when i try to drain one of my upstairs radiators to remove it and flush it through i have a problem! I turn off the lockshield valve fine but when I turn my thermostat valve down to the lowest (frost protection i think) it won't stop the water flowing through. it is about 18-20 years old, so will I need to drain my primary radiator circuit before I can remove it? Are all TRV's like this or is mine just an older type?? Thanks for your help !!