Inspired by a helpful YouTube guide. We based this walk-through on BestHeating’s video “How to Drain Down a Pressurised Heating System”. BestHeating is a UK heating supplier, and the guide is tailored to the sealed, pressurised systems found in most homes with a combi boiler - which covers the vast majority of properties in the UK built or updated since the 1990s.
1. Switch off the boiler and let the system cool
Never drain a system that is still hot. Switch the boiler off at the programmer or thermostat and let the water cool for at least an hour, ideally two. Scalding water coming out of a drain-off valve is not a pleasant experience and the risk is entirely avoidable.
While you are waiting, locate the drain-off valve. On most systems it is a small brass tap with a square or slotted head, usually found at the lowest point of the pipework - often near the boiler, under a downstairs radiator, or by the airing cupboard. You will need a hosepipe or drain hose that fits the spigot, and a bucket for the initial flow.
2. Turn off the water supply to the boiler
On a pressurised system fed directly from the mains, find the filling loop - the braided flexible hose that connects the cold mains supply to the heating circuit. Close the isolation valve on the filling loop by turning it 90 degrees so the slot is across the pipe. This prevents the system refilling as you drain it.
If you are not sure which valve is the filling loop isolation, look for a pressure gauge on or near the boiler. It will read around 1–1.5 bar when the system is pressurised. That is the circuit you are dealing with.
3. Attach the hose to the drain-off valve
Push a hosepipe firmly onto the spigot of the drain-off valve and run the other end to a drain outside, a large bucket, or a bath - somewhere that can take a significant volume of water. The system may hold 50–100 litres depending on the number of radiators.
Hold the hose on securely or use a jubilee clip. When you open the valve, there will be a rush of water initially and a loose hose will spray the floor.
4. Open the drain-off valve
Use a flat-bladed screwdriver or a radiator bleed key to turn the drain-off valve anti-clockwise. Open it slowly at first to confirm the water is going where you intended before opening it fully.
The water will run out reasonably quickly at first, then slow to a trickle as the pressure drops. At this point, you need to let air into the system from above to keep it flowing.
5. Bleed the upstairs radiators to let air in
Open the bleed valves on the upstairs radiators using a radiator bleed key. Start at the highest point in the house and work down. You will hear air entering and the water will start flowing freely again from the drain-off valve below.
You do not need to bleed every radiator at once - just open enough to keep the flow going. Leave the bleed valves open while the system drains. The system will tell you when it is empty: the flow from the drain-off valve will stop, and you will hear no more gurgling from the pipes.
6. Drain the final water from stubborn radiators
Even after the main drain is finished, individual radiators may hold a litre or two of water. If you are removing a radiator, close both its lockshield and thermostatic valves before undoing the connections, and have a towel and a bowl ready for the residual water inside the body of the radiator itself.
To be fair, most people underestimate how much mess this step makes. Old, black heating water stains towels permanently. Use rags or old towels you do not mind losing.
7. Refill and repressurise the system
Once the work is done, close the drain-off valve and all the bleed valves. Reopen the filling loop isolation valve slowly to let water back into the system. Watch the pressure gauge - stop when it reaches 1.5 bar. Bleed any radiators that feel cold at the top to purge trapped air, then top up the pressure to 1.5–2 bar if it has dropped.
Switch the boiler back on and run it through a heating cycle. Check around the drain-off valve and any joints you worked on for leaks. If the pressure drops over the next few days, there is likely still air in the system - bleed again and top up.
When to call a handyman
Draining down to bleed radiators or add inhibitor is something a confident DIYer can manage. But if you are removing a radiator, replacing a valve, or the system keeps losing pressure even after the drain and refill, it is worth having Richard take a look. Persistent pressure loss usually means a leak somewhere - and that is better found and fixed before it gets worse.
Need heating maintenance help?
The Sandwich Handyman can assist with radiator removal, valve replacement, inhibitor dosing, and general heating system maintenance in Sandwich and East Kent.
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