Draining down and re-filling an open vented heating system

In this section we will go through the steps required to drain down a heating system. Some of the main reasons for draining a system are to add/remove a radiator, change valves on radiators, to flush the system through to get rid of Limescale and sludge and also to inhibit the system. We go through the process of how to close off the feed and expansion tank valve, drain down your radiators using the outlet on your radiator valve (or lowest point drain off valve) and then how to refill the system once you’ve finished your jobs.

Vented or unvented?

Before we can continue we need to make sure which system you have, an open-vented or a pressurised system. The easiest way to tell if you have an open vented system is to check for a feed and expansion cistern (also called header tank) the loft. This is a small cistern which has two pipes connected to it, the mains water supply which enters through a float valve near the top and the other pipe near the bottom of the cistern which feeds the central heating. It also has a third pipe called the vent, this pipe is bent over the top of the cistern. It’s the combination of these pipes that keeps the heating system topped up with water when it needs it and also gives the water in the system room to expand through the vent pipe if it gets too hot.

Why Drain Down my Heating System?

Some of the main reasons for draining a system are to add/remove a radiator, change valves on radiators, to flush the system through to get rid of Limescale and sludge and also to inhibit the system. In all events it is necessary to turn off the boiler.

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

Draining your system

  1. Check that you have an open vented heating system (consult with a qualified plumber if not).
  2. Turn off the boiler and also turn the gas or electricity supply to the boiler. If you have a solid fuel boiler make sure the fuel is extinguished.
  3. Lock off the ball valve in your feed and expansion tank. This can be done by placing a piece of timber across the tank and then tying the arm valve to the timber to hold it in the up position [insert pic]. It is possible to stop the feed to this tank by turning off the stopcock but doing this will also stop all the other water outlets in the house, including the toilet cistern.
  4. Connect a hose pipe to one of the drain off valves on the downstairs radiators [insert pic] (sometimes the lowest drain point on a system might be away from a radiator [insert pic]) and run it outside to a suitable place away from any plants, also be careful to not create a tripping hazard.
  5. Use a wrench to open up the drain valve and let the water drain out of the system [insert pic]. Opening up some of the bleed valves on upstairs radiators will speed things up as this lets more air in.
  6. At this point it is possible for your heating system to not be fully drained even tho there is no longer any water running from your hose. This can happen when air gets locked within the pipes and creates a barrier that the water cannot pass through. To check if you have an airlock within your pipes at this point, go back to the feed and expansion cistern, loosen the valve arm and let a good 6 inch or so of water in and then secure up again. If the 6 inches of water doesn’t run away you have an airlock and can remedy this by attaching the hose from the drain off valve to a cold tap and blasting cold water back into the radiator. A word of warning, make sure the hose is well secured to the drain off before attempting this.
  7. Your heating system should now be fully empty and you’re good to get going on the task at hand.

Refilling your system

  1. Close the drain off valve on the radiator and also make double sure all the bleed valves that you have opened have been securely closed.
  2. This is of course optional but at this point I recommend that while your system is empty to add some inhibitor into your system via the feed and expansion cistern as this will protect the inside of your heating system from corrosion and Limescale.
  3. Untie the string holding up the valve arm in the feed and expansion cistern and let the water fill up the system.
  4. Wait until the cistern stops filling and then bleed all the radiators in order from the lowest to the highest.
  5. Now is a good time to double check the work you’ve done, so check all bleed valves, joints and new pipework for signs of leaks.
  6. Turn on the gas/electric supply on to the boiler and then power up and relight the boiler. You may hear some banging at this point as any air still in the system expands as the water heats up. Once the system is fully heated all the radiators will most likely need bleeding again
  7. Now the system is at full heat and pressure check your work one last time to check for leaks.

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