Hi, I'm very new to plumbing but try anything and have replaced all the TRV's in my house without too much problem. I have a question regarding draining the system and TRV's though. So far I've drained the system to replace the TRV's and spent some time reading about whether it's possible without draining the system. My understanding (which I'm guessing is wrong) is that if there is no pressure in the boiler (combi) that if the radiator is isolated, at that point the water is locked into the radiator. I have pipes coming through the floor into the TRV. The TRV (and lockshield) is turned off to prevent the water going into the radiator, so this means if I remove the TRV then water from the pipe going into it from the floor will be exposed. However, if the boiler has no pressure then wouldn't the water level remain at the top of the pipe and not spill out? I hope that makes sense. Then it would be a case of just draining the radiator itself, replacing the TRV and reconnecting everything back. I'm guessing that the pipe from the floor will squirt water everywhere which is why everyone says to drain the system, but with no pressure I don't get how (as no water is being pumped in when the boiler is switched off, right?!). Could someone please clarify for me? The problem is fixed, it's more I'm trying to understand why it wouldn't be possible this way.