In short: the safety valve protects your heating system against excessive pressure. In a single-family home, it usually opens at 3 bar. If it drips, the pressure is almost always rising too much as the water heats up, most often because of an expansion vessel that has lost its pre-charge or is too small. Less often, the system is overfilled or the valve itself is dirty or worn.
What does the safety valve do?
As water heats up, it expands and the pressure rises. The expansion vessel takes up this extra water. If the pressure still exceeds the valve's set pressure, the valve opens, releases a little water and closes again. It is the system's last line of defence: never plug it, never remove it without replacing it, and never fit a shut-off valve between the boiler and the safety valve.
Don't confuse it with the safety valve on the water heater, usually set at 6 bar, which protects the drinking water: it is covered further down.
First, read the pressure gauge
Before changing anything, note the pressure cold, with the heating off for a few hours, then hot, after a good spell of heating. With a healthy expansion vessel, the pressure only rises by a few tenths of a bar and stays well below 2.5 bar. The difference between cold and hot points to the cause.
| What you see | Likely cause |
|---|---|
| Normal pressure when cold (1 to 1.5 bar), climbing towards 3 bar when hot | Expansion vessel without pre-charge, ruptured membrane or vessel too small |
| Pressure already too high when cold, rising day by day | System overfilled, or water entering through the filling valve or the boiler |
| Normal pressure cold and hot, but the valve still drips | Safety valve dirty or worn |
Cause no. 1: the expansion vessel
This is by far the most common cause. Over time, the vessel slowly loses its pre-charge pressure: it no longer absorbs the expansion of the water, and the pressure rises with every heating cycle until the valve opens. When the system cools down, the pressure drops lower than before, because water has escaped.
- Quick test: briefly press the vessel's air valve (like a tyre valve). If water comes out, the membrane is ruptured and the vessel must be replaced.
- Checking the pre-charge: with the vessel isolated from the system and drained on the water side, measure the pressure at the valve with a pressure tester, then top it up if needed with a pump or an aerosol filler.
- Vessel too small: this is common when the system has been extended or a buffer tank added. An additional vessel is then fitted, for example a Reflex N or a Zilmet Zilflex H.
The full check procedure and how to calculate the right size are explained in our guide How to choose an expansion vessel. To check or change the vessel without draining the whole circuit, fit it with a capped shut-off valve with drain.
Cause no. 2: too much water in the system
- Overfilling: if you recently topped up and the pressure was already high when cold, there is no longer enough room for expansion. With the heating off and cold, let a little water out through a drain valve or a radiator bleed screw until you are back at the right value.
- Filling valve no longer closing properly, or filling hose left connected: mains water keeps flowing in and the pressure rises, even when cold. Always remove the hose after filling and replace a leaking valve, for example with a DN 15 fill and drain valve.
- Combi boiler (heating and instantaneous hot water in the same appliance): an internal leak in the hot water heat exchanger can let mains water into the heating circuit. Typical sign: the pressure rises mainly after hot water has been drawn. This repair is done inside the boiler: it is a job for a heating engineer.
Cause no. 3: the valve itself
Once it has opened, a valve can stay slightly open because of a particle of dirt or limescale on its seat. It then drips even at normal pressure.
- Many valves have a test knob or cap: turning it briefly opens the valve and flushes the seat. Do this with the system cold and a bucket under the outlet, then check the pressure.
- If it still drips, or if it is old and scaled up, replace it with a valve of the same set pressure (usually 3 bar), the same connection sizes and suited to the boiler's output, as specified by the manufacturer.
Examples of diaphragm safety valves for heating: CALEFFI female × female, Evenes female × female, CALEFFI CST 3 bar, or the safety valve with built-in gauge. The AFRISO boiler safety group combines valve, gauge and air vent on a single fitting.
Important: a valve that drips because the pressure rises too much is not faulty; it is doing its job. Replacing it without fixing the cause achieves nothing: the new one will drip too.
A properly connected discharge
Water from the valve must discharge visibly into a drain, for example through a tundish connected to the waste pipe or a collection hose. That way you see straight away when the valve opens, without water damage on the floor. The discharge must never be closed off or reduced.
After the repair: top up with demineralised water
Every drop lost through the valve lowers the pressure. Once the cause is fixed, top up to the right cold pressure, usually 1 to 1.5 bar in a single-family home. In Switzerland, guideline SICC BT 102-01 requires demineralised water, including for top-ups.
- For an occasional top-up: the Purotap single-use cartridge, fitted between the tap and the filling hose.
- For several years of filling and top-ups: the Purotap micro DS filling station.
The step-by-step method is in our guide Topping up your heating system with water. After a few hours of heating, bleed the radiators: see Bleeding a radiator: when and how to do it.
What about the water heater's safety valve?
On the cold water inlet of a water heater or hot water cylinder, the safety valve (often set at 6 bar) lets a little water out while heating: this is the water expanding. A few drops while the water heats up are normal if there is no potable-water expansion vessel. A continuous flow is not.
- To stop this water loss, fit a potable-water expansion vessel, approved for drinking water, such as the Reflex Refix DD or the Flamco Airfix A.
- Excessive mains pressure also makes the valve drip: a pressure reducer set at around 3 to 4 bar solves the problem.
- Replacement valves for drinking water: CALEFFI or Evenes.
Frequently asked questions
Is a dripping safety valve dangerous?
Not immediately: it is doing exactly its job. But the system loses water, the pressure drops, and every top-up brings in oxygen, which encourages corrosion. Above all, it is the sign of a problem to fix, most often the expansion vessel.
Can I plug the outlet or shut off the valve?
Never. Without a working safety valve, nothing limits the pressure in the system any more. Fix the cause rather than the symptom.
What is the maximum pressure when hot?
With a 3 bar valve, the pressure while heating should stay below about 2.5 bar. If it approaches 3 bar, check the expansion vessel and the fill pressure.
Should the valve be replaced as soon as it drips?
No. First check the pressure cold and hot. If the pressure stays normal and the valve keeps dripping even after flushing, then yes, replace it.
My valve only drips when the heating is on. Why?
That is the typical case of an expansion vessel that no longer absorbs the water's expansion: lost pre-charge, ruptured membrane or too small.
Everything for stable pressure
Browse our safety valves and boiler safety groups, our expansion vessels and accessories, our Purotap filling stations and our pressure gauges.
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