The safety valve is one of the most important parts of a storage water heater. Although it is small and is often seen as an ordinary connecting piece on the water inlet, its function is directly linked to the mechanical safety of the tank.
Its basic task is simple: not to let the pressure in the heater rise to a value that could damage or destroy the tank.
A safety valve, however, is not necessarily just a valve that „starts dripping“ at a certain pressure. Depending on the design, one body can combine several functions.
Why is a safety valve needed at all?
A storage water heater is a closed vessel connected to the water mains.
When the tank is full, the water pressure in it approximately matches the pressure of the plumbing system. Once the heating element switches on, however, the water starts to heat up.
And when heated, water expands.
In an open vessel that is not a problem, its volume increases and the water level rises a little.
In a closed tank the situation is different. If the expanded water has nowhere to go, the pressure rises.
That is exactly why the system must have a way to take up or relieve the thermal expansion of water.
Where is the safety valve fitted?
The safety valve is fitted on the cold water inlet of the heater, in line with the design of the appliance and the manufacturer's instructions.
That is not by chance.
The valve has to be in direct hydraulic connection with the tank so that it can react to the pressure that exists inside it.
For that reason no element that can isolate it from the heater may be fitted arbitrarily between the tank and its safety device.
A safety valve is not the same thing as an ordinary non-return valve
This matters, because the two ideas are often confused.
An ordinary non-return valve has one basic task:
it allows flow in one direction and prevents it in the other.
A safety valve for a water heater has an additional task, it has to protect the tank from excessive pressure.
The designs of safety valves differ in this respect.
In some systems the return of water towards the mains is practically prevented and thermal expansion is handled through an expansion vessel and safety relief.
In others, including certain types of valve often used in our region, a controlled return of a small amount of water from the heater into the water mains.
That is why the function of a particular valve cannot be judged correctly from its external appearance alone.
What happens while the heater fills up?
When the pressure of the mains is higher than the pressure in the tank, the valve allows flow:
water mains → safety valve → heater
The tank fills with water.
When the pressures are equal, the flow stops.
Up to this point the safety valve behaves practically as a part of the supply line.
The real role of its safety functions becomes visible only when the water is heated or when an abnormal rise of pressure occurs.
What happens when the water is heated?
Let us imagine that the pressure in the water mains is 4 bar.
The heater fills up and the element starts working.
As the water temperature rises, so does its volume.
If the system allows a controlled return of water, at a sufficient pressure difference the valve can let a small amount of water through:
heater → safety valve → plumbing system
In this way part of the thermal expansion can be taken up without opening the main safety outlet.
For one kind of domestic safety valve the manufacturer describes, for example, a first stage of protection in which a small amount of water from the heater can return into the mains in a controlled way once the pressure in the heater exceeds the mains pressure enough.
That is still not the same thing as the main safety relief.
What if the water cannot return into the mains?
A return of water is not possible in every installation.
On the water connection of a building there can be a non-return valve, a pressure reducer, a meter assembly or another element that prevents backflow.
The part of the installation in which the heater sits then practically becomes a closed system.
The thermal expansion of water then has to be taken up in another way, for example by a suitable expansion vessel, or it will lead to a rise of pressure until the safety system reacts.
That is why it is not enough to look only at the heater and the valve. How the whole plumbing installation is built matters.
The second stage of protection, relief of excessive pressure
If the pressure in the tank keeps rising, the safety valve has to be able to release water out of the system.
The spring that holds the valve closed is then no longer able to overcome the force created by the water pressure.
The valve opens the discharge path and part of the water is released.
The pressure in the tank is thereby reduced.
On different heaters and safety valves the opening values can differ, so a valve whose characteristics match the particular appliance must always be used.
On certain safety valves of a domestic manufacturer, for example, the main safety relief is set at a pressure of about 9 bar.
That does not mean that „9 bar is the rule for every heater“, it means the safety device has to be chosen according to the design and the permitted working pressure of the particular heater.
Why does a safety valve sometimes drip?
Dripping from the discharge part of a safety valve does not automatically mean the valve is broken.
While the water is being heated, its volume increases.
If the thermal expansion cannot be returned into the mains or taken up by an expansion vessel to a sufficient degree, the pressure rises and the valve can release a small amount of water from time to time.
In such a situation it is in fact doing exactly what it is meant to do.
A continuous leak from a safety valve, on the other hand, can point to another problem: mains pressure that is too high, a fault or dirt in the valve itself, a problem with the expansion of the water or another irregularity in the system.
That is why it is not correct to replace a valve automatically just because water drips from it, without first establishing why that is happening.
Why must the outlet of a safety valve never be blocked?
Dripping water sometimes annoys users.
That is why improvised solutions can be seen: closing the outlet, adding a plug, over-tightening the mechanism or another alteration meant to stop the leak.
That is a dangerous intervention.
The discharge part exists precisely so that the heater has somewhere to relieve excessive pressure.
If that path is blocked mechanically, the problem is not solved.
One of the protections of the tank has been removed.
The outlet of a safety valve must therefore stay functional and be arranged so that any water released cannot cause damage or endanger the user.
What is the lever or handle on some safety valves for?
Some models have a lever or another mechanism with which the valve can be opened by hand.
It is used to check that the mechanism moves freely and to open the valve in a controlled way, following the manufacturer's instructions.
It is not a tap meant for the user to „adjust“ the pressure of the heater with.
The safety pressure is determined by the design and the calibration of the valve itself.
A safety valve does not regulate the normal temperature of a heater
One more important difference:
the thermostat controls the temperature, while the safety valve controls the consequences of excessive pressure.
In normal operation the thermostat should switch the element off long before the temperature and the pressure become dangerous.
If the working thermostat fails, a sound heater has an additional thermal cut-out that should interrupt the supply to the element.
The safety valve is one more line of protection, a mechanical one.
That is why the safety of a heater does not depend on a single part but on several independent functions.
What does a safety valve actually protect?
Its main task is not to protect the tap, the hose or the water mains.
Above all it protects the tank of the heater from excessive pressure.
The tank is designed to work under pressure, but it is not designed for an unlimited rise of pressure.
If the heating kept putting energy into the water while every path for relief was blocked, the pressure could exceed the mechanical strength of the vessel.
We would then no longer be talking about an ordinary fault of a valve or a heating element, but about a potentially dangerous failure of a pressure vessel.
That is exactly what brings us to the next question:
What actually has to fail for a storage water heater to be able to explode?
We will deal with that in the next text.