Draining a Heating System

People often ask whether the heating should be drained every year before winter. It should not. Water that has circulated in the system for years has lost its oxygen, and the limescale has already dropped out of it. That makes it gentler on the pipes and boiler than fresh tap water. Every refill brings in new oxygen and more limescale, which encourages corrosion and sludge. Draining without a reason does more harm than good.

Draining makes sense mainly in these cases:

• Replacing or moving a radiator that cannot be isolated with its own valves
• Repairing or replacing pipes, valves, the pump or the boiler
• Flushing a sludged-up system, for example when an old boiler is replaced with a new one
• Shutting down a property for winter when it will not be kept frost-free

To change a single radiator, you often do not need to drain the whole house. If the radiator has a valve at each end that can be closed, the heating engineer shuts both and drains just that one radiator.

Radiator central heating

The water comes out through a drain-off valve at the lowest point of the system. It is usually near the boiler, in a cellar, or at the bottom of the lowest radiator. A hose goes onto the valve and runs to a drain or outside. The engineer then opens the bleed valves on the radiators one by one, starting upstairs and working down. This lets air in so the water can flow out.

Before that, the water supply has to be shut off. On a sealed system with a combi or system boiler, this means making sure the filling loop is closed. On an older open-vented system with a feed and expansion tank in the loft, the supply to that tank is shut off, otherwise the system keeps refilling itself while you drain it.

Underfloor heating

Underfloor heating has long loops of pipe lying flat under the floor, so gravity alone will not empty them. If the system must be drained because of frost, the engineer blows each loop out with compressed air or flushes it from the manifold. In most other cases the underfloor loops are simply closed off at the manifold and left full.

Flats

Many flats have their own boiler, and then the system is yours to drain like a small house. If your flat is connected to a communal heating system for the whole building, do not drain it yourself. The risers are shared pipework, so draining and refilling them is arranged by the building management or owners' association. Without their agreement you could leave a whole section of the building without heat. Before you change a radiator, contact them first.

The job is quick when the system is simple and the drain-off valve is easy to reach, the valves work properly and everything is on one floor. These situations take more time:

• Many radiators or several floors
• A mix of radiators and underfloor heating
• Seized or leaking valves that may break when they are turned
• No drain-off valve, so a radiator coupling has to be loosened instead
• Heavily sludged water that blocks the valves and calls for a flush
• Antifreeze (glycol) in the system, which must not go down the drain and has to be collected and taken for proper disposal

A practical homeowner can drain a simple radiator system. Do not touch the gas supply or the boiler's electrics: that work belongs to a qualified, registered gas engineer or an electrician. If you smell gas, leave the property and call the gas emergency line or 112. If you are unsure where the lowest point is or what a valve does, call a heating engineer.

1. Switch off the boiler at the controls or the fused spur, and wait until the radiators feel only lukewarm.
2. Close the filling loop, or on an open-vented system shut off the water supply to the feed and expansion tank.
3. Get a hose, buckets, old towels and a radiator bleed key ready.
4. Fit the hose to the drain-off valve at the lowest point and run the other end to a drain or outside, lower than the valve.
5. Open the drain-off valve.
6. Open the bleed valves on the radiators one at a time, starting on the top floor.
7. Wait until the water stops flowing, and check the lowest radiators, where water can stay behind.
8. For a winter shutdown, also loosen the couplings on the lowest radiators and leave the system open so no water remains anywhere.
9. Before refilling, close all bleed valves and the drain-off valve, and check the expansion vessel pressure as described in the boiler manual.
10. Refill slowly, with water treatment or corrosion inhibitor as the boiler manufacturer recommends, and bleed the radiators from the lowest to the highest.
11. Bring the pressure up to the value stated by the manufacturer, in a house usually around 1 to 1.5 bar when cold.
12. Switch the boiler on, let the system heat up, bleed it again and recheck the pressure.

• Draining a hot system. Hot water under pressure can scald you and also damage seals and floors.
• Forgetting the water supply. Water keeps running and you wonder why the system never empties.
• Water left at low points. During a winter shutdown, a little water in a radiator or a dip in the pipe is enough for frost to split it.
• Forcing old valves. A seized valve snaps easily. It helps to have a spare part ready, or a heating engineer on hand.
• Poor bleeding. Air gurgles in the pipes, radiators heat only at the bottom and the pump suffers.
• Wrong pressure. Too low and the boiler shows a fault; too high and the pressure relief valve discharges.

After refilling and switching on, keep an eye on the system for a few hours and check the pressure again over the next few days. Air comes out of the water gradually, so it is normal to bleed the radiators a second time.

• The pressure gauge reads within the boiler manufacturer's range and does not drop once the system is warm
• The boiler shows no fault and the pump runs quietly
• Every radiator heats evenly across its whole surface
• There is no gurgling or hissing in the system
• Joints, valves and the drain-off valve are dry
• The pressure relief valve is not dripping
• With underfloor heating, all loops are open at the manifold and the flow meters show flow
• If parts were replaced, the heating engineer has told you what was changed and to what pressure the system was filled