Insulatorpro:
When you want to reconnect the bathroom radiator, you'll have to drain the water level in the system down to below the elevation of that radiator.
SAVE the water in 5 gallon pails. Adding new water to a heating system unnecessarily isn't good for it. The new water will contain dissolved oxygen and calcium and iron ions (which make water "hard water"). The dissolved oxygen you add with new water goes either into making O2 gas once the water heats up, or making Fe2O3, which is rust inside the iron boiler or iron piping. The calcium ions in the water you add go into making "scale" in the hottest parts of the heating system (which would be the bottoms of the sections in the boiler).
So, it's better to save the old oxygen depleted an ionically dead water than to put in new water.
In my humble opinion, the BEST way to fill a hot water heating system is to:
1. Remove the highest elevation air vent in the heating system and screw in a 1/8 inch NPT nipple.
2. Now screw a 1/8 inch NPT ball valve on to that nipple.
3. Now screw in a standard coin air vent into the top of that nipple, but don't use any teflon tape or pipe dope.
Most of the time that assembly can be used as an air vent to release accumulated air in the radiator by opening the ball valve and opening the air vent until it starts to leak water.
However, during those times when you want to refill your heating system with old water, you just:
A. Stand a chair next to that air vent
B. remove the air vent from the ball valve and screw in a 1/8" NPT by 1/4" hose barb fitting
C. set a 5 gallon pail with old heating system water on the chair and siphon that water into the hose barb fitting (with the ball valve open).
D. Now, open all the other air vents on the heating system until water bleeds out of them.
That way, there's never any chance of the water pressure in the heating system becoming too high.
After siphoning the old water back into the heating system, add sufficient water from the feedwater make up line to raise the system pressure to about 12 psi (as shown on the boiler's pressure gauge). That's a recommended pressure. Going too much below that MIGHT result in cavitation of your circulating pump's impeller.
AND, if you ever have the opportunity to redo the water make-up line to your boiler, the best way to do it in my opinion is with a pressure gauge, garden hose silcock (pointing up instead of down) and ball valve all immediately down stream of the pressure reducing valve. (Also, install a bypass line to add water quickly by bypassing the pressure reducing valve.)
The advantage of this piping is that it avoids the common problems associated with setting the pressure reducing valve. Often, it will take hours for the pressure reducing valve to fill the system to the correct pressure, and so people aren't sure if the pressure setting on the reducing valve is correct. They can open the main valve on the feed water make up line and come back 4 hours later to find the pressure is 28 psi!. By having a pressure gauge and ball valve immediately down stream of the PRV, you simply have to close that ball valve and turn the adjusting screw on the PRV until the pressure reads 12 psi. Because of the very tiny volume of water that has to be added, the response on the pressure gauge when the PRV screw is turned will be instantaneous.
And, since most PRV's have check valves built into them to prevent cross contamination of boiler water with potable water, if your PRV setting is too high, you can simply open the sillcock downstream of the PRV (but upstream of the ball valve) to release the water pressure, unscrew the adjusting screw on the PRV a bit, and try again. Once you have the PRV raising the water pressure to 12 psi in that tiny volume upstream of the ball valve, you can then open the ball valve and be sure that the PRV will raise the whole system's pressure to exactly 12 psi.
Instead of a silcock, you could also just use a ball valve with a drain cap on it. Just make sure the drain cap is pointing up so that you never introduce air into the piping when releasing the pressure.