How to Drain Down a Central Heating System, and When Not To

When a drain down is necessary, and when it is not

Draining a heating system is a bigger intervention than it looks. It removes the inhibited water the system has been protected by, it introduces oxygen, and it disturbs joints that have been undisturbed for years. Sometimes it is unavoidable. Often it is not, and a partial isolation does the job with a fraction of the consequences.

The question to ask before starting is narrow: does the work require the whole circuit to be empty, or only one part of it? Most radiator work, most valve work and a good deal of pipe work can be done with the rest of the system still full.

Job	Full drain down	Better alternative
Removing one radiator to decorate	Not needed	Close both valves, drain the radiator only
Replacing a radiator valve	Usually not	Freeze the pipe or isolate the branch, then drain locally
Replacing a boiler	Yes	Combine with a chemical clean and fresh water flush
Chemical clean or power flush	Yes, as part of the process	None, the water has to be changed
Tracer gas leak detection on the circuit	Yes	None, the pipe has to be empty to take gas
Work on buried pipework in a screed	Usually	Isolate the affected zone at a manifold where one exists
Adding a magnetic filter	Partial	Drain to below the fitting position only

There is a cost to the unnecessary drain down that is easy to overlook. Fresh mains water is oxygenated, and oxygen is what drives the corrosion that produces magnetite sludge in a steel radiator system. Every refill is a fresh dose. Our article on central heating sludge and corrosion covers how that damage accumulates and why a system drained repeatedly ages faster than one left alone.

Sealed and open vented systems drain differently

Before touching anything, establish which type of system you have, because the isolation step is completely different and getting it wrong means refilling a system that is still being fed.

Identifying which you have

A sealed system has a pressure gauge on or near the boiler, a filling loop, and no water tanks in the loft beyond a domestic cold water cistern if one exists at all. A combination boiler is always on a sealed system. An open vented system has a small feed and expansion cistern in the loft, separate from any larger cold water storage tank, with an open vent pipe curling over it and a ball valve to top it up. That cistern is the reason the isolation step differs: leave it live and it will simply refill the system as fast as you drain it.

On a sealed system you isolate by closing the filling loop valves and disconnecting the loop. On an open vented system you tie up the ball valve in the feed and expansion cistern, or close the valve on its supply, so nothing replaces what you take out.

The drain down sequence

Preparation, before a single valve is turned

Let the system go cold. Heating water at operating temperature will scald, and a drain cock opened on a hot system is a genuine injury risk rather than a theoretical one. Switch the heating off and leave it for at least a couple of hours, longer on a system with a large cylinder and a lot of pipework.

• Turn off the boiler and, on an open vented system, the immersion heater if one is fitted
• Isolate the water feed: close and disconnect the filling loop, or tie up the ball valve in the feed and expansion cistern
• Locate the drain cocks, usually at the lowest points of the system and often on the downstairs radiators or at the bottom of a pipe drop
• Open every thermostatic and lockshield radiator valve fully, so nothing traps water above a closed valve
• Have a hose long enough to reach a drain or gully, a jubilee clip to secure it, towels, and a container for the first dirty flush
• Photograph the lockshield valve positions, and count the turns as you open them, so the balance can be restored

That last point is the one most often skipped and the one most often regretted. Lockshield valves are set to balance flow across the system, and reopening them all without recording where they were means rebalancing every radiator afterwards.

Draining the system

Attach the hose to the lowest drain cock, secure it with the clip, and run it to a gully or drain outside. Heating water is black, it stains, and it carries inhibitor and corrosion products, so it does not belong on a lawn, in a flower bed or down a soakaway.

Open the drain cock and let it run. The flow will slow as the system empties down to the level of the vents, and at that point air has to be admitted for the rest to come out. On an open vented system this happens naturally through the vent pipe. On a sealed system, open the bleed screws on the upstairs radiators first, then work down, which lets air in behind the water and allows the lower parts of the system to empty. Opening downstairs bleed screws early achieves nothing except a wet floor.

Where there is more than one drain cock, use the others in turn to clear sections the first one cannot reach, and expect residual water in horizontal runs that have no fall to a drain point. A system is never completely empty; it is empty enough to work on.

Refilling, venting and recommissioning

Refilling is where most of the avoidable problems are created, almost always by doing it too quickly. Water entering fast pushes air into pockets rather than ahead of itself, and the airlocks that result can take longer to clear than the whole drain down took.

Close every drain cock and every bleed screw first. On a sealed system, reconnect the filling loop and open it slowly, watching the gauge, until the cold fill pressure specified by the boiler manufacturer is reached. On an open vented system, release the ball valve and let the feed and expansion cistern fill the system by gravity. In both cases, work round the radiators from the lowest to the highest, bleeding each one until water appears, then move on. The correct technique for that, including why a radiator that is cold at the top is not always full of air, is covered in our guide to bleeding a radiator properly.

Once the system is full and vented, three things remain. Re-dose the inhibitor. Reset the lockshield valves to the positions you recorded. And bring the pressure back to the manufacturer's figure once the air has finished coming out, which usually means a second top-up after the first heating cycle; our guide to repressurising a boiler covers that step and the mistakes around it.

The inhibitor requirement is not optional

BS 7593:2019, the code of practice for the preparation, commissioning and maintenance of domestic central heating and cooling water systems, sets the expectation clearly: the system is chemically cleaned and flushed with fresh water before inhibitor is added, inhibitor is topped up after any work that drains the system, its level is checked annually, and the water is re-dosed or fully tested every five years. The same standard expects a permanent in-line filter capable of capturing magnetic and non-magnetic debris on new and replacement boiler installations, and the energy efficiency guidance in Approved Document L points at BS 7593 for water treatment when a system is installed or a boiler replaced.

The practical reading of that is simple. A system refilled with plain mains water and left is unprotected, and the corrosion clock starts again from the moment the last drain cock closes.

What a drain down cannot do

A drain down empties a system. It does not clean one, and the difference matters because the two are routinely confused.

Sludge is heavy and settles. Magnetite lies in the bottom of radiator panels, in horizontal pipe runs and in the low points of the circuit, and draining by gravity carries away the water above it while leaving a good deal of the deposit exactly where it was. Removing that requires a chemical clean with a circulating cleanser, or a power flush that moves water at velocity through each emitter in turn, or in bad cases the removal and hosing out of individual radiators. A drain and refill that is described as a clean is neither.

Nor does a drain down fix a leak, and it can reveal several. The cycle of emptying, cooling, refilling and reheating works every joint in the system through a full range of thermal movement, and compression fittings that had been holding at the edge of their tolerance for years frequently choose that moment to weep. Old radiator valve glands are the classic case: a gland that has been static for a decade will often pass once the spindle is turned. This is not damage caused by carelessness; it is a system revealing its condition. But it does mean a drain down should be scheduled with time in hand rather than at four on a Friday afternoon. Where a leak is the reason for the drain down in the first place, our guide to finding a central heating leak covers the tests that should come before it.

Problem	Why it happens	Prepare by
Drain cock will not open, or will not close again	Corroded or scaled after years unused	Having a replacement drain cock on hand before starting
Radiator valve gland weeps after being turned	Packing hardened around a static spindle	Carrying gland packing or replacement valves
System will not vent, radiators stay cold at the top	Refilled too fast, air trapped in pockets	Filling slowly and bleeding from the bottom upward
Pressure drops after the first heating cycle	Dissolved air coming out of solution, not a leak	Expecting a second top-up and re-bleed
Radiators unbalanced, some hot and some cool	Lockshield settings lost when valves were opened	Recording turns or photographing positions first
Joint weeps on refill	Thermal movement through the cycle	Allowing time in the schedule for remedial work

Checks once the system is back in service

The job is not finished when the radiators get hot. A refilled system is still shedding dissolved air, its joints have just been through a full thermal cycle, and its protection has been replaced rather than proven. The following week is when problems declare themselves, and a short round of checks catches nearly all of them while they are still small.

• Read the pressure gauge cold on the first, third and seventh days and write the numbers down; a drop after the first cycle is usually air coming out of solution, a drop that continues is not
• Re-bleed the radiators once after the first full heating cycle, then again a few days later if any are cold at the top
• Run a dry tissue round the underside of every radiator valve union and gland nut, because a weep shows there before it shows on the floor
• Check the floor beneath any joint that was disturbed, and under the boiler, at the end of the first day
• Confirm every drain cock is closed and dry, including the ones you did not use
• Check the external pressure relief discharge pipe after the first few cycles, since an overfilled system will lift the valve
• Feel each radiator across its full face and reset any lockshield that is now out of balance

The weep at a radiator union is the most common find, and it is usually a fitting that needs cleaning and remaking rather than a component that has failed. Our guide to radiator leaks and their causes covers which of these are tightenable, which need new packing, and which mean the valve or the panel has reached the end of its life.

One thing worth resisting: the urge to raise the cold pressure above the manufacturer's figure so that it does not have to be topped up again. An overfilled sealed system has less room for expansion, reaches the relief valve setting sooner, and dumps water outside, which produces exactly the recurring loss the higher fill was meant to avoid.

Where the competence line sits

The wet side of a heating system, meaning the radiators, valves, pipework and drain cocks, is plumbing. The appliance is not. Under the Gas Safety (Installation and Use) Regulations 1998, work on a gas appliance must be carried out by a person who is competent and registered with the Gas Safe Register, and that boundary does not move because the task sounds like plumbing.

Task	Who
Draining and refilling the wet circuit	Competent plumber, or a capable householder on a straightforward system
Removing and refitting a radiator	Plumber or capable householder
Dosing inhibitor and fitting an in-line filter on pipework	Plumber
Anything behind the boiler casing	Gas Safe registered engineer only
Expansion vessel or pressure relief valve inside the appliance	Gas Safe registered engineer only
Recommissioning the boiler after a system refill	Gas Safe registered engineer

Two further cautions. On an unvented hot water system, the stored cylinder is a separate installation with its own safety devices and its own competence requirements, and it is not part of the heating drain down. And in a flat or a block served by communal heating or a heat interface unit, the circuit may not be yours to drain at all; the managing agent decides, and draining a shared riser without permission is a conversation nobody wants to have afterwards.