How to Drain Down a Central Heating System

These instructions are for an open vented system only! If your system is sealed please find a qualified plumber.

For more ideas and information regarding central heating look at the rest of the central heating projects on this website that can be accessed from our "Heating projects page". Before reading this project it is worth thinking about the heating system you have. If your boiler is over 10 years old you could be wasting half the gas you buy!

Why Would I Need to Dain Down my Heating System?

Sometimes it is necessary to drain down a heating system. This may be because you need to introduce an agent to clean it and flush it through, removing sludge etc or simply to change a radiator.

In all events it is necessary to turn off the boiler. Turn off the gas or electricity supply to the boiler or in the case of a back boiler or solid fuel, make sure it is out.

If this is necessary you might like to have a look at the effectiveness of your boiler.

The Feed and Expansion Tank

In your loft you will have a tank called a feed and expansion tank. Its recognizable by a pipe coming in at the top which is the expansion part of the equation. If the water gets too hot in your system the steam can expand, via the pipe, into this tank.

The water is fed into this tank from the mains via a ball valve. This valve is the same as the one in your toilet cistern ( see our tanks and cisterns project), When the water rises it lifts the ballcock.

The ball is attached to an arm which closes the valve when it is lifted. When the tank is full of water, no more can get in because the valve is closed. When some water is drawn off, the ball drops, the valve opens and water rushes in to replace it.

To ensure no water comes in while you are draining the system down it is an easy job to place a piece of timber across the tank, lift up the valve arm and tie it to the timber. This will close the valve.

Draining Down Your Heating System

Now connect a hosepipe to the drain nozzle (marked outlet in the below image) and run it outside to a suitable point. Make sure, especially in winter, that the water does not run onto the road or pavement where it could freeze and cause accidents.

Using an adjustable wrench, open up the drain valve and let the water from the system run through.

The water will run faster if you open any bleed valves on the radiators starting with the radiators at the top of the building. As the water level drops you can open the bleed valves in the downstairs radiators also. New valves and tools can be bought from the tool store below.

When no more water comes out of the hose you must check that the system has completley finished draining before you remove radiators or start work.

There is a chance that some air has got into the system and locked the water from escaping. To remedy this go into the loft and fill the tank with about 6 inches of water by loosening the arm you have tied up.

This should, in a few seconds, start running out of the hose. If it does not you have an air lock and should connect the other end of the hose to the cold tap and send a short blast of water back into the radiator you are draining from. Make absolutely sure the hose is well attached to the radiator drain nozzle.

Quick Step Checklist for Draining Down

1. Check that you have an open vented heating system (consult with a qualified plumber if not)
2. Turn off the gas or electricity supply to your boiler or make sure that any solid fuel based boilers are out
3. Lock off the ball valve in your feed and expansion tank
4. Connect a hosepipe to the drain nozzle on one of your downstairs radiators and run it to a suitable location outside
5. Open up the drain valve and let the water drain out of the system
6. Open up some of the bleed valves on upstairs radiators to aid drainage
7. Check that you have no airlocks in the system preventing full drainage
8. Once fully drained, carry out your desired task

Refilling Your Heating System After Draining Down

Close the drain cock on the radiator and all the bleed valves that you have opened.

Untie the string in the feed tank and let the water fill up the system via the tank. Wait until the tank stops filling and go downstairs to bleed the bottom level radiators.

When they have been bled repeat with the top radiators upstairs. This should ensure your system is filled.

You may also wish to add an inhibitor to your system to prevent corrosion and limescale.

Double check the work you have done to ensure you have re-tightened all nuts etc and turn on your power supply and relight the boiler.

As the water heats up you may hear some knocking sounds as any air expands in the system. The radiators will probably need bleeding again once the system is fully heated.

Once the system has reached full heat/pressure, look at your work again to check for leaks.

Quick Step Checklist for Refilling Your Heating System

1. Close off and tighten up any radiator drain cocks and bleed valves that you have loosend
2. Add any inhibitor to your system as required
3. Unlock the feed and expansion tank valve in your loft and allow the system to refill
4. Once full, bleed all the bottom level radiators
5. Now, bleed all top level radiators
6. Double check all valves, joints and bleed valves and ensure that they are tight
7. Turn on power supply and relight boiler
8. Once system fully heated, bleed all radiators once more
9. Check all valves and joints once more for leaks

You might like to go to our video section on draining down and watch the draining down film that shows how to drain a central heating radiator.