There are other ways, but I’ve found this way easiest.
Ensure that the boiler and pump will NOT fire up while performing the steps below. Locate the boiler thermostat and turn it to the OFF or minimum position, and all control switches to the off position.
Remove the lagging and covers from the header tank, and inspect the float- valve to ensure it is operative (i.e. it allows some water to flow in when pushed down, and water flow stops when it rises to its normal level point. If water is inside the plastic ball, replace it. You can get a replacement ball from SCREWFIX for about 32p, but a local plumbers merchant will most likely only stock the complete valve. Place the wooden stick across the top of the tank, and make an “S” shaped hook with a piece of stiff copper wire, large enough to hook over the stick and under the ball valve rod. This will prevent it refilling the system, without having to turn off the cold water supply to the tank. Of course, if you have an isolator (stop-tap, gate valve, or lever valve) in the “rising cold water pipe” feeding the tank, then just use that instead. Place the hose pipe on the lowest drain-cock on the system, and lead the pipe out to a drain, ensuring that the hose is not higher than any radiator you might be changing. Tighten up with a Jubilee clip if a loose fit.
Open the drain-valve. Using the radiator key, open the upstairs radiator bleed valves a few turns, to allow air into the system, to allow water to flow out. When the water level is below the top of the downstairs radiators, open those too (unless you are only changing upstairs radiators, or fitting new valves). While the system is draining, check the header tank to ensure that no water is being fed back into it from the float-valve.
If the inside of the tank is very sludged up with brown algae ridden water, it is well worth getting rid of it. There is always about 2 to 3cm. water left in the bottom of these tanks, because the outlet pipe is raised up deliberately, to reduce the risk of foreign objects and old builders debris getting into the system. if you DO clean this out, CAREFULLY remove the old water with a small container, ensuring you do not disturb any large bits in the sediment. Remove any debris, and wipe clean. A good quality inhibitor should reduce algae build-up.
Check that draining has finished, remove the hose, close the drain-valve. Close off all the DOWNSTAIRS radiator bleed valve screws. If changing radiators or valves, or adding THERMOSTATIC RAD VALVES (TRV’s), now is the time to do it. If adding inhibitor, cleanser, descaler, or sludge remover, now is the time to add it to the system, in the header tank. FOLLOW THE MANUFACTURERS INSTRUCTIONS, AND ANY SAFETY RELATED LEAFLETS.
If you are sure that all the downstairs bleed valves are closed, unhook the wire from the wooden stick holding the ball-valve up, and allow water to flow into the system. You can now add the inhibitor (or cleanser, or descaler, depending upon the state of your system) into the header tank, while the system is being refilled. DO NOT WAIT UNTIL IT FILLS COMPLETELY, OTHERWISE THE INHIBITOR WILL NOT BE PUMPED AROUND THE SYSTEM IF YOU JUST FILL THE TANK WITH IT, IT NEEDS TO GET INTO THE PIPES AND RADIATORS. Although you will hear air still coming out of the upstairs radiators at this point, if you left them open while you were in the loft, screw all the bleed screws home now, so you can control the air bleed from each one in turn.
You will need to bleed all radiators, ensuring no air is in the system. This can be aided by keeping the boiler in the OFF position, but keeping the room and tank thermostats higher than normal, to make the pump run. This also helps to eliminate airlocks. Most modern systems include an “AIR SEPARATION MANIFOLD”, usually fitted near to the the pump. It will have a riser-pipe going into the loft. This curves over the top of the header tank and doubles up as a pressure relief pipe and air disperser. Do not worry if one or two of the smaller radiators stay cold for a day or so after refilling and bleeding, you should find that this will rectify itself by the air separator doing its job.
Only when you are sure that all the radiators are bled of air, should you allow the boiler to fire-up (put it on a cool boiler thermostat setting at first). After a couple of hours circulating, go around all the radiators again, and them put your boiler thermostat up from cool, to its normal setting. Some cleansers and descalers require the system to run hot for a number of hours before draining down again. FOLLOW THE MANUFACTURERS INSTRUCTIONS CAREFULLY.
If you have used a cleanser or descaler, follow the instructions as regards emptying the system, ensuring that there is no cleanser or descaler present in the water flowing out. You can let the header tank attempt to fill with fresh water at this point, as this will aid the flushing out of any “deflocculated” sludge, and speed up the cleanser removal. Ensure that your final system fill includes the correct quantity of INHIBITOR. NEVER RUN A SYSTEM FOR LONG PERIODS WITHOUT AN INHIBITOR. This would otherwise reduce the life of the pump, and promote rust and corrosion inside radiators, and boiler noises. HINT:Make sure you pour your inhibitor in when the system is around half full (but with the header tank still empty), it should then get circulated throughout the system once completely full, when you fire up your pump and boiler. DON’T put the inhibitor in when the header tank is nearly full, it will take MONTHS to all get into the system, as the concentrate will all be in the tank!
You may want to rebalance your radiators at this point, especially if you have fitted new radiators or valves.