Two different jobs called the same thing
"Draining down" means one of two things, and in an emergency you almost always want the first.
- Draining the cold water system. Stop tap off, every cold tap open, toilets flushed. Two minutes, no tools, and it empties the pipework that is currently emptying itself into your ceiling. This is the emergency move, and it is covered in what to do if a pipe bursts.
- Draining the heating circuit. A slower, deliberate job for replacing a radiator, working on pipework, or leaving a property empty over winter. That is what the rest of this page is about.
It is a job a competent person can do. The parts that go wrong are the parts people rush.
Know which system you have first
| Sign | System | What it changes |
|---|
| Pressure gauge on the boiler and a filling loop | Sealed | Nothing tops it up as you drain. Refilling is done through the loop afterwards. |
| A small tank in the loft, separate from the big cold water cistern | Open-vented | The feed-and-expansion tank will refill the system while you drain it. Its supply must be stopped first. |
The sequence
- Turn the heating off and let it go cold. Off at the programmer, and off at the fused spur beside the boiler if the spur is dry and accessible. Then wait. Heating water runs well above a temperature that scalds, and a drain cock opened on a hot system is how people end up in A and E. Give it a couple of hours, and check a radiator with the back of your hand before you start.
- Stop the system refilling. On a sealed system, make sure the filling loop is closed and detached. On an open-vented system, turn off the service valve on the feed to the loft tank, or tie the float arm up to a batten across the tank so the valve stays shut.
- Find the lowest drain-off cock. A small brass fitting with a square spindle and a hose nozzle, usually on the tail of a downstairs radiator, sometimes on the pipework at the lowest point of the property, occasionally outside on an external wall. If there is more than one, use the lowest.
- Hose on, and clipped. Push a garden hose onto the nozzle and secure it with a hose clip. Run it to an outside gully or a drain, downhill the whole way. Do not run it into a bath, a shower tray or a white sink you care about: the water coming out is black with magnetite and it stains.
- Open the drain cock. A square drain-cock key, or a small adjustable spanner. A quarter turn is usually enough.
- Let air in from the top. Once water is flowing, open the bleed valve on the highest radiator in the property, then work down floor by floor. The system drains far faster with air behind it. Have a cloth under each bleed valve.
- Wait it out. The last of it dribbles. Some water always stays in the bottom of downstairs radiators and in low sections of pipe, so put towels down before you disconnect anything.
Refilling, which is where the damage usually gets done
- Close the drain cock and every bleed valve you opened.
- Add inhibitor. Fresh mains water is full of dissolved oxygen, and oxygen plus steel radiators is how you get sludge and then pinholes. Dosing the system on refill is the difference between this being routine maintenance and this being the reason you are back here in three years.
- Refill slowly. On a sealed system, open the filling loop gently and watch the gauge. On an open-vented system, release the float arm and let the tank do it.
- Bleed the radiators from the bottom up this time, downstairs first, working to the top floor.
- Bring a sealed system to 1.0 to 1.5 bar cold — Worcester Bosch puts the cold target in that band, with the needle in the green — and close the loop fully. A loop left cracked open lets mains pressure creep in.
- Walk every joint you disturbed with a dry piece of kitchen paper, then fire the boiler and walk them again once it is hot. Joints that are dry cold and weeping hot are common.
What goes wrong
- A seized drain cock. Old ones shear off. If it will not move under normal spanner pressure, stop — you are one snapped fitting away from an uncontrolled drain with no way to close it.
- A drain cock that will not reseat. Grit from the system lodges in the seat and it drips permanently afterwards. It needs replacing, which means draining again.
- Airlocks on refill. Usually an upstairs circuit that will not warm. Bleeding in the wrong order is the common cause.
- A leak that only appears once the system is back up to pressure. Draining relieves a joint that had been holding; refilling finds it out. Frustrating, but better found now with the tools out.
- A gauge that will not hold afterwards. If the pressure drops overnight, that is a loss, not an air pocket. Start at the boiler pressure hub.
- Leaving a system drained in freezing weather. An empty system cannot freeze, but an empty system in a cold property means the cold water pipework is the thing at risk instead. Drain that too, or leave background heat on.
What you must not do yourself
- Anything behind the boiler casing. Taking the case off is gas work. Under the Gas Safety (Installation and Use) Regulations 1998, work on gas fittings in a home must be done by a Gas Safe registered engineer.
- Anything on an unvented hot water cylinder. Unvented systems come under Part G3 of the Building Regulations and are installer-certified work. Draining, discharging or adjusting one is not a DIY job, and its discharge pipework must never be capped or blocked.
- Work with live electrics in a wet area. Isolate first, or leave it.
When you do not need to drain down at all
Plenty of jobs get a whole system drained when they did not need to. A radiator can be swapped on its own two valves, as set out in how to drain a radiator without draining the system. A basin, toilet or washing machine has a service valve on the feed. A single length of pipework can be pipe-frozen locally so the rest of the system stays full — which is what we do rather than drain a whole block's heating to change one valve.
And if you are draining down because the system keeps losing water, the drain-down is not the fix, it is a delay. That loss has a location. We isolate and pressure-test to size it, then trace it with thermal imaging and, on a drained circuit, tracer gas, so one board comes up instead of a floor. That is emergency leak repair, charged at £150 per hour agreed before we attend, with nothing to pay for the search time if we do not find it. Where the floor has to be opened, our own handymen put it back — see reinstatement.
