Hiya.
First thing:
something to note (assuming you've got a thermostatic valve on one end of the radiator) - the valve on the other end (I think it's called a lock-shield valve - it's been a long time) is actually used to control the flow of water through the radiator, so when you close it - count the number of turns it takes and make a note somewhere - and when finished putting the new rad on, open it by the same amount
That is actually wrong. It would have been the case if the rad did not have TRV's on, and would have been used to balance the system, but the TRV will control the flow of water to the rad. The lockshield is on the exit side of the rad (with a TRV fitted), while the TRV is on the incoming side (usually, although there are either side TRV's available)
The first question to ask is why you want to replace the rad, and what type of rad is there at the moment! If the rad is a single panel or one panel one K at the moment, then you could keep the same rad size, but move to double panel or double panel double K. This will improve the heating output from the rad, but may allow you to keep the pipework as it is. If this were the case, then you would only need to turn off the locksheild valve, and FULLY turn off the TRV (feel free to PM on how to do this, as its not just a case off turning down the TRV), this could mean that you would not lose the existing fernox in the system (assuming there is some there). When you come to refill the system, get it up to 1 to 1.5 bar via the filling loop, and the bleed the sulphur/air out of the new rad. once this is done, top up the pressure to the desired amount, and that is is. If the pressure drops slightly over the next few days/weeks, then you have a micro leak, which means air is getting into the system (rather than water out!), and this may be cured by just tightening the couplings on the rad valves a bit more.
A few things to note are:
If you are going to install new valves into the new rad, always put at least 13 turns of PTFE around the threads, remember to go in the opposite direction to the way you will screw in the valve, to prevent tape slippage.
The air vent for the rad needs to go on the highest side of the rad, to ease bleeding and filling. A difference of 0.5 to 1mm in height is often not noticed by any visitors.
Due to disturbance of the system, all the rads will need to be bled after the new one has been connected, remembering to re check the system pressure between each bleeding to avoid total loss of pressure.
Once you have refilled the system, you will need to purge the system pump, as it will become full of air when emptying the system. If you do not purge the pump, then it will dramatically reduce its lifespan, or the system may just not fill properly. To purge the pump, there is normally a slot on the front of the housing (the pump will be inside your boiler casing, if the boiler is a combi as you describe) large enough for a 2 pence piece. Run your heating and unscrew the slot very slightly until water dribbles from it, then re screw it.
Those are just a few things off the top of my head lol
Edited to add. Yes, as you say, you will use the filling loop to re pressurise the system. On the incoming side of the filling loop, there should ideally be a stop tap (rather than just an isolation valve) and it is this that you can open to control the re pressurisation. The filling loop should not be permanently attached between the incoming supply and boiler, as this breaks plumbing law, but in truth most are lol
First thing:
something to note (assuming you've got a thermostatic valve on one end of the radiator) - the valve on the other end (I think it's called a lock-shield valve - it's been a long time) is actually used to control the flow of water through the radiator, so when you close it - count the number of turns it takes and make a note somewhere - and when finished putting the new rad on, open it by the same amount
That is actually wrong. It would have been the case if the rad did not have TRV's on, and would have been used to balance the system, but the TRV will control the flow of water to the rad. The lockshield is on the exit side of the rad (with a TRV fitted), while the TRV is on the incoming side (usually, although there are either side TRV's available)
The first question to ask is why you want to replace the rad, and what type of rad is there at the moment! If the rad is a single panel or one panel one K at the moment, then you could keep the same rad size, but move to double panel or double panel double K. This will improve the heating output from the rad, but may allow you to keep the pipework as it is. If this were the case, then you would only need to turn off the locksheild valve, and FULLY turn off the TRV (feel free to PM on how to do this, as its not just a case off turning down the TRV), this could mean that you would not lose the existing fernox in the system (assuming there is some there). When you come to refill the system, get it up to 1 to 1.5 bar via the filling loop, and the bleed the sulphur/air out of the new rad. once this is done, top up the pressure to the desired amount, and that is is. If the pressure drops slightly over the next few days/weeks, then you have a micro leak, which means air is getting into the system (rather than water out!), and this may be cured by just tightening the couplings on the rad valves a bit more.
A few things to note are:
If you are going to install new valves into the new rad, always put at least 13 turns of PTFE around the threads, remember to go in the opposite direction to the way you will screw in the valve, to prevent tape slippage.
The air vent for the rad needs to go on the highest side of the rad, to ease bleeding and filling. A difference of 0.5 to 1mm in height is often not noticed by any visitors.
Due to disturbance of the system, all the rads will need to be bled after the new one has been connected, remembering to re check the system pressure between each bleeding to avoid total loss of pressure.
Once you have refilled the system, you will need to purge the system pump, as it will become full of air when emptying the system. If you do not purge the pump, then it will dramatically reduce its lifespan, or the system may just not fill properly. To purge the pump, there is normally a slot on the front of the housing (the pump will be inside your boiler casing, if the boiler is a combi as you describe) large enough for a 2 pence piece. Run your heating and unscrew the slot very slightly until water dribbles from it, then re screw it.
Those are just a few things off the top of my head lol
Edited to add. Yes, as you say, you will use the filling loop to re pressurise the system. On the incoming side of the filling loop, there should ideally be a stop tap (rather than just an isolation valve) and it is this that you can open to control the re pressurisation. The filling loop should not be permanently attached between the incoming supply and boiler, as this breaks plumbing law, but in truth most are lol
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