit deposits its hardness. Normally a sealed system deposits its hardness once and stops; a continuously topped-up one does not.
- Masking the real fault, which continues and gets worse.
- Water damage, if the leak is somewhere that matters.
The point is that topping up is not neutral maintenance. It is an intervention with a cost, and the cost compounds.
When to stop topping up and call
- More than once every few months without an obvious explanation.
- The discharge pipe outside is wet or stained.
- Pressure swings widely between cold and hot.
- You can see damp anywhere near pipework, or a stain on a ceiling.
- The pressure rises when you run hot water on a combi.
- It drops to zero repeatedly.
- You cannot find the filling loop, or the gauge is not moving when you fill – which may mean the gauge has failed.
Topping up regularly? Look at the outside discharge pipe first, then stop topping up and get the cause found.
Questions we get asked about this
What pressure should my boiler be?
Typically around one to one and a half bar when cold, and it should stay there. The exact figure is usually marked on the gauge or given in the manual. It will rise modestly when hot, which is normal.
Why does my boiler pressure keep dropping?
Either a leak, or the pressure relief valve discharging because the expansion vessel has failed. Check the outside discharge pipe first – it is free and it explains most of these.
Is it safe to keep topping it up?
Occasionally, yes. Regularly, no – each top-up brings oxygenated water into a sealed system, which drives corrosion, dilutes the inhibitor and in hard water brings in fresh scale. Find the cause instead.
My pressure goes very high when the heating is on.
The expansion vessel has almost certainly lost its charge, so there is nothing absorbing the expansion. It is a common fault and a modest repair.
Should I leave the filling loop connected?
Many are designed to be disconnected after use, because a permanently connected loop is a cross-connection between mains and system water. Check what type yours is – and if it is left connected, make sure both valves are fully closed.
The pressure rises when I run a hot tap.
On a combi, that points at the plate heat exchanger letting mains pressure into the heating circuit. It is a distinctive symptom and worth mentioning specifically.
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