Drain the central heating: how-to guide

If you need to replace or remove a radiator, you’ll need to drain the central heating system. Fortunately this isn’t a difficult task. You won’t need any tools beyond a screwdriver, and in most cases, you won’t need to go into the loft. This article will tell you what to do.

How to drain the central heating

1. Shut off the power supply

The first thing you need to do is shut off the power supply and leave the system to cool down. This should take an hour or so.

2. Shut off the water supply

If you have a combi or system boiler, you won’t need to do this step. That’s because, so long as the filling loop is closed, the system is already isolated.

If you have a feed and expansion cistern in the loft, you’ll need to shut off its water supply. You can do this by turning off the mains, which in most cases, you’ll find under the kitchen sink. However, this will shut off the water supply to the entire house.

If you don’t want to do that, you can isolate the feed and expansion tank individually. You’ll need to go into the loft and close the isolation valve on its supply pipe. If there isn’t an isolation valve, you can lay a piece of string across the cistern and tie the ball valve to it with some string or garden twine.

3. Find the draincock

The next thing to do is to find the draincock or drain-off valve. This will be at the lowest point on the system, adjacent to a downstairs radiator and hopefully near a door.

Once you’ve found the valve, attach a garden hose to it and run the hose outside. You can use a jubilee clip to make sure the hose is securely connected to the valve. It’s also worth putting an old towel underneath it to catch any drips.

4. Open the draincock

Using a pair of pliers, open the draincock. The system will now start draining. While this is happening, open all of the radiator bleed valves using a radiator bleed key, and open all of the thermostatic radiator valves. This will help the system drain completely.

Job well done – that’s how you drain the central heating system.

5. After draining the central heating system – how to refill it

Once you’ve completed your task, refilling the system is easy. Close the draincock and all of the bleed valves. Restore the water supply to the feed and expansion cistern. If you have a system boiler or a combi boiler, use the filling loop to refill the system. Most sealed systems require a water pressure of around 1 – 1.5 bar.

To keep your system free from rust and corrosion, you should always add inhibitor fluid. Click here for our guide how to do this.

6. Bleed the radiators

Once you’ve refilled the system, you’ll need to bleed the radiators. Start with the downstairs radiators first and work your way up. Click here for our guide how to do this.