Isolate electrically, then the fuel, then the water, in that order. Never leave an immersion heater or a boiler running on a system that is losing water, and remember that a stored system keeps supplying water after the mains is off.
The order, and why
- Electrical firstTurn the system off at the programmer, then at the fused spur or isolator. This stops the boiler firing and stops any pump running. Doing this first means nothing is trying to operate while you isolate everything else.
- Then the fuelGas: the emergency control valve at the meter, handle turned so it is across the pipe. Oil: the valve at the tank, and check the fire valve position. Only do this if the situation calls for it – it is not needed for a simple water leak.
- Then the waterWhich water depends on what is leaking – see below. The heating circuit and the domestic water are separate systems and isolating one does not isolate the other.
- Then relieve pressureOpen a drain point or a tap, as appropriate, so what is left runs out under control rather than under pressure.
An immersion heater with no water around it burns out within seconds – the element relies on water to carry heat away. A boiler firing on a system that has lost its water can damage the heat exchanger. If you have drained anything, make absolutely sure both are off before you walk away.
Which water to isolate
A house has several separate water systems and they are commonly confused. Knowing which one is leaking tells you what to shut.
| What is leaking | What to isolate |
|---|---|
| A cold tap, a toilet, the incoming main | The internal stopcock |
| A hot tap, on a combi | The internal stopcock – a combi has no store |
| A hot tap, on a cylinder system | The stopcock, plus the cold feed to the cylinder. Stored hot water keeps coming until the cylinder empties |
| A radiator or heating pipe | The heating circuit – the radiator’s own valves if it is one radiator, otherwise the system’s isolation valves or the filling loop plus a drain point |
| A pipe in the loft, or the cold tank overflowing | The stopcock, and the tank’s outlet valve if it has one |
| The boiler itself | Electrically first, then the valves on the boiler – most have isolation on flow, return and cold feed |
| Something you cannot identify | The stopcock, then open all cold taps and see what continues |
The key insight: turning the stopcock off stops water arriving. It does not stop water already stored in a loft tank, a cylinder or the heating circuit from continuing to come out. On a gravity system, several hundred litres can keep flowing after the mains is off.
Isolating a single radiator
The commonest controlled isolation, and worth knowing how to do.
- Close the control valve – the thermostatic valve or the hand wheel – by turning it fully clockwise, or to the closed position on a TRV.
- Close the lockshield at the other end. Take the cap off, and turn the spindle fully clockwise with a small spanner or pliers. Count the turns – you will need to reopen it exactly the same amount to preserve the balancing.
- Put a tray and towels underneath before doing anything else.
- Open the bleed valve slowly to let air in and water out under control.
- The radiator can now be drained or left isolated.
Counting the lockshield turns matters. That valve was set to balance the system, and reopening it fully will take flow away from the rest of the house. See balancing a heating system.
Draining a system down properly
Sometimes necessary, and worth doing deliberately rather than urgently.
- Turn everything off first – electrics, and any immersion heater.
- Close the filling loop and isolate the cold feed.
- Attach a hose to the drain point, which is normally at the lowest point of the system, and run it somewhere the water can go. System water is black and it stains – not onto a light-coloured patio, and not into a pond.
- Open the drain valve, then open bleed valves upstairs to let air in. Without air entering at the top, the system will not drain properly.
- Work from the top down on the bleed valves as it empties.
- Expect it to be dirty. Heating system water is full of magnetite.
- Refilling needs care – slowly, bleeding as you go, and the system needs re-dosing with inhibitor because the old inhibitor has gone down the drain.
Every full or partial drain-down removes the corrosion inhibitor along with the water. Refilling without re-dosing leaves the system unprotected, and that is how magnetite forms. It is the most commonly forgotten step after any heating work – see magnetic system filters.
Isolating for a long absence
A property standing empty, particularly over winter, needs a decision rather than a default.
| Approach | Suits | Watch out for |
|---|---|---|
| Heating on frost setting, water on | Most absences, and commonly a policy condition | Somebody needs to check the property. A failure while you are away is unattended |
| Water off at the stopcock, heating on frost setting | Longer absences | Stored water remains in tanks and cylinders. A leak from stored water can still occur |
| Fully drained down | Long absences, or an unheated property | Needs doing properly – draining, not just switching off. Refilling needs care and re-dosing |
| Everything off, nothing drained | Nothing. Do not do this in winter | This is how empty properties are destroyed |
Check your insurance wording too – many policies impose conditions on unoccupied properties, frequently including a minimum temperature or a requirement to drain down beyond a certain period.
Going away over winter? Frost setting and somebody checking, or a proper drain-down. Not everything switched off and hoped for the best.
Questions we get asked about this
How do I turn my heating off in an emergency?
Electrically first – at the programmer and then the fused spur or isolator. That stops the boiler firing and the pump running, which is what matters. Fuel and water isolation come after.
The stopcock is off but water is still coming out.
It is stored water – a loft tank, the hot cylinder, or the heating circuit. Each needs its own isolation. Open the taps and let it drain, and identify which system it is coming from.
Can I isolate one radiator myself?
Yes. Close the control valve, then the lockshield, counting the turns so you can restore the balance afterwards. Have a tray ready before opening the bleed valve.
Do I need to re-dose inhibitor after draining?
Yes. The inhibitor goes down the drain with the water. Refilling without re-dosing leaves the system to corrode, and it is the most commonly forgotten step after heating work.
What should I do with the heating if the house is empty in winter?
Either leave it on a frost setting with somebody checking the property, or drain it down properly. Check your insurance conditions for unoccupied periods as well.
Is it safe to leave the immersion heater on with the water off?
No. If the cylinder empties, the element burns out in seconds. Turn it off before isolating or draining anything.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.