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How to Drain a Radiator
There are two jobs here and people arrive looking for one without knowing which. Draining a single radiator so it can come off the wall is a twenty minute job you can do with the rest of the system untouched. Draining the whole system is a different exercise entirely. Establishing which you need is the first step.
The Short Answer
To take one radiator off, you do not drain the system. Close the valves at both ends, put towels and a bowl underneath, crack the union nut at one end and let that radiator empty into the bowl. Twenty minutes and the rest of the house keeps its water. You only drain the whole system when you are working on the pipework itself, replacing a component in the circuit, or flushing it, and that is a longer job that ends with refilling, rebalancing and adding inhibitor.
One Radiator, or the Whole System?
Almost everyone asking this wants the first one, so be clear about the difference before going near a spanner.
Draining one radiator means isolating that radiator at its two valves and emptying only what is inside it. The rest of the system stays full and under pressure. This is what you do to take a radiator off for decorating, to move it, to replace it, or to flush it out.
Draining the system means emptying the entire circuit through a drain-off cock, so every radiator and every pipe run is empty. You need this for work on the pipework, for replacing a pump or a valve in the circuit, and for a proper flush.
The distinction matters because draining the system is not just more work, it has consequences. Refilling introduces fresh oxygenated water into a sealed circuit, which is what drives corrosion, so a full drain-down needs inhibitor putting back afterwards. Doing that unnecessarily to take one radiator off is a genuine cost to the system.
Draining One Radiator
The job most people want. You need an adjustable spanner, a radiator bleed key, old towels, a low bowl or roasting tin, and a second container to empty into.
- Turn the heating off and let the radiator go cold. Working on a hot radiator is unpleasant and the water in it will be scalding.
- Close both valves. One end has a control valve, a wheel head or a thermostatic head, which turns clockwise to shut. The other end has the lockshield, usually under a plastic cap, which needs a spanner or pliers. COUNT the turns as you close the lockshield and write the number down, because that setting is what balances the radiator against the rest of the system and putting it back wrong leaves this room cold or another room cold.
- Towels down, generously, and the bowl under the valve you are going to open. Radiators hold more water than they look like they do and the water inside is black.
- Loosen the union nut at the bowl end, slowly, a quarter turn at a time. Water will start to come out. Let it run into the bowl rather than opening the nut fully.
- Open the bleed valve at the top once the flow slows. That lets air in and the rest drains out. Without it the radiator holds its water the way a finger over a straw does.
- Empty the bowl as often as needed. This is the part that takes the time, and have the second container so you are not carrying a full bowl through a room you are about to decorate.
- Once it stops, undo the union nut at the other end, lift the radiator off its brackets, and tilt it as you carry it because there will still be water inside.
Draining the Whole System
Longer, and worth understanding before starting because the refill is the part people underestimate.
Turn the heating off and let everything cool. Then shut off the water feed to the system: on a sealed system that means making sure the filling loop is closed and disconnected; on an open vented system it means tying up the float valve in the feed and expansion tank in the loft so it cannot refill as you drain.
Find the drain-off cock, usually at the lowest point of the system, often on a downstairs radiator tail or near the boiler. Attach a hose, run it to an outside drain, and open it. Then open the bleed valves on the upstairs radiators first, working down, which lets air in and speeds the whole thing considerably.
It takes longer than people expect, and the water is black and stains. Where the hose runs across anything you care about, protect it.
Refilling is the half that matters. Close the drain cock, restore the feed, and fill slowly with a bleed valve open at the highest radiator so air escapes rather than being trapped. Then work round every radiator bleeding air out, top the pressure back to about 1.2 bar on a sealed system, and add inhibitor.
The inhibitor is not optional maintenance advice. You have just replaced the entire water content of the system with fresh oxygenated mains water, and oxygen is what corrodes radiators from the inside and produces the black sludge you have just drained out. Refilling without inhibitor starts that process again from scratch.
The Lockshield Setting
Worth its own section because it is the single most common thing to get wrong, and the consequence appears weeks later when nobody connects it to the radiator that came off.
The lockshield valve is not an on-off tap. It is set, once, to control how much flow that particular radiator gets relative to every other radiator on the system. That balancing is what stops the radiator nearest the boiler getting all the hot water while the one at the end of the run stays lukewarm.
So when you close it to remove a radiator, count the turns precisely and write the number somewhere that will still exist when you refit. Half turns count. On refitting, open it exactly that far and no further.
If you have already closed one without counting, the recovery is to open it fully, then close it fully, then open it about half way as a starting point, and rebalance by feel over a few heating cycles. That works but it is considerably more effort than writing down a number.
Putting It Back
The refit is the reverse and there are three things worth doing properly.
Check the union nut olives and the tail threads before reassembling. If a nut was difficult to undo or the olive looks distorted, replace it now while the radiator is off rather than after it is full and hung.
Hang the radiator, tighten both union nuts firmly but without forcing them, and open the valves slowly. Open the lockshield to its counted setting first, then the control valve.
Then bleed it, and watch both unions for several minutes as the system comes back up. Dry them with a cloth and look again after ten minutes, because a weep shows as a bead on dry metal long before it drips onto a floor.
Check the boiler pressure afterwards on a sealed system. Removing and refitting a radiator loses a little water, and the gauge will usually want topping back to around 1.2 bar.
Getting a Full Radiator Out of a Tenement Flat
The plumbing part of this job is the same anywhere. The logistics are not, and in a Glasgow flat the logistics are most of the difficulty, which is why it is worth planning before the first nut is loosened.
The water is the problem. A double panel radiator in a decent sized room holds a serious volume, it is black, and it stains everything it touches. In a house you carry a bowl to an outside tap or a garden. In a second floor flat you are carrying it to a bathroom along a hall, which is exactly the journey on which it gets spilled. Two containers rather than one, and a route planned before you start, prevents the commonest disaster of this job.
Original floors make a spill worse than it looks. Tongue and groove boards with gaps and no membrane pass water straight into the void and onward to the ceiling below, so a bowl tipped in a hallway is not a mopping-up job, it is potentially a neighbour's ceiling. Old towels down over a wider area than you think you need is the cheapest insurance available.
And the radiator itself has to leave the room. Tenement halls are narrow, often with a turn, and a full-height double panel radiator is heavy and awkward with two people let alone one. If it is going out of the flat rather than back on the wall, work out whether it fits down the close stair before it is off the brackets, because a radiator standing in a hall for a week is a common outcome.
One local advantage worth knowing: the water in the west of Scotland is soft, so radiators here scale up far less than they do in harder water areas. What accumulates instead is magnetite sludge from internal corrosion, which is a different problem with a different fix, covered in the flushing guide in this cluster.
When to Call Someone Out
Draining one radiator is a customer job and worth doing yourself. Worth calling when a valve will not close or is seized, when a union nut will not move without the pipework twisting, or when you want the system drained down properly for pipework or a component replacement, since that ends with refilling, rebalancing and dosing rather than simply emptying. Also worth it if a radiator is cold after being refitted and you cannot get the balance back. Mo, Muhammad or David cover heating work. If you closed a lockshield without counting the turns, say so, because it saves an engineer working out why the balance is off.
Heating Engineer in GlasgowCommon Questions
No, and you should not. Close both valves, isolate that radiator, and drain only what is inside it into a bowl. The rest of the system stays full. Draining down unnecessarily means refilling with fresh oxygenated water, which drives corrosion, so it costs the system something for no benefit.
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