How to Drain Down an Open Vent Central Heating System

Author: dpinning  //  Category: DIY Tips, Heating, Radiators

Whether you’re starting on a property development overhaul devised by architects or just wanting to remove a radiator to do some painting and decorating or plastering, you’re going to need to drain down your hot water system. Thankfully, you’re not going to need to be an expert in the building trade for this – it’s a ‘doable’ DIY job.

Turn off the electricity or gas to your boiler and locate the expansion tank in the loft – it’ll be the smaller of two tanks. Place a wooden batten across it and tie the float-operated valve to it to stop water inflow. Now find the drain nozzle (there might be more than one, which is good). It’ll be in the lowest part of the system and looks like a little brass barrel pointing downwards from a pipe. That’s where you attach a length of hose, running the other end outside where water can drain away. Turn the little square key on the nozzle with a spanner to open it.

Starting upstairs, open all the bleed valves on your radiators. If you have an airlock, undo the float-operated valve and let about 150mm of water flow into the tank – it should drain out immediately (tie the valve back up again).

Then, complete the work you want to do, and reverse the steps you took earlier: remove the hose, close the drain valve and the bleed valves on the radiators, release the float-operated valve and let the system refill. Bleed all the radiators (beginning downstairs this time) once the tank has filled, and switch the boiler back on.

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