How to Drain a Central Heating System
Problems with central heating systems are generally rare, but there are occasions when repairs need to be carried out, which may involve draining the entire system.
If you are not familiar with your heating system, it is recommended that you contact a qualified plumber to undertake any repairs. You should also remember that if any work is required on the boiler or the connection to the mains gas supply, it must be done by a gas safe registered technician.
If it is a relatively easy job such as changing a pump or a radiator, and you feel confident working with central heating systems, then you will probably need to drain the system first.
Only deal with any problems if you are familiar with your system.
Advice on how to drain central heating systems is given below as a general guide, but if you are in any doubt about the system or the components within it, you should consult a qualified professional.
Why drain the system
Many of the typical problems with central heating require the system to be fully drained before repairs can be carried out. If the system needs a flush to clear sludge and debris from the pipe work, the boiler, and the radiators, it must be fully drained first, so the cleansing agents can be pumped through the system.
If a radiator or a leaking length of pipe need to be replaced, it is essential to drain the system so the offending part can be removed.
Most central heating pumps are installed with an isolation valve either side, so they can be easily taken out of the system; if yours is not fitted with the valves, it will be necessary to drain the entire system before replacing the pump.
When a new pump is fitted, make sure it is installed with isolation valves; it will save you coming up against the same problem next time around.
Can you do it yourself?
You can drain the system yourself and carry out any repairs as necessary. However, this is only recommended if you have some experience with heating systems, or are particularly confident with DIY.
It is a legal requirement that any work on a gas boiler or the pipe work supplying it should be carried out by a gas safe registered technician.
It is also a requirement that work on a hot water cylinder is only undertaken by an engineer with a UHWSS (Unvented Hot Water Storage Systems) certificate. If any work is required to either of those parts of the system, you should contact a professional.
Legally any work on a gas boiler or the pipe work supplying it, should be carried out by a gas safe registered technician.
How to drain a central heating system
If you are draining central heating it is vital to know exactly what type of system you have, and how it works. The guide below will give you a rough idea of the process of draining a standard pumped system with a standard boiler, not a combination boiler.
The first thing to do is to make sure that neither the boiler nor the pump will operate while you are working on the system. Set the thermostat on the boiler to the off position, and turn off any controls.
Locate the header tank (the smaller of the two tanks in the loft), and check that the float valve is operating correctly.
You need to isolate the water supply, which can be done via an isolation valve on the cold water supply if you have one, or by securing the float in a closed position.
You should connect a piece of hose to the lowest drain off valve in the system, and run it to the nearest drain; once the hose is secure, the drain valve can be opened. You can open the radiator bleed valves on the upstairs radiators only to begin with, which will allow air to enter the system and water to flow out.
When the water level is below the top of the ground floor radiators, open the bleed valves on those too. While the system is draining down, be sure to check the header tank to confirm that no new water is entering the system.
When the system is completely drained repairs can be carried out or the system can be flushed with chemicals to clean it. After you have completed the necessary work, the system needs to be refilled. Remove the hose and close off the drain valve.
With the bleed valves still open, reinstate the cold water supply and allow the system to start filling. When you get down from the loft close off all the bleed valves, and bleed each radiator in turn to purge all the air from the system.
When you are completely sure that each of the radiators is bled of air, allow the boiler to run at a reduced setting for a few hours. Go around each radiator again to check that there is no more air to remove, before turning the boiler back up to your desired setting.
Rebalancing the system
After a central heating system has been drained, it makes sense to rebalance the system to make sure it runs at its most efficient. The idea of balancing a system is to get an equal temperature drop of around 12oC across all the radiators, which is measured from inlet to outlet. This means that the system is using the least energy to produce the desired heat.
If a pump is too fast for the system, the temperature drop will be lower, meaning less heat is emitted into the room. Balancing requires the adjustment of the pump, the boiler, the room thermostat, and individual thermostatic radiator valves to achieve the correct temperature drop; but once it is done, it should not need changing again unless an alteration is made to the system.
Summary: Draining Central Heating
Draining a central heating system is not a simple task, but it may be required if there is a problem with the heating. If you have any doubts about how the system works or where each part of the system is, it is advisable to contact a professional plumber to quote for the repairs.
